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Cyclopentadienyl Molybdenum(II) N,C-Chelating Benzothiazole-Carbene Complexes: Synthesis, Structure, and Application in Cyclooctene Epoxidation Catalysis

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posted on 27.05.2014, 00:00 by Zhe Wang, Sin Wee Benny Ng, Lu Jiang, Wen Jin Leong, Jin Zhao, T. S. Andy Hor
A series of new benzothiazolyl-imidazolium salts 13 (1-(benzothiazol-2-yl)-3-methylimidazolium chloride (1); 1-(benzothiazol-2-yl)-3-benzylimidazolium chloride (2); 1-(benzothiazol-2-yl)-3-allylimidazolium chloride (3)) have been prepared from nucleophilic substitution of 2-chlorobenzothiazole by imidazoles. They serve as the precursors of hybrid N-heterocyclic carbene (NHC) ligands L1L3 (L1 = 1-(benzothiazolin-2-yl)-3-methylimidazol-2-ylidene; L2 = 1-(benzothiazolin-2-yl)-3-benzylimidazol-2-ylidene; L3 = 1-(benzothiazolin-2-yl)-3-allylimidazol-2-ylidene). Reactions of 13 with Ag2O result in Ag­(I) NHC complexes 46 [Ag­(L)­(μ-Cl)]2 (L = L1, 4; L = L2, 5) and Ag­(L3)Cl (6), in which L1L3 act as monodentate carbene ligand with a benzothiazolyl pendant. Subsequent transmetalation of 46 with CpMo­(CO)3Br (Cp = cyclopentadienyl) and anion exchange reaction with AgX (X = BF4, PF6, or OTf) give complexes [CpMo­(CO)2(L)]2[Ag2Br4] (L = L1, 7a; L = L2, 8a; L = L3, 9a) and [CpMo­(CO)2(L)]­[X] (L = L1, X = BF4, 7b; L = L2, X = BF4, 8b; X = PF6, 8c; X = OTf, 8d; L = L3, X = BF4, 9b), whose structures are reported herein. The ligands L1L3 show their versatility by switching to be N,C-chelating in these Mo­(II) complexes. The chelation of the hybrid NHC ligand results in shorter Mo–Ccarbene bonds (2.14–2.16 Å) comparing with known Mo­(II) NHC complexes in the literature. These complexes are active toward cyclooctene epoxidation with tert-butyl hydroperoxide (TBHP) affording up to 90% yield of epoxide (for 8b) in 3 h at 55 °C.