Copper/alpha-ketocarboxylate chemistry with supporting peralkylated diamines: reactivity of copper(I) complexes and dicopper-oxygen intermediates

Theoretical rationalization for the equilibrium between (μ-η2:η2-peroxido)CuIICuII and bis(μ-oxido)CuIIICuIII complexes: perturbational effects from ligand frameworks

DFT calculations are carried out to investigate the geometric effects of the supporting ligands in the relative energies of the (μ-η2:η2-peroxido)CuIICuII complex 1 and the bis(μ-oxido)CuIIICuIII complex 2. The N3-tridentate ligand bearing acyclic propane diamine framework La preferentially provided 1, whereas the N3-tridentate ligand with cyclic diamine framework such as 1,4-diazacycloheptane Lb gave 2 after the oxygenation of the corresponding CuI complexes as reported previously [S. Itoh, et al., Inorg. Chem., 2014, 53, 8786-8794]. Calculations at the B3LYP*-D3 level of theory can reasonably explain the experimental results in relative energies, structures and harmonic frequencies of 1 and 2. Perturbational effects of the diamine chelates of La and Lb especially on the equilibrium of 1 and 2 are investigated in detail. In the range from 2.30 Å to 3.40 Å of the N-N distance in the diamine moiety, 1 is more stable than 2 by 8.4 kcal mol-1 at the distance of 3.40 Å. Calculated potential energies indicate that the decrease in the N-N distance is associated with a decrease in energy of 2, leading that 2 can be most stabilized at the N-N distance of 2.60 Å. Furthermore, molecular orbitals analyses are performed to explain that the energy gaps between the σ* orbital of the O-O bond and the dx2-y2 orbitals of the CuII ions of 1 get small as the diamine moiety is shrunk, leading to facilitate the O-O bond cleavage from 1 to 2.

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.

Perturbing the Copper(III)-Hydroxide Unit through Ligand Structural Variation

Two new ligand sets, (pipMe)LH2 and (NO2)LH2 ((pipMe)L = N,N'-bis(2,6-diisopropylphenyl)-1-methylpiperidine-2,6-dicarboxamide, (NO2)L = N,N'-bis(2,6-diisopropyl-4-nitrophenyl)pyridine-2,6-dicarboxamide), are reported which are designed to perturb the overall electronics of the copper(III)-hydroxide core and the resulting effects on the thermodynamics and kinetics of its hydrogen-atom abstraction (HAT) reactions. Bond dissociation energies (BDEs) for the O-H bonds of the corresponding Cu(II)-OH2 complexes were measured that reveal that changes in the redox potential for the Cu(III)/Cu(II) couple are only partially offset by opposite changes in the pKa, leading to modest differences in BDE among the three compounds. The effects of these changes were further probed by evaluating the rates of HAT by the corresponding Cu(III)-hydroxide complexes from substrates with C-H bonds of variable strength. These studies revealed an overarching linear trend in the relationship between the log k (where k is the second-order rate constant) and the ΔH of reaction. Additional subtleties in measured rates arise, however, that are associated with variations in hydrogen-atom abstraction barrier heights and tunneling efficiencies over the temperature range from -80 to -20 °C, as inferred from measured kinetic isotope effects and corresponding electronic-structure-based transition-state theory calculations.

Formation of hybrid guanidine-stabilized bis(μ-oxo)dicopper cores in solution: Electronic and steric perturbations

A series of new hybrid peralkylated-amine-guanidine ligands based on a 1,3-propanediamine backbone and their Cu-O2 chemistry is reported. The copper(I) complexes react readily with O2 at low temperatures in aprotic solvents with weakly coordinating anions to form exclusively bis(μ-oxo) dicopper species (O). Variation of the substituents on each side of the hybrid bidentate ligand highlights that less sterically demanding amine and guanidine substituents increase not only the thermal stability of the formed O cores but enhance inner-sphere phenolate hydroxylation pathways. TD-DFT analysis on selected guanidine-amine O species suggest that the additional visible feature observed is a guanidine π*→ Cu2O2 LMCT, which appears along with the classic oxo-ζu*→Cu(III) and πζ*→ Cu(III) LMCT transitions.

[CuO](+) and [CuOH](2+) complexes: intermediates in oxidation catalysis?

