Electrochemical oxidation of H2 catalyzed by ruthenium hydride complexes bearing P2N2 ligands with pendant amines as proton relays

Abstract

Two Ru hydride complexes, Cp*Ru(P^Ph₂N^Bn₂)H (1-H) and Cp*Ru(P^tBu₂N^Bn₂)H (2-H) supported by cyclic P^R₂N^R′₂ ligands (Cp* = η⁵-C₅Me₅; P^R₂N^Bn₂ = 1,5-dibenzyl,-3,7-R-1,5-diaza-3,7-diphosphacyclooctane, where R = Ph or ^tBu) have been developed as electrocatalysts for oxidation of H₂ (1.0 atm, 22 °C). The turnover frequency of 2-H is 1.2 s⁻¹ at 22 °C (1.0 atm H₂) with an overpotential at E_cat/2 of 0.5 V in the presence of exogenous base, DBU (1,8-diazabicyclo[5.4.0]undec-7-ene), while catalysis by 1-H has a turnover frequency of 0.6 s⁻¹ and an overpotential of 0.6 V at E_cat/2. Addition of H₂O facilitates oxidation of H₂ by 2-H and increases its turnover frequency to 1.9 s⁻¹, while H₂O slows down the catalysis by 1-H. In addition, studies of Cp*Ru(dmpm)H (where dmpm = bis(dimethylphosphino)methane), a control complex lacking pendent amines in its diphosphine ligand, confirms the critical roles of the pendant amines of the P₂N₂ ligands as proton relays in the oxidation of H₂.