Metallodithiolate ligands as building blocks for molecular constructions

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2009-06-02

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Abstract

NiN2S2 moieties have been used as a unique class of ligand in organometallic chemistry behaving as innocent mono- and bidentate ligands to metals bound via their thiolate sulfur donor atoms. We have established the donor ability of these ligands with respect to conventional ligands, e.g., diphosphines and diimines, by synthesizing a series of (NiN2S2)W(CO)4 complexes and using infrared spectroscopy to obtain the ?(CO) stretching frequencies as a report of the electron density on the metal. In comparison to the analogous tungsten complexes utilizing traditional ligands, the NiN2S2 ligands were found to be far better donors than the diphosphines as evidenced by significantly lower ?(CO) values, and much closer to diimine ligands. Sulfur?s ability to form aggregates is well documented. The metallodithiolate ligands Ni-1 and Ni-1' have been used resulting in numerous molecular constructions, specifically C3 and C4 paddlewheels of the composition M2Nix. Whereas many paddlewheels in the literature employing NiN2S2 ligands were formed largely unintended, a synthetic approach was designed utilizing multiply-bonded dimetal units of various bond orders. These explorations have extended the range of metal-metal distances accommodated by NiN2S2 units in our library from 2.14 ? to 4.35 ?. The complexes discussed in this dissertation are polymetallic clusters with high positive charges associated with them. Electrochemical studies reveal that the redox activity of the NiN2S2 unit can be deconvoluted from the dimetal unit and that sulfur metallation causes the reduction potential of the NiN2S2 ligand (approximately -2.0 V) to become more positive. With each subsequent reduction, the overall positive charge is lessened which causes the corresponding reduction potentials to be more negative. Solvent dependent studies suggest a partial dissociation of the NiN2S2 ligand reminiscent of the mechanism calculated for Acetyl CoA Synthase. Mixed-ligand species proved unstable in any solvent. As rhodium is an intrinsically catalytic metal, investigations were performed to observe if stable complexes could be prepared to serve as models to study industrially relevant processes. The metalloligands Ni-1 and Ni-1' were found to stabilize multiple oxidation states of rhodium resulting in structural forms such as a heterobimetallic (RhI), a C4 paddlewheel (RhII), and tetrametallic analog to Rubpy (RhIII).

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