Electrocatalytic H2 Evolution by Supramolecular RuII–RhIII–RuII Complexes: Importance of Ligands as Electron Reservoirs and Speciation upon Reduction

The supramolecular water reduction photocatalysts [{(Ph2phen)2Ru(dpp)}2RhX2](PF6)5 (Ph2phen = 4,7-diphenyl-1,10-phenanthroline, dpp =2,3-bis(2-pyridyl)pyrazine X = Cl, Br) are efficient electrocatalysts for the reduction of CF3SO3H, CF3CO2H, and CH3CO2H to H2 in DMF or DMF/H2O mixtures. The onset of catalytic current occurs at −0.82 V versus Ag/AgCl for CF3SO3H, −0.90 V for CF3CO2H, and −1.1 V for CH3CO2H with overpotentials of 0.61, 0.45, and 0.10 V, respectively. In each case, catalysis is triggered by the first dpp ligand reduction implicating the dpp as an electron reservoir in catalysis. A new species with Epc ∼ −0.75 V was observed in the presence of stoichiometric amounts of strong acid, and its identity is proposed as the Rh(H)III/II redox couple. H2 was produced in 72–85% Faradaic yields and 95–116 turnovers after 2 h and 435 turnovers after 10 h of bulk electrolysis. The identities of Rh(I) species upon reduction have been studied. In contrast to the expected dissociation of halides in the Rh(I)...