Reversible hydrogen atom exchange between phenoxyl radical and phenol at cobalt(ii): role of coordination-induced bond activation.

Understanding how transition metals influence net hydrogen-atom exchange is pivotal. This study reports isolation and comprehensive characterization of phenol complexes (N2SArOH)MIIBr2 (1-M; where, M = Co/Ni), supported by an unsymmetrical tripodal ligand N2SArOH. Application of Bordwell relationship affords the O-H bond dissociation free energy (BDFEOH) values as 71.25 for 1-Co and 80.57 kcal mol-1 for 1-Ni, thereby underscoring the critical role of the metal site in governing the thermochemistry of the coordinated phenol. H-atom exchange reaction between phenol in 1-Co and isolated 2,4,6-tri-tert-butylphenoxyl ( t Bu3ArO˙) results in a rare cobalt(ii)-phenoxyl radical complex 4-Co with S = 1 ground state. Reactivity of the phenoxyl functionality in 4-Co with H˙(/H++e-) donor reagents smoothly regenerates the phenol complex 1-Co, thereby illustrating reversible H-atom exchange. Detailed studies on these H-atom exchange reactions provide the kinetic isotope effect (KIE) and activation parameters (ΔH ‡, ΔS ‡, E a), thereby enabling an experimental distinction between dichotomous H-atom transfer (HAT) and proton-coupled electron-transfer (PCET) pathways. Together, this complete set of isolated phenol, phenolate, and phenoxyl radical complexes delineates the relay of protons and H-atoms within the coordination sphere of the metal site, thereby showcasing the strong potential of the phenol-tethered coordination motifs in PCET/HAT based catalysis.