New aspects of the cobalt-dioxygen complex chemistry opened by hydrotris(pyrazoly)borate ligands (TpR):: unique properties of TpRCo-dioxygen complexes

Recent advances in the chemistry of cobalt-dioxygen and related complexes supported by hydrotris(pyrazolyl)borate ligands (Tp(R)) are reviewed. The unique properties of inorganic and organometallic Tp(R)Co complexes clearly indicate that hindered Tp(R) ligands carrying alkyl substituents on the pyrazolyl groups can stabilize the coordinatively unsaturated, low valent cobalt species, and that the reactivity of the coordinatively unsaturated species is influenced by steric hindrance of Tp(R). The unusual low-valent metal-peroxo and high-valent metal-ore species such as Co(II)-superoxo, -alkylperoxo and dinuclear Co(III)-bis(mu-oxo) complexes are characterized. The high-valent metal-ore species, [Tp(R)Co(III)](2)(mu-O)(2), are capable of abstracting the H atom from the alkyl groups proximal to the bimetallic bis(mu-oxo) core. In the hydrotris(3,5-diisopropyl-1-pyrazolyl)borate ligand system (Tp(Pr2)), oxygenation of the proximal isopropyl substituents on Tp(Pr2) is mediated by the Co(2)(III)-(mu-O)(2) and Co(II)-OOX (X = alkyl, H) species. (C) 2000 Elsevier Science S.A. All rights reserved.