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

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-iPr₂ C₆ H₃ )] is reported. It results from oxidation of the corresponding [Li(thf)₃ ]₂ [LNi^II Br₂ ] complex M with excess H₂ O₂ in the presence of Et₃ N. 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.