Coordination Chemistry and Reactivity of Bis(aldimino)pyridine Nickel Complexes in Four Different Oxidation States

A series of nickel
complexes with potentially redox active bis­(aldimino)­pyridine
ligands [NNN] ([NNN] = 1,1′-(pyridine-2,6-diyl)­bis­(N-arylmethanimine), where aryl = 2,6-diisopropylphenyl,
mesityl, 4-methoxyphenyl, 4-trifluoromethylphenyl, and 3,5-bis­(trifluoromethyl)­phenyl)
were synthesized, and their properties and reactivities were investigated
as a function of the overall oxidation state of the system. (Ni­[NNN])2+ complexes of ligands featuring bulky electron-rich substituents
(1a-Br2 and 1b-Br2, [NNN] = 1,1′-(pyridine-2,6-diyl)­bis­(N-(2,6-diisopropylphenyl)­methanimine) and 1,1′-(pyridine-2,6-diyl)­bis­(N-mesitylmethanimine), respectively) demonstrated five electrochemical
reduction events, the first three of which were quasi-reversible.
In contrast, only two quasi-reversible reductions were observed for
the less bulky and electron-deficient N-aryl substituents
4-(trifluoromethyl)­phenyl and 3,5-bis­(trifluoromethyl)­phenyl. Chemical
reduction of 1a-Br2 and 1b-Br2 with 1 equiv of KC8 or CoCp*2 forms (Ni­[NNN])+ complexes of the
general formula Ni­[NNN]Br (2a-Br and 2b-Br). Structural, spectroscopic, and theoretical studies reveal that
these complexes feature significant unpaired spin density on the metal,
consistent with “nickel­(I)” character. This behavior
is in contrast with previously reported bis­(ketimino)­pyridine systems,
in which at the (Ni­[NNN])+ state the unpaired electron
resided exclusively in the ligand. Further reduction forms a series
of (Ni­[NNN])0 complexes, in which all of the potentially
tridentate [NNN] ligands bind via only one iminopyridine unit; the
second arm is left unbound in most complexes. Variable temperature
NMR spectroscopy demonstrates that bound and unbound arms exchange
via a postulated tridentate intermediate. Electrochemical reduction,
via three sequential one-electron reductions, of 1a-Br2 and 1b-Br2 in the presence of CO2/H+ forms an active
catalyst for H2 evolution at a glassy-carbon electrode
surface, again emphasizing the unique redox chemistry of the bulky
bis­(aldimino)­pyridine nickel complexes.