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Thesis Title: Gas phase studies of metal complexes, isomeric carbanions and distonic radical anions under soft ionization mass spectral conditions


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Gas phase ion Chemistry of CrIII(Salen) complex under electrospray
ionization conditions

Proton and alkali metal ion affinities of bidentate bases: spacer
chain length effects

Generation of regiospecific carbanions under electrospray
ionization conditions and characterization by ion-molecule
reactions with carbon dioxide

Chapter 4
Generation of distonic dehydrophenoxide radical anions under
electrospray and atmospheric pressure chemical ionization

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Thesis Title: Gas phase studies of metal complexes, isomeric carbanions and distonic radical anions under soft ionization mass spectral conditions

  1. 1. Gas phase studies of metal complexes, isomeric carbanions and distonic radical anions under soft ionization mass spectral conditions THESIS SUBMITTED TO OSMANIA UNIVERSITY FOR THE DEGREE OF DOCTOR OF PHILOSOPHY IN CHEMISTRY By M. Kiran Kumar, M.Sc. NATIONAL CENTRE FOR MASS SPECTROMETRY INDIAN INSTITUTE OF CHEMICAL TECHNOLOGY Hyderabad -500 007, INDIA April, 2007
  2. 2. Dedicated ToMy Beloved Parents and wife
  3. 3. DECLARATION The research work presented in this thesis entitled “Gas phase studiesof metal complexes, isomeric carbanions and distonic radical anionsunder soft ionization mass spectral conditions” was carried out by meindependently in this institute under the supervision of Dr. M. Vairamani,Scientist-in-Charge, National Centre for Mass Spectrometry, Indian Institute ofChemical Technology, Hyderabad. This work is original and has not beensubmitted in part or full, for any degree or diploma of this or any otheruniversity. Dt : (M. Kiran Kumar) ) National Center for Mass Spectrometry Indian Institute of Chemical Technology Hyderabad, AP-500 007.
  4. 4. Dr. M. Vairamani National Centre for Mass Spectrometry Scientist ‘G’, Head Indian Institute of Chemical Technology Analytical Division Council of Scientific & Industrial Research Hyderabad-500 007, A.P., India CERTIFICATE This is to certify that the research work incorporated in this thesisentitled “Gas phase studies of metal complexes, isomeric carbanions anddistonic radical anions under soft ionization mass spectral conditions”submitted by Mr. M. Kiran Kumar was carried out by the candidate under mysupervision. This work is original and has not been submitted for any otherresearch degree or diploma of this or any other university.Dt : (Dr. M. Vairamani) Tel : +91-40-27193482 Fax : +91-40-27193156 e-mail :
  5. 5. ACKNOWLEDGEMENTS I am very much thankful to my guide and supervisor Dr. M Vairamani, Head AnalyticalChemistry Division, National Centre for Mass Spectrometry (NCMS), for welcoming me into hisresearch group and providing me enough impetus to carry out my work independently. My heartfeltgratitude to him for his guidance and valuable suggestions throughout my research My stepping into the arena of research remains incomplete without mentioning about Prof. G.L. David Krupadanam and Dr. D. Sitha Ram, whose inspiration and guidance encouraged me to stepinto the world of Mass Spectrometry. Special thanks go to Dr. G Narahari Shastry for introducing me to the theoretical chemistryand also for his constant encouragement. I express my heartfelt gratitude to Dr. S. Prabhakar, for his timely help and valuablesuggestions throughout the course of my work. I am very thankful to him as he listened to all myproblems with utmost patience and suggested me solutions in an appropriate manner. I cannot imagineanybody like him taking better care of me and he has been a great source of inspiration for me. My sincere thanks to Dr. R. Srinivas, Mr. L. K. Rao, Dr. N. S. Swamy, Mr. R. Narsimha,Dr. N. P. Raju,, Dr. U. V. R. V. Saradhi, V. V. S. Lakshmi and M. R. V. S. Murty for theircooperation and encouragement. My special thanks to Dr. N. P. Raju for reading my thesis with patience. I would also like to thank my inter and degree college classmates, Ravi,, Nagi Reddy andHari, who stood beside me all the way to keep me in the right path. It is my pleasure to thank all mypast and present collegues, Shama, Veni, Srikanth, Jagadeshwar Reddy, Bhaskar, Ramu, Murali,Shivaleela, Ramesh and Sangeeta, for making my stay at NCMS a pleasant experience. I thank mycolleagues Srinivasa Rao, Sateesh Kumar and Nagaraju.from Molecular Modeling Division. I am grateful to my entire family for their support and encouragement throughout my studies. There are many, many people who have helped me along the way. My regrets to those whom Ihave forgotten if any, but one can be assured that, his help has been greatly appreciated! Lastly, I would like to thank CSIR, New Delhi for the financial support in the form ofResearch Fellowship (JRF/SRF). I take this opportunity to thank Dr. J. S. Yadav Director, IICT,and Dr. K. V. Raghavan, former director for providing the facilities to carryout my research work. -Morishetti Kiran Kumar
  6. 6. Abbrevations ACN : Acetonitrile BSSE : Basis Set Super position Error CID : Collision-Induced Dissociation ∆E : Binding energy difference EI : Electron Ionization EPR : Electron Paramagnetic Resonance ESI : Electrospray Ionization ESR : Electron Spin Resonance FA : Flowing Afterglow FAB : Fast Atom Bombardment FTICR : Fourier transform ion-cyclotron resonance FTMS : Fourier transform mass spectrometer FWHM : Full Width at Half Maximum HF : Hartree-Fock LAMMA : Laser Microprobe Mass Analysis MALDI : Matrix Assisted Laser Desorption Ionization MO : Molecular Orbital NMR : Nuclear Magnetic Resonance Pc/Pd : Precursor/Product ratio QITMS : Quadrupole ion trap mass spectrometers SORI : Sustained off-resonance irradiation Teff : Effective Temperature
  7. 7. IndexContents Page NoCHAPTER 1Gas phase ion Chemistry of CrIII(Salen) complex under electrosprayionization conditions1. Prologue 1 1.1. Brief introduction of ESIMS 5 1.2. Metal-salen complexes analysis by ESIMS 72. Scope of the work 123. Results and discussion 12 3.1. Source experiments 13 3.2. Ligand-pickup experiments 20 3.3. Collision induced dissociation (CID) experiments 254. Conclusions 285. Experimental 296. References 31CHAPTER 2Proton and alkali metal ion affinities of bidentate bases: spacerchain length effectsPart 1: Proton and alkali metal ion affinities of α,ω-Diamines: Spacer chain length effects1. Prologue 35 1.1. The Kinetic method 402. Scope of the work 433. Results and discussion 44 3.1. Li+ ion affinity ladder construction 45 3.2. Na+ and K+ ion affinity ladder construction 48 3.3. Proton affinity ladder construction 51 3.4. Relative alkali metal ion binding energy calculations 52 3.5. Comparison between the proton and alkali metal ion affinity 53 orders 3.6. Theoretical studies 544. Conclusions 59
  8. 8. Part 2: Proton and alkali metal ion affinities of α,ω-Diols: Spacer chain length effects1. Prologue 612. Scope of the work 623. Results and discussion 63 3.1. Proton affinity ladder construction 63 3.2. Li+, Na+ and K+ ion affinity ladder construction 674. Conclusions 725. Experimental 736. References 74CHAPTER 3Generation of regiospecific carbanions under electrosprayionization conditions and characterization by ion-moleculereactions with carbon dioxidePart 1: Generation of regiospecific carbanions from aromatic hydroxyacids and dicarboxylic acids1. Prologue 79 1.1. The generation of carbanions in the gas phase 80 1.1.1. Proton abstraction method 81 1.1.2. Fluorodesilylation method 82 1.1.3. Collision induced decarboxylation method 83 1.2. Characterization of carbanions 84 1.3. Stability studies of carbanions 86 1.4. Generation and characterization of specific carbanions 882. Scope of the work 953. Results and discussion 96 3.1. Geometrical isomers 99 3.2. Positional isomers 102 3.2.1. Aromatic dicarboxylic acids 102 3.2.2. Aromatic hydroxy acids 106 3.3. Effect of desolvation temperature 110 3.4. Theoretical calculations 1124. Conclusions 118Part 2: Generation of regiospecific carbanions from sulfobenzoic acids1. Prologue 1192. Scope of the work 1203. Results and discussion 121
  9. 9. 3.1. Isomeric sulfobenzoic acids 122 3.2. Isomeric benzenedisulfonic acids 1274. Conclusions 1305. Experimental 1316. References 133Chapter 4Generation of distonic dehydrophenoxide radical anions underelectrospray and atmospheric pressure chemical ionizationconditionsPart 1: Generation of distonic dehydrophenoxide radical anions fromsubstituted phenols under Electrospray ionization conditions1. Prologue 140 1.1. Formation of radical anions in the gas phase 141 1.1.1. Electron attachment 141 1.1.2. Electron transfer 142 1.1.3. Ion-molecule reactions 144 1.2. Distonic radical anions 145 1.3. Characterization of radical anions 1482. Scope of the work 1493. Results and discussions 150 3.1. Isomeric nitrobenzoic acids 150 3.2. Isomeric hydroxytoluenes 154 3.3. Isomeric nitrophenols and hydroxy benzaldehydes 157 3.3. Ion-molecule reactions in the collision cell with CO2 1604. Conclusions 163Part 2: Generation of distonic dehydrophenoxide radical anions fromsubstituted nitrobenzenes under atmospheric pressure chemicalionization mass spectral conditions1. Prologue 164 1.1. Atmospheric pressure chemical ionization 1652. Scope of the work 1663. Results and discussions 167 3.1. Isomeric nitrobenzaldehydes 168 3.2. Isomeric nitroacetophenones 1724. Conclusions 1765. Experimental 1776. References 179ℵ Abstract
