A cw-ENDOR investigation of metal- ligand interactions in solution Submitted in canditure for the degree of Doctor of Philosophy By Richard James Tucker, M.Chem Department of Chemistry University of Wales, Cardiff September 2005 UMI Number: U585544 All rights reserved INFORMATION TO ALL USERS The quality of this reproduction is dependent upon the quality of the copy submitted. In the unlikely event that the author did not send a complete manuscript and there are missing pages, these will be noted. Also, if material had to be removed, a note will indicate the deletion. Dissertation Publishing UMI U585544 Published by ProQuest LLC 2013. Copyright in the Dissertation held by the Author. Microform Edition © ProQuest LLC. All rights reserved. This work is protected against unauthorized copying under Title 17, United States Code. ProQuest LLC 789 East Eisenhower Parkway P.O. Box 1346 Ann Arbor, Ml 48106-1346 Declaration This work has not previously been accepted in substance for any degree and is not being concurrently submitted in candidature for any degree. Signed ^ ...... (candidate) Date n - 1 - oC This thesis is the result of my own investigations, except where otherwise stated. Other sources are acknowledged by footnotes giving explicit references. A bibliography is appended. Signed (candidate) Date 0 ( 0 I hereby give consent for my thesis, if accepted, to be available for photocopying and for inter-library loan, and for the title and summary to be made available to outside organisations. Signed .... (7/.. .. .^V^T?^r777T (candidate) D ate 9 .^ . Summary Electron Nuclear DOuble Resonance (ENDOR) spectroscopy has been employed in this Thesis to study a range of paramagnetic metal-ligand complex systems. The investigations focussed on the observation of conformational changes, solvatochromic effects and weak diastereomeric interactions of the complexes in frozen solution. Using a combination of angular selective ENDOR spectroscopy and DFT calculations, the structure and conformations of the [Vlv=0(acac)2] complex in coordinating and non-coordinating solvents was examined. In the non-coordinating solvent (CD 2 CI2 ) the complex was found to adopt the expected square pyramidal structure, where the VO H|lg distances and coordinates obtained by DFT and ENDOR were in excellent agreement with each other confirming the expected structure. More importantly, in coordinating solvents (such as pyridine) two different stereoisomers of the resulting [Vlv=0(acac)2(C5D5N)] adduct, which differed in energy by only 3 kJmol'1, were readily identified and discriminated by ENDOR (PCCP 2002). Subtle changes to metal-ligand structures by solvatochromic effects were also examined using [Vlv=0(salen)] in both coordinating (DMF) and non-coordinating (CD2CI2) solvents. In the non-coordinating solvent (CD2CI2), the expected square pyramidal symmetry o f the V = 0 ion above the NNOO plane of the ligand was confirmed both by ENDOR and DFT. However, in the coordinating solvent (DMF), a subtle perturbation from square pyramidal structure was observed suggesting than DMF coordinated trans to the vanadyl oxo- ligand, pulling Vlv= 0 back into the ligand plane. This was evidenced through analysis of the VO....H|jg distances determined by ENDOR and confirmed by DFT (Chem. Phys. Lett. 2003). ENDOR was also used to explore the weak enantioselective binding between vanadyl based chiral salen complexes (abbreviated to [VO(Jacobsen)]) and chiral propylene epoxide. Differences in epoxide binding by enatiomers of the complex was evidenced by changes to the 'H epoxide derived peaks in the ENDOR spectra. These changes were assigned to the small structural differences between the diastereomeric metal-epoxide adducts. Simulation of the spectra revealed differences in the VO.-.'Hepoxide distances for the diastereomeric pairs, which was confirmed by DFT. While the epoxide molecule was very weakly coordinated, ENDOR measurements of the racemic complex in racemic epoxide nevertheless indicated the preferential coordination of the /?-[VO(Jacobsen)] to /?-epoxide. This demonstrates the unique power of the ENDOR technique to resolve weak chiral interactions for which EPR spectroscopy alone lacks sufficient resolution (./ACS 2004). Finally, the diastereomeric interactions between chiral amines and copper based chiral salen complexes (abbreviated to [Cu(Jacobsen)]) was investigated, as a comparison with the previous weak interactions in the VO(Jacobsen)-epoxide case. Diastereomeric discrimination was once again evidenced by ENDOR. The slight differences in the Cu-amine distances, constrained by the chirality of the anchoring site, resulted in subtle difference in the spin densities to the ligand nitrogens, which was detected in the ENDOR experiment. Acknowledgments