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) (L2– = 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)–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 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.