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 = diphenylphosphinobenzenethiolate, a noninnocent ligand) complex catalytically reduces protons and oxidizes H-2 (J. Am. Chem. Soc. 2015, 137, 9238). Direct calculations of redox potentials (E-0), acidity constants (pK(a)), and free energies of activation (Delta G(double dagger)) 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 H-2 addition to form the Re-hydride/thiol, [(ReH)(S1H)L-2](2+). Under basic conditions, deprotonation of this complex produces [Re(S1H)L-2](+) with the thiol as S1, rather than the proposed S3. Further deprotonation of [Re(S1H)L-2](+) closes the catalytic cycle to regenerate [ReL3]. For the HER, DFT calculations predict that [ReL3] is reduced to [ReL3](-), followed by protonation of [ReL3](-) at the Re center to produce metal-protonated [(ReH)L-3] in accordance with the E-0 = -1.45/-1.60 V (cal./exp.). The E-0 calculations suggest a reassignment of the experimentally observed peak at -1.70 V to the singly protonated E([ReL3 center dot H](0/-)) rather than the doubly protonated E([ReL3 center dot H-2](+/0)). This reassignment and the low relative pK(a)'s of [ReL3 center dot H-2](+) illustrate that addition of the second proton must follow the second electrochemical reduction of [(ReH)L-3] to [(ReH)L-3](-), which is basic enough to be protonated at the Re center making the formation of the Re-dihydride [(HReH)L-3]. Production of H-2 occurs via reductive elimination through a Re-H-2 adduct. The nature of the key intermediates from the DFT calculations is confirmed by the complete active space calculations (CASSCF). This comprehensive investigation into the HOR and HER mechanisms can guide further experimental and theoretical efforts on rationally designing the effective electrocatalysts by comparing how various ligand modifications may shift the mechanistic steps.