Cyclopentadienyl Ruthenium-Nickel Catalysts for Biomimetic Hydrogen Evolution: Electrocatalytic Properties and Mechanistic DFT Studies

The new dinuclear nickel-ruthenium complexes [Ni(xbsms)RuCp(L)][PF6] (H(2)xbsms=1,2-bis(4-mercapto-3,3-dimethyl-2-thiabutyl)benzene; Cp- = cyclopentadienyl; L=DMSO, CO, PPh3, and PCy3) are reported and are bioinspired mimics of NiFe hydrogenases. These compounds were characterized by X-ray diffraction techniques and display novel structural motifs. Interestingly, [Ni(xbsms)RuCpCO][PF6] is stereochemically nonrigid in solution and an isomerization mechanism was derived with the help of density functional theory (DFT) calculations. Because of an increased electron density on the metal centers [Eur. J. Inorg. Chem. 2007, 18, 2613-2626] with respect to the previously described [Ni(xbsms)Ru(CO)(2)Cl-2] and [Ni-(xbsms)Ru(p-cymene)Cl](+) complexes, [Ni(xbsms)RuCp(dmso)][PF6] catalyzes hydrogen evolution from Et3NH+ in DMF with an overpotential reduced by 180 mV and thus represents the most efficient NiFe hydrogenase functional mimic. DFT calculations were carried out with several methods to investigate the catalytic cycle and, coupled with electrochemical measurements, allowed a mechanism to be proposed. A terminal or bridging hydride derivative was identified as the active intermediate, with the structure of the bridging form similar to that of the Ni-C active state of NiFe hydrogenases.