Aliphatic C−H Bond Oxygenation by the Co^IIOOX Species with the Hindered Hydrotris(pyrazolyl)borate Ligand (X = Co(II), Alkyl, H)

Abstract

Aliphatic C−H bond oxygenation is achieved by Co(II)−peroxo species. The dinuclear Co(II)−μ-peroxo complex, {Co[HB(3,5-Pr i 2 pz) 3 ]} 2 (μ-O 2 ) ( 2 ), is yielded by reaction of the bis(μ-hydroxo)−Co(II) complex, {Co(OH)[HB(3,5-Pr i 2 pz) 3 ]} 2 ( 1 ), with an equimolar amount of H 2 O 2 . Spontaneous decomposition of the μ-peroxo complex 2 yields a mono-oxygenated μ-alkoxo-μ-hydroxo in which one of the six 3-isopropyl methine carbon atoms is oxygenated and the resulting alkoxo ligand bridges the two Co(II) centers. In contrast, decomposition of 2 in the presence of an excess amount of H 2 O 2 results in further oxygenation to give a mixture of the dinuclear Co(II)−bis(μ-alkoxo) and the mononuclear Co(II)−hydroxo alcohol ( 6 ). In the bis(μ-alkoxo) complex 5, one of the three 3-isopropyl groups in each ligand is functionalized to give the alkoxide group, which bridges the two metal centers. In the mononuclear hydroxo alcohol complex 6, all of the three 3-isopropyl groups are oxygenated. The Co(II) center is coordinated by the resulting alcohol ligand as well as an hydroxide. Reaction of 1 with 2 equiv of ROOH (R = Bu t and PhMe 2 C) at low temperature yields the blue thermally unstable alkylperoxo compound 3 . The HB(3-Bu t -5-Pr i pz) 3 derivative, Co(OOCMe 2 Ph)[HB(3-Bu t -5-Pr i pz) 3 ] ( 9 ), characterized successfully by X-ray crystallography contains the tetrahedral Co center. The monomeric alkylperoxo complexes 3 also decomposes to give the bis(μ-alkoxo) complex 5, but neither mono-oxygenated 4 nor fully oxygenated 6 is obtained. These observations suggest that the present aliphatic C−H bond oxygenations may proceed via homolysis of the O−O bond of the Co(II)−peroxo species.