Nucleophilic versus Electrophilic Reactivity of Bioinspired Superoxido Nickel(II) Complexes.

The formation and detailed spectroscopic characterization of the first biuret-containing monoanionic superoxido-Ni intermediate [LNiO ] as the Li salt [2; L=MeN[C(=O)NAr) ; Ar=2,6-iPr C H )] is reported. It results from oxidation of the corresponding [Li(thf) ] [LNi Br ] complex M with excess H O in the presence of Et N. The [LNiO ] core of 2 shows an unprecedented nucleophilic reactivity in the oxidative deformylation of aldehydes, in stark contrast to the electrophilic character of the previously reported neutral Nacnac-containing superoxido-Ni complex 1, [L'NiO ] (L'=CH(CMeNAr) ). According to density-functional theory (DFT) calculations, the remarkably different behaviour of 1 versus 2 can be attributed to their different charges and a two-state reactivity, in which a doublet ground state and a nearby spin-polarized doublet excited-state both contribute in 1 but not in 2. The unexpected nucleophilicity of the superoxido-Ni core of 2 suggests that such a reactivity may also play a role in catalytic cycles of Ni-containing oxygenases and oxidases.