Synthesis and characterization of novel cyclopentadienyl molybdenum imidazo[1,5-a]pyridine-3-ylidene complexes and their application in olefin epoxidation catalysis

• Novel cyclopentadienyl imidazo[1,5-a]pyridine-3-ylidene MoII complexes are reported.
• Epoxidation of cis-cyclooctene with these complexes resulted in TOFs up to 53,100 h−1.
• Good conversions were obtained by epoxidation of more challenging substrates.
• Recycling in ionic liquids shows reusability for 10 runs without activity loss.

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 CHCl3 at 55 °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) [C8mim]NTf2 (C8mim = 1-methyl-3-octylimidazolium, NTf2 = bis(trifluoromethanesulfonyl)imide) shows reusability of 1 in catalytic epoxidation for several runs without loss of activity.

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