This document describes Ekaterina Bolbat's doctoral dissertation on the development of novel PEPPSI-type complexes of palladium and platinum and their applications in catalysis. The dissertation investigates the properties of these complexes through experimental X-ray techniques and examines their use in catalytic C-H functionalization and hydrosilylation reactions. The goal of the work is to characterize these complexes and explore approaches for their heterogenization to develop supported homogeneous catalysts.
End on template method for meta C-H activationRahul B S
This document discusses an end-on template method for meta C-H activation in pharmaceutical chemistry. The method uses a temporary helper molecule or template to mediate the addition of functional groups to carbon atoms. A metal catalyst like palladium helps facilitate the process. The template approach allows for easier modification of drug molecules by attaching biologically active functional groups at meta positions. Several drug targets that could benefit from this meta C-H activation are discussed, including antihyperlipidemic, GABA agonist, anti-cancer and antihypertensive drugs.
The DNA cleavage and antimicrobial studies of Co(II), Ni(II), Cu(II) and Zn(I...IOSR Journals
This document summarizes a study on the synthesis and characterization of Schiff base complexes of Co(II), Ni(II), Cu(II) and Zn(II) using 4-pyridinecarboxaldehyde and 4-aminopyridine. The complexes were characterized using elemental analysis, magnetic susceptibility, IR spectroscopy, XRD and SEM. The complexes showed antimicrobial activity against bacteria and fungi. The metal complexes exhibited higher antimicrobial activity than the Schiff base ligand. Gel electrophoresis studies showed the complexes were able to cleave DNA, indicating their potential as chemical nucleases.
This document is the final report for a chemistry project investigating phosphido-borane stabilised tetrylenes. Key findings include:
1) The dimesitylphosphido-borane and dimesitylphosphido-bis(borane) lithium salts were successfully synthesized and characterized by X-ray crystallography.
2) The diphenylphosphido-borane substituted stannate was synthesized and characterized, revealing agostic interactions between the borane groups and lithium cation but not the tin center.
3) Synthesis of a phosphido-borane stabilised carbanion complex was achieved but attempts to isolate a tin-containing product were unsuccessful.
The Development of Bulky Palladium NHC Complexes for the Most-Challenging Cro...DrMAdamSayah
Palladium-catalyzed cross-coupling reactions are important tools for forming carbon-carbon bonds. While phosphine ligands have been widely studied, N-heterocyclic carbene (NHC) ligands have attracted attention due to their ability to strongly bind to palladium and stabilize the catalyst. PEPPSI palladium precatalysts using bulky NHC ligands such as IPr and IPent have proven effective for challenging cross-coupling reactions. This review evaluates PEPPSI complexes containing increasingly bulky NHC ligands for difficult couplings including Suzuki-Miyaura, Negishi, and Stille-Migita reactions to form tetra-ortho-substituted biaryls, as well as amination
Preparation of pyrimido[4,5 b][1,6]naphthyridin-4(1 h)-one derivativeselshimaa eid
This document describes the preparation of pyrimido[4,5-b][1,6]naphthyridin-4(1H)-one derivatives using a zeolite-nanogold catalyst. An efficient one-pot synthesis is developed involving the cyclocondensation of 6-amino-2-thioxo-2,3-dihydropyrimidin-4(1H)-one, aromatic aldehydes, and 1-benzylpiperidin-4-one in ethanol at 80°C. The nanogold catalyst is characterized and found to contain 4-6 nm gold nanoparticles dispersed on zeolite. Several derivatives are synthesized in good yields and characterized. Molecular dock
1) Radical retrosynthesis uses one-electron disconnections to simplify synthesis, avoiding protecting groups, functional group interconversions, and redox steps. This enables more direct and minimal syntheses.
2) Radical cross-coupling reactions allow forming C-C and C-X bonds through hydrogen atom transfer or coupling of radicals with redox-active esters, sulfones, or other species. This provides unique chemoselectivity advantages over polar pathways.
3) Case studies demonstrate strategic benefits of radical cross-coupling for synthesis ideality, efficiency, selectivity, and modularity by opening new retrosynthetic opportunities not accessible through two-electron analysis.
End on template method for meta C-H activationRahul B S
This document discusses an end-on template method for meta C-H activation in pharmaceutical chemistry. The method uses a temporary helper molecule or template to mediate the addition of functional groups to carbon atoms. A metal catalyst like palladium helps facilitate the process. The template approach allows for easier modification of drug molecules by attaching biologically active functional groups at meta positions. Several drug targets that could benefit from this meta C-H activation are discussed, including antihyperlipidemic, GABA agonist, anti-cancer and antihypertensive drugs.
The DNA cleavage and antimicrobial studies of Co(II), Ni(II), Cu(II) and Zn(I...IOSR Journals
This document summarizes a study on the synthesis and characterization of Schiff base complexes of Co(II), Ni(II), Cu(II) and Zn(II) using 4-pyridinecarboxaldehyde and 4-aminopyridine. The complexes were characterized using elemental analysis, magnetic susceptibility, IR spectroscopy, XRD and SEM. The complexes showed antimicrobial activity against bacteria and fungi. The metal complexes exhibited higher antimicrobial activity than the Schiff base ligand. Gel electrophoresis studies showed the complexes were able to cleave DNA, indicating their potential as chemical nucleases.
This document is the final report for a chemistry project investigating phosphido-borane stabilised tetrylenes. Key findings include:
1) The dimesitylphosphido-borane and dimesitylphosphido-bis(borane) lithium salts were successfully synthesized and characterized by X-ray crystallography.
2) The diphenylphosphido-borane substituted stannate was synthesized and characterized, revealing agostic interactions between the borane groups and lithium cation but not the tin center.
3) Synthesis of a phosphido-borane stabilised carbanion complex was achieved but attempts to isolate a tin-containing product were unsuccessful.
The Development of Bulky Palladium NHC Complexes for the Most-Challenging Cro...DrMAdamSayah
Palladium-catalyzed cross-coupling reactions are important tools for forming carbon-carbon bonds. While phosphine ligands have been widely studied, N-heterocyclic carbene (NHC) ligands have attracted attention due to their ability to strongly bind to palladium and stabilize the catalyst. PEPPSI palladium precatalysts using bulky NHC ligands such as IPr and IPent have proven effective for challenging cross-coupling reactions. This review evaluates PEPPSI complexes containing increasingly bulky NHC ligands for difficult couplings including Suzuki-Miyaura, Negishi, and Stille-Migita reactions to form tetra-ortho-substituted biaryls, as well as amination
Preparation of pyrimido[4,5 b][1,6]naphthyridin-4(1 h)-one derivativeselshimaa eid
This document describes the preparation of pyrimido[4,5-b][1,6]naphthyridin-4(1H)-one derivatives using a zeolite-nanogold catalyst. An efficient one-pot synthesis is developed involving the cyclocondensation of 6-amino-2-thioxo-2,3-dihydropyrimidin-4(1H)-one, aromatic aldehydes, and 1-benzylpiperidin-4-one in ethanol at 80°C. The nanogold catalyst is characterized and found to contain 4-6 nm gold nanoparticles dispersed on zeolite. Several derivatives are synthesized in good yields and characterized. Molecular dock
1) Radical retrosynthesis uses one-electron disconnections to simplify synthesis, avoiding protecting groups, functional group interconversions, and redox steps. This enables more direct and minimal syntheses.
2) Radical cross-coupling reactions allow forming C-C and C-X bonds through hydrogen atom transfer or coupling of radicals with redox-active esters, sulfones, or other species. This provides unique chemoselectivity advantages over polar pathways.
3) Case studies demonstrate strategic benefits of radical cross-coupling for synthesis ideality, efficiency, selectivity, and modularity by opening new retrosynthetic opportunities not accessible through two-electron analysis.
This document describes the synthesis and application of new π-allylpalladium precatalysts containing biaryl- and bipyrazolylphosphine ligands. Two classes of complexes were developed - neutral Pd(allyl)(L)Cl complexes and cationic [Pd(allyl)(L)]OTf complexes. The cationic complexes allowed for incorporation of extremely bulky ligands. Both classes of complexes were found to be air-stable and highly active precatalysts for challenging cross-coupling reactions. Their high activity is attributed to facile activation to a 12-electron "L-Pd(0)" species and suppression of dimer formation, supported by structural and kinetic studies. A broad scope
This document discusses a study on the inhibitive properties and quantum chemical analysis of 1,4-benzothiazine derivatives for corrosion inhibition of mild steel in acidic medium. Two derivatives, compounds P3 and P4, were synthesized and their structures were confirmed through various analytical techniques. Electrochemical tests including polarization, electrochemical impedance spectroscopy, and weight loss measurements were used to evaluate the corrosion inhibition efficiency of P3 and P4 in 1 M HCl solution. Quantum chemical calculations based on density functional theory were also performed to correlate inhibition efficiency with molecular properties. The results showed that P3 and P4 acted as efficient corrosion inhibitors for mild steel in acidic solution, with inhibition efficiency increasing with increasing concentration. Adsorption
This document summarizes research on the use of amidinate group 4 metal complexes as catalysts for olefin polymerization. Key points include:
- Bis(amidinate) zirconium and titanium complexes can polymerize ethylene and propylene into elastomeric polymers through an intramolecular epimerization mechanism.
- The symmetry of complexes containing one, two, or three amidinate ligands influences the stereochemistry of the resulting polypropylene, with C2 and C3 complexes producing isotactic and atactic polymers, respectively.
- The nature of the co-catalyst and reaction conditions, such as pressure, temperature and solvent, can alter the active catalytic species
1) The document discusses the effect of water content in hydrogen peroxide on the structure of HTPB produced via the radical polymerization of butadiene.
2) The study found that decreasing the water content of hydrogen peroxide increases the effectiveness of the catalyst in the polymerization process. This leads to increased cis-1,4 HTPB structure and decreased vinyl 1,2 structure in the HTPB product.
3) Kinetic studies showed the reaction is first order with respect to monomer concentration. The formation rates of cis, trans, and vinyl structures could be expressed by rate equations, and decreasing water content had different effects on each rate depending on the power index.
The document discusses transition metals and their properties and uses. It defines transition metals based on their electronic configuration and partially filled d subshells. It describes how transition metals can adopt multiple oxidation states, form complexes, exhibit catalytic activity, and be used in organic reactions like cross-coupling reactions. Common transition metal catalysts used in coupling reactions include palladium and nickel. Organocatalysis is also discussed as an alternative to metal-based catalysis.
This document discusses solvent extraction, which is a versatile separation method used in analytical chemistry. It can be used to separate, purify, enrich, and analyze both tracer and macro amounts of metal ions. The key principles discussed include the phase rule, which describes solvent extraction as a two-phase system, and the Nernst distribution law, which defines the partition or distribution coefficient. Different types of extraction systems are classified, including chelate extraction involving complex formation, extraction by solvation, and ion-pair formation. Factors that affect metal complex stability such as ligand basicity and ring size are also outlined.
Structural and Morphological Studies of Nano Composite Polymer Gel Electroly...vivatechijri
The document summarizes research on a nano composite polymer gel electrolyte containing SiO2 nanoparticles. Key points:
1. Polyvinylidene fluoride-co-hexafluoropropylene polymer was used as the base polymer mixed with propylene carbonate, magnesium perchlorate, and SiO2 nanoparticles to synthesize the nano composite polymer gel electrolyte.
2. The electrolyte was characterized using XRD, SEM, and FTIR which confirmed the homogeneous dispersion of SiO2 nanoparticles and increased amorphous nature of the electrolyte, enhancing its ion conductivity.
3. XRD showed decreased crystallinity and disappearance of polymer peaks upon addition of SiO2. SEM revealed
STUDY OF A CATALYST OF CITRIC ACID CROSSLINKING ON LOCUST BEAN GUMUniversitasGadjahMada
HCl, H2SO4, and potassium persulfate (PPS) were studied as catalysts of the process of citric acid (CA) crosslinking on
locust bean gum (LBG). The copolymer (CA-c-LBG) obtained was characterized by its viscosity, pH, FTIR, NMR and SEM.
It was found that the protonation of the hydroxyl groups at C6 atom of mannose and galactose in LBG and the hydrogen
atoms of CA carboxylic group was accelerated. The best catalytic effect was obtained in presence of HCl.
Spectral studies of 5-({4-amino-2-[(Z)-(2-hydroxybenzylidene) amino] pyrimidi...IOSR Journals
Some transition metal ions Complexes with 5-({4-amino-2-[(Z)-(2-hydroxybenzylidene) amino]
pyrimidin-5-yl} methyl)-2,3,4-trimethoxybenzene were prepared and characterized by elemental analyses,
Infrared , magnetic moment, electronic spectra , mass spectra, X-ray powder diffraction, molar conductance
and thermal analysis (TGA). The complexes have general formulae [ML2.2H2O] {where M = Mn (II), Co (II), Ni
(II), Cu (II), Zn (II), Pd (II) and Pt (II). The coordination behavior of the metal ions towards to the investigated
Schiff base takes place through –C=N,-NH2 and –OH groups. The obtained C, H and N elemental analysis data
showed the Metal: Ligand ratio is 1:2 [M: L] ratio. The molar conductance data reveal that all the metal
complexes are non-electrolytic in nature. From the magnetic moments the complexes are paramagnetic except
Zn metal ion complexes have octahedral geometry with coordination number eight. The thermal behavior of
these complexes shows that, the hydrated complexes have loses two water molecules and immediately followed
by decomposition of the anions and ligand molecules in the second and third stage. The Schiff bases and metal
complexes show good activity against some bacteria. The antimicrobial results indicate that, the metal
complexes have better antimicrobial activity as compared to the prepared Schiff base.
Deactivation and regeneration of catalysts and heterogeneous reaction kinetic...Bapi Mondal
In this Assignment file i try to easily describe the Deactivation mechanism of any catalysis reaction .Furthermore i will describe some Regeneration and prevention method of deactivated catalysts. and in the last part of this assignment i will show very easily the heterogeneous reaction kinetics.
The document describes the synthesis and characterization of a Schiff base ligand and its transition metal complexes with Cu(II), Co(II), and Ni(II).
The Schiff base ligand was synthesized by the condensation of 3-(3-nitrophenyl)-1-phenyl-1H-pyrazole-4-carbaldehyde and o-amino phenol. Metal complexes were prepared by reacting the Schiff base ligand with acetate salts of Cu(II), Co(II), and Ni(II).
The complexes were characterized using elemental analysis, conductivity measurements, IR, UV-Vis, 1H NMR, 13C NMR, mass spectrometry, magnetic susceptibility measurements, and thermal analysis. Spectroscopic data
Highly stable pt ru nanoparticles supported on three-dimensional cubic ordere...suresh899
The cost of the catalysts used in the direct methanol fuel cell
poses a challenge to its widespread use as an energy efficient and environment
friendly fuel conversion technology. In this study, two types of highly ordered
mesoporous carbon CMK-8 (I and II) with high surface area and 3-D
bicontinuous interpenetrating channels were synthesized and deposited with
PtRu nanoparticles using the sodium borohydride reduction method. The
electrocatalytic capabilities for methanol oxidation were investigated using
cyclic voltammetry and chronoamperometry, and the results were compared
with that of PtRu deposited on Vulcan XC-72 using the same preparation
method as well as with commercial PtRu/C (E-TEK) catalyst. Pt Ru/CMK-8-I synthesized by the method developed in this work revealed an
outstanding specific mass activity (487.9 mA/mg) and superior stability
compared with the other supports, thus substantiating its potential to reduce
the costs of DMFC catalysts.
This document describes the synthesis, spectral characterization, and biological screening of transition metal complexes of the ligand 2-(5-mercapto-1,3,4-oxadiazol-2-yl)phenol. A series of complexes were prepared using Cu(II), Co(II), Ni(II), and Zn(II) and characterized using analytical data, magnetic susceptibility, IR, UV-Vis, 1H NMR, 13C NMR, EPR, and other techniques. Spectral data suggest an octahedral geometry around the metal ion. Biological screening showed the complexes have antibacterial, antifungal, and DNA cleavage activities.
The document provides an overview of catalysis. It defines a catalyst as a substance that speeds up a chemical reaction but is not consumed by the reaction. It discusses different types of catalysis including homogeneous catalysis where the catalyst is in the same phase as the reactants, and heterogeneous catalysis where the catalyst is in a different phase. The document also covers catalyst characterization techniques, factors that can lead to catalyst deactivation, and methods for catalyst regeneration. Examples are provided throughout to illustrate catalysis concepts and applications.
This document summarizes various catalytic mechanisms used by enzymes, including acid-base catalysis, covalent catalysis, metal ion catalysis, electrostatic catalysis, proximity and orientation effects, and preferential transition state binding. It provides examples of each mechanism, such as acid-base catalysis lowering the transition state energy of hydrolysis reactions and coenzymes functioning as covalent catalysts. Metal ions are involved in substrate orientation, oxidation-reduction reactions, and stabilizing charges. Enzyme active sites optimize proximity, orientation and transition state binding to greatly increase reaction rates.
The document provides an introduction to heterogeneous catalysts and their importance in industry and academia for producing fuels and chemicals. It discusses supported metal catalysts, with a focus on their role in hydrogenation reactions. Specifically, it summarizes the common industrial applications of hydrogenation, including the production of edible fats from vegetable oils. It also discusses the selective hydrogenation of nitrobenzene, an important reaction for producing the commodity chemical aniline. The key industrial process is the catalytic hydrogenation of nitrobenzene, which can produce aniline with over 99% selectivity.
This document provides an overview of catalysis and catalytic principles. It defines catalysis as the science of catalysts and catalytic processes, which plays an important role in industries like petrochemicals. A catalyst enhances the rate and selectivity of a chemical reaction while being regenerated in the process. Key points discussed include:
- Catalysts are composed of an active phase (e.g. metal), support, and optional promoters. Common supports include metal oxides like alumina.
- Reaction rate depends on temperature, pressure, concentration, and contact time according to rate laws. Temperature particularly impacts reaction rates through its exponential effect in the Arrhenius equation.
- Mass transfer and internal diffusion limitations can
The document describes using reversible addition-fragmentation chain transfer (RAFT) polymerization to synthesize novel block copolymers containing both a polyolefin block and a poly(styrene-co-maleic anhydride) block. Specifically, it details:
1) Using a commercially available polyolefin (Kraton L-1203) modified with a dithioester group to serve as a macroinitiator for RAFT polymerization and form the polyolefin block.
