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

Abstract

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‐iPr2C6H3)] is reported. It results from oxidation of the corresponding [Li(thf)3]2[LNiIIBr2] 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‐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.

Experimental Description of Biomimetic NiII–Superoxo δ-Bond: Franck–Condon Analyses on Its Vibronically-Resolved Spectrum

“…The Mulliken spin distribution was calculated to be localized on O 2 adduct by 0.97e (Figure 3b). This correlates well with an experimental value (0.87e) that was predicted by the 17 21 This spectral feature correlates well with that observed in the present study; their interpretations are not inconsistent with our assignment.…”

“…Taking account of reaction , we can assign the bands at 8328.7 and 8332.2 eV to transitions of the T-shaped Ni I –carbonyl site, whereas the bands at 8329.6 and 8335.8 eV are assigned as the transitions of the square planar Ni II –superoxo site. It was reported that the β-diketiminate Ni II –superoxo complex shows a pre-edge peak at 8333.2 eV and an additional peak at 8338.5 eV . This spectral feature correlates well with that observed in the present study; their interpretations are not inconsistent with our assignment.…”

“…The Ni–superoxo species plays a pivotal role in biological oxidations, for example in quercetin-2,4-dioxygenase . Therefore, the Ni–superoxo complex is one of the attractive synthetic targets for studying the mechanism of the biomimetic enzymatic transformation process. − However, due to their instability, their structural characterization has proven challenging. So far, there is only one report regarding the crystal structure of the Ni II –superoxo complex: β-diketiminato Ni II –superoxo complex .…”

A comprehensive insight into aldehyde deformylation: mechanistic implications from biology and chemistry

“…191 Nucleophilic deformylation of aldehydes has also been reported for a monoanionic nickel(II)-superoxo intermediate, [Ni II O2(L)]  , L= MeN{C(=O)NAr}2; Ar= 2,6-iPr2C6H3]. 192 Cho and co-workers have synthesized a nickel(III)-peroxo complex with the Me6trien ligand, which is an analogue of the macrocyclic 12-TMC ligand but with an open chain structure, Scheme 15. They have compared the reactivity of nickel(III)-peroxo intermediates of both Me6-trien and 12-TMC ligands and found that open-chain nickel(III)-peroxo complex is more reactive than the macrocyclic analogue towards aldehyde deformylation.…”