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

Abstract

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, -SCCS-, 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 dxy charge transfer transition. This is a bonding-to-antibonding transition, and its intensity directly probe...