Hydrogen Bond-Assisted PCET and Formation of W─OH in Bis(Dithiolene) Complex.

The redox non-innocent nature of dithiolene ligands is well known for stabilizing high-valent metal ions and facilitating proton-coupled electron transfer (PCET) processes. Until now, proton reactivity at the dithiolene site has been primarily associated with low-valent metal centers, as high-valent metal-dithiolene complexes were not considered viable for such reactivity. This study introduces high-valent bis(dithiolene) tungsten (W)-oxo complexes featuring hydrogen-bonding interactions, unveiling a novel proton reduction mechanism mediated by the dithiolene moiety. The process begins with a nucleophilic W-oxo, forming a hydrogen bond, followed by a second hydrogen bond at the dithiolene-sulfur (S) site. These hydrogen-bonding interactions significantly modulate the molecular orbital energy levels, enabling the W reduction at -1.75 V (E) and allowing, for the first time, the acquisition of an EPR spectrum of a W─OH intermediate species. In contrast, direct electron transfer into the W═O state would populate the dithiolene π* orbital, demanding substantially larger energy (E = -3.45 V). For catalytic proton reduction, the proton transfer through the dithiolene-S site was identified as the energetically most favorable pathway for generating the W─H catalytic species.