Abstract
The formation and detailed spectroscopic characterization of the first biuret-containing monoanionic superoxido-Ni-II intermediate [LNiO2](-) as the Li salt [2; L=MeN-[C(=O)NAr)(2); Ar=2,6-iPr(2)C(6)H(3))] is reported. It results from oxidation of the corresponding [Li(thf)(3)](2)[(LNiBr2)-Br-II] complex M with excess H2O2 in the presence of Et3N.
The [LNiO2](-) 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-II complex 1, [L'NiO2] (L'=CH(CMeNAr)(2)). 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-II core of 2 suggests that such a reactivity may also play a role in catalytic cycles of Ni-containing oxygenases and oxidases.