  11. 11. Chapter 1 Chemistry of CrIII-Salen complex… CHAPTER 1 CHAPTER 1 Gas phase ion Chemistry of CrIII(Salen) complex under electrospray ionization conditions1. PROLOGUEI t is important to characterize the metal complexes and to identify the crucial intermediates in metal-mediated reactions in order to understand the natureand reactivity of metal complexes and their reaction pathways.1-4 Variety oftechniques based on X-ray diffraction, infrared spectra, nuclear magneticresonance (NMR), and electron paramagnetic resonance (EPR) have been usedto gather coordination structure information.5 For example, the use of NMR islimited for characterization of metal complexes that contain a paramagneticmetal atom; this technique is less applicable if metal complexes are present atlow concentrations or as complex mixtures.5 Consequently, researchers havechosen the advantage of using mass spectrometry (MS) as a technique ofchoice to gather coordination structure information of metal complexes. Thestudy of metal complex systems using MS (i.e., in the gas phase) is a rapidlyexpanding field of research.1,2 As the mass spectrometer is operated in eitherof the positive or negative ion mode, metal complexes can readily be isolatedand studied without interferences from counter ions, solvent or additionalcomplexes those are usually present in solution.1,2 These experimental 1
  12. 12. Chapter 1 Chemistry of CrIII-Salen complex…conditions are ideally suited for studying the intrinsic properties and reactivityof various chemical entities may be clearly unrevealed. Knowledge of the gas-phase structures of metal complexes is importantfor analytical applications, as evidenced by several reviews.1,2,6,7 Recent massspectrometric experiments have drawn direct correlations to metal complexmediated catalytic processes involved in various reactions.2,8-13 Massspectrometric investigations benefit from the ability to evaluate catalyticallyactive species in the gas phase that are too transient to study in solution. Thespecies with short lifetimes in solution do not pose a problem in the high-vacuum environment of a mass spectrometer. Complexes of N,N-bis(salicylidene)ethylenediamine, commonly knownas H2Salen (Figure 1), belong to a fundamental class of compounds incoordination chemistry, known since 1933.15 This compound also belongs to theclass of Schiff base ligands, because of the preparation of this compound is bythe condensation of salicylaldehyde and ethylene diamine. Schiff base ligandsare able to coordinate metals through imine nitrogen and also the through thehydroxyl oxygen in the case of Salen complexes. In fact, Schiff bases are ableto stabilize many different metals in various oxidation states, controlling theperformance of metals in a large variety of useful catalytic transformations. TheSalen type complexes have been extensively studied and more than 2500complexes have been synthesized.16 Interest in Salen type complexesintensified in 1990 when the groups of Jacobsen17 and Katsuki18 discovered the 2
  13. 13. Chapter 1 Chemistry of CrIII-Salen complex…enantioselective epoxidation of unfunctionalised alkenes using chiralMnIII(Salen) complexes as catalysts (Scheme 1). N N OH HO Figure 1: H2Salen. III + O [M (Salen)] + PhIO + PhI Scheme 1: Epoxidation of olefins with Metal(Salen) complex. Since that time, an extremely wide variety of reactions catalyzed bySalen complexes have been investigated. These include oxidation ofhydrocarbons,19 aziridination of alkenes,20 Diels Alder reaction,21 hydrolytickinetic resolution of epoxides,22 alkylation of aldehydes23 and oxidation ofsulfides to sulfoxides.24 Different mass spectrometry techniques have beenused to characterize the Metal-Salen complexes in the gas phase.8,25-28 Ingeneral, application of the traditional method of ionization i.e. electronionization (EI), the earlier ionization method of MS, was limited to some metalcomplexes, because most of the metal complexes are non-volatile andthermally labile.25,27 However, there are few reports on the EI studies on a fewmetal Salen complexes.25 The reported complexes include Co, Ni and Cu Salencomplexes. The EI spectra of these complexes showed abundant molecular 3
  14. 14. Chapter 1 Chemistry of CrIII-Salen complex…ions and fragment ions. Rohly et al.27 compared the EI mass spectra of metalSalen complexes with the laser microprobe mass analysis (LAMMA) spectra,wherein they report positive ion LAMMA spectra failed to provide theinformation that is obtained in EI and negative ion LAMMA spectra weredominated with only the carbon cluster ions. The soft ionization methods such as chemical ionization, field desorption,plasma desorption, secondary ion and fast atom bombardment (FAB) havebeen developed to overcome the drawbacks encountered in EIMS towards theanalysis of metal-complexes. The FAB ionization technique has been extendedto new areas of inorganic and organometallic chemistry.28-30 Zhao at al.28analyzed metal Salen complexes by using positive ion FAB technique. Theyfound that among many solvents tried to dissolve the complexes, the use oftrifluoro acetic acid (TFA) was crucial for producing good FAB spectra. Though,much of the researchers used the FAB technique to analyze various metalcomplexes, still there are some problems, like complications arising fromrecombination of fragments, or interactions with the matrix used.2 Recent developments of ionization methods like matrix assisted laserdesorption ionization (MALDI)31-34 and electrospray ionization (ESI),1,7-14,35-37have also been applied for the characterization of the metal complexes. MALDItechnique, though often used for characterization of high molecular weightcompounds, is relatively not explored much in characterization of metalcomplexes. The selection of a suitable matrix is crucial in MALDI experiments,and the ligand exchange reactions by MALDI matrix are known to complicate 4
  15. 15. Chapter 1 Chemistry of CrIII-Salen complex…the spectra. Relatively more studies are available for the analysis of metalcomplexes using the ESI technique. The major impact of ESIMS to date is that itcan be used in identification of metal complexes, because it has allowedobservation of mass spectra for low as well as high molecular weightcompounds of ionic and nonvolatile, such as salts. In addition to the ionizationof the analytes, ESI process also transfers pre-existing ions in solution, if any, tothe gas phase, and hence is ideal for inorganic and organometallic compounds.Further, ESI has proven to be a soft ionization method that keeps intact anyweakly bound ligands in a complex ion.36,37 Moreover, the ionizationtechniques like ESI need very small amounts of sample to generate reasonablygood spectra. Use of low level quantities of samples for ESI enables thetechnique for the analysis of environmental or biological samples, where thesamples are precious. With these advantages, ESI has become increasinglypopular as an analytical tool in inorganic/organometallic chemistry. Thistechnique, in combination with tandem mass spectrometry (MS/MS), has beenemployed to study mechanistic pathways of reactions.1,7-14,35-371.1. BRIEF INTRODUCTION OF ESIMS ESI technique involves spraying of a solution of the sample through aelectrically charged needle the so-called capillary which is at atmosphericpressure (Figure 2). The spraying process can be streamlined by using anebulizing gas. The charged droplets are produced where the positive ornegative ions are solvated with solvent molecules. Hot gas or a dry gas, usuallycalled as desolvation gas, is applied to the charged droplets to cause solvent 5
  16. 16. Chapter 1 Chemistry of CrIII-Salen complex…evaporation. The desolvation process decreases the droplet size, leads to thecolumbic repulsion between the like charges present in the droplet and furtherthe droplet fission leads to the formation of individual gas phase analyte ions.The charged ions are then focused into the mass analyzer. Figure 2: Schematic diagram of a typical ESIMS source phenomenon operating in positive mode. Solvent evaporation of the charged droplet generated in the source can be clearly seen (courtesy reference Gaskell SJ, J. of Mass Spectrom., 1997)38 Application of an electrostatic field in the region between the capillaryexit and cone causes collisional activation of the solvated analyte ions. Theelectrostatic field can be easily varied and provides control over the amount of 6
  17. 17. Chapter 1 Chemistry of CrIII-Salen complex…collisional activation. At low levels of cone voltage, the generated ions can besampled without causing any fragmentation. At higher levels of cone voltages,the generated ions can be induced to undergo dissociation to give structurallyinformative fragment ions. Such fragmentation in the source is called ‘cone-voltage fragmentation’ or ‘source fragmentation’.1.2. METAL-SALEN COMPLEXES ANALYSIS BY ESIMS It is well known that Mn- and Cr-Salen complexes catalyze the oxidationof organic substrates through the formation of a high-valent metal-oxo species,(Salen)M=O. Kochi et al. used metal-Salen complexes as versatile epoxidationcatalysts in the 1980s.39-41 A typical mechanism is shown in Scheme 2. N N N O N III V M M O O +PhIO O O -PhI [MIII(Salen)]+ [O=MV(salen)]+ O V + III + [O=M (salen)] + + [M (salen)] Scheme 2: The mechanism for olefin epoxidation with Metal(salen) complex. Epoxidation of various alkenes was successfully carried out withiodosylbenzene in the presence of catalytic amounts of CrIII(Salen), and theepoxidation reaction failed in the absence of the chromium complex. Theysuccessfully isolated the catalytically active oxo-chromium(V) (O=CrV)complexes in the condensed phase by careful recrystalization andcharacterized by X-Ray and ESR studies.39 The successful isolation and 7