2) Conducting RAFT polymerizations of styrene and styrene-co-maleic anhydride using this macroinitiator and a small molecule RAFT agent to form the second block and yield polyolefin
Calix Assisted Palladium Nanocatalyst: A Reviewijtsrd
The article reviews recent advances in c c cross coupling area as calix protected palladium nanocatalyst. The extensive use of palladium complex as catalyst in the calix chemistry is newly emerging field which deals with Suzuki, Heck, Stille and Sonogashira cross coupling reactions. The brief survey of cross coupling reactions also includes yield, catalyst loading, Recyclability. Keyur D. Bhatt | Krunal Modi "Calix Assisted Palladium Nanocatalyst: A Review" Published in International Journal of Trend in Scientific Research and Development (ijtsrd), ISSN: 2456-6470, Volume-4 | Issue-1 , December 2019, URL: https://www.ijtsrd.com/papers/ijtsrd29654.pdf Paper URL: https://www.ijtsrd.com/chemistry/other/29654/calix-assisted-palladium-nanocatalyst-a-review/keyur-d-bhatt
A 19-year-old man from Virginia, Michael Ryan, scaled Trump Tower in New York City using suction cups to try to meet with Donald Trump. He made it to the 21st floor before police pulled him into the building. Ryan had recently dropped out of high school and worked at a gardening center. His parents were out of town on vacation when he took a trip to New York City to perform the stunt. Ryan was charged with reckless endangerment and criminal trespass for the illegal climb.
Howard Gardner proposed multiple intelligences in 1983 including spatial or picture smart intelligence. Picture-smart learners can visualize experiences even when not present through mental manipulation like zooming or changing angles. These visualization skills help with directions, maps, and not all spatially intelligent people are artistic. A logo quiz activity tests students' spatial intelligence by having groups identify logos within 2 minutes to see which group can correctly identify the most.
This document describes the synthesis and application of new π-allylpalladium precatalysts containing biaryl- and bipyrazolylphosphine ligands. Two classes of complexes were developed - neutral Pd(allyl)(L)Cl complexes and cationic [Pd(allyl)(L)]OTf complexes. The cationic complexes allowed for incorporation of extremely bulky ligands. Both classes of complexes were found to be air-stable and highly active precatalysts for challenging cross-coupling reactions. Their high activity is attributed to facile activation to a 12-electron "L-Pd(0)" species and suppression of dimer formation, supported by structural and kinetic studies. A broad scope
This document discusses a study on the inhibitive properties and quantum chemical analysis of 1,4-benzothiazine derivatives for corrosion inhibition of mild steel in acidic medium. Two derivatives, compounds P3 and P4, were synthesized and their structures were confirmed through various analytical techniques. Electrochemical tests including polarization, electrochemical impedance spectroscopy, and weight loss measurements were used to evaluate the corrosion inhibition efficiency of P3 and P4 in 1 M HCl solution. Quantum chemical calculations based on density functional theory were also performed to correlate inhibition efficiency with molecular properties. The results showed that P3 and P4 acted as efficient corrosion inhibitors for mild steel in acidic solution, with inhibition efficiency increasing with increasing concentration. Adsorption
This document summarizes research on the use of amidinate group 4 metal complexes as catalysts for olefin polymerization. Key points include:
- Bis(amidinate) zirconium and titanium complexes can polymerize ethylene and propylene into elastomeric polymers through an intramolecular epimerization mechanism.
- The symmetry of complexes containing one, two, or three amidinate ligands influences the stereochemistry of the resulting polypropylene, with C2 and C3 complexes producing isotactic and atactic polymers, respectively.
- The nature of the co-catalyst and reaction conditions, such as pressure, temperature and solvent, can alter the active catalytic species
1) The document discusses the effect of water content in hydrogen peroxide on the structure of HTPB produced via the radical polymerization of butadiene.
2) The study found that decreasing the water content of hydrogen peroxide increases the effectiveness of the catalyst in the polymerization process. This leads to increased cis-1,4 HTPB structure and decreased vinyl 1,2 structure in the HTPB product.
3) Kinetic studies showed the reaction is first order with respect to monomer concentration. The formation rates of cis, trans, and vinyl structures could be expressed by rate equations, and decreasing water content had different effects on each rate depending on the power index.
The document discusses transition metals and their properties and uses. It defines transition metals based on their electronic configuration and partially filled d subshells. It describes how transition metals can adopt multiple oxidation states, form complexes, exhibit catalytic activity, and be used in organic reactions like cross-coupling reactions. Common transition metal catalysts used in coupling reactions include palladium and nickel. Organocatalysis is also discussed as an alternative to metal-based catalysis.
This document discusses solvent extraction, which is a versatile separation method used in analytical chemistry. It can be used to separate, purify, enrich, and analyze both tracer and macro amounts of metal ions. The key principles discussed include the phase rule, which describes solvent extraction as a two-phase system, and the Nernst distribution law, which defines the partition or distribution coefficient. Different types of extraction systems are classified, including chelate extraction involving complex formation, extraction by solvation, and ion-pair formation. Factors that affect metal complex stability such as ligand basicity and ring size are also outlined.
Structural and Morphological Studies of Nano Composite Polymer Gel Electroly...vivatechijri
The document summarizes research on a nano composite polymer gel electrolyte containing SiO2 nanoparticles. Key points:
1. Polyvinylidene fluoride-co-hexafluoropropylene polymer was used as the base polymer mixed with propylene carbonate, magnesium perchlorate, and SiO2 nanoparticles to synthesize the nano composite polymer gel electrolyte.
2. The electrolyte was characterized using XRD, SEM, and FTIR which confirmed the homogeneous dispersion of SiO2 nanoparticles and increased amorphous nature of the electrolyte, enhancing its ion conductivity.
3. XRD showed decreased crystallinity and disappearance of polymer peaks upon addition of SiO2. SEM revealed
STUDY OF A CATALYST OF CITRIC ACID CROSSLINKING ON LOCUST BEAN GUMUniversitasGadjahMada
HCl, H2SO4, and potassium persulfate (PPS) were studied as catalysts of the process of citric acid (CA) crosslinking on
locust bean gum (LBG). The copolymer (CA-c-LBG) obtained was characterized by its viscosity, pH, FTIR, NMR and SEM.
It was found that the protonation of the hydroxyl groups at C6 atom of mannose and galactose in LBG and the hydrogen
atoms of CA carboxylic group was accelerated. The best catalytic effect was obtained in presence of HCl.
Spectral studies of 5-({4-amino-2-[(Z)-(2-hydroxybenzylidene) amino] pyrimidi...IOSR Journals
Some transition metal ions Complexes with 5-({4-amino-2-[(Z)-(2-hydroxybenzylidene) amino]
pyrimidin-5-yl} methyl)-2,3,4-trimethoxybenzene were prepared and characterized by elemental analyses,
Infrared , magnetic moment, electronic spectra , mass spectra, X-ray powder diffraction, molar conductance
and thermal analysis (TGA). The complexes have general formulae [ML2.2H2O] {where M = Mn (II), Co (II), Ni
(II), Cu (II), Zn (II), Pd (II) and Pt (II). The coordination behavior of the metal ions towards to the investigated
Schiff base takes place through –C=N,-NH2 and –OH groups. The obtained C, H and N elemental analysis data
showed the Metal: Ligand ratio is 1:2 [M: L] ratio. The molar conductance data reveal that all the metal
complexes are non-electrolytic in nature. From the magnetic moments the complexes are paramagnetic except
Zn metal ion complexes have octahedral geometry with coordination number eight. The thermal behavior of
these complexes shows that, the hydrated complexes have loses two water molecules and immediately followed
by decomposition of the anions and ligand molecules in the second and third stage. The Schiff bases and metal
complexes show good activity against some bacteria. The antimicrobial results indicate that, the metal
complexes have better antimicrobial activity as compared to the prepared Schiff base.
Deactivation and regeneration of catalysts and heterogeneous reaction kinetic...Bapi Mondal
In this Assignment file i try to easily describe the Deactivation mechanism of any catalysis reaction .Furthermore i will describe some Regeneration and prevention method of deactivated catalysts. and in the last part of this assignment i will show very easily the heterogeneous reaction kinetics.
The document describes the synthesis and characterization of a Schiff base ligand and its transition metal complexes with Cu(II), Co(II), and Ni(II).
The Schiff base ligand was synthesized by the condensation of 3-(3-nitrophenyl)-1-phenyl-1H-pyrazole-4-carbaldehyde and o-amino phenol. Metal complexes were prepared by reacting the Schiff base ligand with acetate salts of Cu(II), Co(II), and Ni(II).
The complexes were characterized using elemental analysis, conductivity measurements, IR, UV-Vis, 1H NMR, 13C NMR, mass spectrometry, magnetic susceptibility measurements, and thermal analysis. Spectroscopic data
Highly stable pt ru nanoparticles supported on three-dimensional cubic ordere...suresh899
The cost of the catalysts used in the direct methanol fuel cell
poses a challenge to its widespread use as an energy efficient and environment
friendly fuel conversion technology. In this study, two types of highly ordered
mesoporous carbon CMK-8 (I and II) with high surface area and 3-D
bicontinuous interpenetrating channels were synthesized and deposited with
PtRu nanoparticles using the sodium borohydride reduction method. The
electrocatalytic capabilities for methanol oxidation were investigated using
cyclic voltammetry and chronoamperometry, and the results were compared
with that of PtRu deposited on Vulcan XC-72 using the same preparation
method as well as with commercial PtRu/C (E-TEK) catalyst. Pt Ru/CMK-8-I synthesized by the method developed in this work revealed an
outstanding specific mass activity (487.9 mA/mg) and superior stability
compared with the other supports, thus substantiating its potential to reduce
the costs of DMFC catalysts.
This document describes the synthesis, spectral characterization, and biological screening of transition metal complexes of the ligand 2-(5-mercapto-1,3,4-oxadiazol-2-yl)phenol. A series of complexes were prepared using Cu(II), Co(II), Ni(II), and Zn(II) and characterized using analytical data, magnetic susceptibility, IR, UV-Vis, 1H NMR, 13C NMR, EPR, and other techniques. Spectral data suggest an octahedral geometry around the metal ion. Biological screening showed the complexes have antibacterial, antifungal, and DNA cleavage activities.
The document provides an overview of catalysis. It defines a catalyst as a substance that speeds up a chemical reaction but is not consumed by the reaction. It discusses different types of catalysis including homogeneous catalysis where the catalyst is in the same phase as the reactants, and heterogeneous catalysis where the catalyst is in a different phase. The document also covers catalyst characterization techniques, factors that can lead to catalyst deactivation, and methods for catalyst regeneration. Examples are provided throughout to illustrate catalysis concepts and applications.
This document summarizes various catalytic mechanisms used by enzymes, including acid-base catalysis, covalent catalysis, metal ion catalysis, electrostatic catalysis, proximity and orientation effects, and preferential transition state binding. It provides examples of each mechanism, such as acid-base catalysis lowering the transition state energy of hydrolysis reactions and coenzymes functioning as covalent catalysts. Metal ions are involved in substrate orientation, oxidation-reduction reactions, and stabilizing charges. Enzyme active sites optimize proximity, orientation and transition state binding to greatly increase reaction rates.
The document provides an introduction to heterogeneous catalysts and their importance in industry and academia for producing fuels and chemicals. It discusses supported metal catalysts, with a focus on their role in hydrogenation reactions. Specifically, it summarizes the common industrial applications of hydrogenation, including the production of edible fats from vegetable oils. It also discusses the selective hydrogenation of nitrobenzene, an important reaction for producing the commodity chemical aniline. The key industrial process is the catalytic hydrogenation of nitrobenzene, which can produce aniline with over 99% selectivity.
This document provides an overview of catalysis and catalytic principles. It defines catalysis as the science of catalysts and catalytic processes, which plays an important role in industries like petrochemicals. A catalyst enhances the rate and selectivity of a chemical reaction while being regenerated in the process. Key points discussed include:
- Catalysts are composed of an active phase (e.g. metal), support, and optional promoters. Common supports include metal oxides like alumina.
- Reaction rate depends on temperature, pressure, concentration, and contact time according to rate laws. Temperature particularly impacts reaction rates through its exponential effect in the Arrhenius equation.
- Mass transfer and internal diffusion limitations can
The document describes using reversible addition-fragmentation chain transfer (RAFT) polymerization to synthesize novel block copolymers containing both a polyolefin block and a poly(styrene-co-maleic anhydride) block. Specifically, it details:
1) Using a commercially available polyolefin (Kraton L-1203) modified with a dithioester group to serve as a macroinitiator for RAFT polymerization and form the polyolefin block.
2) Conducting RAFT polymerizations of styrene and styrene-co-maleic anhydride using this macroinitiator and a small molecule RAFT agent to form the second block and yield polyolefin
Calix Assisted Palladium Nanocatalyst: A Reviewijtsrd
The article reviews recent advances in c c cross coupling area as calix protected palladium nanocatalyst. The extensive use of palladium complex as catalyst in the calix chemistry is newly emerging field which deals with Suzuki, Heck, Stille and Sonogashira cross coupling reactions. The brief survey of cross coupling reactions also includes yield, catalyst loading, Recyclability. Keyur D. Bhatt | Krunal Modi "Calix Assisted Palladium Nanocatalyst: A Review" Published in International Journal of Trend in Scientific Research and Development (ijtsrd), ISSN: 2456-6470, Volume-4 | Issue-1 , December 2019, URL: https://www.ijtsrd.com/papers/ijtsrd29654.pdf Paper URL: https://www.ijtsrd.com/chemistry/other/29654/calix-assisted-palladium-nanocatalyst-a-review/keyur-d-bhatt
A 19-year-old man from Virginia, Michael Ryan, scaled Trump Tower in New York City using suction cups to try to meet with Donald Trump. He made it to the 21st floor before police pulled him into the building. Ryan had recently dropped out of high school and worked at a gardening center. His parents were out of town on vacation when he took a trip to New York City to perform the stunt. Ryan was charged with reckless endangerment and criminal trespass for the illegal climb.
Howard Gardner proposed multiple intelligences in 1983 including spatial or picture smart intelligence. Picture-smart learners can visualize experiences even when not present through mental manipulation like zooming or changing angles. These visualization skills help with directions, maps, and not all spatially intelligent people are artistic. A logo quiz activity tests students' spatial intelligence by having groups identify logos within 2 minutes to see which group can correctly identify the most.
Cecil County is experiencing rapid population growth that threatens its rural character. It is implementing growth management programs and working towards nutrient and sediment reduction goals to protect water quality in the Chesapeake Bay. Agriculture contributes most of the nitrogen and phosphorus runoff in the county. The Northeast River Advanced Wastewater Treatment Plant is upgrading to Enhanced Nutrient Removal, using membrane bioreactors, to further reduce nutrients and meet new regulatory limits. Water quality trading could help lower costs of nutrient reductions by allowing point sources like the treatment plant to purchase credits from non-point sources like farmers implementing best management practices.
1) The study examines the relationship between age and generalized trust levels in British society using survey data from 3075 participants. 2) The results show that while overall trust levels are high in Britain, younger people tend to trust strangers less than older people, with only 35.4% of young people indicating they can trust strangers compared to 53% of older people. 3) Qualitative interview data suggests this relationship may be due to older people having more social connections and awareness of their community, making them more likely to trust strangers.
Este documento lista y describe las partes principales del cuerpo humano en papiamento y español. Se divide en cuatro secciones: la cabeza, el pecho y brazos, las manos, y las partes inferiores del cuerpo como los pies. Proporciona los nombres de órganos, articulaciones y otras partes anatómicas clave en ambos idiomas.
Este proyecto final propone actividades para fomentar el trabajo interdisciplinar y en equipo entre las matemáticas y otras asignaturas. Se desarrollará a lo largo de tres evaluaciones con tareas como la búsqueda de noticias sobre matemáticas, ver una película y resolver enigmas, e investigar sobre temas matemáticos en grupos. El objetivo es que los estudiantes valoren las matemáticas para analizar problemas de la vida diaria y desarrollen habilidades como el razonamiento y trabajo cooperativo.
Los alumnos de la secundaria José Vasconcelos están aprendiendo sobre la historia de México y realizando proyectos de investigación sobre figuras históricas mexicanas como Benito Juárez y Sor Juana Inés de la Cruz. El director de la escuela espera que estos proyectos ayuden a los estudiantes a apreciar más la rica herencia cultural de México.
1. SWA faces both externally and internally driven risks that threaten its financial stability and operations. Externally, it relies on suppliers for fuel, parts, and air traffic control. Internally, it has high debt, low liquidity, minimal retained earnings, and dissatisfied employees due to job cuts. The risk management consultant should identify these risks and recommend controls.
2. Y Ltd faces strong competition exporting children's car seats but has advantages from its home country's safety laws and experience. Benchmarking production against competitors could help improve quality and productivity for exporting.
3. Various information systems serve different organizational levels from transaction processing to decision support. Airlines have low profits due to industry forces like competition and substitutes
The Synthesis and Characterization of Highly Fluorescent Polycyclic Azaborine...Nicolle Jackson
This document summarizes the synthesis and characterization of six new highly fluorescent polycyclic azaborine chromophores. The impact of incorporating a nitrogen-boron-hydroxy (N-BOH) unit into heteroaromatic polycyclic compounds was investigated in comparison to N-carbonyl analogs. Insertion of the N-B(OH)-C unit resulted in strong visible absorption, sharp fluorescence, and efficient quantum yields. The solid-state fluorescence displayed a large Stokes shift compared to solution, offsetting self-quenching effects in the solid state. X-ray crystallography confirmed the presence of the N-BOH moiety and an intramolecular hydrogen bond. DFT calculations explained the electronic
This document provides information about the acetoxylation of olefins, specifically the conversion of ethylene to vinyl acetate. It discusses two processes - a solution-based process and a gas-phase process. The solution-based process uses palladium chloride and copper chloride catalysts in acetic acid. The gas-phase process uses palladium metal catalysts with alkali salts like potassium acetate. Both processes form vinyl acetate as the main product, with acetaldehyde and carbon dioxide as byproducts. The document provides details on the industrial development and operation of these processes, and proposed reaction mechanisms.
Electrosynthesis is the synthesis of chemical compounds through electrochemical reactions in an electrochemical cell. It has advantages over ordinary redox reactions like avoiding undesired side reactions and precisely controlling reaction potentials. The basic experimental setup involves a galvanic cell, potentiostat, and electrodes. Common reactions include anodic oxidations and cathodic reductions. Electrosynthesis has applications in producing inorganic/organic compounds, batteries, water treatment, and more. It is an important industrial process, for example in chlor-alkali production and aluminum refining.
Industrial process of bio butanediol – from renewable sourcesrita martin
BDO is an important raw material for basic organic chemicals and fine chemicals. The principal products are Tetrahydrofuran (THF) and Gamma-butyrolactone (GBL), which in turn both have solvents applications and further high-value derivatives. BDO is used as a cross-linking agent for thermoplastic urethanes, polyester plasticizers, paints and coatings, copolyester hot melt and solvent-borne adhesives
This document summarizes research on PEPPSI-type complexes of palladium and platinum and their applications in catalysis. Specifically, it discusses:
1) The synthesis and characterization of novel PEPPSI-type platinum complexes and their use in hydrosilylation catalysis.
