Heterobimetallic Multi-Site Concerted Proton Electron Transfer (MS-CPET) Promotes Coordination-Induced O–H Bond Weakening

Abstract

Coordination-induced bond weakening of X–H bonds (X = O, N, C) has been observed in a number of low-valent transition metal compounds. However, the impact of an appended electron reservoir on the bond dissociation free energy of the O–H bond (BDFE_O–H) of a substrate bound to a d⁰ metal is poorly understood. To gain insight into the ability of separated deprotonation and oxidation sites to decrease the BDFE_O–H during proton-coupled electron transfer (PCET) reactions, a bimetallic system in which the sites of proton and electron loss are two distinct metal sites is described. Herein, the interconversion of tris(phosphinoamide) Zr/Co complexes HO–Zr(MesNPⁱPr₂)₃CoCN^tBu and O≡Zr(MesNPⁱPr₂)₃CoCN^tBu via hydrogen atom addition/abstraction was studied. Since the Zr center remains in the d⁰ Zr^IV state throughout these transformations, the electron transfer process is mediated by the appended redox-active Co^0/I center. A series of open-circuit potential (OCP) measurements on the HO–Zr(MesNPⁱPr₂)₃CoCN^tBu and O≡Zr(MesNPⁱPr₂)₃CoCN^tBu complexes was performed, from which the BDFE_O–H was found to be 64 ± 1 kcal/mol. The BDFE_O–H value was further verified through a series of stoichiometric H atom transfer reactions, stoichiometric protonation/deprotonation reactions, and computational studies.