Tuning Proton Reduction Efficiencies of Copper Corrole in Electrocatalysis <i>via</i> Multiple β-Chloro Substitution
Kolanu Sudhakar, Pradeepta K. Panda
Abstract
Electrocatalytic hydrogen evolution reaction (HER) is worthy research to produce fuel for human need. Although copper corroles were first reported two decades ago, still there is a great scope to tune their redox potentials and other fundamental properties. β-Chloro-substituted copper corroles are highly stable under electrolysis in contrast to porphyrin analogues having bromo/iodo substituents. A series of copper corroles β-substituted with chlorines denoted as Cu–Cl2, Cu–Cl3, Cu–Cl4, Cu–Cl5, and Cu–Cl8 along with mixed multi-halogenated derivatives, namely, Cu–Cl4–Br4, Cu–Cl4–I2, and Cu–Cl4–I3 were synthesized and structurally characterized by various spectroscopic methods, including single-crystal X-ray diffraction analysis. Presence of eight chloro substituents upon copper corrole led to positively shift its second reduction potential by 850 mV, occurring at nearer to the thermodynamic reduction potential of trifluoroacetic acid proton (−0.9 V) to H2 in acetonitrile. Among all the compounds investigated, Cu–Cl8 displayed higher catalytic performance toward HER: catalytic activity icat/ip value was 215 and low overpotential at 390 mV with addition of 160 mm trifluoroacetic acid in acetonitrile and obeyed the diffusion-controlled process under homogeneous conditions. A higher turnover frequency value of about 2.45 × 104 s–1 for Cu–Cl8 was observed in about 500 mM acid concentration. A HER mechanistic study was performed by both experimental and theoretical methods to identify the intermediates formed during the catalytic process, and obtained results led us to conclude that for β-chloro copper corroles, the H2 evolution operates via the heterolytic cleavage pathway.