Concentration-Dependent Transition in the Rate-Determining Step of Hydrogen Evolution Driven by Pyridinium-Tethered...

Previous studies on the hydrogen evolution reaction (HER) catalyzed by Pt(II)-based molecular catalysts have often demonstrated a second-order dependence of the HER rate on the catalyst concentration, suggesting the importance of stabilizing dinuclear platinum intermediates. However, some Pt(II)-based HER catalysts instead exhibit a first-order dependence, and the key factors governing this selection have received limited attention. Here, we discovered a new class of Pt(II)-based HER photocatalysts, abbreviated as [PtCl2(pBp)]2+, [Pt2Cl4(OpBp)]4+, and [Pt2Cl4(UpBp)]4+. Specifically, pBp2+ is a pyridinium-functionalized bipyridine, while OpBp4+ and UpBp4+ are dinucleating ligands wherein two pBp2+ fragments are doubly or singly bridged by phenylene moieties, respectively. Remarkably, all these photocatalysts exhibit two successive pBp2+-based 1-electron reductions with a potential separation of 290-320 mV. These complexes serve as efficient photocatalysts for the aqueous-based HER at pH 5.0 in the presence of EDTA. Importantly, the HER rate exhibits a second-order dependence at lower concentrations, indicating that a 2-electron-reduced species is generated via a rate-determining disproportionation reaction (2Cat- → Cat + Cat2-) and serves as a key intermediate for HER (Cat2- + 2H+ → Cat + H2). Conversely, a first-order dependence is observed at higher concentrations, where the rate-determining step shifts to the photoinduced electron transfer process. Notably, the Cat2- intermediate exists as a minor component (e.g., 0.2%) due to the large comproportionation constants. Nevertheless, this low-abundance species dictates the overall reaction rate because its HER pathway possesses a ca. 0.3 eV higher thermodynamic driving force than that doubly driven by the Cat-/Cat couple, rendering the process kinetically highly favorable.