2) The development of a highly selective palladium-PEPPSI catalyzed C-H acetoxylation reaction.
3) The synthesis of palladium complexes with alkoxysilyl linker groups and their immobilization on mesoporous silica for use in C-H functionalization catalysis. Characterization of the immobilized complexes using XAS and XPS is also summarized.
This document discusses electrosynthesis, which is the synthesis of chemical compounds in an electrochemical cell. It provides details on experimental setup, types of reactions like anodic oxidations and cathodic reductions, and applications to inorganic compound synthesis and organic compound synthesis. It also discusses energy storage using electrosynthesis, including advantages and applications of redox flow batteries for large-scale energy storage.
Could coal be the answer to global plastics shortagesPlatts
The document discusses the potential for coal-to-olefins (CTO) and methanol-to-olefins (MTO) processes to produce ethylene and propylene as alternatives to traditional naphtha cracking. It provides an overview of the CTO and MTO processes, current projects in China, the economics and challenges of these processes compared to naphtha cracking, and the potential impact on global ethylene and polyethylene markets if numerous planned CTO/MTO projects come online by 2020.
This document discusses reaction mechanisms and their importance. It explains that reaction mechanisms show the step-by-step elementary reactions that make up the overall chemical change. Determining reaction mechanisms allows conditions to be optimized to favor certain reaction pathways and increase product yields. It also helps predict new reactions. The document then discusses how to determine reaction mechanisms using rate experiments and identifying which reactants are involved in the rate-determining step based on reaction orders. An example mechanism is given for the reaction of NO2 with CO, where the rate equation indicates the rate-determining step involves two NO2 molecules and NO3 is identified as the intermediate.
One pot synthesis of chain-like palladium nanocubes and their enhanced electr...tshankar20134
This document describes a one-pot synthesis of chain-like palladium nanocubes and their enhanced electrocatalytic activity. A simple aqueous approach is used to produce anisotropic cubic chain-like Pd nanostructures using the neurotransmitter 5-hydroxytryptamine. Scanning electron microscopy images show the nanocubes have sizes between 140-210 nm and form chain-like branched structures. Testing shows the cubic chain-like nanostructures have over 11 times greater electrocatalytic activity for oxidizing formic acid, methanol, and ethanol compared to spherical nanoparticles and commercial Pd/C catalysts. The enhanced performance makes them promising multipurpose catalysts for direct fuel cells.
This document describes an approach to rapidly generate molecular diversity using photoassisted diversity-oriented synthesis. Linear photoprecursors containing amino ketone and unsaturated pendant groups are assembled using high-yielding coupling reactions. These precursors undergo efficient intramolecular photocyclizations upon irradiation to form polyheterocyclic scaffolds with increased saturation, reduced rotatable bonds, and new structural frameworks. The primary photoproducts contain reactive functional groups that allow for further post-photochemical modifications, growing molecular complexity and accessing elaborated three-dimensional structures. This modular approach aims to generate drug-like molecules with properties known to correlate with clinical success, overcoming limitations of traditional combinatorial chemistry methods.
Stability of Transition Metal Complexes Halides of the Nickel Metalijtsrd
The stability of coordination complex is an important factor that decides the stability and reactivity of a metal complex. The stability of metal complex is governed by two different aspects such as thermodynamic and kinetic stabilities. The stability of metal complex generally means that it exists under favorable conditions without undergoing decomposition and has a considerable shelf life period . The term stability of metal complex cannot be generalized since the complex may be stable to one reagent condition and may decompose in presence of another reagent condition. The stability of metal complexes can be explained with the help of two different aspects, namely, thermodynamic stability and kinetic stability . Nevertheless, a metal complex is said to be stable if it does not react with water, which would lead to a decrease in the free energy of the system, i.e., thermodynamic stability. On the other hand, the complex is said to possess kinetic stability if it reacts with water to form a stable product and there is a known mechanism through which the reaction can proceed. For example, the system may not have sufficient energy available to break a strong bond, although once the existing bond is broken it could be replaced by new bond which is stronger than the older one. Stability of complex compound is assigned to be its existence in aqueous solution with respect to its bond dissociation energy, Gibbs free energy, standard electrode potential, pH of the solution, and rate constant or activation energy for substitution reactions.The crystal field stabilization energy CFSE is an important factor in the stability of transition metal complexes. Complexes with high CFSE tend to be thermodynamically stable i.e., they have high values of Ka, the equilibrium constant for metal ligand association and are also kinetically inert. They are kinetically inert because ligand substitution requires that they dissociate lose a ligand , associate gain a ligand , or interchange gain and lose ligands at the same time in the transition state. These distortions in coordination geometry lead to a large activation energy if the CFSE is large, even if the product of the ligand exchange reaction is also a stable complex. For this reason, complexes of Pt4 , Ir3 both low spin 5d6 , and Pt2 square planar 5d8 have very slow ligand exchange rates.There are two other important factors that contribute to complex stability Hard soft interactions of metals and ligands which relate to the energy of complex formation The chelate effect, which is an entropic contributor to complex stability. Chandrashekhar Meena "Stability of Transition Metal Complexes Halides of the Nickel Metal" Published in International Journal of Trend in Scientific Research and Development (ijtsrd), ISSN: 2456-6470, Volume-6 | Issue-6 , October 2022, URL: https://www.ijtsrd.com/papers/ijtsrd51833.pdf Paper URL: https://www.ijtsrd.com/chemistry/other/51833/stability-of-transition-metal-complexes-halides-of-the
Cross-Coupling of Unactivated Arenes: Direct Arene C-H Bond Arylation (Concepts of C-H Activation/Functionalization and its Recent Developments), Importance in the Drug Discovery Research
1) The document discusses various reagents used in organic synthesis, including phase transfer catalysts, crown ethers, and methods for converting alkenes to epoxides and diols.
2) Phase transfer catalysts facilitate reactions between substances in different phases by shuttling reactants between phases, while crown ethers selectively bind to specific ions due to their ring structure.
3) Common methods for converting alkenes include epoxidation, which introduces an oxygen atom to form an epoxide, and dihydroxylation, which adds two hydroxyl groups to form vicinal diols.
Dr. Raj K. Das is seeking a research position utilizing his skills in chemical industry. He holds a Ph.D. in Organometallic Chemistry from Indian Institute of Technology Kanpur and has extensive experience in synthesis, characterization, and catalytic transformations of inorganic and organometallic compounds. His research has focused on bimetallic cooperative catalysts for C-H bond activation and biomass gasification. He has published 5 papers and presented research at several conferences.
The document discusses bio-inspired catalysts for hydrogen production. It begins by noting the importance of hydrogen as an energy carrier and limitations of existing platinum-based catalysts. It then discusses how hydrogenase enzymes provide an efficient model but have limitations as well. Recent research has focused on developing bio-inspired catalysts that incorporate features of the hydrogenase active site and outer coordination sphere to improve catalytic efficiency. Some promising systems discussed include macrocyclic cobalt complexes and nickel bis(diphosphine) complexes containing amino acid groups to mimic the outer coordination sphere, which have shown activity under broader conditions than hydrogenases. Evaluation of catalytic performance focuses on turnover frequency and overpotential.
Annotated BibliographyYour annotated bibliography should be .docxfestockton
Annotated Bibliography
Your annotated bibliography should be prepared according to ACS format, using The ACS Style Guide: Effective Communication of Scientific Information, 3rd edition. One or more copies of this book are held on reserve in the library.
The Purdue Online Writing Lab (OWL) at http://owl.english.purdue.edu/owl/resource/614/01/ offers further information regarding annotated bibliographies.
This annotated bibliography, along with a correctly formatted citation, should include a summary of the content of the source and a two-pronged critical analysis of the source. The first part of the critical analysis will be your objective evaluation of the source and the second part will be your subjective evaluation. Even if a source is found to be credible, if it does not contribute to your research question, it should not be included.
Prepare your annotated bibliographic entry according to the following guidelines:
1. Bibliography Entry: Include the complete bibliographic information correctly formatted according to the ACS style guidelines
1. Summary of Content: Include a descriptive paragraph summarizing the source. Include key concepts and quotations when appropriate.
Objective Evaluation: Objectively evaluate the credibility of the source using the criteria that are most relevant. Use the questions presented in the TRAAP criteria found under “Evaluate Sources” at
https://youtu.be/zzTBBm1HXvM
1. to stimulate your ideas, but don’t feel as if you need to address each criteria as a checklist. Use the criteria that are appropriate for your source. When relevant, address such things as bias or lack of bias, outdated material or current material, author’s point of view, and author’s credentials and qualifications to write on the topic. What is the author’s purpose in writing the information? Is the information presented without prejudice? Or does the author, publisher, or research funding organization have a stake in the outcome or the controversy you are investigating?
1. Subjective Evaluation: Include a summary of the relevance of the source to your research topic or question. How will the source contribute to your research, and how useful will it be? Does it offer a unique perspective? Does it offer a contradictory viewpoint to another source?
Photochemistry Assignment #2
This assignment covers material from Chapter 2 section 1 to Chapter 2 Section 14.
1) According to the principles of quantum mechanics, what is the wave function?
2) What is the Born-Oppenheimer approximation?
3) Under what two types of interactions does the approximation given in equation 2.4
break down?
4) (a) What is the four-letter abbreviation for the highest energy occupied molecular
orbital?
(b) What is the four-letter abbreviation for the lowest energy unoccupied molecular
orbital?
5) To what does the square of a wave function relate?
6) What is an expectation value?
7) What is meant by an electronic configuration?
8) An alken ...
Annotated BibliographyYour annotated bibliography should be .docxSHIVA101531
This document provides guidelines for preparing an annotated bibliography entry for a photochemistry assignment. The annotated bibliography should include a bibliography entry in ACS format, a summary of the source content, and a two-part critical analysis consisting of an objective evaluation and subjective evaluation of how the source relates to the research topic. The guidelines specify how to structure each part of the annotated bibliography entry. A sample assignment on photochemistry is also provided that requires answers to multiple questions.
Adam B. Powell developed a heterogeneous catalyst composed of palladium, bismuth nitrate, and tellurium metal that promotes the aerobic oxidative esterification of aliphatic alcohols with high yields. The addition of bismuth and tellurium additives significantly increased the rate of product formation and overall yield compared to the catalyst without additives. The catalyst was shown to esterify a variety of activated and aliphatic alcohols, expanding the scope of this transformation. Future work includes adapting the catalyst for other oxidative reactions and developing a robust Pd-Bi-Te catalyst for flow applications.
This document summarizes a research article that describes a visible-light-induced dual carbon-carbon bond formation reaction for the synthesis of alkylated oxindoles from alkenes and simple ethers via selective sp3 carbon-hydrogen bond cleavage under metal-free conditions. The reaction uses a photoredox catalyst, tert-butyl hydroperoxide as an oxidant, and proceeds at room temperature to provide the products in good to excellent yields from a variety of starting materials. Optimization studies established that conducting the reaction in neat tetrahydrofuran using these conditions gave the best results. This method provides an efficient way to synthesize biologically active alkylated oxindole compounds through a mild, atom-economic process.
1) The document discusses new methods for selectively functionalizing meta C-H bonds on aromatic rings, which was previously difficult.
2) One method uses a nitrile-containing template that positions a palladium catalyst in a way that selectively activates the meta C-H bond. The nitrile group binds linearly to the catalyst and relieves steric strain, improving selectivity.
3) The template provides over 90% meta-selectivity for a variety of substituted aromatic substrates. It can be easily removed after the reaction. However, it is not compatible with heterocyclic substrates due to the catalyst binding preferentially to heteroatoms.
Igor Busygin is a postdoctoral fellow in the Department of Chemical Engineering at the University of California, Berkeley. He received his Ph.D. in Chemistry from Åbo Akademi University in Finland in 2007, with a focus on heterogeneous catalysis. His current research at UC Berkeley involves developing novel catalytic materials using metal cluster precursors and testing their performance in gas-phase hydrogenation and hydroformylation reactions. He has over 15 publications in peer-reviewed journals and several years of experience in academia and industry researching catalysis, polymer chemistry, and analytical techniques.
Literature Seminar on Expansion of Pd chains through Beta-Carotene & Tetrapho...Nina Saraei
The document summarizes two papers on the expansion of palladium chains using β-carotene and tetraphosphine ligands. The first paper discusses the stepwise expansion of homonuclear and heteronuclear palladium chains from binuclear Pd(I) complexes supported by tetraphosphine ligands. Characterization data showed the complexes adopt linear structures with reversible metalation/demetalation. The second paper examines a bis-β-carotene ligand's ability to bind decanuclear palladium and palladium-platinum complexes into infinite π-stacked columns. Both papers demonstrate the use of multidentate ligands to control the assembly of extended metal atom chains.
Recent Progress in Synthesis of Nano- and Atomic-Sized CatalystsDevika Laishram
Well-defined nano-and atomic-sized heterogeneous catalysts with extremely high
catalytic activities and unique selectivities show promise in addressing the critical
energy- and environment-related challenges of this century. The exceptional
properties of these catalysts, such as their electronic and geometric structures and
the effective interactions between metals and supports, give rise to unprecedented
catalytic efficiency over that of conventional catalysts. The facile prospects for
tuning the active sites of these catalysts pave the way to optimizing their activities,
selectivities, and stabilities, thus offering extensive application possibilities in
significant industry-related catalytic reactions. A prerequisite for synthesizing
nano- and atomic-sized catalyst is to prepare extremely disperse nano- and
subnanoscale atoms on suitable supports. This book chapter summarizes various
synthesis methods employed to synthesize nano- and atomic-scale catalysts.
Cobalt-entrenched N-, O-, and S-tridoped carbons as efficient multifunctional...Pawan Kumar
We report the synthesis of sustainable and reusable non-noble transition-metal (cobalt) nanocatalysts
containing N-, O-, and S-tridoped carbon nanotube (Co@NOSC) composites. The expensive and benign
carrageenan served as the source of carbon, oxygen, and sulfur, whereas urea served as the nitrogen
source. The material was prepared via direct mixing of precursors and freeze-drying followed by carbonization
under nitrogen at 900 °C. Co@NOSC catalysts comprising a Co inner core and outer electron-rich
heteroatom-doped carbon shell were thoroughly characterized using various techniques, namely, TEM,
HRTEM, STEM elemental mapping, XPS, BET, and ICP-MS. The utility of the Co@NOSC catalyst was
explored for base-free selective oxidative esterification of alcohols to the corresponding esters under
mild reaction conditions; excellent conversions (up to 97%) and selectivities (up to 99%) were discerned.
Furthermore, the substrate scope was explored for the cross-esterification of benzyl alcohol with longchain
alcohols (up to 98%) and lactonization of diols (up to 68%). The heterogeneous nature and stability
of the catalyst facilitated by its ease of separation for long-term performance and recycling studies
showed that the catalyst was robust and remained active even after six recycling experiments.
EPR measurements were performed to deduce the reaction mechanism in the presence of POBN
(α-(4-pyridyl-1-oxide)-N-tert-butylnitrone) as a spin-trapping agent, which confirmed the formation of
•CH2OH radicals and H• radicals, wherein the solvent plays an active role in a nonconventional manner.
A plausible mechanism was proposed for the oxidative esterification of alcohols on the basis of EPR
findings. The presence of a cobalt core along with cobalt oxide and the electron-rich N-, O-, and
S-doped carbon shell displayed synergistic effects to afford good to excellent yields of products.
3. 3
PEPPSI-type Complexes of
Palladium and Platinum:
Investigation of Properties and Applications in
Catalysis
Ekaterina Bolbat
DOCTORAL DISSERTATION
to be publicly defended for the degree of PhD at the Faculty of Science, Lund
University, Sweden on Thursday 22nd
of September 2016 at 09:15 in lecture hall C
at Kemicentrum
Faculty opponent
Prof. Freddy Kleitz, Université Laval, Canada
4. 4
Organization
LUND UNIVERSITY
Document name
DOCTORAL DISSERTATION
Date of issue
August 26, 2016
Author(s)
Ekaterina Bolbat
Sponsoring organization
Title and subtitle
PEPPSI-type Complexes of Palladium and Platinum: Investigation of Properties and Applications in Catalysis
Abstract
The development of catalytic reactions for direct conversion of unreactive carbon-hydrogen (C−H) bonds into
bonds with a variety of elements remains a critical challenge. The traditional approach used by organic chemists
to functionalize a molecule consists of several costly chemical steps including functionalization of an inactivated
starting material and its subsequent transformation to the final product with a desired chemical function. An
alternative method is the direct functionalization of the C−H bonds of the molecule with the help of catalysis. This
approach represents not only a faster and more atom-economical synthetic approach but is also preferred from a
green chemistry perspective, since substantially less waste is produced. Thus, catalytic C−H activation reactions
can be used for the environmentally friendly production of fine chemicals. Another important application is to
provide an efficient way to the natural gas utilization by the low temperature oxidative conversion of methane to
liquid fuel for transportation purposes.
The selectivity and activity of homogeneous catalysts under mild reaction conditions is unbeaten by their
heterogeneous counterparts. But unfortunately, the problem of separation of the single-site-catalysts from the
reaction medium is still an important drawback which often blocks large-scale applications in industry. Therefore
the development of well-defined catalyst systems that allow rapid and selective chemical transformations and at
the same time can be completely recovered from the product phase is a paramount challenge. A promising
approach is the attachment of homogeneous catalysts to polymeric organic, inorganic or hybrid supports.
The present thesis describes the development of novel homogeneous PEPPSI-type complexes of palladium and
platinum and possible approaches for their further heterogenization, together with a thorough investigation of their
properties and subsequent applications in catalysis. For full characterization of the obtained compounds
synchrotron radiation X-ray techniques such as XAS and XPS were used. As benchmark reactions of catalytic
activity, C−H functionalization as well as hydrosilylation reactions were examined.
Key words: PEPPSY-type complexes, palladium, platinum, C−H activation, supported homogeneous catalysts
Classification system and/or index terms (if any)
Supplementary bibliographical information Language: English
ISSN and key title ISBN 978-91-7422-471-9
Recipient’s notes Number of pages
70
Price
Security classification
I, the undersigned, being the copyright owner of the abstract of the above-mentioned dissertation, hereby grant to all
reference sourcespermission to publish and disseminate the abstract of the above-mentioned dissertation.
Signature Date
7. 7
“Anything you dream is fiction, and anything you
accomplish is science, the whole history of mankind
is nothing but science fiction.”
