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.