  18. 18. Chapter 1 Chemistry of CrIII-Salen complex…characterization of O=CrV(Salen) revealed the basis for oxygen activation inthe O=Cr(V) functionality. Kochi et al.39 further developed the use ofMnIII(Salen) complexes as much more versatile oxidation catalysts. Anenatioselective version of the reaction was developed later by Jacobsen et al.17and Katsuki et al.18 by using chiral MnIII(Salen) complexes (Schemes 1 and 2).However, the mechanistic studies on the MnIII(Salen) systems were hamperedby the fact that the catalytically active oxomanganese (O=MnV) species appearonly as short-lived putative intermediates.40 At that time, it was suggested thatthe concentration of MnV-oxo complex was regulated by an equilibriuminvolving µ-oxo-manganese (V) as depicted in Scheme 2. However, thereactive species were neither isolated nor characterized in the condensedphase. Plattner et al. successfully applied ESI technique to give a direct prooffor the epoxidation reactions using MnIII(Salen) complexes.4 They used ESImethod in combination with tandem mass spectrometry to study themechanistic pathway for oxygen transfer to organic substrates in the gas phase.The [MnIII(Salen)]+ salts with iodosylbenzene were electrosprayed and theresulted ESI spectrum showed two oxidized species, i.e. [(Salen)Mn=O]+ at m/z337, [PhIO(Salen)Mn–O–Mn(Salen)OIPh]2+ at m/z 549.4,11 The collision-induceddissociation (CID) of the ion at m/z 549 resulted in the decomposition productsof MnIII and MnV-oxo derivatives [Scheme 3]. These findings represented thefirst experimental evidence for the formation (conproportionation) anddecomposition (disproportionation) of a µ-oxo bridged MnIV dimer acting asreservoir of the catalytically active species involved in the oxidation reaction.12 8
  19. 19. Chapter 1 Chemistry of CrIII-Salen complex… [PhIO(Salen)Mn-O-Mn(Salen)OIPh]2+ m/z 549 [PhIO(Salen)MnIII]++ [PhIO(Salen)MnV=O]+ m/z 541 m/z 557 Scheme 3 The capability of MnV-oxo species to transfer oxygen to suitable organicsubstrates in the gas phase was also demonstrated by collision experiments.When [(Salen)MnV=O]+ ions were mass selected and submitted to CIDexperiments with either Ar or Xe as collision gases, no fragmentation could beobtained, [Scheme 4]. However, if the inert gas was replaced with oxygenacceptors like sulfides or electron-rich olefins, formation of [(Salen)MnIII]+ ions,that is, the reduction product of the oxidation reaction, was detected, [Scheme4]. Ar/Xe [(Salen)MnV=O]+ gas [(Salen)MnV=O]+ (1) m/z 337 m/z 337 O [(Salen)MnV=O]+ [(Salen)MnIII]+ + (2) m/z 337 m/z 321Scheme 4: Collision cell experiments for [(Salen)MnIII]+ ions with (1) Ar/Xe gas (2) electron rich olefin gas. Further, Plattner et al.12 evaluated the effects of various substituents inthe 5- and 5-positions of the Salen and found that the electron-withdrawingsubstituents enhance the reactivity of the Mn=O moiety. The importance of theaxial positions in MnIII(Salen) complexes was also demonstrated by the 9
  20. 20. Chapter 1 Chemistry of CrIII-Salen complex…enhancement of epoxidation reaction yields with addition of a donor ligand thatstabilizes the oxometal-Salen complex. Further, the significance of axial ligandswas demonstrated by their studies on the coordination chemistry of MnIII(Salen)and oxo MnV(Salen) complexes by applying ion-molecule reactions in thecollision cell (ligand-pick up experiments).10 Thus, the role of axial ligation ongeometry and reactivity of the high-valent oxo complex appeared to be quitedrastic. Study of solvent clusters with ionic species in the gas phase providesbasic insights into the chemical reactivity and dynamics of ions in thecondensed phase. Such studies also provide a wealth of information oninteraction between singly charged metal ions and small ligands such as water,methanol, acetonitrile etc. Beauchamp et al.37 studied the evaporation kineticson hydrated Cr and Mn Salen complexes by using ESI technique in a “softsampling” mode. In this study, they observed that the kinetics of waterevaporation from solvated Salen complexes is highly dependent on the centralmetal ion. The clusters of CrIII(Salen) ions with two water molecules attachedexhibit special stability, indicated by their prominence in the overall clusterdistribution. These results were in accord with the solution phase chemistryand with the ligand field theory. Madusudanan et al.36 studied the axial interactions of CrIII(Salprn), whereSalprn = N,N-bis(salicylidene)propanediamine complexes with nucleotidesand nucleosides using ESI-MS. The nucleosides formed 1:1 and 2:1 adductswith [CrIII(Salprn)]+ and dinucleotides formed only the 1:1 adducts. The CID of 10
  21. 21. Chapter 1 Chemistry of CrIII-Salen complex…these adducts revealed the attachment of Cr+ ion to the bases in nucleosidesand to both the phosphate and base in nucleotides. It is well known that the complexes of transition metal ions are known toundergo redox reactions during the ESI process (Scheme 4).42-47 Within thetransition metal ions, only copper ion is shown as an oxidant in severalexamples for peptides and amino acids.42-46 O’Hair et al.47 studied the redoxprocesses in various metal ions other than copper by taking the advantage ofvacant axial positions of the metal(Salen) complexes (metal = Cr, Mn, Fe andCo). In this process they have generated singly charged metal(Salen) ternarycomplexes with hexapeptides under ESI conditions. The CID experiments onthese ternary complexes produced peptide radical cations (P+.) by redoxprocess (Scheme 5). The authors suggested that the redox process occureither by a homolytic cleavage or by a heterolytic process followed bysubsequent electron transfer. In the fragmentation reactions of ternarycomplexes, produced P+. were found to be highly dependent on the metal ionused. The redox pathway was favored with FeIII or MnIII complexes when Salenligand contained an electron withdrawing group. The resulting peptide radicalcations are odd electron species of nonvolatile precursors, and are nottypically available under ESI or MALDI processes. [MII(Salen)] + P+. [MIII(Salen)(P)]+ Redox process [MIII(Salen)]+ + P Scheme 5: Redox process of [MIII(Salen)(P)]+ in the CID experiment. 11
  22. 22. Chapter 1 Chemistry of CrIII-Salen complex…2. SCOPE OF THE WORK Since all of the metal-Salen complexes generally are used as a catalyst inthe reactions, which specifically give enantioselective products, here, in thisstudy we have selected [CrIII(Salen)]PF6 complex for gas phase chiraldiscrimination of enantiomeric compounds. It is well known that, CrIII(Salen)complex have two free axial positions, hence, we started to make use of thesepositions towards chiral recognition by using the kinetic method.Unfortunately, we are unsuccessful in achieving the chiral discrimination withR- and S- phenyl ethyl amines and napthyl ethyl amines in the gas phase. In thisexperiment, we found that the affinity of these amines towards the axialpositions of metal-Salen complexes is fair, hence, we attempted to check theinteraction among the mono and bidentate ligands. To the best of ourknowledge, detailed studies on the behavior of [CrIII(Salen)] complexes at itsaxial positions and coordination chemistry in the gas phase are not available inthe literature. The use of CoIII-Schiff base complexes with two amines in theaxial positions as antimicrobial agents was reported earlier. Therefore weemployed in our work the ESI method in combination with tandem massspectrometry to study the coordination chemistry of axial positions on theunsubstituted [CrIII(Salen)] complex with amines and diamines.3. RESULTS AND DISCUSSION In the process of analysis of [CrIII(Salen)]PF6 complex in acetonitrile(ACN) under ESI conditions with mono and diamines; we applied different 12
  23. 23. Chapter 1 Chemistry of CrIII-Salen complex…types of experiments, i.e. i) Source, ii) Ligand pick-up, and iii) CIDexperiments.3.1. SOURCE EXPERIMENTS The positive ion ESI mass spectrum of [CrIII(Salen)]+ complex in ACNshows major ions at m/z 318, 359 and 400, corresponding to [CrIII(Salen)]+,[CrIII(Salen)(ACN)]+ and [CrIII(Salen)(ACN)2]+ ions, respectively (Figure 3).Such attachment of solvent molecules to a central metal atom in the ESI processis well documented.5,48 However, it was found that the relative abundances ofthese ions are very much dependent on experimental conditions, especiallythe cone voltage. Hence, we recorded the spectra at cone voltages of 10, 20and 30 V (Figure 3(a-c)) to help understand the effect of solvent co-ordinationin the gas-phase. The spectrum recorded at a cone voltage of 10 V showedmainly two peaks at m/z 359 and 400, corresponding [CrIII(Salen)(ACN)]+ and[CrIII(Salen)(ACN)2]+, respectively, in addition to a minor peak at m/z 318 thatcorresponds to [CrIII(Salen)]+ ion. The [CrIII(Salen)(ACN)2]+ ion was found to bethe base peak in the spectrum under these conditions. It is interesting to notethat the [CrIII(Salen)]+ did not pick up more than two acetonitrile molecules. Ifthe ions at m/z 359 and 400 had resulted from simple solvation of the[CrIII(Salen)]+ complex ion by acetonitrile, one would have expected a series ofions corresponding to [CrIII(Salen)(ACN)n]+ where n = 1,2,3 etc. The absence ofsuch higher adducts (n > 2) indicates that the [CrIII(Salen)]+ complex is able toaccept only two acetonitrile molecules, and that the central metal ion can adopta maximum of six as co-ordination state in the gas-phase. 13
  24. 24. Chapter 1 Chemistry of CrIII-Salen complex… H2(Salen) is a tetra-dentate ligand that occupy four coordination sites ofthe central metal ion through the N, N, O, O atoms. The ions observed at m/z359 and 400 represent the occupation of the axial positions of the CrIII(Salen)complex by one and two acetonitrile molecules, respectively, proving thecapability of the [CrIII(Salen)]+ complex to form five- or six-coordinated speciesin the gas-phase. Similar behavior has been reported for the [CrIII(Salprn)]+complex under ESI conditions.36 It is also in good agreement with reportedcrystallographic studies in which [MnIII(Salen)] complexes were shown to bindwith one or two solvent molecules such as acetone, ethanol etc., that were usedfor recrystallization, usually in axial positions.49 The ESI spectrum recorded at a cone voltage of 20 V showed the ion atm/z 318 as the base peak with the acetonitrile adducts present at reasonableabundance. However, the spectrum obtained at cone voltage of 30 V containsmainly the ions at m/z 318 (base peak) and 359, and the ion at m/z 400 isabsent. This demonstrates that, at higher energies (i.e. at high cone voltages, ≥30V), the solvent molecules are dissociated from the complex to leave the[CrIII(Salen)]+ ion. This result prompted us to study the coordination chemistryof the [CrIII(Salen)]+ complex with mono- and bi-dentate ligands (amines anddiamines) in detail. 14
