Synthesis of Alkyl and Alkylidene Complexes of Tungsten Bearing Imido and Redox-Active a-Diimine or o-Iminoquinone Ligands and Their Application as Catalysts for Ring-Opening Metathesis Polymerization of Norbornene

摘要

We report a new strategy to synthesize tungsten imido complexes bearing bidentate redox-active ligands through reduction of high-valent tungsten imido complexes by 1-methyl-3,6-bis(trimethylsilyl)-1,4-cyclohexadiene (abbreviated MBTCD) without forming any metal salt waste. Reaction of W(-NC6H3-2,6-Pr-i(2))Cl-4 and MBTCD in the presence of redox-active ligands, such as alpha-diimine and o-iminoquinone, produced tungsten imido complexes with the corresponding redox-active ligands, (alpha-diimine)W(-NC6H3-2,6-Pr-i(2))Cl-2 (1), [(o-iminoquinone)W(-NC6H3-2,6-iPr(2))Cl](2)(mu-Cl)(2) (3), and (o-iminoquinone)W(-NC6H3-2,6-Pr-i(2))Cl-2(THF) (4), along with Me3SiCl and toluene as whole byproducts. Reaction of the brown complex 1 with [nBu(4)N][Cl] afforded intensely green single crystals of [nBu(4)N][(alpha-diimine)W(-NC6H3-2,6-Pr-i(2))Cl-3] (2). The versatile coordination modes of the alpha-diimine and o-iminoquinone ligands were clarified by spectroscopic methods and X-ray diffraction studies. Treatment of complex 1 with 1 equiv of Mg(CH2Ph)(2)center dot Et2O resulted in the formation of (alpha-diimine)W(-NC6H3-2,6-Pr-i(2))(CH2Ph)(2) (5), and thermolysis of 5 in the presence of PMe2Ph at 80 degrees C afforded the alkylidene complex (alpha-diimine)W(-NC6H3-2,6-Pr-i(2))(-CHPh)(PMe2Ph) (6). On the other hand, thermolysis of 5 in the presence of CCl4 afforded the dissymmetric benzylidene complex (Cl3C-amido-imino)W(-NC6H3-2,6-Pr-i(2))(-CHPh)Cl (7) via reductive cleavage of the C-Cl bond of CCl4. Isolated alkylidene complexes 6 and 7 served as catalysts for ring-opening metathesis polymerization of norbornene with 1 mol% of catalyst loading in toluene at 80 degrees C. Treatment of o-iminoquinone complex 4 with 2 equiv of LiCH(2)CM(e)2Ph afforded the dialkyl complex (o-iminoquinone)W(-NC6H3-2,6-Pr-i(2))(CH2CMe2Ph)(2) (8). Dialkyl complexes 5 and 8 at 80 degrees C served as catalysts to give poly(norbornene) with rather broad polydispersity.