Perturbing the Copper(III)-Hydroxide Unit Through Ligand Structural Variation
Dhar, D.; Yee, G. M.; Spaeth, A. D.; Boyce, D. W.; Zhang, H.; Dereli, B.;
Cramer, C. J.; Tolman, W. B.
J. Am Chem. Soc. 2016, 138, 356 (doi:10.1021/jacs.5b10985).
Two new ligand sets, pipMeLH2 and NO2LH2 (pipMeL = N,N'-bis(2,6-diisopropylphenyl)-1-methylpiperidine-2,6-dicarboxamide, NO2L = 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 logk (where k is the second order rate constant) and BDE of the C-H bond broken. 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 –80 to –20° C, as inferred from measured kinetic isotope effects and corresponding electronic-structure-based transition-state theory calculations.