Mechanistic Elucidation of an Alcohol Oxidation Reaction Promoted by a Nickel Azophenolate Complex

Borrowing hydrogen (BH) reactions by base metals have
attracted
significant focus with a perspective in green and sustainable chemistry.
In a BH reaction, dehydrogenation of the alcohol motifs is a fundamental
step, where several well-established catalysts exhibit metal–ligand
cooperativity. We showcase here that the redox noninnocence of the
azophenolate backbone in an earth-abundant nickel catalyst drives
the alcohol dehydrogenation by a radical pathway. Detailed mechanistic
probation including saturation kinetics supports the binding of the
substrate alcohol to the nickel center. Thermodynamic data derived
from such analysis also proves the azophenolate hemilability to pave
the further steps. An Eyring analysis quantifies the kinetic barrier
associated with a crucial hydrogen atom transfer step that is driven
by the azo radical. Furthermore, both Hammett correlation and linearity
in Evans–Polanyi plot implicate that C–H abstraction
by radical mechanism as the rate-determining step. The overall mechanism
offered by the azophenolate catalyst closely resembles the galactose
oxidase-based copper phenoxide chemistry.