Synthetic, structural, spectroscopic and computational studies of metal-dithiolates as models for pyranopterindithiolate molybdenum and tungsten enzymes: Dithiolate folding effect

摘要

Coordination by an axial oxo and an equatorial ene-dithiolate group is a salient feature of the active sites of the mononuclear pyranopterin Mo/W enzymes. Discrete mononuclear model complexes encompassing these features are important in understanding the metal-ligand interactions in these active sites. The compounds (Tp*)ME(S-S) (M = Mo, W; E = O, NO) and Cp₂M(S-S) (M = Ti, Mo, W) (where Tp* is hydrotris(3,5-dimethyl-1-pyrazolyl)borate, Cp is η⁵-cyclopentadienyl, S-S represents a generic ene-1,2-dithiolate ligand for example 1,2-benzenedithiolate and 3,6-dichloro-1,2-benzenedithiolate) provide access to three different electronic configurations of the metal, formally d¹, d² and d⁰, respectively. These compounds also allow the study of two metal, two axial ligand and two equatorial ene-dithiolate perturbations. X-ray crystallography, density functional theory and photoelectron spectroscopy are utilized to understand the metal-sulfur interaction in the above complexes. Subtle differences in the geometry of these compounds are observed, including the metal-dithiolate fold angle which is sensitive to the electronic occupation of the metal in-plane orbital. This orbital is presumably the "host" orbital to the electrons during catalysis. The work in this area has resulted in the development of a dithiolate-folding-effect. This effect relates to the experimental verification of the Lauher and Hoffmann bonding model for the metal-dithiolate interaction in these complexes. This "dithiolate-folding-effect" is proposed to account for the electronic buffering at the metal center. This effect may provide a regulatory mechanism for the metal-sulfur interactions and could be a factor in the electron transfer reactions that regenerate the active sites of molybdenum and tungsten enzymes. The structure and properties of these compounds are correlated with those of the enzyme active sites.