Promoting hydrogen evolution reaction with a sulfonic proton relay

Enzymes can activate otherwise unreactive substrates by using residues precisely located in the active sites. We herein report on a Ga porphyrin which bears a second-sphere sulfonic group (named as 1) and its high efficiency for the electrocatalytic hydrogen evolution reaction (HER). Complex 1 can achieve a large TOF value of 1.3 × 105 s-1 at low 295-mV overpotential with weak acetic acid (HOAc) as the proton source. Notably, 1 can catalyze H2 generation at potentials close to the thermodynamic HER equilibrium, but the sulfonic-free analogues require >250 mV more cathodic potentials to initiate HER. Mechanistic studies showed that two-electron reduced Ga porphyrins were protonated to form GaIII-H. For 1, its GaIII-H underwent rapid protonolysis with HOAc to evolve H2, but for sulfonic-free analogues, their GaIII-H required further reduction at more cathodic potentials to trigger catalysis. This work presents a synthetic molecular catalyst to achieve high HER rates under low overpotentials by promoting the protonolysis of metal hydrides with otherwise unreactive weak acids.