Mimicking of Tunichlorin: Deciphering the Importance of a β‑Hydroxyl Substituent on Boosting the Hydrogen Evolution Reaction

Mimicking
of tunichlorin is of importance to correlate its biological
function to the unusally similar structure to chlorophylls but with
a nickel cofactor. Benefiting from the facile derivatization of porpholactones,
we herein constructed a tunichlorin mimic 6 carrying
a β-hydroxyl group ([meso-tetrakis­(pentafluorophenyl)-3-hydroxy-2-oxaisobacteriochlorinato]­nickel­(II)),
which exhibits significant hydrogen evolution reaction (HER) rate
acceleration of ca. 56-fold compared to its porphyrin
analogues. Importantly, in the presence of water, the TOF of 6 is up to 6.1 × 104 s–1 with the lowest overpotential, ranking the best among the catalysts
described. Coating catalyst 6 on a Ni foam electrode
showed good HER performance in a two-electrode alkaline (1 M KOH)
electrolyzer (η20 = 540 mV). The functional roles
of the β-hydroxyl group on the acceleration of electron transfer
and the formation of the hydrogen bond network with water has been
suggested in either chemical or electrochemical reductions and H/D
kinetic isotope effects (KIEs), combined with DFT calculations. Interstingly,
the DFT model suggested that the formation of the hydrogen bond renders
more electron density on the Ni center (ρNi = 0.91)
in a one-electron reduced intermediate [6H2O]−, which helps the stabilization of both one- or two-electron reduced
intermediates and dramatically enhances the HER rates.