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 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.

Nucleophilic reactivity of a copper(II)-hydroperoxo complex

“…Natural population analysis (NPA) depicted atomic charges for 1, which demonstrates that the proximal oxygen atom possesses more negative charge than the distal oxygen atom ( Supplementary Fig. 9) 39 . Thus, overall calculations are indicative of a trigonal bipyramidal Cu(II)hydroperoxo intermediate for 1.…”

Generation and Oxidative Reactivity of a Ni(II) Superoxo Complex via Ligand-Based Redox Non-Innocence

“…The solid state spectrum of 3 shows a isotope sensitive feature assigned as an O-O stretch which appears at 1045 cm −1 upon labeling, similar to other published vibrational data for superoxo species. [37][38][39][40][41][42][43][44][45][46][47][48][49] The feature with natural abundance O2 is difficult to directly assign due to overlap with other features in that area, but is likely underneath a feature at ~1105 cm −1 ( Figure S26) to give a shift of 60 cm −1 that is within experimental error of the predicted shift of 64 cm −1 from a simple harmonic oscillator approximation. In an effort to obtain a better subtraction dataset, the IR spectrum of 3 was also recorded as a C6H6 solution.…”

“…These values are consistent with other reported Ni(II) superoxo complexes. [37][38][39][40][41][42][43][44][45][46][47][48][49] We have extensively attempted to collect Raman data on 3 with no success. We have been, however, able to record infrared (IR) spectra of 3 as a KBr pellet and in solution with both 16 O2 and 18 O2.…”

“…35,36 Nickel-superoxo species are generally rare even in synthetic systems, and Ni mediated oxygen activation without accessing Ni(I) or Ni(III) oxidation states has little synthetic precedent. [37][38][39][40][41][42][43][44][45] Therefore, the proposed enzymatic mechanism for quercetin dioxygenase motivates studies to examine whether ligand cooperativity is a viable strategy for Ni systems to activate O2 and mediate oxidative transformations.…”

Cupric-superoxide complex that induces a catalytic aldol reaction-type C–C bond formation

“…Nucleophilic and electrophilic reactivity of a nickel (II)-superoxide was recently reported. 29 The intermediate generated in the reaction of 1 and 3a at the low temperature (blue spectrum in Fig. 4a) may be a superoxide adduct of benzaldehyde IM1 (detailed reaction mechanism is discussed below, Fig.…”