(Dimethyl)dioxomolybdenum(VI) complexes:: syntheses and catalytic applications

Reaction of MoO2Br2S2 complexes [S=THF, CH3CN] with bidentate nitrogen donor ligands (L-2) leads to complexes of the type MoO2Br2L2 in good yields, L-2=substituted bipyridylphenantroline, 1,4-R-2-diazabutadiene and bipyrimidine. Treatment of the latter complexes with Grignard reagents at low temperatures yields complexes of the general formula MoO2 (CH3)(2)L-2 and MoO2(C2H5)(2) (diphenylphenantroline). H-1 NMR and IR data are comparatively indifferent to the ligand changes. The Mo-95 NMR data of selected complexes reflect the donor capability of the organic ligands. Mass spectroscopy and temperature-dependent Mo-95 MMR spectroscopy show a significant stability of the Mo-N bond. The compound MoO2(CH3)(2)(bipyrimidine) was additionally examined by single crystal X-ray analysis. The catalytic activity of the MoO2R2L2 complexes in olefin epoxidation with t-butyl hydroperoxide as oxidizing agent is strongly influenced by the nature of the ligand L and its steric bulk in the equatorial plane. The title complexes with a Mo(CH3)(2) moiety are slightly less active in catalysis than the MoBr2 precursor compounds. Increase of both reaction time and/or temperature lead to a significant increase in the product yield in all examined cases. At about 90 degreesC catalyst decomposition hampers further product yield increase. (C) 2000 Elsevier Science B.V. All rights reserved.