Oxygen Reactivity of a Nickel(II)−Polyoximate Complex

The ligand tris(2-hydroxyiminopropyl)amine (Ox3H3) binds to
nickel(II) in multiple protonation states. In the neutral state, the
X-ray crystal structure of the monomeric complex [Ni(Ox3H3)(NO3)(H2O)](NO3)·(H2O), 1, has six-coordinate pseudo-octahedral geometry, with binding of the amine and three oxime nitrogens, a
nitrate, and a water. In the mono-deprotonated form, the X-ray
crystal structure shows a dimer, [Ni(Ox3H2)(CH3CN)]2(ClO4)2, 2,
which has bridging oximate groups and a Ni−Ni distance of 3.575
Å. The fully deprotonated complex, 3, shows significantly low Ni(II)
oxidation potentials at −390 and +165 mV (versus Fc+/Fc).
Complex 3 shows reactivity when exposed to O2, consuming
multiple O2 equivalents and turning from the purple 3 to a dark
brown complex, 4. Complex 4 has an EPR spectrum consistent
with Ni(III), but spin quantitation accounts for only about 10% of
the total Ni, consistent with turnover of the Ni oxidation states.
This Ni(II)/O2 system oxidizes triphenylphosphine to its oxide, with
incorporation of the isotopic label from O2.