Identification of an “End-on” Nickel−Superoxo Adduct, [Ni(tmc)(O₂)]⁺

Structural Characterization of a Hydroperoxo Nickel Complex and Its Autoxidation: Mechanism of Interconversion between Peroxo, Superoxo, and Hydroperoxo Species

“…[14] For the latter system, the superoxo complex is the final thermodynamic product of the reaction of the Ni I complex with an excess of O 2 , whereas in our case, 1,2-m-peroxo complex 2 was found to be the major component in a dynamic, oxygen-pressure-dependent equilibrium with what appears to be an end-on coordinated superoxo species 3, as reported for palladium. [19] Peroxo complex 2 a further reacted with hydrogen peroxide to give the corresponding hydroperoxo nickel(II) complex 4 a, whereas controlled hydrolysis at low temperature gave Scheme 1.…”

“…[20] A notable spectroscopic feature is the 1 H NMR chemical shift of the hydroperoxo proton, which was observed at 3.62 ppm (CD 2 Cl 2 , 295 K). In accordance with the observed diamagnetism of the compound, the molecular structure determined by single-crystal X-ray structure analysis is characterized by a square-planar-coordinated nickel ( (2) is in the range of the OÀO bond lengths of other metal hydroperoxo complexes (1.397-1.549 ) [4b-e] and significantly longer than expected for a superoxo species [14] (a value of 1.307 was calculated for end-on complex 3 a, see below). The crystallographic data were of sufficient quality to refine the position of the hydrogen atom, which was found to point towards the face of one of the phenyl substituents of the pincer ligands with a torsion angle Ni-O-O-H of À89 (3)8.…”

“…[12a] In the presence of O 2 , the peroxo complex is slowly converted into the mononuclear end-on superoxo species. [14] Recently, we developed a family of chiral pincer ligands, the iso-pmbox [15] and boxmi ligands, [16] which have displayed considerable potential in asymmetric catalysis [17] and the stabilization of reactive T-shaped nickel(I) complexes, in particular. [18] These iso-pmbox nickel(I) complexes 1 were found to react readily with molecular oxygen at low temperatures to form dinuclear 1,2-m-peroxo-bridged nickel(II) complexes 2 (Scheme 1), similar to Riordans previously described nickel tmc system.…”

A “Side‐on” Superoxonickel Complex [LNi(O2)] with a Square‐Planar Tetracoordinate Nickel(II) Center and Its Conversion into [LNi(μ‐OH)2NiL]

“…[6] An attractive alternative for the synthesis of {NiO 2 } peroxo complexes is the two-electron reduction of O 2 by Ni 0 complexes. [7] Recently, Riordan and co-workers reported the synthesis of the remarkable nickel superoxo complexes A [8] and B [9] with five-coordinate Ni II sites (Scheme 1), which resulted from one-electron reduction of O 2 by the respective Ni I precursors at low temperature. Because A and B are only stable at low temperatures, they could only be characterized by spectroscopy and DFT calculations.…”

Overview of ligand versus metal centered redox reactions in tetraaza macrocyclic complexes of nickel with a focus on electron paramagnetic resonance studies

“…The difference among the three forms seen by EPR is relatively slight; we can only speculate the one form corresponds to one isomer in a given geometry, whether distorted tetrahedral or tbp (due to nitrile coordination), and the other two to the other isomer in each of these geometries (or tbp with both axial and equatorial nitrile coordination). + is optimal at low temperature (6 K) and decreases with higher temperature (see Figure S9 in Kieber-Emmons et al 108 ). In contrast, EPR spectra for authentic Ni III complexes are readily observed even at room temperature.…”