Ray Bradbury
8. 8
Content
Content 8
Popular science summary 10
List of publications 12
Abbreviations 14
Introduction 15
References 17
Chapter 1. C−H bond activation 19
1.1 Introduction 19
1.2 Classification 20
1.3 Ligand-directed C−H functionalization catalyzed by palladium 22
1.4 References 26
Chapter 2. N-heterocyclic carbenes as ligands for transition metals 27
2.1 Introduction 27
2.2 Electronic and steric properties 28
2.3 Family of PEPPSI complexes 29
2.4 References 31
Chapter 3. Supported homogeneous complexes 33
3.1. Introduction 33
3.2. Immobilization strategies 33
3.3 Mesoporous silica SBA-15 as a support in catalysis 35
3.4 Functionalization of mesoporous silica 36
3.5 Main challenges for supported systems 37
3.6 References 37
Chapter 4. Experimental X-ray techniques 39
4.1 Introduction 39
4.2 X-ray Photoelectron Spectroscopy 39
4.3 X-ray absorption spectroscopy 42
9. 9
4.4 References 42
Chapter 5. Summary of key results 45
5.1 Novel platinum(II) NHC complex: synthesis and spectroscopic
characterization [Paper I] 45
5.2 Catalytic activity of the Pt-IPr complex [Paper I] 48
5.3 Different Pd-PEPPSI complexes in selective ligand-directed C−H
acetoxylation [Paper II] 49
5.4 Development of Pd-NHC catalysts supported on SBA-15 [Paper III] 54
5.5 Catalytic activity of supported Pd-NHC catalysts [Paper III] 60
5.6 X-ray spectroscopic characterization of supported Pd-NHC complexes
[Paper IV] 61
5.7 References 63
Conclusions and outlook 67
Acknowledgments 69
10. 10
Popular science summary
Catalysis is a powerful tool to highly efficient production of desired new
chemicals by acceleration of a chemical reaction rate. Special compounds used for
this purpose are called catalysts; they induce a change in the chemical
environment without being consumed during the process. Catalysis affects many
fields of life leading to a decrease of the energy use, less pollution, fewer side
products and lower starting materials cost.
The broad variety of the catalysts can be divided into two major types –
homogeneous, that are in the same phase as the reaction mixture, and
heterogeneous, which are presented in a different phase than the reactants. Both
groups have their own advantages as well as drawbacks. Researchers worldwide
put a lot of efforts in creating of the so-called ideal catalyst that will combine
positive features of both kinds of catalytic systems: the high selectivity and
activity of homogeneous catalysts with recyclability and ease of separation for
heterogeneous ones. A promising candidate for such a title is a supported
homogeneous catalyst where a metal complex is anchored by chemical bonding to
a suitable support that can be inorganic oxide, zeolites, organic polymers or carbon
nanotubes.
Among the myriads of chemical processes the direct conversion of unreactive raw
materials, such as hydrocarbons, to functional molecules containing diverse
functionalities such as halogen, nitro, acetoxy, alkyl or aryl groups is highly
desirable. The main challenge here is to break a very strong carbon-hydrogen
(C−H) bond to replace the hydrogen with a desired functional group. Another
problem is the selectivity of the process as the molecule of the reactant often
contains a number of C−H bonds with the same reactivity. These issues can be
overcome with the help of transition-metal catalysis: a metal center can coordinate
the starting material forming an intermediate complex that will selectively deliver
a functional group to a proximal position in the molecule. This transformation is
also known as a C−H bond activation.
N-heterocyclic carbene (NHC) complexes with transition metals are a fascinating
class of compounds where the strong bond between the carbon atom of the carbene
ligand and the metal center is the reason for its high stability. These compounds
find application across the chemical field including their use in materials, as
11. 11
metallopharmaceuticals and as homogeneous catalysts, showing great catalytic
activity in a range of reactions.
Therefore in the present thesis we decided to investigate and discuss the following
topics: development and catalytic activity of a novel platinum-NHC complex of a
PEPPSI type, application of palladium NHC complexes to a selective ligand-
directed C−H bond acetoxylation, development of approaches for immobilization
of the palladium-N-heterocyclic carbene complexes on mesoporous silica support,
application of supported palladium-NHC complexes in C−H bond activation
catalysis and characterization of the functionalized materials.
12. 12
List of publications
I. Ekaterina Bolbat, Karina Suarez-Alcantara, Sophie E. Canton, Ola F.
Wendt: “Synthesis, spectroscopic characterization and catalytic activity of
platinum(II) carbene complexes”, Inorg. Chim. Acta 2016, 445, 129-133.
Contribution: I participated in planning of the project and performed most of the
experimental work, except for the X-ray absorption spectroscopy measurements. I
participated in the NEXAFS experiment and collaborated on the interpretation of
the data. I wrote most of the article.
II. Ekaterina Bolbat, Ola F. Wendt: “Ligand Control in Selective C–H
Oxidative Functionalization Using Pd-PEPPSI-Type Complexes”, Eur. J.
Org. Chem. 2016, 3395-3400.
Contribution: I participated in planning of the project, performed all the
experimental work presented in the article and I was the main responsible for the
data analysis. I wrote most of the article.
III. Ekaterina Bolbat, Maitham H. Majeed, Axel R. Persson, L. Reine
Wallenberg, Ola F. Wendt: “PEPPSI-type Pd-NHC catalysts for C−H
functionalization supported on mesoporous silica SBA-15”, in manuscript.
Contribution: I participated in planning of the project, performed all the
experimental work presented in the manuscript, except for the TEM, BET, TGA,
ICP and SS NMR spectroscopy experiments and I was the main responsible for the
data analysis. I wrote most of the manuscript.
13. 13
IV. Olesia Snezhkova*, Ekaterina Bolbat*, Fredric Ericson, Payam Shayesteh,
Shilpi Chaudhary, Niclas Johansson, Ashley Head, Petter Persson, Ola F.
Wendt, Joachim Schnadt: “Structure, stability and catalytic activity in
C−H activation of supported Pd-NHC complexes”, in manuscript.
Contribution: I took a part in planning of the project, performed all the synthetic
work and participated in the X-ray photoelectron and absorption spectroscopy
measurements and the data analysis. I wrote a part of the manuscript.
* Equal contribution.
14. 14
Abbreviations
BET Brunauer, Emmet and Teller theory for surface area determination
BJH Barret-Joyner-Halenda theory for pore size distribution
C-H Carbon-Hydrogen
DFT Density Functional Theory
EXAFS Extended X-ray Absorption Fine Structure
HP Hybridization Peak
ICP Inductively Coupled Plasma
IUPAC International Union of Pure and Applied Chemistry
MCM-41 Mobil Composition of Matter No. 41 mesoporous silica
NEXAFS Near Edge X-ray Adsorption Fine Structure
NHC N-Heterocyclic Carbene
NMR Nuclear Magnetic Resonance
PEPPSI Pyridine-Enhanced Precatalyst Preparation Stabilization and
Initiation
SBA-15 Santa Barbara Amorphous No. 15 mesoporous silica
SS NMR Solid-State Nuclear Magnetic Resonance
TEM Transmission Electron Microscopy
TGA Thermogravimetric Analysis
UHV Ultra-High Vacuum
WL White Line
XANES X-ray Absorption Near Edge Structure
XAS X-ray Absorption Spectroscopy
XP X-ray Photoemission
XPS X-ray Photoelectron Spectroscopy
15. 15
Introduction
Over the past decades research interest of many scientific groups has been
particularly focused on the activation of strong carbon-hydrogen bonds. The first
question arising is what makes it such an important topic. The traditional and
general approach used by organic synthetic chemists to functionalize a molecule
consists of several steps: at the beginning, commonly unfunctionalized starting
material is equipped with a first functional group (FG1) for both reactivity and
selectivity. Further, the obtained derivative is readily transformed to the final
product with a desired chemical function (FG2) (Figure 1). Thus, the
transformation of C−H bonds into C−X functionality adds several, often costly,
synthetic steps to the overall construction of a required molecule [1,2].
Figure 1. Functionalization of unactivated organic molecule: organic synthesis vs.
catalysis.
An alternative method is the direct functionalization of the C−H bonds with the
help of catalysis [3-5]. This approach represents not only a faster and overall more
atom-economical synthetic approach but also is preferred in green chemistry: the
catalytic method can be used for environmentally friendly production of fine
chemicals due to the reduced amount of generated waste and the avoidance of
halogenated agents.
The difficulty lies in two main fundamental challenges arising for direct C−H
bond functionalization. Firstly, the relatively inert nature of most carbon-hydrogen
bonds. Another important issue is selectivity of the process of C−H
functionalization in a complex molecule containing a variety of C−H bonds. A
solution consists in the utilization of transition metal catalysis [6-9]. Complexes of
FG1 FG2 FG2
[M]
vs
16. 16
transition metals can coordinate a substrate and activate and cleave a proximal
C−H bond.
The main aim of this thesis was to investigate the possible applicability of late
transition metal complexes in C−H bond activation reactions. The discussion starts
with the literature background, followed by a section with description of X-ray
experimental techniques, extensively used in the project, and finally summarizes
the key results of the research and shows the perspective of the accomplished
work. A number of scientific papers based on the obtained research results can be
found in the attachment at the end of the book.
Paper I deals with the synthesis and investigation of a novel PEPPSI-type
platinum-N-heterocyclic carbene complex, its characteristic NEXAFS behavior
and catalytic activity. Paper II is focused on the application of a range of Pd-
PEPPSI complexes, containing different NHC ligands, to selective ligand-directed
C−H acetoxylation. PEPPSI complexes of palladium are well known for their high
catalytic activity in cross-coupling reactions but their application in C−H
activation processes was almost not studied.
The selectivity and activity of homogeneous catalysts under mild reaction
conditions is unsurpassed by their heterogeneous counterparts. But unfortunately,
issues related to the separation of the single-site-catalysts from the reaction media
as well as its recycling is still an important drawback which in many cases blocks
the large scale application in industry. The development of well-defined catalytic
systems that allow effective and selective chemical transformations and at the
same time can be completely recovered from the product phase is still a principal
challenge. A promising approach consists in the attachment of homogeneous
catalysts to polymeric organic, inorganic or hybrid supports (Figure 2).
Figure 2. Immobilized Pd-NHC complexes studied in this thesis.
N N Si
Si
Si
Si
O
O
OEt
O
O
O
O
O
Cl-
+
Si
Si
N N Si
Si
Si
Si
O
O
OEt
O
O
O
O
O
Cl-
+
Si
Si
17. 17
Therefore Paper III describes the development of new supported palladium-NHC
homogeneous catalysts, covalently immobilized on mesoporous silica SBA-15 as a
support, and study of their catalytic activity in C−H activation reactions. For
characterization of obtained systems a range of analytical techniques like solid-
state NMR spectroscopy, thermogravimetric analysis, BET measurements as well
as X-ray spectroscopy was used. X-ray techniques can be used for investigating
the interactions of molecules with surfaces as well as for exploring the mechanism
of reactions from characterization of the elementary steps and intermediates.
Paper IV covers the investigation of the stability, geometric properties and
surface orientation as well as catalytic activity of two palladium complexes with
different N-heterocyclic carbene ligands grafted on a silicon wafer surface,
employing X-ray Photoemission and Absorption spectroscopy and DFT
calculations.
References
1. R. Jazzar, J. Hitce, A. Renaudat, J. Sofack-Kreutzer, O. Baudoin, Chem. Eur. J.
2010, 16, 2654.
2. D. F. Taber: “Organic synthesis-State of the Art 2003–2005”, Wiley, 2006.
3. J. A. Labinger, J. E. Bercaw, Nature 2002, 417, 507.
4. J. Wencel-Delord, T. Dröge, F. Liu, F. Glorius, Chem. Soc. Rev. 2011, 40, 4740.
5. K. Godula, D. Sames, Science 2006, 312, 67
6. P. B. Arockiam, C. Bruneau, P. H. Dixneuf, Chem. Rev. 2012, 112, 5879.
7. M. Lersch, M. Tilset, Chem. Rev. 2005, 105, 2471.
8. H. M. L. Davies, J. R. Manning, Nature 2008, 451, 417.
9. D. A. Colby, R. G. Bergman, J. A. Ellman Chem. Rev. 2010, 110, 624.
19. 19
Chapter 1. C−H bond activation
1.1 Introduction
Alkanes, alkenes, arenes and alkynes are the constituents of a widespread class of
organic compounds that can be found in excess in natural gas and petroleum -
hydrocarbons. The major difficulty for the direct application of hydrocarbons lies
in their relatively inert nature: the molecule is held together in total by strong
carbon-carbon and carbon-hydrogen bonds. The average dissociation energy of the
C−H bond is 90-110 kcal/mol [1], which leads to a need for harsh reaction
conditions for functionalization such as high temperature and use of strong
oxidants (Figure 1.1). From an economical perspective, finding an efficient
approach for direct conversion of hydrocarbons to more valuable products under
mild conditions is of substantial importance.
Figure 1.1. Bond dissociation energies of some organic molecules.
Inspired by the Nature’s enzyme-catalyzed pathways for C−H bond breakage, the
problem of controlled activation of inert hydrocarbons has been addressed by
transition-metal chemistry since the late 1950s [2]. Metal insertion into a carbon-
hydrogen bond, leading to the formation of a far more reactive carbon-metal bond,
or so-called “C−H activation” became a powerful tool for direct functionalization
of unactivated substrates [3-7] (Figure 1.2).
There are four general mechanisms for the metal C−H bond insertion step:
oxidative addition in electron-rich late transition-metal complexes, σ-bond
metathesis in early transition metal complexes, electrophilic activation in electron
deficient late transition metal complexes and 1,2-addition (Scheme 1.1).
H H3C HCH2 H Cl H3C Cl
105 84113 90 97
Bond:
Bond dissociation energies:
(kcal/mol)
20. 20
Figure 1.2. Schematic representation of a C−H bond activation process.
Scheme 1.1. Main C−H activation mechanisms: oxidative addition (a), σ-bond metathesis
(b), electrophilic substitution (c), 1,2-addition (d).
1.2 Classification
Transition-metal catalyzed homogeneous C−H functionalization might be roughly
separated in two major fields: the first covers reactions of the completely
unfunctionalized hydrocarbons, where the interactions between a substrate and a
metal center are quite weak – so-called “first functionalization” directory; the
C H
R1
R2
R3
M
C M
R1
R2
R3-H
C-H activation
C + LnM C
H
MLn
H
C
H
MLn
C
H
+ LnM R C
H
R
MLn
C MLn +
R
H
C
H
+ LnM X C
MLn
H
X C MLn +
X
H
C
H
+ LnM X C
H
X
MLn
C XH
MLn
a)
b)
c)
d)
21. 21
second one includes conversion of hydrocarbons containing one or more pre-
existing functional groups – “further functionalization” field [8,9].
In the case of first functionalization, to overcome the weak affinity between the
substrate and the metal catalyst, the starting material is often used in large excess,
even as a solvent, and control of the site-selectivity is complicated (Scheme 1.2).
Additionally, as often both the substrate and the resulting product represent rather
low-value chemicals, the development of competitive low-cost catalysts with high
catalytic activity is vastly preferred.
Scheme 1.2. Examples of the first functionalization C−H activation reactions [10,11].
The main advantage of “further functionalization” is that an existing functionality
can chelate the metal center, which can selectively deliver the functional group to
the molecule [12-15]. Coordination can also help to overcome the inertness of a
C−H bond, increasing the effective concentration of the substrate at the metal
center. The directing group can initially be a part of the substrate molecule or can
be installed to promote the process, but the latter is undesirable, as it will add extra
synthetic steps to the overall construction of a molecule. Therefore, the main
challenge arising is to develop highly selective reaction with commonly occurring
intrinsically functionalized molecules (Scheme 1.3).
+ Ph
40 mol % Pd(OAc)
2 equiv AgOAc
HOAc, reflux, 8h
55%
(solvent)
Fujiwara et al., 1968
Ph
PtII
CH3R
H+
PtII
PtIV
Cl
Cl
Cl
Cl
CH2R
ClCl
Cl
Cl
Cl
Cl
Cl
CH2R
Cl
PtIV
PtII
ClCH2R
Cl
Shilov system
22. 22
Scheme 1.3. Pd(II)-catalyzed examples of further C−H functionalization [16,17].
1.3 Ligand-directed C−H functionalization catalyzed by
palladium
Direct transformation of a C−H into a C−X bond, where X is oxygen, nitrogen,
halogen, sulfur or carbon, is an efficient process for formation of novel
pharmaceuticals, polymers and fine chemicals with less waste generated and under
lower energy consumption conditions. However, there are two major limitations.
Firstly, the already discussed issue is the relatively inert nature of C−H bonds.
Another challenge is selectivity. First of all, the starting material should not
contain any functionality that may irreversibly react with the metal center.
Secondly, most of the complex molecules have a number of similar C−H bonds
from the standpoint of relative reactivity.
The most widespread strategy for controlling site selectivity is by means of
directing groups – pre-existing functionalities in substrate that are able to
coordinate to a metal center and selectively deliver the catalyst to a proximal C−H
bond in a molecule. Commonly, this has been accomplished by nitrogen-,
phosphorus- or sulfur-containing directing groups, displaying strong σ-donor
and/or π-acceptor properties. They commonly form thermodynamically stable five
or six-membered metallacycles with the catalyst metal center in a process known
as cyclometalation (Scheme 1.4) [8].
N
N
Ph
Cl2
3 mol % PdCl2
1,4-dioxane/H2O
90 oC, 35 h
N
N
Ph
Cl
68%
HN t-Bu
PhI
1.5 mol % Pd(OAc)2
2 equiv AgOAc
TFA, 120 oC, 3h
HN t-Bu
Ph Ph
91%
Fahey, 1971
Zaitsev et al., 2005
23. 23
Scheme 1.4. Schematic illustration of the cyclometalation process.
The concept of “cyclometalation” was initially introduced by Trofimenko in his
work “Some Studies of the Cyclopalladation Reaction” published in 1973 [18].
Transformations of this type have been known for several decades with first
examples of the cyclometalated compounds appearing as early as the 1960s
[19,20].
Different transition metals, such as Ru, Rh, Pt, Ir and Pd readily participate in
cyclometalation reactions. Particularly attractive catalysts among them are
palladium complexes due to a few special reasons [21]:
(i) Ligand-directed C−H functionalization at Pd centers might be applied
to form a variety of linkages, including carbon-oxygen, carbon-
halogen, carbon-nitrogen and carbon-carbon bonds. This feature is
quite unique and is mostly due to the following characteristics: the
ability of Pd(II) catalysts to interact with many different oxidants, and
opportunity for selective functionalization of the created
palladacycles.
(ii) The variety of directing groups that can be used for cyclometalation
with palladium is significantly broad.
(iii) Palladium can be used for C−H activation of sp2
as well as more
challenging sp3
C−H sites.
(iv) Commonly Pd-catalyzed directed C−H functionalization reactions can
be performed in the presence of moisture and do not require the use of
inert gas techniques, representing a significant advantage for their
practical application in synthesis.
