Investigation of metal-dithiolate fold angle effects: Implications for molybdenum and tungsten enzymes

Gas-phase photoelectron spectroscopy and density functional theory have been used to investigate the interactions between the sulfur π-orbitals of arene dithiolates and high-valent transition metals as minimum molecular models of the active site features of pyranopterin Mo/W enzymes. The compounds (Tp*)MoO(bdt) (compound 1), Cp₂Mo(bdt) (compound 2), and Cp₂Ti(bdt) (compound 3) [where Tp* is hydrotris(3,5-dimethyl-1pyrazolyl)borate, bdt is 1,2-benzenedithiolate, and Cp is η⁵cyclopentadienyl] provide access to three different electronic configurations of the metal, formally d¹, d², and d⁰, respectively. The gas-phase photoelectron spectra show that ionizations from occupied metal and sulfur based valence orbitals are more clearly observed in compounds 2 and 3 than in compound 1. The observed ionization energies and characters compare very well with those calculated by density functional theory. A "dithiolate-folding-effect" involving an interaction of the metal in-plane and sulfur-π orbitals is proposed to be a factor in the electron transfer reactions that regenerate the active sites of molybdenum and tungsten enzymes.