Effects of Charged Ligand Substituents on the Properties of the Formally Copper(III)-Hydroxide ([CuOH]) Unit.

With the goal of understanding how distal charge influences the properties and hydrogen atom transfer (HAT) reactivity of the [CuOH] core proposed to be important in oxidation catalysis, the complexes [M][LCuOH] (M = [K(18-crown-6)] or [K(crypt-222)]) and [LCuOH]X (X = BAr or ClO) were prepared, in which SO or NMe substituents occupy the para positions of the flanking aryl rings of the supporting bis(carboxamide)pyridine ligands. Structural and spectroscopic characterization showed that the [CuOH] cores in the corresponding complexes were similar, but cyclic voltammetry revealed the E value for the [CuOH]/[CuOH] couple to be nearly 0.3 V more oxidizing for the [LCuOH] than the [LCuOH] species, with the latter influenced by interactions between the distal -SO substituents and K or Na counterions.

Reactivity of the copper(iii)-hydroxide unit with phenols.

Kinetic studies of the reactions of two previously characterized copper(iii)-hydroxide complexes (LCuOH and LCuOH, where L = ,'-bis(2,6-diisopropylphenyl)-2,6-pyridine-dicarboxamide and L = ,'-bis(2,6-diisopropyl-4-nitrophenyl)pyridine-2,6-dicarboxamide) with a series of substituted phenols (ArOH where X = NMe, OMe, Me, H, Cl, NO, or CF) were performed using low temperature stopped-flow UV-vis spectroscopy. Second-order rate constants () were determined from pseudo first-order and stoichiometric experiments, and follow the trends CF < NO < Cl < H < Me < OMe < NMe and LCuOH < LCuOH. The data support a concerted proton-electron transfer (CPET) mechanism for all but the most acidic phenols (X = NO and CF), for which a more complicated mechanism is proposed.

Determination of the Cu(III)-OH Bond Distance by Resonance Raman Spectroscopy Using a Normalized Version of Badger's Rule.

The stretching frequency, ν(Cu-O), of the [CuOH] core in the complexes LCuOH (L = N,N'-bis(2,6-diisopropyl-4-R-phenyl)pyridine-2,6-dicarboxamide, R = H or NO, or N,N'-bis(2,6-diisopropylphenyl)-1-methylpiperidine-2,6-dicarboxamide) was determined to be ∼630 cm by resonance Raman spectroscopy and verified by isotopic labeling. In efforts to use Badger's rule to estimate the bond distance corresponding to ν(Cu-O), a modified version of the rule was developed through use of stretching frequencies normalized by dividing by the appropriate reduced masses. The modified version was found to yield excellent fits of normalized frequencies to bond distances for >250 data points from theory and experiment for a variety of M-X and X-X bond distances in the range ∼1.

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.

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.

Transition metal complexes and the activation of dioxygen.

In order to address how diverse metalloprotein active sites, in particular those containing iron and copper, guide O₂binding and activation processes to perform diverse functions, studies of synthetic models of the active sites have been performed. These studies have led to deep, fundamental chemical insights into how O₂coordinates to mono- and multinuclear Fe and Cu centers and is reduced to superoxo, peroxo, hydroperoxo, and, after O-O bond scission, oxo species relevant to proposed intermediates in catalysis. Recent advances in understanding the various factors that influence the course of O₂activation by Fe and Cu complexes are surveyed, with an emphasis on evaluating the structure, bonding, and reactivity of intermediates involved.

Simple routes to bulky silyl-substituted acetylide ligands and examples of V(III), Fe(II), and Mn(II) complexes.

Synthesis of substituted phenylacetylide ligands 2,6-bis(trimethylsilyl)phenylacetylene (H1) and 2-(triphenylsilyl)phenylacteylene (H2) is reported. Ligand 1 supports tetrahedral complexes of V(III), Fe(II), and Mn(II) (3-5). Complexes 3-5 are high-spin and redox active.