  25. 25. Chapter 1 Chemistry of CrIII-Salen complex… Figure 3: The ESI mass spectra of CrIII(Salen) at different cone voltages. a) 10, b) 20, c) 30V. The ESI mass spectrum of the [CrIII(Salen)]+ complex in the presence ofpropylamine (Pr-NH2) clearly demonstrates that the displacement of solventmolecules present in the axial positions by the stronger ligand. At low conevoltage (10V) the [CrIII(Salen)(Pr-NH2)2]+ ion at m/z 436 is dominant, that of the[CrIII(Salen)(Pr-NH2)(ACN)]+ ion at m/z 418 is of significant abundance, and the[CrIII(Salen)]+ ion is of negligible importance (Figure 4a). 15
  26. 26. Chapter 1 Chemistry of CrIII-Salen complex… [CrIII(Salen)(Pr-NH2)(ACN)2]+Figure 4: The positive ion ESI mass spectra of [CrIII(Salen)]PF6 complex in thepresence of propylamine (Pr-NH2) at the cone voltage of a) 10 eV b) 20 eV andc) 30 eV. The ion at m/z 418 is dominant over the ion at m/z 436 in the spectrumrecorded at a cone voltage 20V (Figure 4b). It may be due to decomposition ofa fraction of [CrIII(Salen)(Pr-NH2)2]+ to [CrIII(Salen)(Pr-NH2)]+ that immediatelypicks up one molecule of acetonitrile, the surrounding solvent molecule, toresult in a stable six-coordinated [CrIII(Salen)(Pr-NH2)(ACN)]+ ion (m/z 418).The ion at m/z 418 is stable even at high cone voltage (30V) but abundant 16
  27. 27. Chapter 1 Chemistry of CrIII-Salen complex…[CrIII(Salen)]+ ions and its acetonitrile adduct ions at m/z 359 and 400 also areobserved in the spectrum as the result of the fragmentation of the[CrIII(Salen)(Pr-NH2)2]+ ions (Figure 4c) (the abundance of the [CrIII(Salen)(Pr-NH2)2]+ ion is considerably reduced at high cone voltages ≥30 V). Thus, therelative abundances of the ions at m/z 318, 359 and 400 in this experiment canbe used as a measure of the stability of the [CrIII(Salen)(Pr-NH2)2]+ ion. Theseexperiments reveal the ability of Pr-NH2 to occupy both the axial positions ofthe complex to form six-coordinate complex ions that survive at low conevoltage. This prompted us to study the coordination of diamines which arebidentate in nature and can occupy both axial positions of the [CrIII(Salen]+complex. We carried out three types of experiments, i.e. source experiments,ligand pick-up experiments and CID experiments, for this purpose. A series of primary α,ω-diamines (DA) were selected to study not onlythe effect of the bidentate nature of diamines but also the effect of chain lengthof the ligand on the occupation of the axial positions of [CrIII(Salen)]+. We used1,2-diaminoethane (1), 1,3-diaminopropane (2), 1,4-diaminobutane (3), 1,5-diaminopentane (4), 1,6-diaminohexane (5), 1,7-diaminoheptane (6) and 1,8-diaminooctane (7) as bidentate ligands. The ESI mass spectra of an equimolar(100 µM) mixture of [CrIII(Salen)]+ and diamines were recorded at conevoltages of 10 and 30 V. All the spectra recorded at 10V show mainly the[CrIII(Salen)(DA)]+ ions, and other ions are negligible. However, the spectrarecorded at 30V still showed the [CrIII(Salen)(DA)]+ ion as the base peak but, inaddition, [CrIII(Salen)]+, [CrIII(Salen)(ACN)]+ and [CrIII(Salen)(ACN)2]+ ions are 17
  28. 28. Chapter 1 Chemistry of CrIII-Salen complex…also present in significant abundances. This indicates that the[CrIII(Salen)(DA)]+ ion is stable at high cone voltage (30V), unlike[CrIII(Salen)(ACN)2]+ and [CrIII(Salen)(Pr-NH2)2]+. Though the[CrIII(Salen)(DA)]+ ion is dominant, the appearance of the other low mass ionsin the spectra recorded at cone voltage 30 V but not at 10 V implies partialdecomposition of [CrIII(Salen)(DA)]+ ion to give unbound [CrIII(Salen)]+ ion(m/z 318) that picks up surrounding solvent molecules in the API interfaceregion to yield the ions at m/z 359 and 400 (Figure 4). + H2N (CH2)n N N ESI N N CrIII + H2N-(CH2)n-NH2 III Cr O O PF6 O O N H2Scheme 6: Reaction of [CrIII(Salen)]+ with a diamines (n=2-8, 1-7) to form abidentate complex. The relative abundances of these ions were found to vary depending onthe size of the ligand used. Hence, we consider the spectra recorded at a conevoltage of 30V for further discussion, as the relative abundances of the ions atm/z 318, 359 and 400 are found to reflect the stability of the diamine co-ordination complexes formed with [CrIII(Salen)]+. The ESI mass spectra of[CrIII(Salen)] in the presence of the chosen diamines are listed in Table 1. It isclear that ligands 1 and 2 are able to form the [CrIII(Salen)(DA)]+ ion with ease,and the other ions due to loss of DA at m/z 318, 359 and 400 are less abundant(<8%). [CrIII(Salen)(DA)2]+ and [CrIII(Salen)(DA)(ACN)]+ ions are absent, 18
  29. 29. Chapter 1 Chemistry of CrIII-Salen complex…consistent with the effective occupation of the two empty axial positions of[CrIII(Salen)]+ by the two amino groups in the case of ligands 1 and 2 (Scheme6). Relative abundance (%) Ion 1 2 3 4 5 6 7 [CrIII(Salen)]+ 2.9 4.3 10 19 44 34 40 m/z 318 [CrIII(Salen)(ACN)]+ 5.8 7.9 20 37 65 61 66 m/z 359 [CrIII(Salen)(ACN)2]+ 4.4 7.1 18 35 56 54 48 m/z 400 [CrIII(Salen)(DA)]+ 100 100 100 100 100 100 100 III + [Cr (Salen)(DA)(ACN)] - - 2.9 5.1 5.1 0.7 0.3 [CrIII(Salen)(DA)2]+ - - 1.5 4.4 5.1 8.7 13Table 1: Positive ion ESI mass spectra (cone voltage 30 V) of mixtures of [CrIII(Salen)]+ (as the PF6- salt) with diamines (DA) ligands (1-7) in acetonitrile (ACN) solvent. In the case of higher diamine homologues (3-6), the [CrIII(Salen)(DA)2]+and [CrIII(Salen)(DA)(ACN)]+ ions are also present, reflecting the decreasingability of the higher homologues to yield stable bidentate complexes. It isinteresting to note that the relative abundances of the ions m/z 318, 359 and 400gradually increase from ligand 1 to 5. In the case of 6 and 7, the relativeabundances of these ions are lower than those found for ligand 5. The gradualincrease of the abundances of the fragment ions at m/z 318, 359 and 400 fromligand 1 to 5 reflects a gradual decrease in the stability of [CrIII(Salen)(DA)]+ion from 1 to 5 under the ESI conditions used here. The stability of 19
  30. 30. Chapter 1 Chemistry of CrIII-Salen complex…[CrIII(Salen)(DA)]+ ion for ligands 6 and 7 seems to be higher than that of ligand5 and lower than those from 1-4. There is a marginal but consistent increase inthe relative abundances of [CrIII(Salen)(DA)(ACN)]+ and [CrIII(Salen)(DA)2]+ions from 3 to 5, reflecting that the ability of the diamine to occupy two axialpositions decreases from 1 to 5. In the case of 6 and 7, the abundance of the[CrIII(Salen)(DA)(ACN)]+ ion is negligible and that of the [CrIII(Salen)(DA)2]+ ionis higher than for the other ligands used. From these observations it can beinferred that the bidentate nature decreases as the chain length increases from1 to 5, while that of ligands 6 and 7 shows mixed behavior. In order tounderstand the stability of these ionic species in the gas phase, we performedligand-pickup experiments in the collision cell.3.2. LIGAND-PICKUP EXPERIMENTS In these experiments the ions of interest were selected using MS1 andallowed to undergo ion-molecule reactions in the collision cell with the ligandof interest introduced into the collision cell. The resultant product ions werethen analyzed by MS2 (Figure 5). Experiments done on [CrIII(Salen)]+ as themass-selected precursor ion using acetonitrile as the collision gas showed thepickup of one and two acetonitrile molecules by [CrIII(Salen)]+ to form five- andsix-coordinated species (ions at m/z 359 and 400, respectively, Figure 6). Thesame behavior was observed when [CrIII(Salen)(ACN)]+ and [CrIII(Salen)(ACN)2]+ are selected as precursor ions; the former ion showed addition of oneacetonitrile and the latter ion did not undergo any further addition ofacetonitrile (Figure 6). 20
  31. 31. Chapter 1 Chemistry of CrIII-Salen complex…Figure 5: Schematic diagram of a triple quadrupole instrument.Figure 6: The spectra obtained from ligand-pickup experiments using acetonitrile as the collision gas for precursor ions: a) [CrIII(Salen)]+ ion, m/z 318 b) [CrIII(Salen)(ACN)]+ ion, m/z 359 and c) [CrIII(Salen) (ACN)2]+ ion, m/z 400. 21
  32. 32. Chapter 1 Chemistry of CrIII-Salen complex… The displacement of weaker ligands (ACN) in the axial positions of[CrIII(Salen)]+ by relatively stronger ligands (amines) was also observed in thecollision cell experiments using propylamine as collision gas. The propylamineformed abundant [CrIII(Salen)(Pr-NH2)]+ and [CrIII(Salen)(Pr-NH2)2]+ ions with[CrIII(Salen)]+ as the precursor ion; the same product ions are also observedwhen [CrIII(Salen)(ACN)]+ and [CrIII(Salen)(ACN)2]+ are selected as precursors(Figure 7). These experiments clearly indicate that the empty axial positions ofunsubstituted [CrIII(Salen)]+ ion are easily occupied by any ligand, and that thedisplacement of weaker ligands by relatively stronger ligands occurs when acomplex with weaker ligands is selected as precursor ion. Similar ligand-pickup experiments were reported previously by Plattner et al.10 for MnIII(Salen) species; they showed that MnIII is able to form only five-coordinatedspecies unless there is an electron-deficient substituent on Salen. However, inthe present case, [CrIII (Salen)]+ is able to form six-coordinate complexeseasily, possibly due to differences in the electronic configurations of MnIII andCrIII ions. Note that it is difficult to achieve equilibrium between the collision(reactant) vapor and the selected ion species in the collision cell under themass spectral conditions used, as the experimental time window for the ion inthe collision cell is about 10-100 milliseconds. However, the results indicatethat it is possible to study the relative efficiencies of ligand exchange in thecollision cell. 22
  33. 33. Chapter 1 Chemistry of CrIII-Salen complex…Figure 7: The spectra obtained from ligand-pickup experiments using propylamine as the collision gas for precursor ions: a) [CrIII(Salen)]+ ion, m/z 318 b) [CrIII(Salen)(ACN)]+ ion, m/z 359 and c) [CrIII(Salen) (ACN)2]+ ion, m/z 400. We extended the ligand-pickup experiments to study of the bidentatenature of diamines and the stability of diamine complexes, using acetonitrile inthe collision cell. From the ESI source mass spectra (Table 1) it is evident thatthe five-coordinated complex [CrIII(Salen)(L)]+ (L = acetonitrile orpropylamine) exists in the gas-phase in addition to the stable six-coordinated 23
