Date of Award

2008

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Chemistry

First Advisor

Geiger, William

Abstract

The chemistry of radical species of organometallic complexes bearing the cyclopentadienyl ligand has been explored using electrochemical and spectroscopic techniques. For the first time, organometallic complexes containing the η5-C5H4N2 ligand have been isolated, and are stable as solids in an inert atmosphere. Specifically, cymantrene diazonium salts ([Mn(η5-C5H4N2)(CO)3][BF4 or PF6]) and cobaltocenium diazonium hexafluorophosphate ([CoCp(η5-C5H4N2)][PF6]2) have been prepared, and their crystal structures have been solved. Attempts at preparations of analogous compounds containing different metals (e.g. Fe, Re, Ru) were undertaken, but the products proved to be extremely unstable and characterization was impossible. It has been shown that electrochemical reduction of diazonium-containing organometallic compounds generates films on electrode surfaces, probably induced by a radical-radical coupling mechanism that results in covalent bond formation. Surface films prepared in this manner have been analyzed using electrochemical, spectroscopic, and physical methods. In the case of cobaltocenium-modified electrodes, multilayer films are generated under most circumstances studied. These films are persistent for weeks under ambient conditions, although some chemical decomposition does occur upon voltammetric scans in the cathodic direction. Cymantrene-modified electrodes are apparently less prone to multilayer formation, although the surface species are extremely unstable and rapidly decompose upon voltammetric scans in the anodic direction. Attempts have been made to stabilize immobilized cationic species using carbonyl substitution displacement reactions. The spectroscopic and electrochemical properties of 17 e- species of the cymantrene family MnCp*(CO)3 (Cp* = η5-C5(CH3)5), Mn(η5-C5H4R)(CO)3, and Mn(η5-C5H5)(CO)(R’)(R”)) have been studied extensively. Electronic transitions in the near-infrared region of the electromagnetic spectrum were determined to be closely related to the oxidation potentials of the compounds, with ESR spectroscopy being employed to elucidate the nature of frontier orbitals as well as to substantiate absorption energies. Two of these radical cations, [MnCp*(CO)3]+ and [Mn(η5-C5H4NH2)(CO)3]+ were isolated as tetrakis(perfluorophenyl)borate [B(C6F5)4]- (TFAB) salts, and analyses of their crystal structures were used for additional insight into their spectroscopic properties. The electrochemistry of half-sandwich compounds of the type M(η5- C5H4PMePh2)(CO)3 (M = Cr, Mo, W) was explored as well, with the Mo and W species undergoing rapid dimerization upon oxidation.

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