Synthesis, structures, and properties of bis(μ-oxo)nickel(III) and bis(μ-superoxo)nickel(II) complexes: An unusual conversion of a Ni(III)2(μ- O)₂ core into a Ni(II)₂(μ-OO)₂ core by H₂O₂ and oxygenation of ligand

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 Ni(III)(μ-O)₂Ni(III) core into a Ni(II)(μ-OO)₂Ni(II) 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 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.