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

Abstract

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-iPr2)Cl4 and MBTCD in the presence of redox-active ligands, such as α-diimine and o-iminoquinone, produced tungsten imido complexes with the corresponding redox-active ligands, (α-diimine)W(═NC6H3-2,6-iPr2)Cl2 (1), [(o-iminoquinone)W(═NC6H3-2,6-iPr2)Cl]2(μ-Cl)2 (3), and (o-iminoquinone)W(═NC6H3-2,6-iPr2)Cl2(THF) (4), along with Me3SiCl and toluene as whole byproducts. Reaction of the brown complex 1 with [nBu4N][Cl] afforded intensely green single crystals of [nBu4N][(α-diimine)W(═NC6H3-2,6-iPr2)Cl3] (2). The versatile coordination modes of the α-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·Et2O resulted in the formation of (α-diimine)W(═NC6H3-2,6-iPr2)(CH2Ph)2 (5), and thermolysis of 5 in the presence of PMe2Ph at 80 °C afforded the alkylidene complex (α-diimine)W(═NC6H3-2,6-iPr2)(═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-iPr2)(═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 °C. Treatment of o-iminoquinone complex 4 with 2 equiv of LiCH2CMe2Ph afforded the dialkyl complex (o-iminoquinone)W(═NC6H3-2,6-iPr2)(CH2CMe2Ph)2 (8). Dialkyl complexes 5 and 8 at 80 °C served as catalysts to give poly(norbornene) with rather broad polydispersity.