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.

A series of complexes {[NBu4]­[LCuII(O2CR)] (R = −C6F5, −C6H4(NO2), −C6H5, −C6H4(OMe), −CH3, and −C6H2( i Pr)3)} were characterized (with the complex R = −C6H4(m-Cl) having been published elsewhere (J. Am. Chem. Soc.201914117236)). All feature N,N′,N″-coordination of the supporting L2– ligand, except for the complex with R = −C6H2( i Pr)3, which exhibits N,N′,O-coordination. For the N,N′,N″-bound complexes, redox properties, UV–vis ligand-to-metal charge transfer (LMCT) features, and rates of hydrogen atom abstraction from 2,4,6,-tri-t-butylphenol using the oxidized, formally Cu­(III) compounds LCuIII(O2CR) correlated well with the electron donating nature of R as measured both experimentally and computationally. Specifically, the greater the electron donation, the lower is the energy for LMCT and the slower is the reaction rate. The results are interpreted to support an oxidatively asynchronous proton-coupled electron transfer mechanism that is sensitive to the oxidative power of the [CuIII(O2CR)]2+ core.

The addition of 1 equiv of KO 2 and Kryptofix222 (Krypt) in CH 3 CN to a solution of LCu(CH 3 CN) [L = N,N′-bis(2,6-diisopropylphenyl)-2,6-pyridinecarboxamide] in tetrahydrofuran at −80 °C yielded [K(Krypt)][LCuO 2 ], the enhanced stability of which enabled reexamination of its reactivity with 2-phenylpropionaldehyde (2-PPA). Mechanistic and product analysis studies revealed that [K(Krypt)][LCuO 2 ] reacts with wet 2-PPA to form [LCuOH] − , which then, deprotonates 2-PPA to yield the copper(II) enolate complex [LCu(OC=C(Me)Ph)] − . Acetophenone was observed upon workup of this complex or mixtures of KO 2 and 2-PPA alone, in support of an alternative mechanism(s) to the one proposed previously involving an initial nucleophilic attack at the carbonyl group of 2-PPA.Elucidating the structure and function of putative monocopper−oxygen intermediates is imperative for understanding many oxygenase enzymes 1 and abiological oxidation catalysts. 2 Common to all mechanistic schemes for the activation of O 2 in such systems is the initial formation of a 1:1 Cu/O 2 species, typically formulated as a copper(II) superoxide comprising a [CuO 2 ] + core. 3 Significant appreciation of the structures and spectroscopic properties of [CuO 2 ] + cores has been achieved through studies of synthetic complexes. 4 Some features of the reactivity of such complexes also have been examined, but many questions concerning detailed mechanisms, supporting ligand effects, 5 and other aspects remain unanswered because of their thermal instability, challenges associated with isolating the complexes in pure form, and complicated side reactions.