Pushing the Limits of the Hydrogen Evolution Reaction: Palladium Calixpyrrole Catalysts That Exhibit Kinetics near...

Controlling catalyst microenvironments using proton shuttles and hydrogen bond donors in the secondary coordination sphere is a promising approach for developing catalysts that can affect multiproton and multielectron transfer processes. In this context, three palladium calixpyrrole complexes with pendent amine (1), amide (2), and carbamate (3) groups were examined as electrocatalysts for the hydrogen evolution reaction (HER). Building on prior studies showing that the palladium complexes generated catalytically active heterogeneous HER catalysts in the presence of p-toluenesulfonic acid monohydrate, 1-3 were evaluated using the significantly milder proton source anilinium tetrafluoroborate. The active catalytic species for all three systems was found to be solution-based, and kinetic analysis uncovered a first-order dependence on acid, as well as large H/D kinetic isotope effect values, which were consistent with proton-coupled electron transfer being rate-limiting. The calixpyrrole complexes displayed exceptional activity, achieving k_obs and turnover frequency values of 4.65 × 10⁶ s^-1, 4.19 × 10⁶ s^-1, and 3.09 × 10⁶ s^-1 for 1, 2, and 3, respectively. These catalytic activities and rate constants approached the diffusion rate limit and ranked among the fastest HER catalysts to date.