Characterization of monocopper intermediates in enzymes and other catalysts that attack strong C-H bonds is important for unraveling oxidation catalysis mechanisms and, ultimately, designing new, more efficient catalytic systems. Because initially formed 1:1 Cu/O2 adducts resulting from reactions of Cu(I) sites with O2 react relatively sluggishly with substrates with strong C-H bonds, it has been suggested that reductive O-O bond scission might occur instead to yield more reactive [CuO](+) or protonated [CuOH](2+) cores. Experimental and theoretical studies of [CuO](+) species in the gas phase have provided key insights into the possible reactivity of these species, but detailed information is lacking for discrete complexes with the [CuO](+) or [CuOH](2+) core in solution or the solid state. We describe herein our recent efforts to address this issue through several disparate approaches. In one strategy based on precedent from studies of enzymes and synthetic compounds with iron-α-ketocarboxylate motifs, reactions of O2 with Cu(I)-α-ketocarboxylate complexes were explored, with the aim of identifying reaction pathways that would implicate the intermediacy of a [CuO](+) species. A second approach focused on the reaction of N-oxides with Cu(I) complexes, with the goal being to elicit O-N bond heterolysis to yield [CuO](+) complexes. For both strategies, the course of the reactions depended on the nature of the supporting bidentate N-donor ligand, and indirect evidence in support of the sought-after [CuO](+) intermediates was obtained in some instances. In the final approach discussed herein, strongly electron donating and sterically encumbered pyridine-dicarboxamide ligands (L) enabled the synthesis of [LCu(II)OH](-) complexes, which upon one-electron oxidation formed complexes with the [CuOH](2+) core that were characterized in solution. Rapid hydrogen atom abstraction (HAT) from dihydroanthracene (DHA) was observed, yielding LCu(II)OH2. The O-H bond dissociation enthalpy (BDE) of ∼90 kcal/mol for this complex was determined through evaluation of its pKa (∼19) and the [LCu(II)OH](-)/LCu(III)OH reduction potential (approximately -0.08 V vs Fc/Fc(+)). Thus, the poor oxidizing power of the complex is offset by the high basicity of the hydroxide moiety to yield a strong O-H bond. This high BDE provided a thermodynamic rationale for the rapid HAT rate from DHA and suggested that stronger C-H bonds could be attacked. Indeed, using an inert solvent (1,2-difluorobenzene), substrates with C-H bond strengths as high as 99 kcal/mol were shown to react with the [CuOH](2+) complex, and a linear log k vs C-H BDE plot supported similar HAT pathways across the series. Importantly, these results provided key evidence in favor of the possible intermediacy of this core in oxidation catalysis, and we suggest that because it is a more energetically accessible intermediate than the [CuO](+) moiety, it should be considered as an alternative in proposed mechanisms for oxidations by enzymes and other synthetic systems.

Conversion of carbon dioxide to oxalate by α-ketocarboxylatocopper(II) complexes

The α-ketocarboxylatocopper(II) complex [{Cu(L1)}{O2CC(O)CH(CH3)2}] can be spontaneously converted into the binuclear oxalatocopper(II) complex [{Cu(L1)}2(μ-C2O4)] upon exposure to O2/CO2 gas. (13)C-labeling experiments revealed that oxalate ions partially incorporated (13)CO2 molecules. Furthermore, the bicarbonatocopper(I) complex (NEt4)[Cu(L1){O2C(OH)}] in an Ar atmosphere and the α-ketocarboxylatocopper(I) complex Na[Cu(L1){O2CC(O)CH(CH3)2}] in an O2 atmosphere were also transformed spontaneously into the oxalato complex [{Cu(L1)}2(μ-C2O4)].

Dinuclear copper complexes: coordination of Group 14 heteroborates

The dinuclear copper(i) complex [Cu₂(μ-dmapm)(CH₃CN)₂][BF₄]₂ was reacted with Group 14 heteroborates. Four different coordination motives have been characterized.

Efficient oxidative coupling of 2,6-disubstituted phenol catalyzed by a dicopper(II) complex

Complexation of a rigid multi-pyridine ligand bis(2-pyridyl)-1,8-naphthyridine (bpnp) with [Cu(2)(TFA)(4)] (TFA = trifluoroacetate) resulted in the formation of a dinuclear copper(II) complex, namely [Cu(2)(bpnp)(μ-OH)(TFA)(3)] (1). This complex has been characterized by X-ray crystallographic, spectroscopic and elemental analyses. Complex 1 is an efficient catalyst for the oxidative coupling of various 2,6-disubstituted phenols with molecular oxygen. Yields and selectivity depend on the reaction conditions employed, the best results being obtained in isopropanol or dioxane at 90 °C with yields of >99%. Mechanistic pathway of the catalysis is discussed.