A series of N-heterocyclic carbene (NHC) complexes CpMo(CO) 2 (NHC)X (NHC = IMe = 1,3-dimethylimidazol-2-ylidene, X = Br, 1; NHC = 1,3-dipropylimidazol-2-ylidene, X = Br, 2; NHC = IMes = 1,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene, X = Br, 3; NHC = IBz = 1,3-dibenzylimidazol-2-ylidene, X = Br, 4a, and X = Cl, 4b; NHC = 1-methyl-3-propylimidazol-2-ylidene, X = Br, 5 ) and [CpMo(CO) 2 (IMes)(CH 3 CN)][BF 4 ] ( 6 ) have been synthesized and fully characterized. The stability of metal−NHC ligand bonds in these compounds under oxidative conditions has been investigated. The thermally stable Mo(VI) dioxo NHC complex [CpMoO 2 (IMes)][BF 4 ] ( 9 ) has been isolated by the oxidation of the ionic complex 6 by TBHP ( tert -butyl hydrogen peroxide). Complex 6 can be applied as a very active (TOFs up to 3400 h −1 ) and selective olefin epoxidation catalyst. While under oxidative conditions (in the presence of TBHP), compounds 1 − 5 decompose into imidazolium bromide and imidazolium polyoxomolybdate. The formation of polyoxomolybdate as oxidation products had not been observed in a similar epoxidation catalyzed by Mo(II) and Mo(VI) complexes. DFT studies suggest that the presence of Br − destabilizes the CpMo(VI) oxo NHC carbene species, consistent with the experimental observations.
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Li et al. (2010) studied this question.