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

Abstract

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 Co III −oxo complex PhB( tBu Im) 3 Co III O. We have been interested in testing the limits of such basicity-dependent reactivity and have synthesized an analogous, more basic complex, PhB( Ad Im) 3 Co III O, and studied its reactivity with H-atom donors. This complex displays a higher degree of imbalanced CPET reactivity than PhB( tBu Im) 3 Co III O 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.

The role of basicity in selective C–H bond activation by transition metal-oxidos

“…Plotting these factors that determine bond strength versus log( k ) have proved valuable in determination of PCET mechanisms exhibiting imbalanced transition states. 22,23,27,30…”

“…However, a growing area of interest in inorganic chemistry are mechanisms of C-H bond activation that fall between these limits off the diagonal often referred to as "asynchronous", "nonsynchronous", or "imbalanced" PCET. [18][19][20][21][22][23][24][25][26][27][28][29][30] These types of reactions are still considered concerted, as compared to stepwise, because a thermodynamically stable intermediate is not formed during the transfer process but differ from balanced CPET as the degree of charge transfer or delocalization of either the proton or electron lags behind the other (Fig. 2).…”

“…Plotting these factors that determine bond strength versus log(k) have proved valuable in determination of PCET mechanisms exhibiting imbalanced transition states. 22,23,27,30 While discussions of semiclassical CPET provide important insight into the mechanisms of C-H bond activation, essential aspects of this reaction cannot be sufficiently explained without a more complete quantum mechanical treatment. Effects like nonadiabatic couplings, multiconfigurational interactions, and tunneling can alter the mechanisms and therefore rates of both the proton and electron transfer.…”

“…The role of nonadiabatic couplings in PCET and C-H bond activation specifically have been extensively theoretically Dalton Transactions Frontier treated. 13,14,[25][26][27]32 However, unambiguous delineation of these mechanisms experimentally is challenging as they are controlled by factors that are hard to isolate and systematically control such as the symmetry and energy differences between the ground and excited states of the reactants and products.…”

“…Tuning of the pK a , on the other hand, is a relatively unexplored yet appealing approach for achieving selective C-H bond activation. 23,27,[44][45][46][47][48] Not only does a more basic metal-oxido decrease the oxidation potential required for a given C-H bond, but also can be used to select for the most acidic C-H bond rather than one with the weakest BDFE. Modulation of the redox potential and pK a are not entirely independent, however there are several strategies that can be employed to achieve greater alteration of one over the other.…”