From the mechanistic point of view, typically a ligand-directed C−H activation is
employing Pd(II) centers that at first participate in cyclometalation with the
substrate. This step is thought to be redox neutral. The so formed intermediate can
further react according to two main mechanistic routes: reductive or electrophilic
functionalization (Scheme 1.5 and Scheme 1.6).
DG
H
R
+[M] DG
H
R
M
DG
R
M
-H+
24. 24
Scheme 1.5. Reductive functionalization mechanistic manifold.
In the first case functionalization is promoted by the reductive process employing
a nucleophilic coupling partners through a Pd(II)/Pd(0) catalytic cycle.
The second type of mechanisms occurs due to a reaction of the cyclometalated
intermediate with an electrophilic agent. It can be done by several particular
mechanistic manifolds: without a change in the oxidation state of palladium by a
direct cleavage of the linkage between Pd and a substrate, by one-electron
oxidation of the palladacycle and through two-electron oxidation. In the last case
the process can go through the formation of either a Pd(IV) intermediate or a
Pd(III)/Pd(III) dimer, regarding the chemical environment at the metal center of
the catalyst.
In general, the step of functionalization with a chemical function in the discussed
catalytic cycles can occur with the help of an external reagent as well as by an
intramolecular mechanism.
Reductive functionalization
DG
C H
PdII
DG
C
PdII
DG
C
PdII
Reductive
elimination
C-H
activation
Ligand
exchange
DG
C FG
Pd0
FG
H
oxidant
FG
25. 25
Scheme 1.6. Electrophilic functionalization: three mechanistic routes.
Electrophilic functionalization
PdII
DG
C
PdII
DG
C H
C-H
activation
oxidant-FG
DG
C FG
Electrophilic
cleavage
Direct functionalization
One-electron oxidation
PdII
DG
C H
DG
C
PdII
DG
C
PdIII
FG
oxidant-FG One-electron
oxidation
C-H
activation
PdI
DG
C FG
Reductive
elimination
One-electron
oxidation
Two-electron oxidation
PdII
DG
C H
DG
C
PdII
DG
C
PdIV
FG
oxidant-FG
Two-electron
oxidation
C-H
activation
DG
C FG
Reductive
elimination
26. 26
1.4 References
1. S. J. Blanksby, G. B. Ellison, Acc. Chem. Res. 2003, 36, 255.
2. J. A. Labinger, J. E. Bercaw, Nature 2002, 417, 507.
3. J. Wencel-Delord, T. Dröge, F. Liu, F. Glorius, Chem. Soc. Rev. 2011, 40, 4740.
4. J.-Q. Yu, Z. Shi: “Topics in Current Chemistry: C-H Activation”, Vol. 292,
Berlin Heidelberg: Springer, 2010.
5. M. Lersch, M. Tilset, Chem. Rev. 2005, 105, 2471.
6. K. Godula, D. Sames, Science 2006, 312, 67.
7. P. B. Arockiam, C. Bruneau, P. H. Dixneuf, Chem.Rev. 2012, 112, 5879.
8. K. M. Engle, T.-S. Mei, M. Wasa, J.-Q. Yu, Acc. Chem. Res. 2012, 45(6), 788.
9. T. Bruckl, R. D. Baxter, Y. Ishihara, P. S. Baran, Acc. Chem. Res. 2012, 45(6),
826.
10. A. E. Shilov, G. B. Shul’pin, Chem. Rev. 1997, 97(8), 2879.
11. Y. Fujiwara, I. Moritani, M. Matsuda, S. Teranishi, Tetrahedron Lett. 1968, 9,
3863.
12. C. Wang, Y. Huang, Synlett 2013, 24, 145.
13. D. A. Colby, R. G. Bergman, J. A. Ellman, Chem. Rev. 2010, 110, 624.
14. G. Rouquet, N. Chatani, Angew. Chem. Int. Ed. 2013, 52, 11726.
15. M. Zhang, Y. Zhang, X. Jie, H. Zhao, G. Li, W. Su, Org. Chem. Front. 2014, 1,
843.
16. D. R. Fahey, J. Organomet. Chem. 1971, 27, 283.
17. O. Daugulis, V. G. Zaitsev, Angew. Chem., Int. Ed. 2005, 44, 4046.
18. S. Trofimenko, Inorg. Chem. 1973, 12 (6), 1215.
19. J. P. Kleiman, M. Dubeck, J. Am. Chem. Soc. 1963, 85, 1544.
20. A. C. Cope, R. W. Siekman, J. Am. Chem. Soc. 1965, 87, 3272.
21. T. W. Lyons, M. S. Sanford, Chem. Rev. 2010, 110, 1147.
27. 27
Chapter 2. N-heterocyclic carbenes as
ligands for transition metals
2.1 Introduction
During the last decade the chemistry of N-heterocyclic carbenes has attracted
considerable attention from the scientific community. The first examples of
transition metal-NHC complexes have been independently reported in the work by
Öfele [1] and Wanzlick [2] and further have been insightfully studied by Lappert
[3]. However, the metal-carbene chemistry gained a tremendous momentum since
the 1991 report by Arduengo et al. [4] describing the first air-stable free carbenes -
imidazol-2-ylidenes, and their preparation and handling. As a result, NHCs
received substantial practical significance and libraries of novel structurally
diverse analogues were synthesized (Figure 2.1).
Figure 2.1. Structural variety of five-membered NHCs.
The unexpected great stability of a free N-heterocyclic carbene originates from
considerable σ-charge electron transfer from the carbenic carbon to the adjacent
N X N N
N
N
R
R1
R R R R
R
.. .. ..
Thiazolylidene
Oxazolylidene
Triazolylidene Pyrrolidinylidene
N NR R
..
Benzimidazolylidene
N N
R R..
Imidazolinylidene
N N
R R..
Imidazolylidene
28. 28
more electronegative nitrogen atoms. The cyclic electron stabilization imparts a
certain aromatic character to the structure [5].
2.2 Electronic and steric properties
The use of NHCs as ligands in transition-metal coordination chemistry has
increased significantly due to their success in olefin metathesis and Pd-catalyzed
cross-coupling reactions [6-8]. The explanation of the suitability of NHCs as
ligands for transition metals lies in their intrinsic σ-donor ability with an electron
lone pair available for donation. Typically, metal-carbene coordination is drawn as
a single bond with a curved line between the two heteroatoms within the
heterocyclic ring, representing delocalized π-contribution. Studies aimed on the
fundamental electronic and steric properties of this class of compounds are
thoroughly discussed in reviews by Diez-Gonzalez [9] and Cavallo [10].
The electron bonding characteristics such as strong σ-donor ability and relatively
weak π-acceptor properties make the NHCs mimic transition metal coordination
chemistry of tertiary phosphines. Nevertheless, the main difference between these
types of organic ligands lies in the significantly higher electron-donating ability of
NHC ligands. Therefore the metal-carbene bond is notably shorter and
thermodynamically stronger compared to phosphines, and NHC metal complexes
are more thermally and oxidatively stable. It is necessary to mark that the
important exception arises when the most sterically demanding carbene ligands
interfere with metal-ligand binding.
Figure 2.2. Difference in the steric bulk shape for the ligands.
The difference also occurs in steric properties: if in phosphines the shape of the
steric bulk forms a cone-like arrangement due to the sp3
hybridization of
phosphorus [11], sterically demanding NHCs form fence- or umbrella-like shapes
with substituent groups oriented towards the metal (Figure 2.2) [12]. Contrary to
P
M
R
R
R
Phosphine
M
NN RR
NHC
29. 29
phosphines, the steric properties of NHCs are highly anisotropic with possible
rotation around the metal-carbene bond, which is often named flexible steric bulk.
Another distinguishing feature of NHCs is their synthetic, structural versatility and
the possibility to tune the properties by changing nitrogen substituent groups,
backbone functionality or the kind of heterocycle (Figure 2.3) [13].
NHC complexes of transition metals found a wide range of applications across the
chemical field including their use in metallopharmaceuticals [14-16],
organometallic materials [17,18] and most extensively as homogeneous catalysts
in a variety of reactions [19,20].
Figure 2.3. Common NHC ligands based on imidazole.
2.3 Family of PEPPSI complexes
The name PEPPSI for a group of transition metal complexes was introduced by
Organ et al. in 2006 and is an acronym for pyridine enhanced pre-catalyst
preparation, stabilization and initiation [21]. A schematic representation of the
family of compounds of this type can be found in Scheme 2.1 and consists of a
palladium metal center coordinating two anionic ligands, a mono-ligated N-
heterocyclic carbene and a substituted pyridine, acting as a throw-away ligand.
The first-generation pre-catalysts are the ones bearing less hindered NHC ligands
such as IMes, IEt, SIPr and IPr. They can be prepared by heating the
corresponding carbene salts with a PdCl2 precursor, applying K2CO3 as a base for
N N N N
N NN N
N N N N
.. ..
.. ..
.. ..
IMe ItBu
IPr IMes
ICy IAd
30. 30
the in situ formation of the free carbene and using 3-chloropyridine as a solvent, a
process that provides the desired complexes in high yields. The obtained
complexes show a remarkable catalytic activity in a range of cross-coupling
reactions such as Suzuki-Miyaura [22-24], Negishi [25,26], Kumada-Tamao-
Corriu [27] and Buchwald-Hartwig-Yagupol’skii amination [28-30]. Moreover,
this pre-catalysts are air and moisture stable and are easy to handle.
Scheme 2.1. Synthesis of Pd-PEPPSI complexes.
In all cases the investigation of the influence of the carbene ligand on the
performance of the catalyst showed a dependence of the high reaction yields on
the steric bulk of the NHC ligand: the best results were reached using Pd-PEPPSI-
IPr pre-catalyst, the most sterically demanding one. As the electronic factors are
similar for all the series of ligands, the improved reactivity is attributed to the
steric effects [31].
Figure 2.4. Two generations of PEPPSI pre-catalysts.
In attempt to further improve the reactivity of the catalyst, a second-generation of
PEPPSI pre-catalysts was introduced, where more sterically demanding functional
groups in places of ortho-substituent groups in NHC ligands were used.
N N
PdCl Cl
N
Cl
N N
PdCl Cl
N
Cl
N N
PdCl Cl
N
Cl
N N
PdCl Cl
N
Cl
Pd-PEPPSI-IMes Pd-PEPPSI-IPr Pd-PEPPSI-IPent Pd-PEPPSI-cPent
First-generation pre-catalysts Second-generation pre-catalysts
N N
R
R R
R
R1R1
Cl
PdCl2, K2CO3
3-ClPy
80 oC, 16-24 hr
N N
R
R R
R
R1R1
PdCl Cl
N
Cl
31. 31
Among the developed catalytic systems, Pd-PEPPSI-IPent showed improved
reactivity and superior performance in the carbon-carbon bond formation cross-
coupling reactions as well as in sulfanation and amination processes (Figure 2.4)
[32-34].
2.4 References
1. K. Öfele, J. Organomet. Chem. 1968, 12, 42.
2. H. W. Wanzlick, H.-J. Schönherr, Angew. Chem. Int. Ed. 1968, 7, 141.
3. D. J. Cardin, B. Cetinkaya, M. F. Lappert, Chem. Rev. 1972, 72, 545.
4. A. J. Arduengo, R. L. Harlow, M. A. Kline, J. Am. Chem. Soc. 1991, 113, 361.
5. M. N. Hopkinson, C. Richter, M. Schedler, F. Glorius, Nature 2014, 510, 485.
6. M. Scholl, S. Ding, C. W. Lee, R. H. Grubbs, Org. Lett. 1999, 1, 953.
7. W. A. Herrmann, M. Elison, J. Fischer, C. Kocher, G. R. J. Artus, Angew. Chem.
Int. Ed. Engl. 1995, 34, 2371.
8. G. C. Fortman, S. P. Nolan, Chem. Soc. Rev. 2011, 40, 5151.
9. S. Diez-Gonzalez, S. P. Nolan, Coord. Chem. Rev. 2007, 251, 874.
10. H. Jacobsen, A. Correa, A. Poater, C. Costabile, L. Cavallo, Coord. Chem. Rev.
2009, 253, 687.
11. C. A. Tolman, Chem. Rev. 1977, 77, 313.
12. J. Huang, H.-J. Schanz, E. D. Stevens, S. P. Nolan, Organometallics 1999, 18,
2370.
13. O. Kuhl: “Functionalised N-Heterocyclic Carbene Complexes”, Wiley, 2010.
14. A. Kascatan-Nebioglu, M. J. Panzner, C. A. Tessier, C. L. Cannon, W. J. Youngs,
Coord. Chem. Rev. 2007, 251, 884.
15. K. M. Hindi, M. J. Panzner, C. A. Tessier, C. L. Cannon, W. J. Youngs, Chem.
Rev. 2009, 109, 3859.
16. W. Liu, R. Gust, Chem. Soc. Rev. 2013, 42, 755.
17. L. Mercs, M. Albrecht, Chem. Soc. Rev. 2010, 39, 1903.
18. R. Visbal, M. C. Gimeno, Chem. Soc. Rev. 2014, 43, 3551.
19. C. S. J. Cazin (ed.): ”N-Heterocyclic Carbenes in Transition Metal Catalysis and
Organocatalysis”, Springer, 2011.
20. H. D. Velazquez, F. Verpoort, Chem. Soc. Rev. 2012, 41, 7032.
21. C. J. O’Brien, E. A. B. Kantchev, N. Hadei, C. Valente, G. A. Chass, J. C.
Nasielski, A. Lough, A. C. Hopkinson, M. G. Organ, Chem. Eur. J. 2006, 12,
4743.
22. S. Yaşar, Ç. Şahin, M. Arslan, İ. Özdemir, J. Organomet. Chem. 2015, 776, 107.
32. 32
23. D. Canseco-Gonzalez, A. Gniewek, M. Szulmanowicz, H. Muller-Bunz, A. M.
Trzeciak, M. Albrecht, Chemistry 2012, 18, 6055.
24. L. Ray, M. M. Shaikh, P. Ghosh, Dalton Trans. 2007, 4546.
25. S. Calimsiz, M. Sayah, D. Mallik, M. G. Organ, Angew. Chem. Int. Ed. Engl.
2010, 49, 2014.
26. M. Organ, G. Chass, D.-C. Fang, A. Hopkinson, C. Valente, Synthesis 2008,
2776.
27. M. G. Organ, M. Abdel-Hadi, S. Avola, N. Hadei, J. Nasielski, J. O'Brien, C.
Valente, Chemistry 2007, 13, 150.
28. Y. Zhang, G. Lavigne, V. Cesar, J. Org. Chem. 2015, 80, 7666.
29. Y. Zhang, V. César, G. Lavigne, Eur. J. Org. Chem. 2015, 2042.
30. A. Chartoire, X. Frogneux, A. Boreux, A. M. Z. Slawin, S. P. Nolan,
Organometallics 2012, 31, 6947.
31. C. Valente, S. Calimsiz, K. H. Hoi, D. Mallik, M. Sayah, M. G. Organ, Angew.
Chem. Int. Ed. 2012, 51, 3314.
32. S. Calimsiz, M. Sayah, D. Mallik, M. G. Organ, Angew. Chem. 2010, 122, 2058;
Angew. Chem. Int. Ed. 2010, 49, 2014.
33. K. H. Hoi, S. Calimsiz, R. D. J. Froese, A. C. Hopkinson, M. G. Organ, Chem.
Eur. J. 2012, 18, 145.
34. M. Sayah, M. G. Organ, Chem. Eur. J. 2011, 17, 11719.
33. 33
Chapter 3. Supported homogeneous
complexes
3.1. Introduction
Well-defined single-site homogeneous catalysis is a thoroughly studied area since
these catalysts possess a number of indisputable advantages: they often allow
transformations under mild reaction conditions, provide good selectivity in the
product formation and give an opportunity to investigate structure-reactivity
relationships for a further rational improvement of the catalytic system.
Nevertheless there are also a few significant drawbacks. Frequently homogeneous
catalysts can be deactivated by dimerization during the catalytic reaction, their
separation from the product phase constantly present considerable difficulties,
leading to a metal contamination, and the recyclability is impossible.
On the other hand, heterogeneous catalysts are easy to handle as they are mostly
presented in a different phase than the products and can be readily reused several
times, which is beneficial from an economical perspective. Moreover, such
catalytic systems can be used in continuous flow processes that are highly
attractive on the industrial scale. However the nature of the active sites in these
systems is hard to evaluate and often the activity of heterogeneous catalysts is
significantly lower compared to the homogeneous counterparts.
The intention to combine the main advantages of homogeneous and heterogeneous
catalysis resulted in the idea of immobilizing well-defined soluble metallic species
on a suitable solid support such as organic polymers, inorganic oxides, zeolites
and hybrid organic-inorganic materials (Scheme 3.1) [1-5].
3.2. Immobilization strategies
Two main synthetic pathways were suggested for construction of materials
containing immobilized single-site catalysts. The first one consists in direct
grafting of the organometallic complex on a surface of the support by a covalent or
34. 34
ionic bond or via Lewis acid-base interaction between the metal center and the
surface of the support. Thus, the surface functions as a macroligand and is directly
involved in the coordination sphere of the metal, where the rest of the ligand
environment influences the activity and stability of the formed material as well as
selectivity of the catalysis. This approach is called surface organometallic
chemistry [6-9].
Scheme 3.1. Immobilization of the organometallic complexes on a support: a) a two-step
process with initial incorporation of a ligand and further formation of the complex; b)
direct grafting of a complex containing pre-functionalized ligand with a linker group; c)
surface organometallic chemistry strategy.
Another technique is based on the design of a homogeneous complex, containing a
ligand with one or more functional groups that can be used as a linker to bind
covalently or via an ionic interaction to the selected support – so-called single-site
supported homogeneous catalysis. This strategy splits into two main synthetic
routes: initial creation of the material, containing an incorporated ligand, and
subsequent synthesis of the desired complex on the surface of the material [10-13]
or development of the organometallic complex containing a ligand with a linker
group and its direct grafting to the surface [14-16].
The main difference between the discussed heterogenization approaches is in a
tendency to loose the molecular character of the catalyst in the case of surface
Support
Support
Support
L L L
M
X
Y L
Support
L
M
X
Y
L
M
X
Y
L
M
X Y
L MX Y
a
b
c
++
+
+
35. 35
organometallic chemistry: often the metal center forms several linkages with the
surface.
3.3 Mesoporous silica SBA-15 as a support in catalysis
Porous materials can be divided into three classes in accordance with IUPAC
definition: microporous (with pore size < 2 nm), mesoporous (2 – 50 nm) and
macroporous (> 50 nm) [17]. The first synthesis of an ordered mesoporous
material was described in the literature as early as in 1969, although due to a lack
of analytical techniques, its outstanding properties were concealed until a 1992
report by Mobil Oil Corporation on the remarkable features of the novel type of
silica – MCM-41 [18]. Since this discovery a large variety of ordered mesoporous
materials was synthesized, thoroughly investigated and used in different areas of
research [19-25].
