Mechanisms for Hydrogen-Atom Abstraction by Mononuclear Copper(III) Cores: Hydrogen-Atom Transfer or Concerted Proton-Coupled Electron Transfer?

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 formally Cu(III) complex supported by the same dicarboxamido ligand, LCu(III)–O 2 CAr 1 ( 3, Ar 1 = 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 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.