The two-electron, one-proton reaction of a family of [CoCp(dxpe)(NCCH3)]2+ complexes (Cp = cyclopentadienyl, dxpe = 1,2-bis(di(aryl/alkyl)phosphino)ethane) to form the corresponding hydride species [HCoCp(dxpe)]+ does not proceed through a simple proton-coupled electron transfer (PCET) process. Mechanistic studies reveal hydride formation proceeds via reduction of the Co(III) species, followed by dissociation of the bound acetonitrile ligand and subsequent reduction of the unligated Co(II) species to form a Co(I) species. In the presence of acid, the Co(I) species then undergoes protonation at the Cp ring, followed by tautomerization to generate the stable Co(III)-hydride product [HCoCp(dxpe)]+. This complex, multi-step PCET mechanism manifests as a complex Brønsted relationship, wherein the apparent proton transfer rate constants, quantified via cyclic voltammetry, correlate with the strength of the proton source for weak acids, but these apparent proton transfer rate constants plateau as the reaction becomes increasingly exergonic. The steric bulk of the dxpe ligand impacts the apparent proton transfer rate constant, with bulkier ligands having slower rate constants. The complexity of the reaction mechanism underpinning hydride formation, encompassing dynamic behavior of the entire ligand set, highlights the critical need to understand elementary reaction steps in proton-coupled electron transfer reactions in order to advance catalyst design.

Not a simple PCET process: Redox-induced structural reorganization and ligand protonation dictate cobalt(III)-hydride formation kinetics

ACS Spring Meeting 2021

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