Infrared spectroelectrochemical studies of coordination compounds.

The discovery of metal-containing constituents in many biological systems has stimulated research into the redox activity of transition metal complexes with non-innocent ligands. Interest has centred upon synthesising model compounds, which have similar spectroscopic or structural characteristics to the metal site of metalloproteins. In this way, a better understanding is gained of the behaviour of metalloenzymes such as the molybdenum-containing co-factors of nitrogenase, the oxotransferases, and the iron-containing ferredoxin electron-transfer agents. The work carried out involved the electrochemical study of inorganic species, supplemented by infrared and ultra-violet/visible spectroelectrochemical experiments. By these means, it was possible to obtain a more precise evaluation of the changes occurring on oxidation or reduction, this being related to the frontier molecular orbitals of the complex. The extremely reactive and short-lived nature of many electrogenerated products required the development of controlled, strictly anaerobic sample-handling procedures for spectroelectrochemical experiments. This was achieved by modifying the design of the spectroelectrochemical cell. The limitations of standard spectroelectrochemical techniques for the study of reactive short-lived species in solution prompted the development of the "Modulation Technique". This involved computer control and synchronisation of spectral data acquisition with the change of potential applied to the electrode. By the reversible generation, for a maximum of 2-3s, of the reactive species reliable spectroscopic results could be obtained. Modification of the working electrode enabled these experiments to be performed on a reduced timescale and at reduced temperatures to those previously possible. Using this technique, spectra were recorded of a large range of previously unobserved and unstable species, such as the tetra-anionic tris maleonitriledithiolate complexes of vanadium, chromium, molybdenum and rhenium. The complicated series of chemical and electrochemical transformations initiated by oxidation of the oxomolybdenum compound MoOmnt22- was also investigated and the intermediates identified. Significant insights were gained into the behaviour upon reduction of the carbonyl- and nitrosyl-containing compound cpMo(CO)2(NO). Spectral data were obtained for Fe4S4(NO)4, a model compound for the iron-sulphur electron transfer proteins, in four different oxidation states, the evidence suggesting that the molecule retains its stable, cubane-like structure throughout all the redox steps