Enhancing the reactivity of nickel(II) in hydrogen evolution reactions (HERs) by β-hydrogenation of porphyrinoid ligands

Fine-tuning of the porphyrin β-periphery is important for naturally occurring metal tetrapyrroles to exert diverse biological roles. Here we describe how this approach is also applied to design molecular catalysts, as exemplified by Ni(ii) porphyrinoids catalyzing the hydrogen evolution reaction (HER). We found that β-hydrogenation of porphyrin remarkably enhances the electrocatalytic HER reactivity (turnover frequencies of 6287 vs. 265 s^-1 for Ni(ii) chlorin (Ni-2) and porphyrin (Ni-1), and of 1737 vs. 342 s^-1 for Ni(ii) hydroporpholactone (Ni-4) and porpholactone (Ni-3), respectively) using trifluoroacetic acid (TFA) as the proton source. DFT calculations suggested that after two-electron reduction, β-hydrogenation renders more electron density located on the Ni center and thus prefers to generate a highly reactive nickel hydride intermediate. To demonstrate this, decamethylcobaltocene Co(Cp∗)₂ was used as a chemical reductant. [Ni-2]^2- reacts ca. 30 times faster than [Ni-1]^2- with TFA, which is in line with the electrocatalysis and computational results. Thus, such subtle structural changes inducing the distinctive reactivity of Ni(ii) not only test the fundamental understanding of natural Ni tetrapyrroles but also provide a valuable clue to design metal porphyrinoid catalysts.