Effects of Charged Ligand Substituents on the Properties of the Formally Copper(III)-Hydroxide ([CuOH]2+) Unit

With the goal of understanding how distal charge influences the properties and hydrogen atom transfer (HAT) reactivity of the [CuOH](2+) core proposed to be important in oxidation catalysis, the complexes [M](3)[(LCuOH)-L-SO3] (M = [K(18-crown-6)](+) or [K(crypt-222)]) and [(LCuOH)-L-Nme3]X (X = BAr4F- or ClO4-) were prepared, in which SO3- or NMe3+ substituents occupy the para positions of the flanking aryl rings of the I 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-1/2 value for the [CuOH](2+)/[CuOH](+) couple to be nearly 0.3 V more oxidizing for the [(LCuOH)-L-NMe3](2+) than the [(LCuOH)-L-SO3](-)species, with the latter influenced by interactions between the distal -SO3- substituents and K+ or Na+ counterions. Chemical oxidations of the complexes generated the corresponding [CuOH](2+) species as evinced by UV vis spectroscopy. The rates of HAT reactions of these species with 9,10-dihydroanthracene to yield the corresponding [Cu(OH2)](2+) complexes and anthracene were measured, and the thermodynamics of the processes were evaluated via determination of the bond dissociation enthalpies (BDEs) of the product O-H bonds. The HAT rate for [(LCuOH)-L-SO3](-) was found to be similar to 150 times faster than that for [(LCuOH)-L-Nme3](2+), despite finding approximately the same BDEs for the product O-H bonds. Rationales for these observations and new insights into the roles of supporting ligand attributes on the properties of the [CuOH](2+) unit are presented.