Computational EPR, ENDOR and DFT studies of catalytic transition metal systems

Landon, James Hugh Pearson 2009. Computational EPR, ENDOR and DFT studies of catalytic transition metal systems. PhD Thesis, Cardiff University.

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Abstract

The following thesis discusses the combined use of electron paramagnetic resonance (EPR) spectroscopy, electron nuclear double resonance (ENDOR) spectroscopy and density functional theory (DFT) calculations to investigate a number of transition metal catalyst systems the binding of epoxide molecules by a vanadyl analogue of the catalytically important metallosalen class of compounds the binding of a chiral aryl amine by a copper salen complex and the incorporation of copper(II) ions in aluminophosphate materials. Two classes of epoxide selectivity by a vanadyl salen derivative are presented here, the discrimination of the geometric isomers of 2,3-epoxybutane, cw-2,3-epoxybutane and /ra 5-2,3-epoxybutane by Ar,Ar'-bis(3,5-di-tert-butylsalicylidene)-l,2-diaminocyclohexa- ne-vanadium(IV) oxide ( VO(l) ) 1 and the stereoselectivity of epoxypropane, 1,2- epoxybutane, chloromethyloxirane and fluoromethyloxirane by VO(l) . In both cases it is shown that hydrogen-bonding interactions, including interactions between the epoxide oxygen atom and a hydrogen atom bonded to a stereocentre carbon atom of the complex are important in determining the binding mode, thus implicating the given stereocentre carbon atom in the transfer of chirality. In the geometric isomerism of 2,3-epoxybutane, steric arguments regarding the obstruction caused by the methyl groups made on the basis of the DFT structures explain the selectivity observed in the EPR/ENDOR spectra. In the chiral selectivity of the other epoxides, more complicated reasoning, based on tripodal weak hydrogen-bonding configurations involving the hydrogen atoms of the epoxide ring and the oxygen atoms of the complex ligand is required to fully explain the selectivity observed, with different selectivity effects in the more electronegative halogenated epoxides compared to the alkyl cases. The coordination of methyl benzyl amine to a series of analogues of Cu(l) with various levels of tert-butylation, to model the steric effects in this interaction is studied here using DFT methods to explain the coordination preference for heterochiral pairings observed in the EPR spectra. Reasoning based on the preference of each enantiomer of the MBA to become involved in % - n interaction with alternate benzene rings of the complex, along with a slightly increased crowding of one ring over the other caused by the same hydrogen atom as implicated in determining selectivity in the epoxide study (above), namely the hydrogen atom bonded to one of the stereocentre C atoms, explains the selectivity observed in terms of n n interactions, also identifying the role of the stereocentre C atoms in conferring chirality. In combination, these studies demonstrate the importance of weak interactions, namely hydrogen-bonding andn-n interactions, in determining the binding configurations, and by extension the selectivity of these transition metal complexes. They also describe the nature of the involvement of the stereocentres of the complex in directing that selectivity, delineating a link between the chirality of the complex and that of the bound species in each case. The importance of using both EPR/ENDOR and DFT techniques in such studies, namely of explaining selectivity observed by EPR in terms of ENDOR and DFT derived geometry parameters is further explored in this thesis in the development of genetic algorithm routines to modify DFT-derived structures, by means of the ENDOR spectra simulated with the hyperfine parameters derived from a simple point-dipole model applied to the coordinates. The application of this process to a sample axial system, VO(acac)2, demonstrates the effectiveness of exploiting the complementary nature of the ENDOR and DFT techniques in this manner. Finally, a second copper study is reported here. This example is of a microporous aluminophosphate material, and concerns the incorporation of Cu11 ions into framework vs. extra-framework sites, a subject of some controversy. Here, evidence is presented for the ability of copper to distort the tetrahedral lattice into a distorted octahedral and a square- based pyramidal environment in which one or both of the remaining coordination sites is/are occupied by the templating molecules and water molecules, without rendering the lattice unstable, arguing in favour of framework site incorporation.

Item Type:	Thesis (PhD)
Status:	Unpublished
Schools:	Chemistry
Subjects:	Q Science > QD Chemistry
ISBN:	9781303213984
Funders:	EPSRC
Date of First Compliant Deposit:	30 March 2016
Last Modified:	12 Feb 2016 23:13
URI:	https://orca.cardiff.ac.uk/id/eprint/54786

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