Carboxylate Structural Effects on the Properties and Proton-Coupled Electron Transfer Reactivity of [CuO2CR]2+ Cores.

A series of complexes {[NBu4][LCuII(O2CR)] (R = -C6F5, -C6H4(NO2), -C6H5, -C6H4(OMe), -CH3, and -C6H2(iPr)3)} were characterized (with the complex R = -C6H4(m-Cl) having been published elsewhere ( Mandal et al. J. Am. Chem. Soc. 2019 , 141 , 17236 )). All feature N,N',N″-coordination of the supporting L2- ligand, except for the complex with R = -C6H2(iPr)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.

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.

Copper Complexes of Multidentate Carboxamide Ligands.

The copper coordination chemistry of two multidentate carboxamido ligands derived from HL1 (offering two quinolyl and one carboxamide donor) and H4L2 (with two pyridine(dicarboxamido) units linked by naphthalene spacers) was explored. The former was chosen because upon deprotonation it would provide a monoanionic mer-coordinating N-donor set that would model the putative deprotonated form of the His-brace in copper monooxygenases, while the latter was designed to bind two copper ions and enable comparisons to other systems with different ligand spacers. Upon reaction with Cu(I)-mesityl, HL1 yielded a symmetric dimer (L1Cu)2 in which each bis(quinolyl)amide ligand binds via two N-donors to one Cu(I) ion and via the third to the other Cu(I) center. Monomeric Cu(II) complexes [L 1 Cu(H 2 O) 2 ](OTf) and L 1 2 Cu were also characterized. Treatment of H4L2 with Cu(OTf)2 and excess Me4NOH (in CH3CN, pyridine/H2O, or MeOH) yielded complexes with anions of general formula [L 2 Cu 2 (X)]n-, where X = CH3CONH- (n = 1), CO3 2- (n = 2), or MeO- (n = 1). X-ray structures of these complexes revealed the (L2)4- ligand binding to two Cu(II) ions in an open paddle-wheel geometry, with an additional bridging ligand (X) completing the square planar coordination sphere of each metal ion. The open paddlewheel motif differs from the more 'open' puckered geometry seen with related ligands with different spacer units.

Revisiting the Synthesis and Nucleophilic Reactivity of an Anionic Copper Superoxide Complex.

The addition of 1 equiv of KO2 and Kryptofix222 (Krypt) in CH3CN to a solution of LCu(CH3CN) [L = N, N'-bis(2,6-diisopropylphenyl)-2,6-pyridinecarboxamide] in tetrahydrofuran at -80 °C yielded [K(Krypt)][LCuO2], the enhanced stability of which enabled reexamination of its reactivity with 2-phenylpropionaldehyde (2-PPA). Mechanistic and product analysis studies revealed that [K(Krypt)][LCuO2] 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 KO2 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.

Copper-Oxygen Complexes Revisited: Structures, Spectroscopy, and Reactivity.

A longstanding research goal has been to understand the nature and role of copper-oxygen intermediates within copper-containing enzymes and abiological catalysts. Synthetic chemistry has played a pivotal role in highlighting the viability of proposed intermediates and expanding the library of known copper-oxygen cores. In addition to the number of new complexes that have been synthesized since the previous reviews on this topic in this journal (Mirica, L. M.; Ottenwaelder, X.; Stack, T. D. P. Chem. Rev. 2004, 104, 1013-1046 and Lewis, E. A.; Tolman, W. B. Chem. Rev. 2004, 104, 1047-1076), the field has seen significant expansion in the (1) range of cores synthesized and characterized, (2) amount of mechanistic work performed, particularly in the area of organic substrate oxidation, and (3) use of computational methods for both the corroboration and prediction of proposed intermediates. The scope of this review has been limited to well-characterized examples of copper-oxygen species but seeks to provide a thorough picture of the spectroscopic characteristics and reactivity trends of the copper-oxygen cores discussed.