Electrocatalytic Hydrogen Evolution and Oxidation with Rhenium Tris(thiolate) Complexes: A Competition between Rhenium and Sulfur for Electrons and Protons

Recent electrochemical experiments reveal that the rhenium-tris(thiolate) [ReL3] (L = DPPBT = diphenylphosphinoben-zenethiolate, a noninnocent ligand) complex catalytically reduces protons and oxidizes H2 (J. Am. Chem. Soc. 2015, 137, 9238). Direct calculations of redox potentials (E0), acidity constants (pKa), and free energies of activation (G‡) by density functional theory (DFT) with the help of high-level ab initio calculations predict that hydrogen oxidation reaction (HOR) thermodynamically and kinetically favors a Re-hydride/monothiol intermediate, while the hydrogen evolving reaction (HER) favors Re-dihydride intermediates, in contrast to the Re-dithiols as proposed previously. The catalytic pathway for HOR involves two oxidation steps from [ReL3] to [ReL3]2+, followed by H2 addition to form the Re-hydride/thiol, [(ReH)(S1H)L2]2+. Under basic conditions, deprotonation of this complex produces [Re(S1H)L2]+ with the thiol as S1, rather than the proposed S3. Further deprotonation of [Re(S1H)L2]+ clos...