Experimental and Theoretical Insight into Electrocatalytic\nHydrogen Evolution with Nickel Bis(aryldithiolene) Complexes as Catalysts

A series\nof neutral and monoanionic nickel dithiolene complexes with p-methoxyphenyl-substituted 1,2-dithiolene ligands have\nbeen prepared and characterized with physicochemical methods. Two\nof the complexes, the monoanion of the symmetric [Ni­{S2C2(Ph-p-OCH3)2}2] (3–) with NBu4+ as a counterion and the neutral asymmetric [Ni­{S2C2(Ph)­(Ph-p-OCH3)}2] (2), have been structurally characterized by\nsingle-crystal X-ray crystallography. All complexes have been employed\nas proton-reducing catalysts in N,N-dimethylformamide with trifluoroacetic acid as the proton source.\nThe complexes are active catalysts with good faradaic yields, reaching\n83% for 2 but relatively high overpotential requirements\n(0.91 and 1.55 V measured at the middle of the catalytic wave for\ntwo processes observed depending on the different routes of the mechanism).\nThe similarity of the experimental data regardless of whether the\nneutral or anionic form of the complexes is used indicates that the\nneutral form acts as a precatalyst. On the basis of detailed density\nfunctional theory calculations, the proposed mechanism reveals two\ndifferent main routes after protonation of the dianion of the catalyst\nin accordance with the experimental data, indicating the role of the\nconcentration of the acid and the influence of the methoxy groups.\nProtonation at sulfur seems be more favorable than that at the metal,\nwhich is in marked contrast with the catalytic mechanism proposed\nfor analogous cobalt dithiolene complexes.