S. Li, C.W. Kee, K.-W. Huang, T.S. Andy Hor, J. Zhao
Organometallics, 29 (8), 1924-1933, (2010)
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)(CH3CN)][BF4] (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 [CpMoO2(IMes)][BF4] (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.