Electronic structure studies of molybdenum and tungsten dithiolate complexes: Electronic structure contributions to reactivity in pyranopterin molybdenum and tungsten enzymes

Solution and solid state electronic absorption (EA), magnetic circular dichroism (MCD) and resonance Raman (RR) spectroscopies have been used to probe in detail the excited state electronic structure of LWO(bdt) and LWO(tdt) (L = hydrotris(3,5-dimethyl-1-pyrazolyl)borate; bdt = 1,2-benzenedithiolate; tdt = 3,4-toluenedithiolate). The energies, intensities, and MCD band patterns appear to be characteristic of the low-symmetry paramagnetic d1 LWVO(S-S) dithiolate compounds, where (S-S) is a 1,2-dithiolene or 1,2-dithiolate ligand forming a five-membered chelate ring with the W(V) ion. This work is important for determining the contribution of the metal (Mo vs. W) on the electronic structure of these systems. These results have been related to the inherent reactivity differences between pyranopterin Mo and W enzymes, as well as small molecule oxygen atom transfer catalysts. These studies have shown that inherent metal dependent differences in the rate of nucleophilic attack on M=O by strong oxo acceptors are not solely a function of the reduction potential, but are modified by the degree of metal induced destabilization of the M=O pi* acceptor orbital. This effect is as dominant as the reduction potential differences between Mo and W in affecting the rate.;In summary, the application of inorganic bonding theories have been directed toward understanding the effects of metal-sulfur covalency on reactivity. We have developed an extensive protocol for understanding the electronic structure of relevant oxo-Mo dithiolates by utilizing electronic absorption, magnetic circular dichroism, and resonance Raman spectroscopies to probe ligand-metal interactions via ligand-to-metal CT transitions. The results of these studies have been used to correlate electronic structure contributions to reactivity. An important contribution in this area was the realization that a unique Mo-dithiolate bonding description was required to explain the intensity of the low energy S → Mo CT bands which dominate the spectral region below ∼28,000 cm -1 in LMoVO(bdt) and related LMoVO(S-S) dithiolate model compounds. (Abstract shortened by UMI.).