New chloro and triphenylsiloxy derivatives of dioxomolybdenum(VI) chelated with pyrazolylpyridine ligands: Catalytic applications in olefin epoxidation

Dioxomolybdenum(VI) complexes of general formula [MoO2X2L2] (X = Cl, OSiPh3; L2 = 2-(1-butyl-3-pyrazolyl)pyridine, ethyl[3-(2-pyridyl)-1-pyrazolyl]acetate) were prepared and characterised by 1H NMR, IR and Raman spectroscopy. The assignment of the vibrational spectra was supported by ab initio calculations. A single crystal X-ray diffraction study of the complex [MoO2Cl2{ethyl[3-(2-pyridyl)-1-pyrazolyl]acetate}] showed that the compound is monomeric and crystallises in the tetragonal system with space group P41. The four complexes are active and selective catalysts for the liquid-phase epoxidation of olefins by tert-butylhydroperoxide. Selectivities to the corresponding epoxides were mostly 100% (for conversions of at least 34%) for the substrates cyclooctene, cyclododecene, 1-octene, trans-2-octene and (R)-(+)-limonene. For styrene epoxidation, the corresponding diol was also formed in significant quantities. The turnover frequencies for cyclooctene epoxidation at 55 °C were around 340 mol molMo−1 h−1 for the chloro complexes and 160 mol molMo−1 h−1 for the triphenylsiloxy complexes. The addition of co-solvents (1,2-dichloroethane or n-hexane) had a detrimental effect on catalytic activities. Kinetic studies for the two complexes bearing the ligand ethyl[3-(2-pyridyl)-1-pyrazolyl]acetate revealed an apparent first order dependence of the initial rate of cyclooctene conversion with respect to cyclooctene or oxidant concentration.

Dioxomolybdenum(VI) complexes bearing substituted pyrazolylpyridines were prepared and characterised by 1H NMR, IR and Raman spectroscopy, and single crystal X-ray diffraction. The complexes are active and selective catalysts for the liquid-phase epoxidation of olefins by tert-butylhydroperoxide. Kinetic studies were carried out to determine the dependence of the initial rate of cyclooctene conversion on either the initial amount of cyclooctene, oxidant or catalyst. Figure optionsDownload as PowerPoint slide