Cyclopentadienyl Molybdenum(II) N,C-Chelating Benzothiazole-Carbene Complexes: Synthesis, Structure, and Application in Cyclooctene Epoxidation Catalysis
datasetposted 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 1–3 (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 L1–L3 (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 1–3 with Ag2O result in Ag(I) NHC complexes 4–6 [Ag(L)(μ-Cl)]2 (L = L1, 4; L = L2, 5) and Ag(L3)Cl (6), in which L1–L3 act as monodentate carbene ligand with a benzothiazolyl pendant. Subsequent transmetalation of 4–6 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 L1–L3 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.