Mechanisms for Hydrogen-Atom Abstraction by Mononuclear Copper(III) Cores: HAT or cPCET?

Mandal, M.; Elwell, C. E.; Bouchey, C. J.; Zerk, T. J.; Tolman, W. B.; Cramer, C. J.
J. Am. Chem. Soc. 2019, 141, 17236 (doi:10.1021/jacs.9b08109).

In a possibly biomimetic fashion, formally copper(III)-oxygen complexes LCu(III)–OH (1) and LCu(III)–OOCm (2) (L^2– = N,N'-bis(2,6-diisopropylphenyl)-2,6-pyridinedicarboxamide, Cm = α,α-dimethylbenzyl) have been shown to activate X–H bonds (X = C, O). Herein, we demonstrate similar X–H bond activation by a newly prepared Cu(III) complex supported by the same dicarboxamido ligand, LCu(III)–O₂CAr¹ (3, Ar¹ = meta-chlorophenyl), and we compare its reactivity to that of 1 and 2. Kinetic measurements revealed a second order reaction with distinct differences in the rates: 1 reacts the fastest in the presence of O–H or C–H based substrates, followed by 3, which is followed by (unreactive) 2. The difference in reactivity is attributed to both a varying oxidizing ability of the studied Cu(III) complexes and to a variation in X–H bond functionalization mechanisms, which in these cases are characterized as either a hydrogen-atom transfer (HAT) or a concerted proton-coupled electron transfer (cPCET). Select theoretical tools have been employed to distinguish these two cases, both of which generally focus on whether the electron (e^–) and proton (H⁺) travel "together" as a true H-atom, (HAT), or whether the H⁺ and e^– are transferred in concert, but travel between different donor/acceptor centers (cPCET). In this work, we reveal that both mechanisms are active for X–H bond activation by 1-3, with interesting variations as a function of substrate and copper functionality.