Chemical Reactivity of Electronically Excited Transition Metal Complexes

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Metal cluster compounds are an important class of catalysts and are also of substantial current interest as models of species which exist at the metal surfaces when they are used as catalysts. Such cluster compounds absorb light strongly and the high-energy (excited state) species which are formed thereby are very reactive in bringing about chemical transformations of organic molecules. This research project is expected to provide information which is essential to effective use of metal clusters as catalysts for light-induced chemical reactions. The proposed research is focused on providing general guidelines for multielectron photochemistry beginning with two classes of transition metal compounds: multiply bonded metal- metal (M-n-M) dimers and hexanuclear clusters of Mo(II) and W(II). The excited states of these compounds are highly luminescent and long-lived. The factors governing the excited state dynamics of the hexanuclear clusters and the M-n-M dimers will be defined using electronic absorption, emission and time-resolved spectroscopies. Preliminary data indicate that the oxidation-reduction chemistry of these species is derived from the metal cores. The studies will be extended by using cyclic voltammetry, chronoamperometry and electrogenerated chemiluminescence (ecl) to identify reaction pathways. Of special importance are experiments in which ecl will be used to probe several mechanistic features of the hexanuclear cluster oxidation-reduction processes. The spectroscopic and electrochemical studies are complementary and will be used to define a framework in which to construct multielectron transformations of excited state metal clusters. Excited state reaction pathways that rely on redox reactions of small molecule reactants and organic substrates at electronically excited, coordinatively unsaturated metal cluster cores will be developed.