  34. 34. Chapter 1 Chemistry of CrIII-Salen complex…species. The observation of [CrIII(Salen)(DA)]+ ion in the ESI mass spectrum ofthe mixture of [CrIII(Salen)]+ and diamine poses the question whether or notboth axial positions are occupied by the two amino groups of the diamineligand. When we selected a complex ion containing a monodentate ligand (Pr-NH2) for ligand-pickup experiments, for example [CrIII(Salen)(Pr-NH2)]+, itpicked up one acetonitrile molecule in the collision cell to yield [CrIII(Salen)(Pr-NH2)(ACN)]+ (Figure 8). This observation is as expected because one axialposition in [CrIII(Salen)(Pr-NH2)]+ is free and acetonitrile can readily occupy thevacant axial position. When we selected [CrIII(Salen)(DA)]+ ion for ligands 1-4in MS1 for ligand-pickup experiments with acetonitrile in the collision cell, noaddition of acetonitrile to the selected species was observed. The sameexperiments for [CrIII(Salen)(DA)]+ ions from 5-7 resulted in[CrIII(Salen)(DA)(ACN)]+ ions of low abundance (5.1, 1.7 and 0.5% for 5, 6 and7, respectively); the spectrum for the case of ligand 5 is shown in Figure 8b asan example. Similar results were obtained from ligand-pickup experimentsbetween [CrIII(Salen)(DA)]+ ions of 1-7 and propylamine in the collision cell.From these experiments it can be concluded that [CrIII(Salen)(DA)]+ ions fromligands 1-4 are stable in the collision cell under the present experimentalconditions. In the case of ligands 5-7, one of the axial coordinate bondsbetween the central metal ion and the amino group of the ligand becomesweaker, so that the DA can be displaced by acetonitrile in the collision cell.Further, these experiments confirm that the [CrIII(Salen)(DA)]+ species is 24
  35. 35. Chapter 1 Chemistry of CrIII-Salen complex…relatively less stable for ligand 5 than for 6 and 7. These observations areconsistent with the results obtained from the ESI mass spectra (source).Figure 8: The spectra obtained from ligand-pickup experiments using acetonitrile as the collision gas for precursor ions: a) [CrIII(Salen)(Pr-NH2)]+ ion, m/z 377 and b) [CrIII(Salen)(hexd)]+ ion, m/z 434.3.3. COLLISION INDUCED DISSOCIATION (CID) EXPERIMENTS With a view to study the stability of diamine complexes with[CrIII(Salen)]+, we also performed CID experiments on [CrIII(Salen)(DA)]+ ionsusing argon as the collision gas at different collision energies (10, 12 and 14 25
  36. 36. Chapter 1 Chemistry of CrIII-Salen complex…eV). All the spectra resulted in only one product ion corresponding to[CrIII(Salen)]+, and the relative abundance of this ion was found to depend onthe nature of the diamine used. It is demonstrated in the literature that theprecursor/product (Pc/Pd) abundance ratio can be used to measure therelative stability of adduct ions, since a stable precursor ion undergoes lessdecomposition.50 The Pc/Pd ratios, i.e. relative abundance ratio[CrIII(Salen)(DA)]+/[CrIII(Salen)]+, obtained from the CID spectra of[CrIII(Salen)(DA)]+ ions from 1-7 at different collision energies, are presented ingraphical form in Figure 9.Figure 9: The plot of Pc/ Pd ratios ([Cr(III)(Salen)(DA)]+/ [Cr(III)(Salen)]+) obtained at collision energies of 10, 12 and 14 eV from CID of [Cr(III)(Salen)(DA)]+ ions for ligands (Diamines) 1-7. The order of stabilities of [CrIII(Salen)(DA)]+ complexes for diamines 1-7can be given as 2> 1> 3> 4 ≈ 7> 6> 5. This agrees well with the similar stabilityorder obtained from source experiments (mass spectra) already discussed, 26
  37. 37. Chapter 1 Chemistry of CrIII-Salen complex…except for ligands 1 and 2. The collision cell experiments show that ligand 2forms a more stable complex with [CrIII(Salen)]+ compared to ligand 1. In the source experiments the behavior of ligands 1 and 2 is reversedbut the difference is marginal. Hence, it can be concluded from both sourceand collision cell experiments that the feasibility of complexation of diamineswith unsubstituted [CrIII(Salen)]+, by occupying the axial positions, decreasesas the chain length increases from ligand 1 to 5. We cannot offer an explanationfrom the available experimental data for the marginal increase in the stabilityof the complexes with 1 and 2 and similarly with ligands 6 and 7. 27
  38. 38. Chapter 1 Chemistry of CrIII-Salen complex…4. CONCLUSIONS The positive ion ESI mass spectra for [CrIII(Salen)]+ complex in thepresence of amines as ligands (propylamine and a series of diamines (1-7))were studied with a view to understand the coordination chemistry of thecomplex in the gas phase. The ESI mass spectra of [CrIII(Salen)]+, either inacetonitrile alone or in the presence of propylamine, showed ionscorresponding to five- and six-coordinated species, respectively. The[CrIII(Salen)]+ in the presence of bidentate ligands (L = diamines) mainlyresulted in [CrIII(Salen)(L)]+ ions in which the two empty axial positions in[CrIII(Salen)]+ species are occupied by the two amino groups of the diamine. Inaddition to five- and six-coordinated complex ions, other ions corresponding to[CrIII(Salen)]+ and its solvent adduct ions are also observed in the ESI massspectra, and the relative abundances of these ions were found to depend on thecone voltage. However, the relative abundances of the above ions at constantcone voltage reflected the stability of the [CrIII(Salen)(L)]+ ions. The[CrIII(Salen)(L)]+ ion is most stable for 1,2-diaminoethane and 1,3-diaminopropane ligands. The stability of the complex ion decreased from 1,4-diaminobutane to 1,6-diaminohexane, and there is a slight increase for 1,7-diaminoheptane and 1,8-diaminooctane. A similar trend was observed from theligand-pickup experiments in the collision cell using acetronitrile orpropylamine as collision gas, and from CID experiments on [CrIII(Salen)(L)]+ions. 28
  39. 39. Chapter 1 Chemistry of CrIII-Salen complex…5. EXPERIMENTAL The [CrIII(Salen)]PF6 was synthesized using a known procedure.51 All theligands (propylamine and diamines, 1-7) used in the present study werepurchased from Sigma-Aldrich (Steinheim, Germany) and were used withoutfurther purification. The solvents (HPLC-grade) were purchased from Merck(Mumbai, India). Stock (1mM) solutions of all ligands and of [CrIII(Salen)]PF6were made in acetonitrile. The stock solutions of the ligand of choice and of[CrIII(Salen)]PF6 were mixed in appropriate volumes (1:1) and diluted withacetonitrile to achieve final concentrations of 100µM each. All the mass spectra were recorded using a Quattro LC triple-quadrupole mass spectrometer (Micromass, Manchester, UK) coupled with anHP1100 series liquid chromatograph (Agilent, Palo Alto, USA); the data wereacquired using Masslynx software (version 3.2). The ESI capillary voltage wasmaintained between 4 and 4.2 kV, and the cone voltage was kept at 30 V unlessotherwise stated. Nitrogen was used as desolvation and nebulization gas. Thesource and desolvation temperatures were kept at 100o C. The ESI mass spectrawere recorded by scanning MS1 and the sample solutions were injectedthrough the Quattro LC injector with a Valco six-port valve with a 10 µL loop,using acetonitrile at a flow rate of 100 µL/min using the HPLC pump. The CIDspectra and ligand-pickup experiments were obtained by selecting theprecursor ion of interest with MS1 and scanning MS2. For these experiments,the sample solutions were introduced into the source of the mass spectrometer 29
  40. 40. Chapter 1 Chemistry of CrIII-Salen complex…using an infusion pump (Harvard Apparatus) at a flow rate of 10 µL/min. Argonwas used as the collision gas for CID experiments and the collision cellpressure was maintained at 9x10-4 mbar. For ligand-pickup experiments,acetonitrile or propylamine was used as the collision gas, maintaining thecollision cell pressure at 9x10-4 mbar. All the spectra reported here wereobtained as the averages of 20 scans. 30
  41. 41. Chapter 1 Chemistry of CrIII-Salen complex…6. REFERENCES1. Cotton R, D’Agostino A, Traeger JC. Mass. Spectrom. Rev., 1995; 14: 79.2. Henderson W, Nicholson BK, McCaffrey LJ. Polyhedron, 1998; 17: 4291.3. Plattner DA. Int. J. Mass Spectrom., 2001; 207: 125.4. Plattner DA, Feichtinger D, El-Bahraoui J, Wiest O. Int. J. Mass Spectrom., 2000; 195/196: 351.5. Chipperfield JR, Clayton J, Khan SJ, Woodword S. J. Chem.Soc., Dalton Trans., 2000; 1087.6. Di Marco VB, Bombi GG. Mass Spectrom. Rev., 2006; 25: 347.7. Operti L, Rabezzana R. Mass Spectrom. Rev., 2006; 25: 483.8. Hinderling C, Plattner DA, Chen P. Angew. Chem., Int. Ed. Engl., 1997; 36: 243.9. Hinderling C, Feichtinger D, Plattner DA, Chen P. J. Am. Chem. Soc., 1997; 119: 10793.10. Feichtinger D, Plattner DA. Angew. Chem., Int. Ed. Engl., 1997; 36: 1718.11. Feichtinger D, Plattner DA. J. Chem. Soc., Perkin. Trans., 2000; 2: 1023.12. Feichtinger D, Plattner DA. Chem. Eur. J., 2001; 7: 591.13. El-Bahraoui J, Wiest O, Feichtinger D, Plattner DA. Angew. Chem. Int. Ed., 2001; 40: 2073.14. Plattner DA. Top. Curr. Chem., 2003; 225: 153.15. Pfeiffer P, Breith E, Lübbe E, Tsumaki T. Liebigs Ann., 1933; 503: 84.16. Dalton CT, Ryan KM, Wall VM, Bousquet C, Gilheany DG, Top. Catal., 1998; 5: 75. 31
  42. 42. Chapter 1 Chemistry of CrIII-Salen complex…17. Zhang W, Loebach JL, Wilson SR, Jacobsen EN. J. Am. Chem. Soc., 1990; 112: 2801.18. Irie R, Noda K, Ito Y, Matsumoto N, Katsuki T. Tet. Lett., 1990; 31: 7345.19. Lee NH, Lee CS, Jung DS. Tetrahedron Lett., 1998; 39: 1385.20. Omura K, Uchida T, Irie R, Katsuki T. Chem. Commun., 2004; 2060.21. Mc Gilvra JD, Rawal VH. Synlett., 2004; 2440.22. Shin CK, Kim SJ, Kim GJ. Tetrahedron Lett., 2004; 45: 7429.23. Maeda T, Takeuchi T, Furusho Y, Takata T. J. Polym. Sci., Part A: Polym. Chem., 2004, 42: 4693.24. Kim SS, Rajagopal G. Synthesis., 2003: 2461.25. Patel KS, Rinehart KL, Bailar JC. Jr. Org. Mass Spectrom., 1970; 4: 441.26. Lacey MJ, Macdonald CG, Shannon JS. Org. Mass Spectrom., 1978; 13: 188.27. Rohly KE, Heffren JS, Douglas BE. Org. Mass Spectrom., 1984; 19: 398.28. Huang SK, Rood MH, Zhao SH. J. Am. Soc. Mass spectrom., 1997; 8: 996.29. Miller JM. Adv. Inorg. Chem. Radiochem., 1984; 28: 1.30. Bruce MI, Liddell MJ. Appl. Organomet. Chem., 1987; 1: 191.31. Dale MJ, Dyson PJ, Suman P, Zenobi R. Organometallics, 1997; 16: 197.32. Dale MJ, Dyson PJ, Johnson BFG, Langridge-Smith PRR, Yates HT. J. Chem. Soc. Dalton Trans., 1996; 771.33. Dale MJ, Dyson PJ, Johnson BFG, Martin CM, Langridge-Smith PRR, Zenobi R. J. Chem. Soc. Chem. Commun., 1995; 1689.34. Huc V, Boussaguet P, Mazerolles P. J. Organomet. Chem., 1996; 521: 253. 32
