Spectroscopic evidence for a unique bonding interaction in oxo-molybdenum dithiolate complexes: Implications for sigma 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 = hydrotri s(3,5-dimethyl-1-pyrazolyl)borate; bdt = 1,2-benzenedithiolate; tdt = 3,4-t oluenedithiolate). The observed energies, intensities, and MCD band pattern s are found to be characteristic of LMoO(S-S) compounds, where (S-S) is a d ithiolate ligand which forms a five-membered chelate ring with Mo. Ab initi o calculations on the 1,2-ene dithiolate ligand fragment, -SC=CS-, show tha t the low-energy S --> Mo charge transfer transitions result from one-elect ron promotions originating from an isolated set of four filled dithiolate o rbitals that are primarily sulfur in character. Resonance Raman excitation profiles have allowed for the definitive assignment of the ene-dithiolate S in-plane -->Mo d(xy) charge transfer transition. This is a bonding-to-antib onding transition, and its intensity directly probes sulfur covalency contr ibutions 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) compl exes are interpreted in the context of the mechanism of sulfite oxidase, th e modulation of reduction potentials by a coordinated ene-dithiolate (dithi olene), and the orbital pathway for electron transfer regeneration of pyran opterin dithiolate Mo enzyme active sites.