The formation and detailed spectroscopic characterization of the first biuret-containing monoanionic superoxido-NiII 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 [LNiII 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-NiII 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-NiII core of 2 suggests that such a reactivity may also play a role in catalytic cycles of Ni-containing oxygenases and oxidases.

Berlin Joint EPR lab Institute Nanospectroscopy Helmholtz Zentrum Berlin für Materialien und Energie Berlin Germany

CWRU Center for Synchrotron Biosciences NSLS 2 Brookhaven National Laboratory Upton NY 11973 USA

Department of Chemistry Humboldt Universität zu Berlin Brook Taylor Strasse 2 12489 Berlin Germany

Department of Chemistry Indian Institute of Technology Bombay Powai Mumbai Maharashtra 400 076 India

Department of Chemistry Metalorganics and Inorganic Materials Technische Universität Berlin Strasse des 17 Juni 135 Sekr C2 10623 Berlin Germany

Department of Organic Chemistry Faculty of Science Charles University Hlavova 2030 8 128 43 Prague 2 Czech Republic

Institute for Molecules and Materials Radboud University Heyendaalseweg 135 6525 AJ Nijmegen The Netherlands

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