  43. 43. Chapter 1 Chemistry of CrIII-Salen complex…35. Cole RB (ed.), "Electrospray Ionisation Mass Spectrometry, Fundamentals, Instrumentation and Applications,” Wiley Interscience, New York, 1997.36. Madhusudanan KP, Katti SB, Vijayalakshmi R, Nair BU. J. Mass spectrom., 1999; 34: 880.37. Lee SW, Chang S, Kossakovski D, Cox H, Beauchamp JL. J. Am. Chem. Soc., 1999; 121: 10152.38. Gaskell SJ. J. Mass Spectrom., 1997; 32: 677.39. Siddall TL, Miyaura N, Huffman JC, Kochi JK. J. Chem. Soc., Chem. Commun., 1983; 1185.40. Samsel EG, Srinivasan K, Kochi JK. J. Am. Chem. Soc., 1985; 107: 7606.41. Srinivasan K, Kochi JK. Inorg. Chem., 1985; 24: 4671.42. Hu P, Loo JA. J. Am. Chem. Soc., 1995; 117: 11314.43. Gatlin CL, Turecek F, Vaisar T. J. Mass Spectrom., 1995; 30: 1605.44. Gatlin CL, Turecek F, Vaisar T. J. Mass Spectrom., 1995; 30: 1617.45. Gatlin CL, Rao RD, Turecek F, Vaisar T. Anal. Chem., 1996; 68: 263.46. Vaisar T, Gatlin CL, Turecek F. Int. J. Mass Spectrom. Ion Processes, 1997; 162: 77.47. Waters T, O’Hair RAJ, Wedd AG. J. Am. Chem. Soc., 2003; 125: 3384.48. Katta V, Choudhury SK, Chait BT. J. Am. Chem. Soc., 1990; 112: 5348.49. Calligaris M, Randaccio L. in “Comprehensive Coordination Chemistry,” Wilkinson G, Mc Cleverty JA. (Eds.), Vol. 2, Peramon, Oxford, 1987, chap. 20.1, pp. 715.50. Cai Y, Cole RB. Anal. Chem., 2002; 74: 985. 33
  44. 44. Chapter 1 Chemistry of CrIII-Salen complex…51. Premsingh S, Venkatramanan NS, Rajagopal S, Mirza SP, Vairamani M, Sambasivarao P, Valavan K. Inorg. Chem., 2004; 43: 5744. 34
  46. 46. Chapter 2.1 Proton and alkali metal ion interactions of Diamines.. CHAPTER 2 CHAPTER 2 PART--1 PART 1 Proton and alkali metal ion affinities of α,ω -Diamines: Spacer chain length effects1. PROLOGUEP rotonated species are central to many chemical and biological processes, such as acid-base phenomena, astrochemistry, radiation chemistry, massspectrometry, catalysis, surface chemistry, protein conformation, membranetransport and enzyme catalysis.1 Since, the proton is having almost comparableattributes with the alkali metal ions, the hydrogen has been placed in the firstgroup of the periodic table. Alkali metal ions are one of the most abundant ionsin biological systems, where they are involved in a variety of processes,including osmotic balance, the stabilization of biomolecular conformations andinformation transfer through ion pumps and ion channels.2-6 They interact withpoly functional molecules, like peptides and proteins to perform suchregulatory and structural functions.2,7 Thus, the knowledge of proton and alkalimetal ion binding interactions with polyfunctional biomolecules is an importantstep in understanding the biochemical processes.8,9 Good correlations existbetween the metal ion and proton binding affinity to the bases, though theproton affinities are much higher.10,11 Alkali metal ion binding interactions tosmall model ligands bearing the heteroatom (oxygen or nitrogen) functionalgroup (binding sites) in the gas phase provides intrinsic information necessary 35
  47. 47. Chapter 2.1 Proton and alkali metal ion interactions of Diamines..for better understanding of the interaction of the metal ions with biologicallyactive macromolecules. The proton/alkali metal ion interactions decrease the space occupied inthree dimensional structures wherever possible, and they can opt to theregulation of enzymatic activity, protein folding and functioning and stability ofbiological systems.2,7 A common structural feature of the proton/alkali metal ionbound complex is the presence of interactions between multiple functionalgroups. For instance, in the protonated polyfunctional ions, protonated part ofthe molecule may interact with an unprotonated group to form intramolecularhydrogen bonds. Many organic reactions also proceed through protonatedintermediates or involve direct hydrogen bonding such as those involved inprotein or DNA complexes. Such hydrogen bondings greatly influence thestructure and the properties of organic compounds. In particular,intramolecular hydrogen bonds are often responsible for determining thepredominant conformers in solution12 as well as in the gas phase.13-17Intramolecular solvation of protonated functional groups influence the gas-phase basicities of polyfunctional molecules. Occurrence of intramolecularsolvation in protonated species was characterized by several authors from aseries of di- and polyfunctional ions such as diols,17-21 diamines,10,13,14,16,20-28diethers,15,16 diketones,15,16,29 amino acid derivatives,30 cyclic and acyclicpolyethers, and open chain and cyclic diols31 and amino alcohols using boththeoretical and experimental studies.14 The intramolecular hydrogen bond 36
  48. 48. Chapter 2.1 Proton and alkali metal ion interactions of Diamines..stabilizes the ion by up to 20 kcal mol-1, and thereby increases the protonaffinity of several bi or polyfunctional compounds.20/21 In biological systems, especially in proteins, several basic motifs exist,separated by varying chainlengths. Polyamines found to be present in the cellsof microorganisms and animal organisms, contribute to the stabilization of thestructure and activity of tRNA and DNA.32 It is well known that polyamines, suchas putrescine (1,4-diaminobutane), spermidine and spermine, are present inmillimolar concentrations in most tissues and microorganisms. Otherpolyamine derivatives including cadaverine (1,5-diaminopentane) and 1,3-diaminopropane are also found in some living cells. Although there wereseveral reports that describe the effects of the polyamines on the higher orderstructure of DNA, the mechanism of the action of polyamines on DNA moleculeshas not been clarified yet.33 α,ω-alkanediamines are compounds of interest in various domains oforganic and organometallic chemistry because these are bifunctional, cancyclize after protonation (Scheme 1).13,22 These are also known as chelatingbidentate ligands in coordination chemistry, as reactants in industrialpolymerization processes, and as synthetic enzymes for complex formationswith target substrates through hydrogen bonding.34 Thermochemicalproperties of α,ω-diamines have been studied by severalresearchers.10,13,14,16,20-27 37
  49. 49. Chapter 2.1 Proton and alkali metal ion interactions of Diamines.. H2 N H+ H2N (CH2)n NH2 (CH2)n H+ N H2 Scheme 1 Estimation of thermo chemical properties to the monofunctionalmolecules is straight forward, whereas to that of molecules with two or morefunctional groups is intresting to investigate, because of possible internalhydrogen bonding between the two like or unlike functional groups. Moleculeswith two or more functional groups may have more proton affinity, greater thanthat expected for either of the individual groups, due to the internal hydrogenbond formation by favorable molecular geometry. Intramolecular hydrogenbonding and the consequent chelating ring size were found to be the keyfactors controlling the stability of the protonated complexes.10,13,14,16,20-28 Thefirst examples and interpretations of this phenomenon were explicated by Aueet al.22 and followed by Yamdagni and Kebarle,13 who found that the protonaffinities of α,ω-diamines are significantly higher than those of monoamineswith the same alkyl chain length. The protonated diamines were proposed tohave cyclic structures, and the ring strain present in the structures wasevaluated with reference to the strain-free structures of proton bound dimers ofmonoamines. Both groups of workers noted that the proton affinity ofH2NCH2CH2NH2 was substantially less than that of its higher diamino analogues,which was attributed to the large strain energy expected for a five membered 38
  50. 50. Chapter 2.1 Proton and alkali metal ion interactions of Diamines..ring (the assumption being that for maximum stability, the N--H+--N bond willbe linear as in proton-bound dimers of monoamines). Later, Bouchoux et al.extensively studied protonation thermo chemistry of α,ω−diamines. Massspectrometric methods and computational techniques were extensively usedfor the protonation studies on α,ω-diamines. 10,13,14,16,20-28 Though there were a numerous reports on the protonationthermochemistry of α,ω-diamines, studies towards the alkali metal ion affinitiesof the diamines are scarce. There was only one report on ab initio molecularorbital (MO) calculations on the stabilities and binding energies of bidentateethylene diamine with alkali metal (Li+ and Na+) ions.10 The computed bindingenergies of Li+ and Na+ ions with ethylene diamine are 66.3 and 42.3 kcal mol-1,respectively. Thermochemical data obtained in the gas phase are of particularvalue both for understanding the nature of metal ion-basic componentinteractions in condensed phase and for explaining solvent phenomenon.35 The solvent-free environment of the mass spectrometer provides anideal medium for measuring the intrinsic properties such as proton/metal ionaffintity in the absence of interfering solvent effects. The kinetic methoddeveloped by Cooks et al.36-39 has been used to estimate thermochemical datafor a wide range of organic and biological molecules for more than 25 yearsand has often been reviewed.36 39