Mesoporous silica SBA-15 (Santa Barbara Amorphous No. 15) presents a well-
studied material with a 2D hexagonal pore framework [26] (Figure . It has thick
walls of 3-7 nm thicknesses and narrowly distributed large mesopores that can be
regulated by the choice of the synthetic conditions between 6 to approximately 15
nm [27]. Owing to its thick walls, the present silica possess high thermal and
hydrothermal stability compared to the related mesoporous materials together with
high mechanical durability [28]. Other specific characteristic of SBA-15 is the
presence of microporous irregular channels inside of the mesopore wall – intrawall
pores, which connect the adjacent mesopores [29].
Figure 3.1. TEM image of SBA-15. Scale bar: 50 nm.
36. 36
SBA-15 by itself is quite inert from the point of view of catalytic activity and
presents much higher interest as a catalyst support due to a large pore size that
allows unrestricted diffusion of bulky reactants and products within the
mesoporous system, even after immobilization of the large catalytically active
sites on the material. In addition, this silica has a very high surface area (800-1000
m2
/g) that allows uniform distribution and high concentration of the active sites on
the surface.
3.4 Functionalization of mesoporous silica
There are three general approaches for functionalization of mesoporous materials:
the first one incorporates organosilane molecules or metal species in a “one-pot”
synthetic process. Commonly it is based on the co-condensation of functionalized
organosiloxane and siloxane moieties directly during the synthesis of the material
[30,31]. The second route consists in the use of bissilylated organic precursors for
construction of periodic mesoporous organosilicas [32]. The last method presents
subsequent grafting of organic or organometallic species onto a pristine silica
matrix.
Although a high degree of functionalization can be achieved applying the first two
approaches, the obtained hybrid materials often suffer from structural
deformations and as a consequence have poor durability parameters. In addition,
synthesis of such materials is frequently quite costly.
The surface of mesoporous silica consists of siloxane bridges and silanols, the
relative concentration of which depends on the temperature of a pre-treatment.
Post-synthesis modification requires a presence of active groups on the material
surface that can function as anchor sites for functional groups. The advantage of
this method is that under applied synthetic conditions the mesostructure of the
starting silica is entirely maintained while the surface of the walls is significantly
changed. Prevalent functionalities are chloroalkylsilane, trialkoxysilane or silazane
derivatives that utilize free silanol groups of the pore surfaces. However, several
research reports show that the density of silanols in ordered mesoporous silica is
lower compared to conventional hydroxylated silica (about 4-6 Si-OH/nm2
) [33].
For SBA-15 pretreated at 200 °C under vacuum a concentration of only around 1
Si-OH/nm2
is reported [34].
As the surface silanols can considerably influence stability of the incorporated
catalyst as well as the substrate interaction with the surface, the silylation methods
of the surface hydroxyls are broadly applied.
37. 37
3.5 Main challenges for supported systems
Despite its potential importance, supported homogeneous catalysts still have not
been used on an industrial scale because of several challenging reasons [35]:
1. First of all, quite often leaching of the immobilized complexes into the
reaction medium occurs, leading to contamination of products and in
some cases resulting in complete deactivation of the catalyst.
2. Prepared supported catalysts are frequently not stable and decompose
under the catalytic reaction conditions and, therefore, can not be recycled.
3. The synthetic pathway for their development is complicated, requires
several synthetic steps and is economically insufficient.
4. For most of the developed heterogenized systems low turnover numbers
and lower selectivity have been observed.
Thus, the current field must be thoroughly investigated and it is very
important to determine reasons leading to the deactivation of the supported
catalysts and their decomposition.
3.6 References
1. C. Coperet, J.-M. Basset, Adv. Synth. Catal. 2007, 349, 78.
2. A. Corma, H. Garcia, Adv. Synth. Catal. 2006, 348, 1391.
3. C. A. McNamara, M.J. Dixon, M. Bradley, Chem. Rev. 2002, 102, 3275.
4. D. E. De Vos, M. Dams, B. F. Sels, P. A. Jacobs, Chem. Rev. 2002, 102, 3615.
5. A. P. Wight, M. E. Davis, Chem. Rev. 2002, 102, 3589.
6. Y. I. Yermakov, B. N. Kuznetsov, V. A. Zakharov, Stud. Surf. Sci. Catal. 1981, 8,
1.
7. D. G. H. Ballard, Adv. Cat. 1973, 23, 263.
8. C. Coperet, M. Chabanas, R. Petroff Saint-Arroman, J.-M. Basset, Angew. Chem.
Int. Ed. 2003, 42, 156.
9. C. Copéret, A. Comas-Vives, M. P. Conley, D. P. Estes, A. Fedorov, V. Mougel,
H. Nagae, F. Núñez-Zarur, P. A. Zhizhko, Chem. Rev. 2016, 116 (2), 323.
10. A. Sattler, D. C. Aluthge, J. R. Winkler, J. A. Labinger, J. E. Bercaw, ACS Catal.
2016, 6, 19.
11. D. Sahu, A. R. Silva, P. Das, RSC Adv. 2015, 5, 78553.
12. T. Iwai, S. Konishi, T. Miyazaki, S. Kawamorita, N. Yokokawa, H. Ohmiya, M.
Sawamura, ACS Catal. 2015, 5, 7254.
13. M. P. Conley, R. M. Drost, M. Baffert, D. Gajan, C. Elsevier, W. T. Franks, H.
Oschkinat, L. Veyre, A. Zagdoun, A. Rossini, M. Lelli, A. Lesage, G. Casano, O.
38. 38
Ouari, P. Tordo, L. Emsley, C. Coperet, C. Thieuleux, Chem. Eur. J. 2013, 19,
12234.
14. D. P. Allen, M. M. Van Wingerden, R. H. Grubbs, Org. Lett. 2009, 11(6), 1261.
15. S. A. Raynor, J. M. Thomas, R. Raja, B. F. G. Johnson, R. G. Bell, M. G. Mantle,
Chem. Commun. 2000, 1925.
16. V
17. K. S. W. Sing, D. H. Everett, R. A. W. Haul, L. Moscou, R. A. Pierotti, J.
Rouquerol, T. Siemieniewska, Pure Appl. Chem. 1985, 57, 603.
18. J. S. Beck, C. T.-W. Chu, I. D. Johnson, C. T. Kresge, M. E. Leonowicz, W. J.
Roth, J. W. Vartuli, WO Patent 91/11390, 1991.
19. J. M. Thomas, Angew. Chem. Int. Ed. 1999, 38, 3588.
20. A. Corma, Chem. Rev. 1997, 97, 2373.
21. X. He, D. Antonelli, Angew. Chem. Int. Ed. 2002, 41, 214.
22. G. J. de A. A. Soler-Illia, C. Sanchez, B. Leveau, J. Patarin, Chem. Rev. 2002,
102, 4093.
23. F. Schuth, Chem. Mater. 2001, 13, 3184.
24. P. Yang, S. Gaib, J. Lin, Chem. Soc. Rev. 2012, 41, 3679.
25. C. Gérardin, J. Reboul, M. Bonne, B. Lebeau, Chem. Soc. Rev. 2013, 42, 4217.
26. D. Zhao, J. F. Q. Huo, B. F. Chmelka, G. D. Stucky, J. Am. Chem. Soc. 1998,
120, 6024.
27. D. Zhao, J. Feng, Q. Huo, N. Melosh, G. H. Fredrickson, B. F. Chmelka, G. D.
Stucky, Science 1998, 279, 548.
28. A. Galarneau, D. Desplantier-Giscard, F. di Renzo, F. Fajula, Catal. Today 2001,
68, 191.
29. M. Imperor-Clerc, P. Davidson, A. Davidson, J. Am. Chem. Soc. 2000, 122,
11925.
30. F. Hoffmann, M. Cornelius, J. Morell, M. Fröba, Angew. Chem. Int. Ed. 2006, 45,
3216.
31. A. Patti, A. D. Mackie, V. Zelenakc, F. R. Siperstein, J. Mater. Chem. 2009, 19,
724.
32. M. Cornelius, F. Hoffmann, M. Froeba, Chem. Mater. 2005, 17(26), 6674.
33. L. T. Zhuravlev, Langmuir 1987, 3, 316.
34. J. Jarupatrakorn, T. D. Tilley, J. Am. Chem. Soc. 2002, 124, 8380.
35. S. Hübner, J. G. de Vries, V. Farina, Adv. Synth. Catal. 2016, 358,3.
39. 39
Chapter 4. Experimental X-ray
techniques
4.1 Introduction
One of the main challenges in the supported homogeneous catalysis is the limited
number of available techniques for characterization of obtained materials. It was
suggested that the specially designed surface science X-ray techniques could be
used for gaining insightful information about the immobilized catalyst structure,
chemical environment, geometry and oxidation state of the elements.
Among the many possible analytical tools to investigate surface properties, the
techniques based on the excitation of a sample surface with X-ray irradiation,
causing photoelectrons to be emitted, are most widely used. The main limitation of
these methods consists in the requirement for ultrahigh vacuum (UHV) conditions
(basic pressure of 10-10
mbar) due to a very thin layer of the surface that can be
probed by electron spectroscopy. Therefore, for the successful experiment it is
necessary to have atomically clean surface during the measurement.
The photon-electron interaction can be described by the photoemission and
photoabsorption processes (Figure 4.1). In X-ray photoemission spectroscopy
(XPS) absorption of a photon with well-defined energy by an atom leads to the
electron emission from either a valence or a core shell. If the energy of the photon
is close to the absorption threshold, the emitted electron may be absorbed into an
unoccupied molecular orbital level of the atomic sample (X-ray absorption
spectroscopy) [1].
4.2 X-ray Photoelectron Spectroscopy
Photoelectron spectroscopy is a technique for the analysis of the surface chemistry
of the material such as composition and electronic state of the elements. The exact
energy of the photoemitted electrons is depending on the nature of the element,
their oxidation state and the chemical environment of the atom under study.
40. 40
Therefore it is often named as Electron Spectroscopy for Chemical Analysis
(ESCA) [2].
Figure 4.1. Schematic illustration of photon-electron interactions induced by X-ray
irradiation: photoemission, X-ray absorption and Auger decay processes.
The kinetic energy distribution (Ekin) of the photoemitted electrons is measured by
an electron energy analyser resulting in a photoelectron spectrum. The binding
energy (BE) defining the energy required to remove the electron from the surface,
can be determined from the following equation:
BE = hv – Ekin – ϕ;
where ϕ is the work function of the sample surface, hv is the fixed energy of the X-
rays. The development of this technique was performed by Siegbahn and his
research group in Uppsala [3] and led to a Nobel Prize awarded in 1981.
An X-ray photoemission (XP) spectrum represents the number of collected
electrons as a function of kinetic energy, from where it is possible to calculate the
binding energy. Photoemission peaks appearing in the spectrum belong to specific
atoms emitting electrons of a characteristic energy. The integrated area under the
peak represents a measure of the relative amount of the element characteristic for
this binding energy. Hence the binding energies and intensities of the peaks allow
identification and quantification of all surface elements [4].
The exact binding energy of the emitting electron depends not only on the core-
level from which photoemission is happening but also on the formal oxidation
state of the atom and the local chemical environment. Difference in these
Core level
HOMOs
LUMOs
hv
X-ray photoemission X-ray absorption Auger decay
41. 41
parameters resulting in slight shifts of the photoemission peak position in the
spectrum also called chemical shifts [5]. For example, atoms of a higher oxidation
state exhibit a higher binding energy due to the extra Coulomb interaction between
the emitted electron and the ion core.
In a standard spectrum besides the main photoemission peak other features can be
observed. It includes a continuous background induced by inelastic losses of the
photoelectrons, additional lines called satellites and in some cases Auger electron
decay lines [6]. Analysis of Auger electrons can be a complimentary tool in X-ray
photoemission spectroscopy [7].
A typical XPS setup commonly consists of a source of X-rays, a vacuum system
(high vacuum or UHV), preparation chamber with a sample stage, analysis
chamber, sample load lock and hemispherical electron energy analyzer (Figure
4.2).
Figure 4.2. Schematic drawing of the XPS setup.
Traditional X-ray sources include lasers, monochromatic anodes and discharge
lamps. Since the 1980s, electron accelerators for production of synchrotron
radiation were found to be highly powerful light sources providing a wide energy
range starting from “hard” X-rays (above 2 keV) to “soft” X-rays (below 2 keV).
Synchrotron radiation consists of electrons with a velocity close to the speed of
light accelerated onto a curved trajectory that is enforced by a strong magnetic
field. The most broadly used electron accelerators for this propose are electron
storage rings that apply bending magnets, undulators and wigglers for the
acceleration of the electrons.
hv
X-ray
source
e-
Electron
energy
analyser
Sample
42. 42
4.3 X-ray absorption spectroscopy
The technique that can provide geometrical orientation and structural
characterization of the material under study called X-ray absorption spectroscopy.
The range of the samples that can be probed by this method is quite broad and
include all three aggregation states – solutions, gases and solid matter.
If the energy of the incident X-ray beam is equal to the energy gap between the
electronic ground state and excited state of the atom, absorption of a photon can
happen. The excited state is represented by a core-level hole and an electron,
situated on a previously unoccupied orbital. Atom in the excited state is very
unstable and easily undergoes electron decay back to the ground state. Therefore
applying the photon energy close to the absorption threshold leads to the
generation of excited core-level electrons and belongs to NEXAFS (Near Edge X-
ray Absorption Fine Structure) spectral region. In the EXAFS (Extended X-ray
Absorption Fine Structure) region slightly higher kinetic energies are used what
causes scattering of the outgoing photoelectrons, leading to the energy-dependent
modulation of the photoelectron intensity due to constructive and destructive
interference. Information about interatomic distances can be obtained from the
spectral lineshape in this region [8].
X-ray absorption spectrum represents a function of excitation photon energy. The
outstanding characteristic of XAS is the possibility to obtain information about the
geometrical orientation of the molecules under study. There are two types of
unoccupied antibonding molecular orbitals: low-energy π* and high-energy σ*.
Creation of a σ bond resulting in appearance of the σ resonance in the absorption
spectrum. The π resonance is assigned to the transition from the core level to the
lowest unoccupied level and presented in spectrum as first several peaks. The
width of the peaks is depended on the transition energy of the resonance. If the
transition energy is lower than the ionization energy, the peak will have a sharp
shape. And if the transition energy is higher than the ionization energy, the excited
states are less bound leading to the broader lineshapes at higher energies in the
spectrum [9].
4.4 References
1. K. Oura, V. G. Lifshits, A. A. Saranin, A. V. Zotov, M. Katayama: “Surface
Science: An Introduction (Advanced Texts in Physics)”, New York, Springer,
2010.
43. 43
2. K. Siegbahn, C. Nordling, R. Fahlman, R. Nordberg, K. Hamrin, J. Hedman, G.
Johansson, T. Bergmark, S.-E. Karlsson, I. Lindgren, B. Lindberg, Nova Acta
Regiae Soc. Sci., Upsaliensis, Ser. IV, Vol. 20, 1967.
3. K. Siegbahn, J. Elec. Spec. Rel. Phen. 1985, 36, 113.
4. S. Hüfner; “Photoelectron Spectroscopy: Principles and Applications”, 3d ed.,
Springer, 2003.
5. J. N. Andersen, D. Henning, E. Lundgren, M. Methfessel, R. Nyholm, M.
Scheffler, Phys. Rev. B 1994, 50, 17525.
6. C. S. Fadley, Surf. Interface Anal. 2008, 40, 1579.
7. M. P. Seah, D. Briggs (eds.): “Practical Surface Analysis by Auger and X-ray
Photoelectron Spectroscopy”, 2nd
ed., Wiley & Sons, Chichester, 1992.
8. J. Stöhr: “X-ray Absorption: Pronciples, Applications, Techniques of EXAFS,
SEXAFS and XANES”, R. Prins, New York, Wiley, 1988.
9. J. Stöhr: “NEXAFS Spectroscopy”, 1st
ed., Springer, 1996.
45. 45
Chapter 5. Summary of key results
5.1 Novel platinum(II) NHC complex: synthesis and
spectroscopic characterization [Paper I]
Transition metal complexes of N-heterocyclic carbenes are a fascinating class of
compounds that find widespread application in different areas of research,
particularly in homogeneous catalysis (see Chapter 2). Although these compounds
have been extensively studied during the last few decades, so far most of the
research interest within the field of catalysis is focused on complexes of palladium
and ruthenium. Along with them platinum complexes of NHCs are highly
significant, but they have been much less investigated. Our study started with the
development of a synthetic approach to a platinum(II) complex bearing an N-
heterocyclic carbene as a ligand. We focused our attention on the so-called
PEPPSI-type complexes, commonly based on the Pd center, that are well-known
for their high activity in a variety of cross-coupling reactions under ambient
conditions. For the synthesis of the platinum analogue, a standard procedure was
used (Scheme 5.1). The desired complex was obtained in 73 % isolated yield.
Scheme 5.1. Synthesis of the Pt-IPr complex.
It is worth noting, that the prolongation of the reaction time up to 64 hours led to
the formation of the bis-chloropyridine complex of platinum, trans-Pt(3-ClPy)2Cl2,
despite the nature of the strong metal-carbene bond.
N N
Cl
PtCl2, K2CO3
3-ClPy
80 oC, 48 h
N N
PtCl Cl
N
Cl
46. 46
To fully characterize and investigate the structural features of the Pt-IPr
compound, a single crystal X-ray diffraction experiment was performed and the
molecular structure of the complex was obtained (Figure 5.1). As the experimental
data shows, the Pt(II) center has a slightly distorted square-planar geometry with a
trans arrangement of the two chloride ligands. Other structural characteristics such
as bond angles and bond lengths were in agreement with the previously published
data for similar complexes [1-3].
Figure 5.1. Molecular structure of the Pt-IPr complex. Hydrogen atoms omitted for clarity.
To extend our knowledge about the influence of the specific chemical environment
around the metal ion on the electronic and geometric structure in the obtained
complexes, it was decided to apply X-ray absorption spectroscopy. Moreover,
such information can be used for the characterization of supported platinum-
carbene complexes. Figure 5.2 displays the X-ray absorption (XA) spectra of the
complexes at the Pt L3-edge.
We focused on the analysis of XANES region of the spectra due to its high
sensitivity to the local coordination environment around the particular absorbing
atom. The first sharp spectral feature at the Pt L3-edge, also called the white line
(WL), is attributed to the dipole-allowed transition from Pt 2p3/2 to the unoccupied
5d5/2 and 5d3/2 levels that lie above the Fermi level. It can be considered as a rather
precise fingerprint of the oxidation state of the Pt center.
