Oxygen Reactivity of a Nickel(II)−Polyoximate Complex

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Abstract
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.