There are many people to whom I am deeply indebted for their advice and continuous support in the preparation of this thesis and execution of this research. This acknowledgement seeks to provide my deepest thanks to these people. Firstly, among these people are Dr. Damien Murphy and Dr. Ian Fallis, who were my supervisors during the project. They provided the highest Evel of support, encouragement, and guidance and scientific advice of the highest quality. I would also like to thank Dr. Robert Farley who was a post doctoral research assistant in the department. Dr. Farley provided excellent technical assistance on both the EPR and ENDOR analysis. I would like to acknowledge Dr. David Willock for his kind assistance in providing the DFT data. Additionally, I would like to acknowledge my fellow postgraduates in both the EPR laboratory and the surface science department, as well as the postgraduates in the coordination chemistry laboratories. Finally, I would like to thank my parents for their support and financial influence whilst undertaking this study. Thank you all 'Nothing exists until it is measured' Niels Bohr, 1930 “I am further inclined to think that when are views are sufficiently to enable us to reason with precision concerning the proportions of elementary atoms we shall find the arithmetical relation alone will not be sufficient to explain their mutual action and that we shall be obliged to acquire a geometrical conception of their relative arrangement in all three dimensions o f solid extension " William Hyde Wo/lison, 1808 Contents Section Title___________________________________________________ Page 1.0 Introduction 1 1.1 Asymmetric homogeneous catalysts 4 1.2 Oxo-transfer oxidants 5 1.3 Stoichiometric oxidants 7 1.4 Characterization of metal-oxo complexes 7 1.5 Ligand design 7 1.6 Kinetic Resolution in catalysis for asymmetric synthesis 10 1.6.1 Chirality 10 1.6.2 Hydrolytic Kinetic Resolution (HKR) 11 1.6.3 Stereoselectivity in salen-type epoxidations 13 1.7 Spectroscopic investigation of metal Schiff-base complexes 15 1.7.1 Paramagnetic cobalt complexes 15 1.7.2 Manganese salen-type complexes 21 1.7.3 Nickel salen-type complexes 25 1.7.4 Oxovanadium (IV) complexes 34 1.7.4.1 Spectral properties 34 1.7.4.2 Electronic structure of oxovanadium (IV) complexes 35 1.7.4.3 Solvent effects 36 1.7.4.4 Structure and mobility aspects 39 1.7.5 Copper salen-type complexes 42 1.8 References 46 2.0 Basic principles of cw EPR and ENDOR spectroscopy 54 2.1 Introduction 55 2.2 Basic principles of EPR spectroscopy 55 2.2.1 Spin populations 57 2.2.2 Relaxation processes 58 2.2.3 Hyperfine structure 59 Section Title Page 2.2.4 Mechanism of hyperfme interaction 60 (a) Dipole-dipole interactions 61 (b) Isotropic or Fermi contact interactions 63 2.2.5 Hyperconjugation 65 2.2.6 The powder spectra of polycrystalline materials: Lineshape considerations 66 2.3 Basic principles of ENDOR spectroscopy 69 2.3.1 Level populations of a simple description of the ENDOR effect 69 2.3.2 Relaxation mechanisms in ENDOR spectroscopy 72 2.3.3 The electric circuit analogy of the ENDOR effect 75 2.2.4 Angle selective ENDOR spectroscopy and simulation of spectra 78 2.3.5 Powder ENDOR simulation procedure 81 2.4 References 83 Experimental 85 3.0 3.1 Synthesis of Inorganic complexes 86 3.2 Manipulation of air-sensitive compounds 100 3.2.1 Inert atmosphere techniques 100 3.3 Experimental conditions 101 3.3.1 EPR and ENDOR experimental 101 3.3.2 Initial ENDOR setup 103 3.4 Gas Chromatography 105 3.4.1 Industrial components 105 3.4.1.1 Carrier gas 105 3.4.1.2 Sample injection port 105 3.4.1.3 Detectors 106 3.5 References 107 Section Title Page 4.0 Conformational changes of a bis(acetylacetonato)oxovanadium(IV) complex-)Vlv=0(acac)2| in coordinating and non-coordinating solvents 108 4.1 Introduction 109 4.2 Experimental 111 4.2.1 Measurement and analysis of EPR / ENDOR spectra 111 4.2.2 DFT calculations 111 4.3 Results and Discussion 1 12 4.3.1 EPR analysis of VO(acac )2 in coordinating and non­ coordinating solvents 112 4.3.2 ENDOR and DFT analysis of VO(acac )2 in non-coordinating solvent (CD 2CI2) 115 4.3.3 ENDOR and DFT analysis of VO(acac )2 in coordinating solvents (C 5 D5N and 2 (CD3)-CsD4N) 121 4.4 Conclusions 130 4.5 References 132 Solvatochromic effects of a A^,Ar,-ethylene-bis(salicylideneamine) 5.0 oxovanadium(lV) complex-[Vlv=0(salen)] 134 5.1 Introduction 135 5.2 Experimental 136 5.2.1 Measurement and analysis of EPR / ENDOR spectra 136 5.3 Results and Discussion 137 5.3.1 EPR analysis of VO(salen) in coordinating and non­ coordinating solvents 137 5.3.2 ENDOR analysis 139 5.3.3 Structure of [Vlv=0(salen)] in non-coordinating solvent (CD2CI2) 139 Section Title Page 5.3.4 Structure of [Vlv=0(salen)] in coordinating solvent (C3D7NO) 144