Two novel cyclopentadienyl molybdenum complexes [CpMo(CO)(2)(ImPyMes)Cl] (1) (ImPyMes = 2-mesitylimidazo[1,5-a]pyridine-3-ylidene) and [CpMo(CO)(2)(ImPyMes)(NCCH3)]BF4 (2) were employed as pre-catalysts in olefin epoxidation catalysis using tert-butylhydroperoxide (TBHP) as oxidant. Turnover frequencies (TOFs) of 40,900 h(-1) (0.005 mol% 1) and 53,100 h(-1) (0.01 mol% 2) were achieved in cis-cyclooctene epoxidation in CHCI3 at 55 degrees C, outperforming most previously reported Mo epoxidation catalysts. The synthesis of 1 proceeds via the silver carbene transmetallation route in 90% yield, and further treatment of 1 with AgBF4 in CH3CN leads to the ionic complex 2 in 87% yield. Even at very low pre-catalyst concentrations, quantitative cis-cyclooctene conversion with high selectivity is achieved. Furthermore, epoxidation of more challenging substrates results in good conversions and recycling experiments in room temperature ionic liquid (RTIL) [C(8)mim]NTf2 (Csmim = 1-methyl-3-octylimidazolium, NTf2 = bis(trifluoromethanesulfonyl)imide) shows reusability of 1 in catalytic epoxidation for several runs without loss of activity. (C) 2014 Elsevier. Inc. All rights reserved.