DIOXOMOLYBDENUM(VI) COMPLEXES STABILIZED BY POLYDENTATE LIGANDS WITH NO3, N2O2, AND NS2 DONOR-ATOM SETS

Dioxomolybdenum(VI) complexes of the general formula LMoO2 (L = (OCH2CH2)2N(CH2CH2OH) (1), O(CH2)2-NMe(CH2)2NMe(CH2)2O (2), HN(CH2CH2S)2 (3) ) have been synthesized and characterized. The structures of all compounds have unambiguously been determined in solution with the use of NMR spectroscopy. Complexes 1 and 2 have a distorted octahedral coordination about Mo with cis oxo groups and two alkoxo oxygen atoms in the apical positions. The coordination sphere is completed by amino and alkoxo oxygen atoms in 1 and by two N atoms in 2. Dissolution of 1 in H2O promotes hydrolysis of the remaining hydroxy arm of the ligand to give the anionic species H[MoO2(OCH2CH2)3N] in which all the alkoxo arms are equivalent. In 3, the cis-MoO2(2+) core is coordinated in a distorted trigonal-bipyramidal arrangement by two sulfur atoms in axial positions, by a nitrogen atom, and by two cis oxygen atoms. As shown by a single-crystal X-ray diffraction analysis, 3 retains the trigonal-bipyramidal structure in the solid state: orthorhombic Pnma, a = 9.814(1) angstrom, b = 11.960(2) angstrom, c = 7.119(1) angstrom, Z = 4, R = 0.031. The reactions of 1-3 with some inorganic (PPh3) and organic substrates (p-thiocresol, thioanisole, benzyl alcohol, benzaldehyde) have been investigated. Only complex 3 undergoes selective reduction of the metal center by triphenylphosphine and p-thiocresol converting to the dimer Mo2O3[HN(CH2CH2S)2]2. Our results confirm that a bulky ligand with sulfur donors is required to make LMoO2 complexes capable of catalyzing oxygen-transfer reactions.