Hydrogen Evolution Catalysis by a Sparsely Substituted Cobalt Chlorin

This is the authors' abstract. We don't add key points for this paper.

A sparsely substituted chlorin macrocycle containing a Co(II) center ( 1-Co ) has been synthesized and structurally characterized. The Co(II) atom resides in a square planar coordination environment and induces significant out-of-plane distortion of the chlorin macrocycle. The paramagnetic Co(II) center resides in the macrocycle in a S = 1 / 2 spin state, which displays an axial doublet signal ( g ⊥ ≈ 2.3 and g ∥ ≈ 2.03) in the X-band EPR spectrum. The open-shell d-orbital configuration is manifest in the transient absorption spectrum, which reveals an excited-state lifetime of 8.6 ± 0.2 ps for 1-Co . The Co(II) chlorin exhibits a rich oxidation–reduction chemistry with five reversible one-electron waves (three oxidative processes and two reductive processes) observed in the cyclic voltammogram. The reduction processes of 1-Co drive hydrogen evolution catalysis. Electrochemical kinetics analysis of HER by 1-Co in trifluoroacetic acid reveals a hydrogen evolution mechanism that proceeds by an ECEC mechanism. Benchmarking the catalytic activity, 1-Co exhibits higher HER activity at low overpotentials, versus its porphyrin congeners.