H2 Evolution from a Thiolate-Bound Ni(III) Hydride

Abstract

Terminal NiIII hydrides are proposed intermediates in proton reduction catalyzed by both molecular electrocatalysts and metalloenzymes, but well-defined examples of paramagnetic nickel hydride complexes are largely limited to bridging hydrides. Herein, we report the synthesis of an S = ½, termi-nally bound thiolate-NiIII-H complex. This species, and its terminal hydride ligand in particular, have been thoroughly characterized by vibrational and EPR techniques, including pulse EPR studies. Corre-sponding DFT calculations suggest appreciable spin leakage onto the thiolate ligand. The hyperfine cou-pling to the terminal hydride ligand of the thiolate-NiIII-H species is comparable to that of the hydride ligand proposed for the Ni-C hydrogenase interme-diate (NiIII-H-FeII). Upon warming, the featured thio-late-NiIII-H species undergoes bimolecular reduc-tive elimination of H2. Associated kinetic studies are discussed and compared with a structurally related FeIII-H species that has been recently reported to also undergo bimolecular H-H coupling.