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.
