Hydrogen Evolution Catalysis by a Sparsely Substituted Cobalt Chlorin

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.