Decoding Proton-Coupled Electron Transfer with Potential–p K a Diagrams: Applications to Catalysis
The applied potential at which [Ni II (P 2 Ph N 2 Bn ) 2 ] 2+ (P 2 Ph N 2 Bn = 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 p K a in acetonitrile. Yet, most molecular catalysts exhibit catalytic onsets at p K a -independent potentials. Using experimentally determined thermochemical parameters associated with reduction and protonation, a coupled Pourbaix diagram is constructed for [Ni II (P 2 Ph N 2 Bn ) 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 p K 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 p K a -dependent catalytic processes involve protonation of the ligand, not the metal center. For proton sources with p K a values in the range 6–10.6, the initial species formed is the doubly reduced, doubly protonated species [Ni 0 (P 2 Ph N 2 Bn H) 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 [HNi II (P 2 Ph N 2 Bn )(P 2 Ph N 2 Bn H)] 2+ that can release hydrogen to close the catalytic cycle. Crucially, 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 p K a -dependent activity might be engendered in other molecular catalysts for multi-electron, multi-proton transformations.