Interesting features of the XAS spectrum can be found when comparing the
behavior of the Pt-IPr (1) and trans-Pt(3-ClPy)2Cl2 (2). As can be seen from Figure
5.2, replacement of 3-chloropyridine with the IPr carbene ligand leads to a
broadening of the white line and to a decrease of its intensity. As the intensity of
the WL reflects the unoccupied density of state at the Pt(II) center it can be
47. 47
concluded that the additional electron density associated with the strong σ-donor
ability of the NHC ligand results in a higher d-electron density at the Pt center.
This finding is in accordance with the crystallography data: the Pt−N bond
distance between platinum and 3-chloropyridine is slightly longer in the case of
the carbene complex 1, indicating a high trans influence of the carbene ligand.
Figure 5.2. X-ray absorption spectra for the Pt-IPr complex - 1 (red), and the trans-Pt(3-
ClPy)2Cl2 - 2 (black) at the Pt L3-edge. The white line and the hybridization peak are
indicated by WL and HP respectively. The inset zooms on the near-edge region and shows
the FEFF 9.0 simulations for complex 1 (orange) and 2 (grey) based on the crystal
structures, with an arbitrary offset in the y axis for clarity
Another important feature of the XANES spectra is the hybridization peak (HP) - a
shoulder that can be found after the WL peak [4]. HP is a peak corresponding to
excitations of unoccupied Cl 3d-states mixed with Pt d-states in Pt−Cl systems,
and for 1 it is more diffuse than for complex 2. This can be considered as a
distinguishable feature of complex 1.
As a conclusion, it was shown that the Pt-NHC bond has its unique features that
can be determined as a sensitive spectral fingerprint in XANES region at the Pt L3-
edge. This information might be used for a further examination of such
compounds e.g. for in situ catalytic studies applying specially-designed analysis
48. 48
cells combined with XANES instrument that may provide insights into the
mechanism of the reaction [5].
5.2 Catalytic activity of the Pt-IPr complex [Paper I]
As the research interest of our group is focused on the investigation of C−H
activation transformations, the initially obtained Pt-IPr complex was applied as a
catalyst in a range of ligand-directed C−H functionalization reactions. In a model
reaction 4-(2-pyridyl)benzaldehyde was reacted with PhI(OAc)2, NCS or Ph2IPF6
and 5 mol % of the catalyst and heated at 100 °C for 24 h. Unfortunately, complex
1 did not show any catalytic activity under the applied reaction conditions.
Catalytic studies were continued with an examination of the possible reactivity of
1 in the hydrosilylation reaction. Carbene complexes of platinum represent
attractive catalysts in such transformations due to the low amounts of isomerized
olefins produced and the undetectable formation of colloidal platinum [6]. As a
benchmark reaction hydrosilylation of styrene in the presence of bis-
(trimethylsilyloxy)methylsilane was chosen (Table 5.1).
Table 5.1. Hydrosilylation of styrene.
Entrya
T o
C Conversion
(styrene) [%]b
Ratio A:Bb
1 100 96 85:15
2 140 87 87:13
a
Reaction conditions: 0.5 mol % of the catalyst, styrene (4 mmol), bis(trimethyl-
silyloxy)methylsilane (4.4 mmol), 6h. b
Determinded by 1
H NMR.
We were pleased to find that the 0.5 mol % of the Pt-IPr complex catalyzes up to
96 % conversion of styrene to the products A and B at 100 °C after 6 h in
accordance with the 1
H NMR data. The selectivity between the hydrosilane
+ SiHMe
O
O
SiMe3
SiMe3
Si
O
Me
SiMe3
SiMe3
O
Si
Me
O O SiMe3Me3Si
+
0.5 mol % Pt-IPr
A B
49. 49
addition products A vs. B was 85 to 15 %, respectively, which is within the range
reported by Strassner et al. [2]. It is important to note, that an increase of the
reaction temperature to 140 °C led to a decrease of the styrene conversion and the
formation of platinum black was observed.
5.3 Different Pd-PEPPSI complexes in selective ligand-
directed C−H acetoxylation [Paper II]
Compounds of Pd(II) are well-known for their ability to take part in the C−H
activation processes. The main issue for such transformations lies in a challenge to
reach a selective functionalization of a desired C−H bond within a complex
molecule containing a number of positions suitable for activation [7-12]. Different
approaches have been suggested to solve this problem. The most prevalent
solution consists in the utilization of substrates containing so-called directing
groups – ligands that are able to coordinate to a metal center, which selectively
delivers the functional group to a proximal C−H site (see Chapter 1). However
even in this case selectivity issues can occur. A more general method to control the
site-selectivity is a catalyst-based control, where the selectivity is determined by
the ligand environment of the catalytic complex. Therefore, the development of
ligated palladium complexes with a tunable ligand environment that are able not
only to promote C–H bond activation but to affect the site-selectivity of the
process is highly desirable.
The family of Pd-PEPPSI complexes represents a range of Pd-NHC compounds
stabilized by 3-chloropyridine as an ancillary ligand. These compounds have been
widely investigated mostly due to their high catalytic activity in cross-coupling
reactions. However there are no reports on their application to C−H bond
functionalization processes. Moreover, there are just a few reports of examples of
any Pd-NHC catalysts applied to C−H activation, mostly focused on methane
oxidation and direct arylation procedures [13-17]. Therefore, we decided to focus
our research efforts on the investigation of applicability of a variety of Pd-PEPPSI
catalyst to the ligand-directed C−H oxygenation reactions. Particularly, we were
interested in examination of the influence of the carbene ligand structure on the
catalytic process.
Previously reported elegant examples of Pd(OAc)2-catalysed ligand-directed C–H
bond oxygenation have been obtained by Sanford and her research group [18-20].
The critical issue in this work is that for substrates possessing two equivalent
positions for aryl ortho-C–H bond functionalization only modest yields of mono-
oxidized products were obtained and difunctionalized derivatives were reported as
the main products.
50. 50
Inspired by above-mentioned research results, our initial studies started with an
examination of the C−H oxidative functionalization reaction. To have a broad
structural variety, palladium complexes with following NHC ligands were
evaluated: IMes, SIPr, IPent, IPr and Ad (Figure 5.3).
Figure 5.3. Family of Pd-PEPPSI complexes: PEPPSI-IMes, PEPPSI-IPr, PEPPSI-Ad,
PEPPSI-IPent, PEPPSI-SIPr.
The complex bearing the Ad ligand was synthesized for the first time in our
laboratory and fully characterized, including a single-crystal X-ray analysis. The
choice of this ligand was based on the difference in electronic effects of the
substituent groups in the imidazole ring in comparison with other NHC ligands in
the series, that potentially could influence the activity of the catalytic complex.
The listed complexes were screened in the benchmark reaction of the 2-
phenylpyridine acetoxylation in an attempt to see the dependence of the catalytic
behavior from the structural diversity. Surprisingly, the comparison showed only a
minor difference in selectivity of the mono-acetoxylation and a slight distinction in
conversion of the substrate for different catalysts, with the best overall result
obtained by PEPPSI-IPr complex (Table 5.2). However, a significant enhancement
in selectivity for mono-functionalized product formation was found compared to
previous work where Pd(OAc)2 was used as a catalyst [19]. After optimization of
the reaction conditions, 2-(2-acetoxyphenyl)pyridine was obtained in 72 %
isolated yield, that is 20 %-units higher than the result reported for Pd(OAc)2.
Thus, it can be concluded, that the NHC ligand on the palladium center plays a
major role in the selectivity of the process.
Trying to understand the observed selectivity, one can assume that during the
catalysis the steric bulk of the NHC ligand blocks the second ortho-position in the
phenyl ring making formation of difunctionalized product disadvantageous.
N N
..
IMes
PdCl Cl
N
Cl
N N
..
SIPr
N N
..
IPr
N N
..
IPent
NHC
[Pd(NHC)(2-ClPy)Cl2]
N N
..
Ad
51. 51
Nevertheless, as previously was mentioned, the comparative study of complexes
containing a range of NHC ligands with various steric properties did not show any
difference in the selectivity of the catalytic process. The electronic factors of the
ligands within the series are also not playing a crucial role as can be concluded
from the comparison of e.g. the reactivity of PEPPSI-IPr and PEPPSI-Ad.
Table 5.2. Catalyst screening for the direct acetoxylation of 2-phenylpyridine.
Entrya
Catalyst Selectivity to Ab
Conversionc
1 PEPPSI-SIPr 79 77
2 PEPPSI-IPr 82 93
3 PEPPSI-IMes 80 85
4 PEPPSI-IPent 82 85
5 PEPPSI-Ad 82 88
a
Reaction conditions: 3 mol-% of catalyst, 1 equiv. of substrate, 1.1 equiv.of PhI(OAc)2 in
MeCN, 92 °C, 12 h. b
Based on results of GC analysis for A and B upon an average of two
runs. c
Determined by GC analysis with mesitylene as the calibrated internal standard
based upon an average of two runs.
To further investigate the catalytic process, the kinetic profile of the reaction was
determined by gas chromatography. As can be seen from Figure 5.4, a
characteristic feature of the kinetic plot is an induction period during the first hour
of the reaction. Its presence may be caused by an activation barrier existing for the
molecular precatalyst. The initial conversion up to 20 % is probably due to a
background reaction of the substrate with the oxidant. However, this induction
period could also be a consequence of the heterogeneous catalytic behavior
induced by Pd nanoparticles. To shed some light on this hypothesis, the kinetic
profile of reaction in the presence of mercury(0) was recorded and almost no
influence on the reaction rate could be observed. Therefore, it can be concluded,
that the catalytic activity in the process under study is attributed to the molecular
catalyst rather than Pd0
species.
N
Pd-PEPPSI
PhI(OAc)2
MeCN, 92 oC
12h
N
AcO
N
AcO
AcO
+
A B
52. 52
Figure 5.4. Kinetic profile of the C–H acetoxylation of 2-phenylpyridine in the precence
of Hg0
(violet) and without Hg0
(green) determined by GC chromatography with
mesitylene as the calibrated internal standard based upon an average of two run
Encouraged by these results we decided to expand our studies of the C−H
acetoxylation on a wider scope of substrates to check the applicability of PEPPSI-
IPr catalyst for activation of both sp2
as well as more challenging sp3
C−H sites
(Table 5.3). We were pleased to find that in most cases it was possible to
selectively obtain a monosubstituted product in good to excellent yields.
Particularly noteworthy is the almost quantitative selectivity for tolylpyridine as a
substrate (entry 1). Conversion of bulky substrates such as azobenzene and N-
benzylideneaniline unfortunately gave no product possibly due to too high
bulkiness of both the substrate and the catalyst. At the same time, several
previously unreported monoacetoxylated products were successfully obtained in
high yields. Thus, compared to the previously reported results, it was possible to
decrease the Pd loading as well as the amount of the rather expensive oxidant in
addition to obtaining significantly improved selectivity for mono-
functionalization.
53. 53
Table 5.3. Substrate scope of the C−H acetoxylation.
a
Determined by GC with mesitylene as the calibrated internal standard based upon an
average of two runs. Isolated yields are in parenthesis.
N
N
N
O
N
O
N
N
N
N
N
N
N
N
N O
N
N
N
N
O
N
O
N
N
N
N
N
N
N
N O
N
AcO
AcO
AcO
AcO
AcO
AcO
AcO
AcO
OAc
OAc
OAc
N AcO
Entry Substarte Product Yielda
1
2
3
4
5
6
7
8
9
10
11
12
96(89)
(77)
89(89)
(15)
62
59(52)
70
0
0
71
65
98
54. 54
5.4 Development of Pd-NHC catalysts supported on
SBA-15 [Paper III]
Remarkable reactivity and, more importantly, selectivity of the Pd-PEPPSI
complexes towards a ligand-directed C−H oxidative functionalization has
prompted the idea of an immobilization of similar complexes on a suitable support
for creation of supported homogeneous catalysts. The presence of a strong metal-
carbene bond in such compounds was a promising starting point for the formation
of the stable catalytic systems. From the multitude of possible supports we chose
mesoporous silica, namely SBA-15, due to its significant mechanical strength and
thermal/hydrothermal stability, relative chemical inertness, high surface area and
considerable pore size.
As a first step to immobilized catalysts, two modified carbene ligands bearing a
linker group were designed (Scheme 5.3). To investigate the impact of the linker
structure on the stability and catalytic activity of a grafted complex, we selected
two structurally different alkoxysilanes: one containing a flexible chain and one
with a phenylene ring in the structure. Alkoxysilanes have an advantage over
chlorosilanes as no acidic byproducts are formed during the reaction with the
surface of silica, which could destroy sensitive immobilizing organic
functionalities or transition metal complexes [21].
Scheme 5.3. Synthesis of the NHC ligands with a linker group.
Through the alkoxysilyl functionalities it is possible to perform a condensation
between obtained ligands and the surface hydroxyl groups of the mesoporous
silica. However, the number of silanol groups on the surface of SBA-15 is
relatively low (see Chapter 3) and accompanied by a substantial amount of
physisorbed water. Thus, prior to the immobilization it was necessary to perform
N N + Cl Si(OEt)3 N N Si OEt
OEt
OEtCl150 oC, 24 h
N N +
Cl
Si(OMe)3
diglyme
120 oC, 24 h
N N
Si OMe
OMe
OMe
Cl
55. 55
an activation of the SBA-15 material. We chose to use calcination at 600 °C for 6
hours to create siloxane bridges which act as reactive chemisorption sites [22-25].
Therefore, the immobilization of the ligands was performed on freshly calcined
SBA-15 mesoporous silica (Scheme 5.4) To prevent possible side reactions and
improve the stability of the obtained complexes we performed a silylation of the
remaining surface hydroxyls with HMDS prior to the coordination of the
palladium source. The obtained materials were characterized by solid-state NMR
spectroscopy, BET measurements and TGA.
Scheme 5.4. Immobilization of modified NHC ligands.
The next step consisted in a synthesis of palladium complexes on the surface of
the functionalized SBA-15 material containing NHC ligands. Pd(PhCN)2Cl2 was
used as a precursor, applying KHMDS as a base for the deprotonation of the
carbene ligands (Scheme 5.5). Unfortunately, as was revealed by TEM studies, the
obtained material was not homogeneous and contained metallic palladium
inclusions that were not soluble in tolerated organic solvents. An attempt to
prepare the supported complexes by analogy with a standard synthetic procedure
for the Pd-PEPPSI-type complexes preparation was also unsuccessful. Therefore,
we concluded that the selected two-step synthetic methods were not suitable for
obtaining the supported homogeneous catalysts.
Si
Si
Si
O
HO
HO
O
OO
N N Si OEt
OEt
OEtCl
+
Si
Si
Si
O
Me3SiO
Me3SiO
O
OO
N N Si
O
EtO
Cl
1. Toluene
105 °C, 24h
Si
Si
Si
O
HO
HO
O
OO
N N
Si OMe
OMe
OMe
Cl
+
Si
Si
Si
Me3SiO
Me3SiO
O
OO
N N
Si O
OMeO
Cl
2. HMDS
RT, 24h
1. Toluene
105 °C, 24h
2. HMDS
RT, 24h
Pr@SBA-15
Ph@SBA-15
56. 56
Scheme 5.5. Two-step synthetic pathway for obtaining immobilized Pd-NHC complexes.
In order to change the synthetic approach, we synthesized homogeneous
complexes of Pd containing the modified NHC ligands (Scheme 5.6). These
complexes were further directly grafted on a freshly calcined SBA-15 surface and
the remaining silanol groups were end-capped with HMDS. For characterization of
the prepared materials, solid-state NMR spectroscopy, TEM, BET measurements
and TGA were utilized. The content of palladium in the functionalized SBA-15
samples was determined by means of ICP analysis.
The structural parameters of the prepared supported complexes such as specific
surface area, pore volume and pore diameter were determined according to
nitrogen adsorption−desorption isotherms and Barrett−Joyner−Halenda (BJH)
plots. Both complexes exhibited the characteristic hysteresis loop found in
mesoporous materials with a pore diameter distribution at an average of 6 nm.
Compared to the pristine SBA-15, the average surface area and pore volume of the
heterogenized complexes were substantially reduced due to the post-synthetic
modification of the surface (Table 5.4).
Si
Si
Si
O
Me3SiO
Me3SiO
O
OO
N N Si
O
EtO
Cl
Si
Si
Si
Me3SiO
Me3SiO
O
OON N
Si O
OMeO
Cl
Pd(PhCN)2Cl2
KHMDS
50 oC, 24 h
Toluene
Si
Si
Si
O
Me3SiO
Me3SiO
O
OO
N N Si
O
EtO
Pd ClCl
N
Cl
Si
Si
Si
Me3SiO
Me3SiO
O
OON N
Si O
OMeO
Pd ClCl
N
Cl
3-ClPy
Pd(PhCN)2Cl2
KHMDS
50 oC, 24 h
Toluene
3-ClPy
Pd-Pr1@SBA-15 Pd-Ph1@SBA-15
57. 57
The 13
C CP/MAS NMR data clearly indicate that the complexes were successfully
immobilized on the surface of SBA-15 silica: the comparison of the solution 13
C
NMR spectrum of the Pd-Pr complex with the corresponding solid-state NMR
spectrum of the Pd-Pr2@SBA-15 material shows an agreement of characteristic
features assigned to the carbene ligand and 3-chloropyridine. TMS signal can be
found at 0.46 ppm (Figure 5.5).
Transmission electron microscopy showed that the overall structure of the
functionalized material was not deformed and represented a 2D mesoporous
framework with homogeneous distribution of palladium species on the surface
(Figure 5.6). Notably, there was no indication of detectable Pd nanoparticles on
the material surface.
The prepared catalysts are thermally stable up to 280 °C as can be seen from TGA
weight loss curve. Above this temperature slow degradation of the material starts
with slightly higher rate for the Pd-Pr2@SBA-15 compared to Pd-Ph2@SBA-15.
Table 5.4. Structural parameters of the materials.
Material BET surface area
m2
/g
Pore volume, cc/g Pore diameter, Å
SBA-15 877 0.75 34
Pd-Pr2@SBA-15 462 0.68 64
Pd-Ph2@SBA-15 378 0.50 58
58. 58
Scheme 5.6. Synthetic approach for the direct grafting of the Pd-Pr and the Pd-Ph
complexes.
