Spectroscopic evidence for a unique bonding interaction in oxo-molybdenum dithiolate complexes:: Implications for σ electron transfer pathways in the pyranopterin dithiolate centers of enzymes

Solution and solid state electronic absorption, magnetic circular dichroism, and resonance Raman spectroscopies have been used to probe in detail the excited state electronic structure of LMoO(bdt) and LMoO(tdt) (L = hydrotris(3,5-dimethyl-1-pyrazolyl)borate; bdt = 1,2-benzenedithiolate; tdt = 3,4-toluenedithiolate). The observed energies, intensities, and MCD band patterns are found to be characteristic of LMoO(S-S) compounds, where (S-S) is a dithiolate ligand which forms a five-membered chelate ring with Mo. Ab initio calculations on the 1,2-ene dithiolate ligand fragment, -SC=CS-, show that the low-energy S --> Mo charge transfer transitions result from one-electron promotions originating from an isolated set of four filled dithiolate orbitals that are primarily sulfur in character. Resonance Raman excitation profiles have allowed for the definitive assignment of the ene-dithiolate Sin-plane -->Mo d(xy) charge transfer transition. This is a bonding-to-antibonding transition, and its intensity directly probes sulfur covalency contributions to the redox orbital (Mo d(xy)). Raman spectroscopy has identified three totally symmetric vibrational modes at 362 cm(-1) (S-Mo-S bend), 393 cm(-1) (S-Mo-S stretch), and 932 cm(-1) (Mo=O stretch), in contrast to the large number low-frequency modes observed in the resonance Raman spectrum of Rhodobacter sphaeroides DMSO reductase. These results on LMoO(S-S) complexes are interpreted in the context of the mechanism of sulfite oxidase, the modulation of reduction potentials by a coordinated ene-dithiolate (dithiolene), and the orbital pathway for electron transfer regeneration of pyranopterin dithiolate Mo enzyme active sites.