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

• Published in: Angewandte Chemie International Edition 2016-04, Vol.55 (18), p.5457-5462
• Main Authors: Han, Yongzhen, Fang, Huayi, Jing, Huize, Sun, Huiling, Lei, Haitao, Lai, Wenzhen, Cao, Rui
• Format: Article
• Language: English
• Subjects: Acetic acid; bimetallic reactions; Bimetals; Catalysis; Communication; Communications; electrocatalysis; Electrochemistry; homolysis; hydrogen evolution; Hydrogen evolution reactions; Hydrogen production; Nickel; nickel porphyrins; Porphyrins; Protonation; Reduction; Trifluoroacetic acid
• ISSN: 1433-7851; 1521-3773
• DOI: 10.1002/anie.201510001

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 H2 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 ic/ip value of 77 with TFA at a scan rate of 100 mV s−1 and 20 °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 H2 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 H2 generation. Learning to understand HER: A nickel porphyrin bearing four meso‐C6F5 groups catalyzed H2 evolution from acetic acid and trifluoroacetic acid by different mechanisms initiated by doubly and singly reduced species, respectively. Experimental and theoretical evidence suggest that bimetallic homolysis of a hydride intermediate formed by oxidative protonation of singly reduced species may be involved in the catalytic cycle.