Testing the Limits of Imbalanced CPET Reactivity: Mechanistic Crossover in H‑Atom Abstraction by Co(III)–Oxo Complexes

Transition metal–oxo complexes are key intermediates in a variety of oxidative transformations, notably C–H bond activation. The relative rate of C–H bond activation mediated by transition metal–oxo complexes is typically predicated on substrate bond dissociation free energy in cases with a concerted proton–electron transfer (CPET). However, recent work has demonstrated that alternative stepwise thermodynamic contributions such as acidity/basicity or redox potentials of the substrate/metal–oxo may dominate in some cases. In this context, we have found basicity-governed concerted activation of C–H bonds with the terminal CoIII–oxo complex PhB­( tBuIm)3CoIIIO. We have been interested in testing the limits of such basicity-dependent reactivity and have synthesized an analogous, more basic complex, PhB­(AdIm)3CoIIIO, and studied its reactivity with H-atom donors. This complex displays a higher degree of imbalanced CPET reactivity than PhB­( tBuIm)3CoIIIO with C–H substrates, and O–H activation of phenol substrates displays mechanistic crossover to stepwise proton transfer–electron transfer (PTET) reactivity. Analysis of the thermodynamics of proton transfer (PT) and electron transfer (ET) reveals a distinct thermodynamic crossing point between concerted and stepwise reactivity. Furthermore, the relative rates of stepwise and concerted reactivity suggest that maximally imbalanced systems provide the fastest CPET rates up to the point of mechanistic crossover, which results in slower product formation.