N N Si OEt
OEt
OEtCl
N N Si OEt
OEt
OEt
Pd ClCl
N
Cl
PdCl2
3-ClPy
K2CO3
80 oC
24 h
Si
Si
Si
O
HO
HO
O
OO
+
Si
Si
Si
O
Me3SiO
Me3SiO
O
OO
N N Si
O
EtO
Pd ClCl
N
Cl
Si
Si
Si
O
HO
HO
O
OO
N N Si
O
EtO
Pd ClCl
N
Cl
Toluene
105 oC, 24 h
HMDS
Toluene
r.t., 24 h
Pd-Pr2@SBA-15
N N
Si OMe
OMe
OMe
Cl
PdCl2
3-ClPy
K2CO3
80 oC
24 h
N N
Si OMe
OMe
OMe
Pd ClCl
N
Cl
Si
Si
Si
O
HO
HO
O
OO
+
Toluene
105 oC, 24 h
Si
Si
Si
HO
HO
O
OON N
Si O
OMeO
Pd ClCl
N
Cl
Si
Si
Si
Me3SiO
Me3SiO
O
OON N
Si O
OMeO
Pd ClCl
N
Cl
HMDS
Toluene
r.t., 24 h
Pd-Ph2@SBA-15
Pd-PhPd-Pr
59. 59
Figure 5.5. Superimposed 13
C NMR spectrum (green) and 13
C CP/MAS NMR spectrum
(red) of the Pd-Pr and the Pd-Pr2@SBA-15, respectively.
Figure 5.6. TEM image of the Pd-Pr2@SBA-15 material. Scale bar: 100nm.
60. 60
5.5 Catalytic activity of supported Pd-NHC catalysts
[Paper III]
By analogy with homogeneous Pd-PEPPSI catalysts, the heterogenized supported
complexes were applied in the ligand-directed C−H acetoxylation of 2-
phenylpyridine. This reaction was successful and led to the formation of mono-
acetoxylated product in moderate yield (Table 5.5). A decrease in reactivity in
comparison with homogeneous systems is a common issue for supported
homogeneous catalysts and may be caused by the difficulty to access the active
sites within the mesoporous material network. However, ICP analysis of the
reaction mixture after the first run of the reaction showed significant leaching of
Pd equal to 17.5 %. Moreover, unfortunately, it was impossible to recycle the
catalyst: the second run of the catalytic reaction resulted in only 4 % GC yield of
the product for the Pd-Pr2@SBA-15. Therefore, a question about the true
heterogeneous nature of the prepared supported systems arises.
To examine the effect of the mesoporous silica support on the performance of
these supported catalysts, a comparative catalytic study of the polymer-based Pd-
NHC catalyst, developed in our laboratory, was performed. The test reaction of the
C−H acetoxylation of 2-phenylpyridine with the polymer-supported Pd-NHC
complex gave similar conversion to the products and, surprisingly, the catalyst
also could not be recycled. Consequently, the issue of recycling can consists in a
possible deactivation of the complexes on a molecular level during the catalytic
cycle.
Table 5.5. C-H acetoxylation of 2-phenylpyridine.
Catalyst Pd loading, % Selectivity to Aa
Yield to Aa
Pd-Pr2@SBA-15 0.03 82 52
Pd-Ph2@SBA-15 0.03 83 50
Poly Pd-NHC 0.05 88 56
a
Based on results of GC analysis with mesitylene as the calibrated internal standard for A
and B upon an average of two runs.
61. 61
Another branch of research in our group is associated with the direct C−H
functionalization of unactivated substrates. We decided to test the catalytic activity
of the Pd-Pr2@SBA-15 complex in the reaction of acetoxylation of biphenyl. The
reaction resulted in a mixture of para, meta and ortho isomers with 31 % GC yield
of the para-substituted product. It was possible to recycle the catalyst and the
second run showed 23 % GC yield of the product. Nevertheless, the leaching test
of the reaction mixture after the first run indicates a significant amount of Pd in the
solution.
Thus, the developed supported Pd-NHC catalytic systems are not optimal and
under applied catalytic reaction conditions gradually decompose. The reason can
be the inefficient approach for immobilization of the complexes or due to the harsh
conditions utilized during the catalysis.
5.6 X-ray spectroscopic characterization of supported
Pd-NHC complexes [Paper IV]
For further full characterization and investigation of the stability of the supported
homogeneous complexes of palladium, it was decided to use a combination of X-
ray photoelectron spectroscopy (XPS), X-ray absorption spectroscopy (XAS) and
density functional theory (DFT). The aforementioned techniques provide
information about electronic and elemental structure of the chemical species, helps
to explore chemical binding in compounds and determine their geometrical
orientation on surfaces. For this study, the developed NHC ligands and
corresponding Pd-NHC complexes were immobilized on a surface of a silicon
wafer.
Figure 5.7 represents the N 1s XAS data for immobilized Pd-Pr and Pd-Ph
complexes. As it can be seen, there are three distinguishable π* resonances for
both complexes with the most intense peak at 402 eV, corresponding to the N 1s to
1π* transition in imidazole [26]. Two smaller peaks at 398.6 and 399.3 eV
assigned to the 1π* resonances of 3-chloropyridine depending on the chemical
environment: the higher energy peak is attributed to chloropyridine coordinated to
palladium in the complex [27], and the lower energy peak corresponds to “free”
chloropyridine on the surface [28]. Such an assignment of the peak positions is
supported by the DFT calculations. The expected ratio of the peak intensities
between imidazole and 3-chloropyridine in the complexes is 1:1 as calculations
suggest. However, the experimentally observed N 1s intensities are far from this
estimation. Therefore, it can be assumed that the complexes immobilized on the Si
62. 62
wafer surface dissociate a chloropyridine ligand and presumably form a linkage
with oxygen atoms of the surface.
Figure 5.7. N 1s X-ray absorption spectra for a) complex Pd-Pr (1) and c) complex Pd-Ph
(2) recorded at different angles; b) and d) integrated XA intensities of π* resonance of
imidazole peak as a function of incidence angle.
The resulting structure of the supported complexes is in agreement with XPS
spectra of Pd 3d5/2: the major contribution to the spectra has been observed at
339.1 eV with a minor shoulder at 338.1 eV (Figure 5.8). If the binding energy of
the low intensity peak corresponds to Pd(II) in organometallic complexes
according to the literature data [29,30], the main peak can be assigned to Pd(II)
bonded to the surface oxygen [31,32]. Further evidence for this hypothesis can be
seen in N 1s XP spectra of the complexes: three components can be found in the
spectrum of the Pd-Pr complex corresponding to the imidazole peak and two
chloropyridine peaks in different chemical environment.
To determine the geometrical orientation of the complexes on the surface of the Si
wafer, the intensity variation of the imidazole 1π* resonance peak with the light
incidence angle was recorded. For both complexes more than one absorbing
structure on the surface can be found. Thorough analysis of the data suggests that
63. 63
the dissociated part of the surface complexes are situated with the imidazole ring
perpendicular to the surface and the intact complexes have an orientation of the
imidazole ring parallel to the surface of the silicon wafer.
Figure 5.8. a) N1s and b) Pd 3d XP spectra of the Pd-Pr complex (top) and the Pd-Ph
(bottom). The grey areas in Pd 3d spectra indicate the range of the literature binding
energies for the PdII
and Pd0
oxidation states.
Thus, the obtained X-ray spectroscopic data indicate the challenges associated
with stability of the immobilized complexes. The observed dissociation of the
complexes could be induced by the immobilization strategy due to a reactive
behavior of the chosen support or be a consequence of the application of UHV
conditions and X-ray irradiation during the measurements.
5.7 References
1. C. J. Adams, M. Lusi, E. M. Mutambi, A. G. Orpen, Chem. Commun. 2015, 51,
9632.
2. M. A. Taige, S. Ahrens, T. Strassner, J. Organomet. Chem. 2011, 696, 2918.
3. G. Berthon-Gelloz, O. Buisine, J.-F. Brière, G. Michaud, S. Stérin, G. Mignani,
B. Tinant, J.-P. Declercq, D. Chapon, I. E. Markó, J. Organomet. Chem. 2005,
690, 6156.
64. 64
4. A. L. Ankudinov, I. I. Rehr, S. R. Bare, Chem. Phys. Lett. 2000, 316(5–6), 495.
5. F. Low, J. Kimpton, S. A. Wilson, L. Zhang, Environ. Sci. Technol. 2015, 49(13),
8246.
6. B. Marciniec (ed.): “Hydrosilylation: A Comprehensive Review on Recent
Advances”, Springer, 2009.
7. B. Sun, T. Yoshino, M. Kanai, S. Matsunaga, Angew. Chem.Int. Ed. 2015, 54,
12968.
8. Y. Xu, G. Yan, Z. Ren, G. Dong, Nature Chem. 2015, 7, 829.
9. I. A. Sanhueza, A. M. Wagner, M. S. Sanford, F. Schoenebeck, Chem. Sci. 2013,
4, 2767.
10. S. R. Neufeldt, M. S. Sanford, Acc. Chem. Res. 2012, 45, 936.
11. T. Brückl, R. D. Baxter, Y. Ishihara, P. S. Baran, Acc. Chem. Res. 2012, 45, 826.
12. M. S. Chen, M. C. White, Science 2010, 327, 566.
13. M. Muehlhofer, T. Strassner, W. A. Herrmann, Angew. Chem. Int. Ed. 2002, 41,
1745.
14. X. Luan, R. Mariz, C. Robert, M. Gatti, S. Blumentritt, A. Linden, R. Dorta, Org.
Lett. 2008, 10, 5569.
15. A. R. Martin, A. Chartoire, A. M. Z. Slawin, S. P. Nolan, Beilstein J. Org. Chem.
2012, 8, 1637.
16. D. Munz, T. Strassner, Angew. Chem. Int. Ed. 2014, 53, 2485.
17. S. Puneet Desai, M. Mondal, J. Choudhury, Organometallics 2015, 34, 2731.
18. D. Kalyani, M. S. Sanford, Org. Lett. 2005, 7, 4149.
19. A. R. Dick, K. L. Hull, M. S. Sanford, J. Am. Chem. Soc. 2004, 126, 2300.
20. L. V. Desai, K. L. Hull, M. S. Sanford, J. Am. Chem. Soc. 2004, 126, 9542.
21. E. F. Vansant, P. Van Der Voort, K. C. Vrancken: ”Studies in Surface Science
and Catalysis: Characterization and Chemical Modification of the Silica
Surface”, Vol. 93, Elsevier, 1995.
22. K. D. Behringer, J. Blumel, J. Liq. Chrom. Rel. Technol. 1996, 19(17-18), 2753.
23. F. Rascon, R. Wischert, C. Coperet, Chem. Sci. 2011, 2, 1449.
24. L. T. Zhuravlev, Colloids Surf. A 2000, 173, 1.
25. A. P. Legrand: “The Surface Properties of Silica”, Wiley-VCH:Weinheim,
Germany, 1998.
26. M. J. Thomason: “Soft X-ray Spectroscopy of Molecular Species in Solution:
Studies of Imidazole and Imidazole/Water Systems”, Ph.D. thesis, University of
Manchester, 2012.
27. R. Arrigo, M. E. Schuster, Z. Xie, Y. Yi, G. Wowsnick, L. L. Sun, K. E. Hermann
M. Friedrich, P. Kast, M. Hävecker, A. Knop-Gericke, R. Schlögl, ACS Catal.
2015, 5, 2740.
28. C. Kolczewski, R. Puttner, O. Plashkevych, H. Agren, V. Staemmler, M. Martins,
G. Snell, A. S. Schlachter, M. SantAnna, G. Kaindl, L. G. M. Pettersson, J. Chem.
Phys. 2001, 115, 6426.
65. 65
29. A. Azua, J. A. Mata, P. Heymes, E. Peris, F. Lamaty, J. Martinez, E. Colacino,
Adv. Synth. Catal. 2013, 355, 1107.
30. I. Pryjomska-Ray, A. Gniewek, A. M. Trzeciak, J. J. Zoilkowski, W. Tylus, Top.
Catal. 2006, 40, 173.
31. M. Hyland, G. Bancroft, Geochim. Cosmochim. Acta 1990, 54, 117.
32. G. Mattogno, G. Polzonetti, G. R. Tauszik, J. Electron Spectrosc. Rel. Phen.
1978, 14, 237.
67. 67
Conclusions and outlook
The present thesis describes the development of novel homogeneous PEPPSI-type
complexes of palladium and platinum and possible approaches for their further
heterogenization, together with a thorough investigation of their properties and
subsequent applications in catalysis. For full characterization of the obtained
compounds synchrotron radiation X-ray techniques such as XAS and XPS were
used. As benchmark reactions of catalytic activity, hydrosilylation as well as C−H
functionalization reactions were examined.
At the beginning of our study, a novel Pt-NHC complex of PEPPSI-type was
prepared and insightfully investigated by means of a combination of the X-ray
absorption spectroscopy and single crystal X-ray diffraction, showing sensitive
distinguishable features for the N-heterocyclic carbene ligand in the structure. The
obtained compound was further studied in application to a ligand-directed
functionalization but did not demonstrate any catalytic activity. The investigation
was continued with an examination of the applicability of the platinum complex to
the hydrosilylation of styrene where it displayed significant activity.
Next we discovered the influence of an NHC ligand on the site-selectivity of the
ligand-directed C−H functionalization. A series of Pd-PEPPSI complexes were
tested in C−H acetoxylation of the 2-phenylpyridine resulting in a significantly
improved yield of mono-functionalized product formation compared to previously
reported studies where Pd(OAc)2 was used as a catalyst, indicating that the ligand
environment of the Pd complex is affecting the catalytic process. The application
of PEPPSI-IPr catalyst was extended to a large variety of substrates including
functionalization of both sp2
as well as more challenging sp3
C−H sites pointing on
the multifunctionality of the catalyst.
Having established a high catalytic activity and considerable selectivity of Pd-
PEPPSI complexes in the ligand-directed C−H functionalization, we focused our
efforts on the creation of supported homogeneous catalysts. For this purpose we
designed and prepared two NHC ligands containing alkoxysilyl linker groups. A
two-step synthesis of the heterogeneous catalysts including initial preparation of a
ligand immobilized on SBA-15 mesoporous silica and subsequent coordination of
the palladium source was unsuccessful and led to formation of a material
containing inclusions of metallic Pd. Direct grafting of the pre-formed Pd
complexes with modified carbene ligands on the SBA-15 surface resulted in stable
68. 68
materials with homogeneous distribution of the immobilized species. Obtained
supported Pd complexes were characterized with the range of techniques including
solid state NMR spectroscopy, TEM, TGA and BET surface measurements.
To examine the applicability of novel immobilized PEPPSI-type complexes in
catalysis, benchmark reactions of the ligand-directed C−H oxygenation of 2-
phenylpyridine and the undirected C−H acetoxylation of biphenyl were performed.
In both cases high catalytic activity was found. However, there was a significant
leaching of palladium in the solution accompanied with gradual deactivation of the
catalysts.
One has to admit, there is a limit of time for any PhD project. Therefore, there are
still a number of research questions that would be interesting to investigate for
further optimization of the preparation of catalytic materials. First of all, the
problem can lie in the inefficient grafting of the immobilized complexes on the
surface of the SBA-15, thermally pre-treated at 600 °C. The reaction between the
silica surface and alkoxysilyl groups of the linker can possibly give only one
covalent bond with the surface of the material, resulting in the unstable moieties
on the surface, prone to dissociation. An alternative pre-activation of the surface
for grafting such as a hydroxylation approach might be tested to check this
hypothesis.
Design development of the NHC ligands with other type of linker groups can also
improve the catalyst performance.
Lastly, the instability of the complexes can be overcome by an optimization of the
catalytic reaction conditions. The milder reaction temperature as well as the
selection of a suitable solvent can improve the stability of the supported catalysts.
In my work it was shown that the X-ray techniques represent a powerful tool for
specific characterization of the organometallic moieties. The combination of
experimental methods such as XAS, XPS and XRD together with theoretical DFT
calculations can give a complete picture of the electronic and elemental structure
of the compounds under study and predict the geometrical orientation of transition
metal complexes on surfaces. Further exploration of the applicability of the
abovementioned techniques to the study of the catalysis would lead to exciting
discoveries.
69. 69
Acknowledgments
To begin with, I would like to express my gratitude to my supervisor Ola Wendt
for giving me an opportunity to be a part of his research group. I truly appreciate
the continuous trust in my abilities and the valuable guidance through all these
years. Most of all, I am grateful for the provided significant freedom and
independence to carry out my research, I’ve really learned a lot.
I would like to say many thanks to my co-supervisor Achim Schnadt for the
opportunity to collaborate with a group of outstanding physicists and for teaching
me the core of the X-ray techniques.
I would like to thank all the members of the Marie Curie Initial Training Network
SMALL for the very intense three years of training, full of exciting scientific
meetings, curiosity and fruitful discussions.
I am grateful for an opportunity to participate in a summer exchange project at
Queens University in Canada and to Cathleen Crudden for providing a warm
welcome and supporting me during my stay.
I would like to acknowledge Viveka Alfredsson for providing insights into the
materials chemistry, Reine Wallenberg for the TEM studies and an interesting
course in HRTEM, Birgitta Lindén for the urgent measurements, Göran Carlström
for the help with solid-state NMR spectroscopy, Ebbe Nordlander for the exciting
course in chemistry of the elements and Sofia Essén for all the mass spectra.
Thank you very much Maria Levin, Bodil Eliasson and Katarina Fredriksson for
making my life so much easier by taking care of so many practical details.
For the financial support I would like to thank the FP7 Marie Curie Actions of the
European Commission, via the Initial Training Network SMALL (MCITN-
238804) and the Swedish Research Council.
Next I would like to thank my bright colleagues. Special thanks to Inus and Naga
for your substantial help and kind support during the uneasy time − at the very
beginning of my PhD: without you both it would be much harder; Olesia for
always been an inspiring example and a great friend over the years, and what
happens in Baden Baden stays in Baden Baden; Sheetal for the countless support,
our fun talks and the exciting night train ride in Bari; Tripta and Shilpi for our
enjoyable trips all over the Europe and effective discussions at the monthly
70. 70
meetings; Maitham for all the scientific and non-scientific talks. Many thanks to
all the present and former members of the Wendt group for the great working
environment, unforgettable summer excursions and group activities: Magnus,
Klara, Misha, Abdel, Sasha, Roma, David – thank you!
I think we have an amazing atmosphere at CAS and I would like to thank Maria,
Michaela, Eira, Irene, Paola, Victor, Merichel, Filip and Björn for your sunny
attitude to the rainy problems, meaningful and not really discussions during the
lunchtime and creation of good vibes.
Finally, the people without whom this thesis would never be written: I am very
grateful to my best friends – Alisa, Anya, Katya, Inga, Evelyn, Alisa phys-math,
Nastya. You always kindly supported me and all the laugher we shared kept me
going. With the team like you I have nothing to be worry about. Vera, thank you
so much for helping me out with the “always-right” advice and for never being
overcritical.
Dear Mum, without your tremendous support and strong belief in me even in the
darkest times I won’t be able to make it. I dedicate my thesis to you.