Department of Chemistry,
Center for Metals in Biocatalysis, Chemical Theory Center, and Minnesota
Supercomputing Institute, University of
Minnesota, 207 Pleasant
Street Southeast, Minneapolis, Minnesota 55455, United States
J. Am. Chem. Soc., Article ASAP
DOI: 10.1021/jacs.5b10985
Publication Date (Web): December 22, 2015
Copyright © 2015 American Chemical Society
Abstract
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.
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