Perturbing the Copper(III)–Hydroxide Unit through Ligand Structural Variation

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.