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

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. Variabletemperature 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(mu-eta1-O,Oprim-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 gemdiolate has been determined crystallographically.