Cyclopentadienyl molybdenum(II/VI) N-heterocyclic carbene complexes: synthesis, structure, and reactivity under oxidative conditions

S. Li, C.W. Kee, K.-W. Huang, T.S. Andy Hor, J. Zhao
Organometallics, 29 (8), 1924-1933, (2010)

Cyclopentadienyl molybdenum(II/VI) N-heterocyclic carbene complexes: synthesis, structure, and reactivity under oxidative conditions

Keywords

 Cyclopentadienyl molybdenum

Abstract

​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 15 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.

Code

DOI:  10.1021/om900980a

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