Decoding Proton-Coupled Electron Transfer with Potential–p Ka Diagrams: Applications to Catalysis

Rountree, Eric S.; McCarthy, Brian D.; Dempsey, Jillian L.

doi:10.1021/acs.inorgchem.8b03368

Decoding Proton-Coupled Electron Transfer with Potential–p K_a Diagrams: Applications to Catalysis

Journal Article · Mon Apr 29 00:00:00 EDT 2019 · Inorganic Chemistry

DOI:https://doi.org/10.1021/acs.inorgchem.8b03368· OSTI ID:1604868

Rountree, Eric S. ^[1]; ^[2]; ^[2]

Univ. of North Carolina, Chapel Hill, NC (United States); University of North Carolina
Univ. of North Carolina, Chapel Hill, NC (United States)

The applied potential at which [Ni^II(P₂^PhN₂^Bn)₂]²⁺ (P₂^PhN₂^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 pK_a in acetonitrile. Yet, 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(P₂^PhN₂^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 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 [Ni⁰(P₂^PhN₂^BnH)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(P₂^PhN₂^Bn)(P₂^PhN₂^BnH)]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 pK_a-dependent activity might be engendered in other molecular catalysts for multi-electron, multi-proton transformations.

View Accepted Manuscript (DOE)

Research Organization:: Univ. of North Carolina, Chapel Hill, NC (United States)

Sponsoring Organization:: USDOE Office of Science (SC), Basic Energy Sciences (BES) (SC-22)

Grant/Contract Number:: SC0015303

OSTI ID:: 1604868

Journal Information:: Inorganic Chemistry, Journal Name: Inorganic Chemistry Journal Issue: 10 Vol. 58; ISSN 0020-1669

Publisher:: American Chemical Society (ACS)Copyright Statement

Country of Publication:: United States

Language:: English

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