  51. 51. Chapter 2.1 Proton and alkali metal ion interactions of Diamines..1.1. THE KINETIC METHOD The best-known application of the kinetic method is for thedetermination of proton affinities, gas phase acidities, metal and chloride ionaffinities, and electron affinities.20,21,36-48 This method37,40,41 is an effectivemethod for estimating the relative binding energies of two similar bases thatbind to a central ion, typically a proton/metal ion. Several series of bidentatemolecules, such as diols, diethers and diamines, have been studied by thismethod for the determination of their proton affinities. The method starts withthe generation of proton/metal ion bound dimer between two bases and issubjected to tandem mass spectrometric experiments to obtain thecorresponding proton/metal ion bound monomeric bases. The ratio of therelative abundances associated with two competitive dissociation channels(heterodimers) is then measured to estimate the relative binding energies. Thelogarithmic value of the relative abundance is proportional to the logarithm ofthe relative rate of dissociation of the two reaction channels. For example, the dissociation of a proton/metal (M) bound heterodimerof L1 and L2 leads to M+ bound monomers (equation 1 and 2) k1 L1 + L2M+ (1) [L1- - -M+- - -L2] k2 L2 + L1M+ (2) 40
  52. 52. Chapter 2.1 Proton and alkali metal ion interactions of Diamines.. Here k1 and k2 are the rate constants for the competitive dissociations ofthe cluster ion to yield L1M+ and L2M+, respectively. Based on transition statetheory, 49 the natural logarithm of rate constant ratio is given by the equation 3. ln(k1/k2) = ln(Q2*/Q1*) + [εo(1) - εo(2)]/RTeff (3) In which Q1* and Q2* are the partition functions of the activatedcomplexes of reaction 1 and 2, respectively; εo(1) and εo(2) are thecorresponding activation energies; R is the gas constant and Teff is the effectivetemperature, a parameter in temperature units that reflects the internal energyof the dissociating heterodimer. Assuming that the abundances reflect rateconstants37, 40,41 and that no reverse activation barriers exist equation 3 tendsto, ln([L2M+]/[L1M+]) = ln(Q2*/Q1*) - ∆HML1/RTeff + ∆HML2/RTeff (4) Where, ∆H°M is the ∆H of the dissociation reaction LM+ L + M+ or themetal ion affinity of L. If L1 and L2 are structurally similar, as expected with usedligands, ∆(∆S M+) should be close to zero, i.e. Q2* ≈ Q1* and fragmentation ofL1M and L2M proceed by simple bond cleavages from the loosely boundcomplex L1--M+--L2, the reverse activation energies for channels L1M and L2Mshould be negligible. In such a case, the difference in proton/metal ionaffinities between the two amino acids of interest would be nearly equals to thebinding energy (∆E) of those amino acids: then the above equation is simplifiedfurther to (equation 5). 41
  53. 53. Chapter 2.1 Proton and alkali metal ion interactions of Diamines.. ln([L2M+]/[L1M+]) ~ (∆HML2 - ∆HML1)/RTeff ~ ∆EM/RTeff (5) ∆EM is the binding energy of central proton/metal ion between the twoheterodimers, where L1M and L2M are the relative abundances of the tworeactions from the fragmentation of the M+ bound heteodimer, and Teff is theeffective temperature of the activated precursor cluster ion. Thus, Teff is ameasuring parameter for internal energy of dissociating cluster ion andprimarily depends on the structure and lifetime of the ion. Severalinvestigations have shown that different dimer ions (of chemically similarmolecules), generated under identical experimental conditions and found tohave the same lifetime, also have fairly similar Teff, independent of the centralion holding them. Hence, Teff of [L1--M+--L2] can be approximated by theeffective temperature of the corresponding H+-bound heterodimers. Using the above assumptions Cerda and Wesdemiotis39 semi-quantitatively evaluated the relative Cu+ ion binding energies of α-amino acids.In these experiments, Teff values for [AA1-Cu-AA2]+ was approximated to the Tefffor H+- bound hetero amino acids, coproduced in the same sample. Applicationof the equation 5 yielded Teff value, and this value was further used to convertln(k2/k1) values in the estimation of Cu+ binding energies of all the 20 commonamino acids. In a similar way, Lee et al.41 also constructed relative Ag+ ionbinding energy ladder for essential α-amino acids using the kinetic method.These binding energies were compared with their relative H+ and Cu+ ionbinding energies. However, there is no systematic study on the alkali metal 42
  54. 54. Chapter 2.1 Proton and alkali metal ion interactions of Diamines..affnities of homologues series of α,ω-diamines, and the effect of spacer chainlength on their binding efficiency.2. SCOPE OF THE WORK The literature reports clearly demonstrate the enhancement of theproton affinities of α,ω-diamines with respect to the primary amines. This wasreadily explained by the formation of a strong internal hydrogen bond in theprotonated form of the diamines. Among homologues series of α,ω-diamines,1,4-butane diamine depicts highest proton affinity, owing to the sevenmembered ring stabilized structure after protonation. However, there were nosystematic studies on the alkali metal ion affinities of α,ω-diamines. Hence, weundertook a systematic experimental and computational study on themeasurement of relative gas phase affinity of alkali metal ions (Li+, Na+ and K+)with a series of α,ω-diamines and compared them with the correspondingproton affinities. In this part, the kinetic method and quantum chemicalcalculations are employed to address the following points. What are the variations in the relative binding affinities of proton and alkali metal ions in the given series? What is the nature of bridging interactions the alkali metal ion complexes have? What are the structural differences between the proton and alkali metal ion complexes of diamines? 43
  55. 55. Chapter 2.1 Proton and alkali metal ion interactions of Diamines..3. RESULTS AND DISCUSSION It is well known that electrospray ionization technique is the best methodto study the interactions between the metal ions and various systems. We haveused the kinetic method for evaluating the relative alkali metal ion [Li+, Na+ andK+] affinities for a series of seven homologues α,ω-diamines, namely 1,2-diaminoethane (1), 1,3-diaminoproane (2), 1,4-diaminobutane (3), 1,5-diaminopentane (4), 1,6-diaminohexane (5), 1,7-diaminoheptane (6), and 1,8-diaminooctane (7). [Li-2]+ 81 100 [1-Li-2]+ CID with Ar gas [1-Li-2]+ 141 % [Li-1]+ 67 0 m/z 40 60 80 100 120 140 160 180 200 +Figure 1: CID mass spectra of Li bound heterodimer of compounds 1 and 2. The study was initiated with Li+ ion binding of 1-7. The typical ESI massspectrum recorded for a methanol/water solution containing two differentdiamines from 1-7 (DA1 and DA2) and lithium chloride show the H+ and Li+bound mono and dimeric cluster ions. The spectrum recorded for a mixture of1 and 2 in the presence of Li+ is shown in Figure 1 as an example. The Li+ 44
  56. 56. Chapter 2.1 Proton and alkali metal ion interactions of Diamines..bound heterodimeric ions, [DA1+Li+DA2]+ formed with various combinations ofdiamines are mass selected by MS1, and dissociated in the collision cell undersimilar experimental conditions. The heterodimeric ions dissociate bycompetitive elimination of neutral diamines yielding two fragment ionscorresponding to [DA1+Li]+ and [DA2+Li]+ (equations 6 and 7). The relativeabundances of the resulted Li+ bound monomers, viz., I(Li+-DA1) and I(Li+-DA2)vary and reflect the Li+ ion affinity of individual diamine. The diamine that hasmore affinity results in higher abundance of its Li+ bound monomer than that ofwith less affinity. For example, the CID spectrum of [1+Li++2] (Figure 1) showshigher abundance of [1+Li+] than [2+Li+], which confirms higher Li+ ion affinityof 1 when compared to that of 2. DA2 + DA1Li+ (6) [DA1--Li+--DA2] DA2Li+ + DA1 (7)3.1. Li+ ION AFFINITY LADDER CONSTRUCTION The CID spectra were recorded for all possible lithiated heterodimers ofdiamines (1-7). The spectra of fifteen out of twentyone heterodimers resulted inboth Li+ bound monomers, [Li-DA1]+ and [Li-DA2]+ with considerableabundance. The other spectra are dominated with only one of the lithiatedmonomer being the other monomer negligible due to large difference in theirLi+ ion affinities. Hence, we consider only those spectra, which resulted in both 45
  57. 57. Chapter 2.1 Proton and alkali metal ion interactions of Diamines..fragment ions, for constructing the Li+ ion affinity order by the kinetic method.The relative abundance ratio of two fragment ions, i.e., I(Li+-DA2)/I(Li+-DA1)ratio values are calculated from the CID spectra of all possible heterodimers,where the Li+ ion affinity of DA2 is higher than DA1. The natural logarithm ofI(Li+-DA2)/I(Li+-DA1) ratio values are used to construct relative Li+ ion affinityladder. A metal ion binding ladder can be constructed with ln[I(Li+-DA2)/I(Li+-DA1)] values in which the ligand of lowest affinity is considered as reference.The experimentally measured ln[I(Li+-DA2)/I(Li+-DA1)] values are summarizedin a relative Li+ affinity ladder shown in Figure 2. In this ladder construction,most of the diamines are compared to at least three others.Figure 2: Measured ln[I(Li+-DA2)/I(Li+-DA1)] values for Li+-bound heterodimers of diamines (1–7). The data presented under the heading ln[I(Li+-DA2)/I(Li+- 1)] are average cumulative values expressed relative to ethylene diamine (1). The numbers given in parentheses are estimated errors resulting from the measurement of abundance ratios. 46
  58. 58. Chapter 2.1 Proton and alkali metal ion interactions of Diamines.. The ln[I(Li+-DA2)/I(Li+-DA1)] values calculated for the successfulcombinations are found to be reproducible. The ln[I(Li+-DA2)/I(Li+-DA1)] valuesare internally consistent for the Li+ bound heterodimers of diamines. Forexample, the value for [4·Li·1]+ is 3.51, a very similar value is obtained byadding the ln[(ILi+-DA2/I(Li+-DA1)] values of three intermediate steps, viz.ln[I(Li+-3)/I(Li+-1)] + ln[I(Li+-2)/I(Li+-3)] + ln[I(Li+-4)/I(Li+-2)] = 1.91 + 0.08 +1.58 = 3.57. The ln[I(Li+-DA2)/I(Li+-DA1)] values for other pairs are alsoconsistent internally with a difference not more than 0.2. Similarly results arealso obtained when the experiments were performed at different collisionenergy values (2, 4, 6, and 8 eV). This accord confirms that entropic effects,which tend to be non-additive, are indeed negligible with the diaminesstudied. From Li+ ion affinity ladder, the relative Li+ ion affinity order for α,ω- + + + + + + +diamines can be drawn as, 1Li < 3Li ≤ 2Li < 4Li < 6Li < 5Li ≤ 7Li . In theLi+ affinity order for α,ω-diamines, the deviation of compound 2 and 5 in theorder indirectly suggests that the structure of the lithiated diamine may beplaying a role. There are two possible structures for the resulted lithiatedspecies. One possibility is acyclic structure in which Li+ ion is bound with oneof the amine group. The other is a cyclic structure where both the amine groupsin diamine coordinate to the Li+ ion. If the lithiated diamines were acyclic, onewould expect gradually increase in Li+ ion affinity order as the chain length ofdiamine increased due to increase of positive inductive effect with increase in 47
