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

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-^<i>i</i>Pr₂)­Cl₄ and MBTCD in the presence of redox-active ligands, such as α-diimine and <i>o</i>-iminoquinone, produced tungsten imido complexes with the corresponding redox-active ligands, (α-diimine)­W­(NC₆H₃-2,6-^<i>i</i>Pr₂)­Cl₂ (<b>1</b>), [(<i>o</i>-iminoquinone)­W­(NC₆H₃-2,6-^<i>i</i>Pr₂)­Cl]₂(μ-Cl)₂ (<b>3</b>), and (<i>o</i>-iminoquinone)­W­(NC₆H₃-2,6-^<i>i</i>Pr₂)­Cl₂(THF) (<b>4</b>), along with Me₃SiCl and toluene as whole byproducts. Reaction of the brown complex <b>1</b> with [^<i>n</i>Bu₄N]­[Cl] afforded intensely green single crystals of [^<i>n</i>Bu₄N]­[(α-diimine)­W­(NC₆H₃-2,6-^<i>i</i>Pr₂)­Cl₃] (<b>2</b>). The versatile coordination modes of the α-diimine and <i>o</i>-iminoquinone ligands were clarified by spectroscopic methods and X-ray diffraction studies. Treatment of complex <b>1</b> with 1 equiv of Mg­(CH₂Ph)₂·Et₂O resulted in the formation of (α-diimine)­W­(NC₆H₃-2,6-^<i>i</i>Pr₂)­(CH₂Ph)₂ (<b>5</b>), and thermolysis of <b>5</b> in the presence of PMe₂Ph at 80 °C afforded the alkylidene complex (α-diimine)­W­(NC₆H₃-2,6-^<i>i</i>Pr₂)­(CHPh)­(PMe₂Ph) (<b>6</b>). On the other hand, thermolysis of <b>5</b> in the presence of CCl₄ afforded the dissymmetric benzylidene complex (Cl₃C-amido-imino)­W­(NC₆H₃-2,6-^<i>i</i>Pr₂)­(CHPh)Cl (<b>7</b>) via reductive cleavage of the C–Cl bond of CCl₄. Isolated alkylidene complexes <b>6</b> and <b>7</b> served as catalysts for ring-opening metathesis polymerization of norbornene with 1 mol% of catalyst loading in toluene at 80 °C. Treatment of <i>o</i>-iminoquinone complex <b>4</b> with 2 equiv of LiCH₂CMe₂Ph afforded the dialkyl complex (<i>o</i>-iminoquinone)­W­(NC₆H₃-2,6-^<i>i</i>Pr₂)­(CH₂CMe₂Ph)₂ (<b>8</b>). Dialkyl complexes <b>5</b> and <b>8</b> at 80 °C served as catalysts to give poly­(norbornene) with rather broad polydispersity