Enhanced Proton-Coupled Electron-Transfer Reactivity by a Mononuclear Nickel(II) Hydroxide Radical Complex

The synthesis, characterization,
and reactivity of a NiOH core
bearing a tridentate redox-active ligand capable of reaching three
molecular oxidation states is presented in this paper. The reduced
complex [LNiOH]2– was characterized by single-crystal
X-ray diffraction analysis, depicting a square-planar NiOH core stabilized
by intramolecular H-bonding interactions. Cyclic voltammetry measurements
indicated that [LNiOH]2– can be reversibly oxidized
to [LNiOH]− and [LNiOH] at very negative reduction
potentials (−1.13 and −0.39 V vs ferrocene, respectively).
The oxidation of [LNiOH]2– to [LNiOH]− and [LNiOH] was accomplished using 1 and 2 equiv of ferrocenium,
respectively. Spectroscopic and computational characterization suggest
that [LNiOH]2–, [LNiOH]−, and
[LNiOH] are all NiII species in which the redox-active
ligand adopts different oxidation states (catecholate-like, semiquinone-like,
and quinone-like, respectively). The NiOH species were found to promote
H-atom abstraction from organic substrates, with [LNiOH]− acting as a 1H+/1e– oxidant and [LNiOH]
as a 2H+/2e– oxidant. Thermochemical
analysis indicated that [LNiOH] was capable of abstracting H atoms
from stronger O–H bonds than [LNiOH]−. However,
the greater thermochemical tendency of [LNiOH] reactivity toward H
atoms did not align with the kinetics of the PCET reaction, where
[LNiOH]− reacted with H-atom donors much faster
than [LNiOH]. The unique stereoelectronic structure of [LNiOH]− (radical character combined with a basic NiOH core)
might account for its enhanced PCET reactivity.