Singly versus Doubly Reduced Nickel Porphyrins for Proton Reduction: Experimental and Theoretical Evidence for a Homolytic Hydrogen-Evolution Reaction

A nickel(II) porphyrin Ni-P (P=porphyrin) bearing four meso-C6F5 groups to improve solubility and activity was used to explore different hydrogen-evolution-reaction (HER) mechanisms. Doubly reduced Ni-P ([Ni-P](2-)) was involved in H-2 production from acetic acid, whereas a singly reduced species ([Ni-P](-)) initiated HER with stronger trifluoroacetic acid (TFA). High activity and stability of Ni-P were observed in catalysis, with a remarkable i(c)/i(p) value of 77 with TFA at a scan rate of 100 mVs(-1) and 20 degrees C. Electrochemical, stopped-flow, and theoretical studies indicated that a hydride species [H-Ni-P] is formed by oxidative protonation of [Ni-P](-). Subsequent rapid bimetallic homolysis to give H-2 and Ni-P is probably involved in the catalytic cycle. HER cycling through this one-electron-reduction and homolysis mechanism has been proposed previously but rarely validated. The present results could thus have broad implications for the design of new exquisite cycles for H-2 generation.