Synthesis, Structures, and Properties of Bis(μ-oxo)nickel(III) and Bis(μ-superoxo)nickel(II) Complexes:  An Unusual Conversion of a Ni^III₂(μ-O)₂ Core into a Ni^II₂(μ-OO)₂ Core by H₂O₂ and Oxygenation of Ligand

Abstract

A six-coordinate bis(μ-oxo)nickel(III) complex, [Ni2(μ-O)2(Me3-tpa)2]2+ (1), was synthesized by the reaction of [Ni2(μ-OH)2(Me3-tpa)2]2+ (2) with 1 equiv of hydrogen peroxide in methanol at −90 °C, where Me3-tpa = tris(6-methyl-2-pyridylmethyl)amine. The 6-methyl groups of Me3-tpa have a significant influence on the formation and stabilization of the high-valent bis(μ-oxo)dinickel(III) species. The reaction of 2 with a large excess of hydrogen peroxide (>10 equiv) afforded a novel bis(μ-superoxo)dinickel(II) complex, [Ni2(μ-O2)2(Me3-tpa)2]2+ (3), thus, the reaction demonstrates a unique conversion of a NiIII(μ-O)2NiIII core into a NiII(μ-OO)2NiII core upon exposure to hydrogen peroxide. Complexes 1, 2, and 3 have been characterized by X-ray crystallography and various physicochemical techniques. Complex 1 has a Ni(μ-O)2Ni core and the average Ni−O and Ni−N bond distances (1.871 and 2.143 Å, respectively) are significantly shorter than those of 2 (2.018 and 2.185 Å, respectively), suggesting that 1 is a bis(μ-oxo)dinickel(III) complex. Complex 3 consists of a Ni(μ-OO)2Ni core with two μ-1,2-O−O bridges to form a six-membered ring with chair conformation and the O−O bond distance is 1.345(6) Å. The resonance Raman spectrum of a powdered sample of 3 measured at ∼110 K showed an isotope-sensitive band at 1096 cm-1 (1044 cm-1 for an 18O-labeled sample), indicating that 3 is a bis(μ-superoxo)dinickel(II) complex. Thermal decomposition of both 1 and 3 in acetone at −20 °C under N2 atmosphere resulted in partial hydroxylation of a methyl group of Me3-tpa in yields of 21−27% for both complexes. For complex 3, a carboxylate complex, [Ni(Me2-tpaCOO)(OH2)]+ (4), where one of the three methyl groups of Me3-tpa is oxidized to carboxylate, was also isolated as a decomposed product under N2 atmosphere. During the decomposition process of 3, dioxygen evolution was simultaneously observed. The electrospray ionization mass spectrometry (ESI-MS) of 3 revealed the formation of 1 during the decomposition process. These results suggest that one possible decomposition pathway of 3 is a disproportionation of two coordinated superoxides to dioxygen and peroxide followed by the O−O bond scission of peroxide to regenerate 1, which is responsible for the hydroxylation and the oxidation of the 6-methyl group of Me3-tpa.

Dioxygen Reactivity of Copper(I) Complexes with Tetradentate Tripodal Ligands Having Aliphatic Nitrogen Donors: Synthesis, Structures, and Properties of Peroxo and Superoxo Complexes

“…Such hydrophobic cavities have also been shown to be effective for stabilization of the unstable ("-peroxo)diiron complexes 49 and high-valent bis("-oxo)-dinickel(III) complex. 50 The methyl groups of L Me,Me also have a similar steric effect toward the stabilization of the peroxo species.…”

Spectroscopic Elucidation of a Peroxo Ni2(μ‐O2) Intermediate Derived from a Nickel(I) Complex and Dioxygen

“…Green crystals suitable for diffraction analysis were obtained by vapor diffusion of pentanes into a saturated THF solution. Elemental analysis calcd (%) for N 4 …”

“…Consequently, we turned our attention to macrocyclic tetradentate ligands that coordinate in a planar array, reasoning that such an attribute would render access to the m-h 2 ,h 2 -peroxo coordination more difficult. Herein, we report on the successful pursuit of this strategy, which led to the discovery of a Ni 2 (m-O 2 ) complex in which the peroxo moiety spans the metals in the m-1,2 mode.Addition of dry O 2 to [Ni(tmc)]OTf [8,9] (1; tmc = 1, 4,8,4,8, OTf = [CF 3 SO 3 ] À ) in THF or acetonitrile at room temperature resulted in a color change from pale blue to clover green. The FT-IR spectrum of the paramagnetic product (m eff = 2.7(1) m B ) included a prominent new feature at 3628 cm À1 assigned to the ñ(OÀH) mode…”

“…The most common synthetic approach, reaction of a nickel(ii) precursor with H 2 O 2 , has yielded bis-m-oxo [1][2][3][4] and bis-msuperoxo complexes. [4] An attractive alternate route utilizes the reaction of nickel(i) complexes with O 2 . This strategy relies on, and is limited by, the ability to prepare suitable nickel(i) species.…”

The First μ6‐Peroxide Transition‐Metal Complex: [Ni8(L)12(O2)]2+

“…[27] In addition, a bis(m-alkylperoxo)dinickel complex was isolated and structurally characterized very recently, [28] the synthesis of which involved a related bis(m-superoxo)dinickel(ii) complex. [29] A monomeric NiÀO 2 adduct was also reported recently and described as Ni II -superoxo species, although no single-crystal X-ray diffraction data were obtained. [30] Despite the scarcity of nickel peroxide complexes, however, recent calculations on [L n Ni 2 (O 2 )] and [L n Ni(O 2 )] complexes suggest that molecular nickel peroxides should be stable.…”