Formation and reactivity of a cationic alkylidene-aryl complex of tungsten

Article abstract of DOI:10.1021/om00135a021

Reaction of [W(η-C₅H₅)₂(CH₃)₂] with NH₄I in THF allows convenient access to the valuable synthetic intermediate [W(η-C₅H₅)₂(CH₃)I] (1). Treatment of 1 with PhMgBr gives [W(η-C₅H₅)₂(CH₃)Ph] (2), which can be oxidized to [W(η-C₅H₅)₂(CH₃)Ph]PF ₆ (3) with ferrocenium hexafluorophosphate. The 17-electron cation in 3 can be reduced back to 2. Treatment of 3 with the trityl radical in acetonitrile generates [W(η-C₅H₅)₂(CH₂Ph)(NCCH ₃)]PF₆ (4), in a reaction proposed to involve initial hydrogen atom abstraction from 3 to give [W(η-C₅H₅)₂(CH₂)Ph]⁺. Migratory insertion within this cationic methylidene-aryl complex then generates a 16-electron benzyl complex, which forms a stable solvent adduct. Adduct 4 can also be prepared by solvolysis of [W(η-C₅H₅)₂(C₂H ₄)(CH₂Ph)]PF₆. The reaction of 3 with trityl radical in CH₂Cl₂ gives the paramagnetic compound [W(η-C₅H₅)₂(CH₂Ph)Cl]PF ₆ (5). The cation in 5 can be reduced to [W(η-C₅H₅)₂(CH₂Ph)Cl] (6), which can be reoxidized to 5. It is proposed that the CH₂Cl₂ in an intermediate solvato-benzyl complex analogous to 4 is activated toward further reaction with the trityl radical. The intermediate methylidene complex can also be formed by hydride abstraction from 2, as demonstrated by the isolation of 4 when 2 is treated with the trityl cation in CH₃CN. The reaction of 2 with the trityl cation in CH₂Cl₂ gives a 1:1 mixture of 3 and 5 at room temperature and 3 at -78°C, consistent with an electron transfer/hydrogen atom abstraction mechanism for the hydride abstraction.