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 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.
Bibliographical noteFunding Information:
We thank the National Institutes of Health (R37GM47365 to W.B.T.) and the National Science Foundation (CHE-1361595 to C.J.C.) for financial support of this research. The authors thank Laura J. Clouston, Benjamin D. Neisen, and Dr. Victor G. Young, Jr. for assistance with X-ray crystallography and/or EPR spectroscopy and Pavel Solntsev for thoughtful discussion. Some X-ray diffraction experiments were performed using a crystal diffractometer acquired through NSF-MRI Award CHE-1229400. This work was carried out in part using computing resources at the University of Minnesota Supercomputing Institute.
© 2015 American Chemical Society.