Cycloaddition and C–H Activation Reactions of a Tantalum Alkylidyne

The alkylidyne complex (C₅Me₅)­Ta­(CPh)­(PMe₃)₂Cl (<b>1</b>) was first reported by Schrock in 1978, but little if any follow-up work on <b>1</b> or other group 5 metal alkylidynes has been reported. This work discloses two avenues of reactivity of <b>1</b>. Treatment of <b>1</b> with 3-hexyne resulted in the formation of a tantalacyclobutadiene. Abstraction of chloride from <b>1</b> led to a mixture of products that included isomers of [(C₅Me₅)­Ta­(CHPh)­(CH₂PMe₂)­(PMe₃)]⁺ (<b>4</b>), in which a C–H bond of a PMe₃ ligand was added across the TaC bond. The C–H activation was found to be reversible, and the equilibrium mixture functioned as an equivalent of a cationic Ta alkylidyne in reaction with 3-hexyne, producing a cationic tantalacyclobutadiene (<b>6</b>). Compounds <b>4</b> and <b>6</b> were structurally characterized in the solid state by XRD methods, with <b>6</b> being the first structurally characterized metallacyclobutadiene in group 5