Research on Soluble Metal Sulfides: From Polysulfido Complexes to Functional Models for the Hydrogenases

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Abstract
Results from this laboratory are surveyed, emphasizing the synthesis of metal sulfides. Four themes are described. Continuing studies exploit the exothermic desulfurization of polysulfido complexes as a means to generate new clusters and rings. Illustrative inorganic rings prepared in this way include 1,5-[L2M]2(S3)2 and 1,4-[L2M]2(S2)2, where L2M = CpRu(PPh3) and Cp2Ti. Fundamentally new clusters prepared in this project included the cubanes [(C5R5)MS]4 for M = Ti, V, Ru, Ir. Associated redox studies led to the discovery of the phenomenon of mobile metal−metal bonds, as manifested in [Cp*4Ir4S4]2+ wherein the localized Ir−Ir bond migrates over the six Ir- - -Ir edges of the cluster. Other desulfurization experiments led to the preparation of the reactive species IrII2S2(PPh3)4 from [IrS16]3- and the synthesis of the first high polymers of ferrocene, [(RC5H3S)2Fe]n (n ∼ 500). A second theme uncovered the useful role of donor solvents on the reaction of metals with sulfur. It was found that pyridine accelerates the low temperature conversion of Cu to crystalline CuS via the intermediacy of the cluster Cu4(S5)2L4. Related synthetic methodology led to a family of amine-stabilized zinc polysulfides, e.g. ZnS6(tmeda), an efficient sulfur-transfer agent. A third theme explored the organic and organometallic chemistry of the tetrathiometalates. The sulfido analogue of OsO4, ReS4- was shown to be broadly reactive toward unsaturated organic substrates such as alkenes, alkynes, nitriles, and isocyanides. The final and still emerging theme focuses on the preparation of functional and structural models for bio-organometallic reaction centers. Studies on models for the Fe-only hydrogenases began with the synthesis of the highly reducing species [Fe2(SR)2(CN)2(CO)4]2- where (SR)2 also includes the proposed azadithiolate cofactor HN(CH2S-)2. Systematic studies on the cyanide substitution process led to the preparation of [HFe2(SR)2(CN)(CO)4(PMe3)], which efficiently catalyzes the reduction of protons to H2. Work on the hydrogenases was expanded to include modeling of acetyl Co-A synthase, leading to the preparation of mixed valence Ni2 models containing bound CO substrate.

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