Synthesis, Structures, and Properties of Bis(μ-oxo)nickel(III) and Bis(μ-superoxo)nickel(II) Complexes: An Unusual Conversion of a NiIII2(μ-O)2 Core into a NiII2(μ-OO)2 Core by H2O2 and Oxygenation of Ligand

A six-coordinate bis( μ -oxo)nickel(III) complex, [Ni 2 ( μ -O) 2 (Me 3 -tpa) 2 ] 2+ ( 1 ), was synthesized by the reaction of [Ni 2 ( μ -OH) 2 (Me 3 -tpa) 2 ] 2+ ( 2 ) with 1 equiv of hydrogen peroxide in methanol at −90 °C, where Me 3 -tpa = tris(6-methyl-2-pyridylmethyl)amine. The 6-methyl groups of Me 3 -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, [Ni 2 ( μ -O 2 ) 2 (Me 3 -tpa) 2 ] 2+ ( 3 ), thus, the reaction demonstrates a unique conversion of a Ni III ( μ -O) 2 Ni III core into a Ni II ( μ -OO) 2 Ni 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) 2 Ni 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) 2 Ni 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 18 O-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 N 2 atmosphere resulted in partial hydroxylation of a methyl group of Me 3 -tpa in yields of 21−27% for both complexes. For complex 3, a carboxylate complex, [Ni(Me 2 -tpaCOO)(OH 2 )] + ( 4 ), where one of the three methyl groups of Me 3 -tpa is oxidized to carboxylate, was also isolated as a decomposed product under N 2 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 Me 3 -tpa.