The complexes [MoO2Cl(HC(bim)(3))]Y (Y = Cl (1), BF4 (2) and PF6 (3)) have been prepared by reaction of MoO2Cl2(THF)(2) (for 1) or [MoO2Cl(THF)(3)]Y (for 2 and 3) with the tridentate ligand HC(bim)(3) = tris(benzimidazolyl)methane, and characterized by IR and Raman spectroscopy, and H-1 NMR. The turnover frequencies for the epoxidation of cis-cyclooctene at 55 degrees C with tert-butyl hydroperoxide (TBHP, in decane) as the oxidant and complexes 1-3 as catalysts are in the range of 70-200 mol mol(Mo)(-1) h(-1). 1,2-Epoxycyclooctane is always the only reaction product for reaction times up to 24 h. With the aim of facilitating the recyclability of the complexes, the ionic liquids (ILs) [BMIM]Y and [BMPy]Y(BMIM = 1-n-butyl-3-methylimidazolium,BMPy = 1-n-butyl-3-methylpyridinium:Y = BF4 or PF6) were applied as ionic solvents. The catalytic performance for cyclooctene epoxidation depends strongly on the catalyst solubility in the IL. Of the 12 catalyst/IL mixtures examined, the systems 1/[BMIM]PF6 and 1/[BMPy]PF6 exhibit the most favorable reaction rates allied with good recyclability. The 1/[BMIM]PF6 system was further applied using different oxidants (aqueous TBHP, aqueous H2O2 and urea-hydrogen peroxide adduct) and olefins (norbornene, cyclohexene, styrene, alpha-pinene). (C) 2009 Elsevier B.V. All rights reserved.