  59. 59. Chapter 2.1 Proton and alkali metal ion interactions of Diamines..the number of methylene groups attached to the amine groups in diamine.However, the observed Li+ ion affinity order shows the possibility of cyclicstructures. The formation of cyclic structures for protonated diamines wasstudied in detail. As mentioned in the introduction, the high proton affinity for 3among series of primary α,ω-diamines (1-7) is due to its stable cyclic structureon protonation. Similarly, the higher Li+ ion affinity for compound 2 and 5 whencompared to their respective higher homologous diamine may also be due tothe stability of the resulted cyclic lithiated species. In fact, the formation ofbicoordinated lithium complexes is known in the literature.50,513.2. Na+ AND K+ ION AFFINITY LADDERS CONSTRUCTION We have extended the experiments towards Na+ and K+ ion affinity orderdetermination for diamines (1-7) by performing similar experiments as weapplied to lithium, to study the effect of metal ion size in the stabilization ofmetal bound diamines. For this purpose, we have generated all possibleheterodimers of Na+/K+ ion bound diamines, [DA1-M+-DA2], where M=Na or K.The same fifteen pairs of diamines that are used for Li+ are also successful forboth the Na+ and K+ experiments. The CID spectra of these [DA1-M+-DA2] ionsare recorded, and the relative abundances of the Na+/K+ bound monomers(i.e., M+-DA1 and M+-DA2 formed during the dissociation) correlated with therelative Na+/K+ ion affinities of the two bases. The natural logarithm ofabundance ratio, ln[I(M+-DA2)/I(M+-DA1)] values are calculated from the CIDspectra of all possible heterodimers at similar experimental conditions, where 48
  60. 60. Chapter 2.1 Proton and alkali metal ion interactions of Diamines..the M+ ion affinity of DA2 is higher than DA1, and are used to obtain the Na+ andK+ ion affinity ladders. The relative Na+ and K+ affinity ladder is shown inFigure 3 and Figure 4, respectively.Figure 3: Measured ln[I(Na+-DA2)/I(Na+-DA1)] values for Li+-bound heterodimers of diamines (1–7). The data presented under the heading ln[I(Na+-DA2)/I(Na+-1)] are average cumulative values expressed relative to ethylene diamine (1). The numbers given in parentheses are estimated errors resulting from the measurement of abundance ratios. From these ladders, the relative Na+ ion affinity order can be given as1Na+ < 2Na+ < 3Na+ < 4Na+ < 5Na+ < 6Na+ < 7Na+, and is not similar whencompared to that obtained for lithium. The sodium ion affinity towards diaminesincreases as the number of methylene groups in diamine is increased. This 49
  61. 61. Chapter 2.1 Proton and alkali metal ion interactions of Diamines..observation suggests that the resulted sodiated diamines either have linearstructure, or do not reflect the ring size effect on their stabilization if there arecyclic. However, from the present data we are unable to propose the correctstructure for the sodiated diamines.Figure 4: Measured ln[I(K+-DA2)/I(K+-DA1)] values for Li+-bound heterodimers of diamines (1–7). The data presented under the heading ln[I(K+- DA2)/I(K+-2)] are average cumulative values expressed relative to propane diamine (2). The numbers given in parentheses are estimated errors resulting from the measurement of abundance ratios. The relative K+ ion affinity orders of the diamines (1-7) were alsodetermined and can be given as 2K+ < 1K+ < 3K+ < 4K+ < 6K+ < 5K+ < 7K+. As inthe case of Li+ ion affinity ladder, the ln[I(M+-DA2)/I(M+-DA1)] values for Na+ 50
  62. 62. Chapter 2.1 Proton and alkali metal ion interactions of Diamines..and K+ ions calculated for successful combinations are found to bereproducible and values are internally consistent for the Na+/K+ boundheterodimers of diamines. The K+ ion affinity order is closely comparable tothat obtained for sodium ion, except 1 and 2 where the potassium ion affinity of1 is higher than 2. The higher affinity of compound 1 compared to that of 2 maybe explained assuming cyclic structures for the potassiated diamines.3.3. PROTON AFFINITY LADDER CONSTRUCTION The present study on the relative affinity of a series of diamines towardsLi+, Na+ and K+ shows that the affinity order is affected by the size of metal atomand diamines. The discrepancies in the Li+ ion affinity order of diamines may beexplained through cyclic structures and their stability. However, the sodiumand potassium ion affinity order of diamines cannot be explained in a similarway. Though all the alkali metals used are known to be bi-dentate in bindingwith ligands, the present experimental results does not give much informationon the structures of the ions. With a view to understand the differences between the alkali metal ionaffinity order of the studied compounds and the proton affinity order, we havealso constructed proton affinity ladder. We applied similar method that wasfollowed for alkali metal ions, for construction of proton affinity ladder byreplacing alkali metal ion with proton. The obtained proton affinity ladder isgiven in Figure 5. From the ladder, the relative proton affinity order can begiven as 1H+ < 2H+ < 7H+ < 6H+< 5H+ < 4H+ < 3H+. The proton affinity order 51
  63. 63. Chapter 2.1 Proton and alkali metal ion interactions of Diamines..obtained in the present method is in good agreement with the literaturevalues.52Figure 5: Measured ln[I(H+-DA2)/I(H+-DA1)] values for H+-bound heterodimers of diamines (1–7). The data presented under the heading ln[I(H+- DA2)/I(H+-1)] are average cumulative values expressed relative to propane diamine (2). The numbers given in parentheses are estimated errors resulting from the measurement of abundance ratios.3.4. RELATIVE ALKALINE METAL ION BINDING ENERGY CALCULATIONS It is well known that, for chemically similar compounds, the naturallogarithm of abundance (I) ratio values are directly proportional to the alkalimetal ion binding energy difference (∆E) (equation 8) between the useddiamines, where the entropy term is close to zero.40,41,53,54 52
  64. 64. Chapter 2.1 Proton and alkali metal ion interactions of Diamines.. ln[I(M+- DA2) /I(M+- DA1)] ~ ∆E /RTeff ~ (8) Attempts were made to convert relative alkali metal ion affinity orders intorelative binding energies by measuring the Teff of the dissociating clusterions.40,41,53,54 It was already shown in the literature that when experiments areperformed at identical conditions, Teff is fairly similar for dimeric ions ofchemically similar molecules, irrespective of the central ion holding the twomolecules. For the measurement of Teff, the dissociation of proton boundheterodimers of diamines was studied at different collision energies (2, 4, 6, and8 eV). For successful measurement of Teff value in this method, heterodimers ofeach diamine with atleast three other diamines should be studied. The diamine 1and 3 could not be used for this purpose because of their extreme low or highproton affinity values when compared to the other diamines. The left outdiamines 2, 4, 5 and 6 also could not be used for this study because thedifference in the proton affinity values among the three diamines (4, 5 and 6) isvery less (± 0. 5 kcal mole-1). This restricts the number of good referencesneeded for the measurement of a reliable Teff value. Consequently, we could notobtain reliable Teff values due to the non availability of enough number ofreferences among the studied diamines. Hence, the present study is limited tothe relative alkali metal ion affinity orders.3.5. COMPARISON BETWEEN PROTON AND ALKALI METAL ION AFFINITY ORDERS Inspection of the relative orders of proton affinities and alkali metal ionaffinities of primary α,ω-diamines (1-7) reveals that the proton affinity order is 53
  65. 65. Chapter 2.1 Proton and alkali metal ion interactions of Diamines..substantially different from the alkali metal ion affinity order. In the case ofproton affinities of diamines, the diamine 3 has higher proton affinity due to itsstable seven membered cyclic structure for the protonated 3. The diamine 1has the least proton affinity due to unstable five membered ring formation afterprotonation, and the proton affinity of diamine 2 is a little higher than 1 with arelatively stable six membered structure for the protonated 2. The protonaffinity of diamines 4-7 are in between 3 and 2, and gradually decrease from 4to 7, which could be due to gradual increase in the ring strains. Whereas, therelative alkali metal ion affinity is always high for diamine 7 for all the alkalimetal ions studied. Although there are few differences among the alkali metalion affinity orders i.e., between 2 and 3; 5 and 6 in Li+ order, and 1 and 2 in K+order, overall the metal ion affinity order decreased from 7 to 1. It suggests thatthe positive inductive effect is playing major role in stabilization of themetallated diamine than those of ring strains. The minor differences among therelative orders of alkali metal ions may be due to the size of alkali metal atom. We seek to explain the observed contrasting order for H+ and Li+ ionaffinities of α,ω-diamines through quantum chemical calculations.3.6. THEORETICAL STUDIES The H+ and Li+ ion affinities are estimated using the equations 9 and 10,respectively. B3LYP/6-31G* method is used for the geometry optimizations andobtaining the thermochemical data. All the structures considered arecharacterized as minima on the potential energy surface. This is followed bysingle point calculations at MP2/6-311++G** level. Counterpoise method was 54