Niobium complexes containing a new chiral heteroscorpionate ligand and the reactivity of such a complex with O2 to give the first gem-diolate niobium complex

Abstract

The synthesis of a novel unsymmetrical bis(pyrazol-1-yl) ligand, (3,5-diphenylpyrazol-1-yl-3′,5′-dimethylpyrazol-1-yl)methane (dpmpzm), 1, has been studied. Deprotonation at the methylene group of 1 with BuⁿLi, followed by reaction with carbon dioxide yielded a racemic mixture of [{Li(dpmpza)(H₂O)}₄], 2 [dpmpza = (3,5-diphenylpyrazol-1-yl-3′,5′-dimethylpyrazol-1-yl)acetate], which is a novel chiral monoanionic NNO tripod ligand. This compound is an excellent precursor for the introduction of this scorpionate ligand into transition metal complexes. The complexes [NbCl₃(dme)(RCCR′)] (dme = 1,2-dimethoxyethane) reacted with 2, to give the corresponding [NbCl₂(dpmpza)(RCCR′)] complexes (R = R′ = Me, 3; R = R′ = SiMe₃, 4; R = Ph, R′ = Me, 5; R = Ph, R′ = Et, 6). The structures of these complexes have been determined by spectroscopic methods. Variable-temperature NMR studies of these complexes were carried out in order to study their dynamic behaviour in solution. The barriers to alkyne rotation have also been calculated. The reactivity of [NbCl₂(dpmpza)(Me₃SiCCSiMe₃)], 4, toward molecular oxygen is particularly noteworthy since it led to the formation of the first gem-diolate niobium species, [(NbCl₂O)₂(μ-η¹-O,O′-tpzpdo)], 7 [tpzpdo = 1,3-bis(3,5-diphenylpyrazol-1-yl)-1,3-bis(3′,5′-dimethylpyrazol-1-yl)-2,2′-propanediolate], in a process that has no precedent in the literature. The molecular structure of this gem-diolate has been determined crystallographically.