Sulfur K-edge XAS of WVO vs. MoVO bis(dithiolene) complexes: Contributions of relativistic effects to electronic structure and reactivity of tungsten enzymes

Abstract

Molybdenum-or tungsten-containing enzymes catalyze oxygen atom transfer reactions involved in carbon, sulfur, or nitrogen metabolism. It has been observed that reduction potentials and oxygen atom transfer rates are different for W relative to Mo enzymes and the isostructural Mo/W complexes. Sulfur K-edge X-ray absorption spectroscopy (XAS) and density functional theory (DFT) calculations on [Mo V O(bdt) 2 ] − and [W V O(bdt) 2 ] − , where bdt = benzene-1,2-dithiolate(2−), have been used to determine that the energies of the half-filled redox-active orbital, and thus the reduction potentials and M=O bond strengths, are different for these complexes due to relativistic effects in the W sites.

Relativistic two-component calculations of electronic g-tensor for oxo-molybdenum(V) and oxo-tungsten(V) complexes: The important role of higher-order spin-orbit contributions

“…This is again significantly more (by a factor 6-7) than for the Mo complex. More pronounced relativistic effects on structure and reactivity in [WO(bdt) 2 ] À compared to [MoO(bdt) 2 ] À have been also recently reported by Solomon et al [60] using sulfur K-edge X-ray absorption spectroscopy. The smaller high-order SO effects compared to the [MOCl 5 ] 2À dianions (cf.…”

“…As for the previously studied [MoOCl 4 ] À system [26,33,60,61], the dominant contributions to Dg || arise from coupling of the SOMO (singly occupied molecular orbital) to a d x 2 À d y 2 -based MO, whereas Dg? is dominated by couplings from the SOMO to d xz -and d yz -based MOs.…”

Substrate and Metal Control of Barrier Heights for Oxo Transfer to Mo and W Bis-dithiolene Sites

“…This energetic difference, along with the structural parameters, suggest that TS2 occurs earlier along the reaction coordinate compared with the Mo-Me 2 SO reaction, indicating that the W-Me 2 SO reaction is more facile in agreement with experimental kinetic studies ( k W =3.9 × 10 −5 M −1 s −1 vs. k Mo =1.3 × 10 −5 M −1 s −1 ). We have previously proposed that such differences in reactivity are due to larger relativistic effects in tungsten, 38 which would faciliate electron transfer from an occupied metal-based orbital to the S-O σ * orbital. 11 Indeed, calculations using all-electron basis sets without and with relativistic effects show that inclusion of relativistic effects destabilizes the redox-active tungsten orbital by ~0.2 eV in both the free [W(OMe)(mdt) 2 ] 1− and the DMSO-bound intermediate (Supporting Information), and thus, the energy of TS2 is be expected to be about ~4.5 kcal/mol lower in energy.…”

Periplasmic nitrate reductase revisited: a sulfur atom completes the sixth coordination of the catalytic molybdenum

“…The absence of this interaction in the tungsten complexes can be explained by their lower redox potential and plausibly by a stronger W=S bond, compared with their molybdenum analogues. Both differences, known experimentally for a long time, have recently been explained for this type of complex on the basis of the relativistic effects that need to be considered when dealing with third-row transition metals [40].…”