Decoding Proton-Coupled Electron Transfer with Potential-pKa Diagrams: Applications to Catalysis

The applied potential at which [Ni-II((P2N2Bn)-N-Ph)(2)](2+) ((P2N2Bn)-N-Ph = 1,5-dibenzyl-3,7-diphenyl-1,5-diaza-3,7-diphosphacyclooctane) catalyzes hydrogen production is reported to vary as a function of proton source pK(a) in acetonitrile. By contrast, most molecular catalysts exhibit catalytic onsets at pK(a)-independent potentials. Using experimentally determined thermochemical parameters associated with reduction and protonation, a coupled Pourbaix diagram is constructed for [Ni-II((P2N2Bn)-N-Ph)(2)](2+). One layer describes proton-coupled electron transfer reactivity involving ligand-based protonation, and the second describes metal-based protonation. An overlay of this diagram with experimentally determined E-cat/2 values spanning 15 pK(a) units, along with complementary stopped-flow rapid mixing experiments to detect reaction intermediates, supports a mechanism in which the proton-coupled electron transfer processes underpinning the pK(a)-dependent catalytic processes involve protonation of the ligand, not the metal center. For proton sources with pK(a) values in the range 6-10.6, the initial species formed is the doubly reduced, doubly protonated species [Ni-0((P2N2H)-N-Ph-H-Bn)(2)](2+), despite a higher overpotential for this proton-coupled electron transfer reaction in comparison to forming the metal-protonated isomer. In this complex, each ligand is protonated in the exo position with the two amine moieties on each ligand binding a single proton and positioning it away from the metal center. This species undergoes very slow isomerization to form an endo-protonated hydride species [HNiII((P2N2Bn)-N-Ph)((P2N2H)-N-Ph-H-Bn)](2+) that can release hydrogen to close the catalytic cycle. Importantly, this slow isomerization does not perturb the initially established proton-coupled electron transfer equilibrium, placing catalysis under thermodynamic control. New details revealed about the reaction mechanism from the coupled Pourbaix diagram and the complementary stopped-flow studies lead to predictions as to how this pK(a)-dependent activity might be engendered in other molecular catalysts for multi-electron, multi-proton transformations.