Photoelectron Spectroscopy of the Doubly-Charged Anions [MIVO(mnt)_2]~(2-) (M = Mo, W; mnt = S_2C_2(CN)_2~(2-)): Access to the Ground and Excited States of the [M~VO(mnt)_2]~- Anion

摘要

Photodetachment photoelectron spectroscopy was used to investigate the electronic structure of the doubly charged complexes [M~(IV)O(mnt)_2]~(2-) (M = Mo, W; mnt = 1,2-dicyanoethenedithiolato). These dianions are stable in the gas phase and are minimal models for the active sites of the dimethyl sulfoxide reductase family of molybdenum enzymes and of related tungsten enzymes. Adiabatic and vertical electron binding energies for both species were measured, providing detailed information about molecular orbital energy levels of the parent dianions as well as the ground and excited states of the product anions [M~VO(mnt)_2]~-. Density functional theory calculations were used to assist assignment of the detachment features. Differences in energy between these features provided the energies of ligand-to-metal charge-transfer transitions from S(π) and S(σ) molecular orbitals to the singly occupied metal-based orbital of the products [M~VO(mnt)_2]~-. These unique data for the MV species were obtained at the C_(2v) geometry of the parent MIV dianions. However, theoretical calculations and available condensed phase data suggested that a geometry featuring differentially folded dithiolene ligands (C_s point symmetry) was slightly lower in energy. The driving force for ligand folding is a favorable covalent interaction between the singly occupied metal-based molecular orbital (a_1 in C_(2v) point symmetry; highest occupied molecular orbital (HOMO)) and the least stable of the occupied sulfur-based molecular orbitals (b_1 in C_(2v) point symmetry, HOMO-1) that is only possible upon reduction to the lower symmetry. This ligand folding induces a large increase in the intensity predicted for the a'S(π)→a'd_(x~2-y~2) charge-transfer transition originating from the HOMO-2 of [M~VO(mnt)_2]~- under Cs point symmetry. Electronic absorption spectra are available for the related species [Mo~VO(bdt)_2]~- (bdt = 1,2-benzenedithiolato) and for the oxidized form of dimethyl sulfoxide reductase. The intense absorptions at ～1.7 eV have been assigned previously to S(σ) → Mo transitions, assuming C_(2v) geometry. The present work indicates that the alternative a'S(π) → a'd_(x~2-y~2) of C_s geometry must be considered. Overall, this study confirms that the electronic structure of the M-dithiolene units are exquisitely sensitive to dithiolene ligand folding, reinforcing the proposal that these units are tunable conduits for electron transfer in enzyme systems.