Photoelectron spectroscopy and electronic structure calculations of d ¹ vanadocene compounds with chelated dithiolate ligands: Implications for pyranopterin Mo/W enzymes

Gas-phase photoelectron spectroscopy and density functional theory have been used to investigate the electronic structures of open-shell bent vanadocene compounds with chelating dithiolate ligands, which are minimum molecular models of the active sites of pyranopterin Mo/W enzymes. The compounds Cp ₂V(dithiolate) [where dithiolate is 1,2-ethenedithiolate (S ₂C₂H₂) or 1,2-benzenedithiolate (bdt), and Cp is cyclopentadienyl] provide access to a 17-electron, d¹ electron configuration at the metal center. Comparison with previously studied Cp ₂M(dithiolate) complexes, where M is Ti and Mo (respectively d ⁰ and d² electron configurations), allows evaluation of d⁰, d¹, and d² electronic configurations of the metal center that are analogues for the metal oxidation states present throughout the catalytic cycle of these enzymes. A "dithiolate-folding effect" that involves an interaction between the vanadium d orbitals and sulfur p orbitals is shown to stabilize the d¹ metal center, allowing the d¹ electron configuration and geometry to act as a low-energy electron pathway intermediate between the d⁰ and d² electron configurations of the enzyme.