Mechanism of Alkyne Insertion of a Cationic Zirconocene Aryl Complex

The reaction of Cp₂Zr(4-F-o-tolyl)(CD₂Cl₂)⁺ (A) with 2-butyne at −85 °C in CD₂Cl₂ proceeds by reversible formation of the alkyne complex Cp₂Zr(4-F-o-tolyl)(2-butyne)⁺ (C), followed by rate limiting insertion to produce Cp₂Zr{CMeCMe(4-F-o-tolyl)}⁺ (D). Analysis of this system by pre-equilibrium kinetics (−85 °C) gives K_eq = 1.4(1) M^-1 for 2-butyne coordination to A and k_insert = 6.3(8) × 10^-4 s^-1 and Δ = 13.5(4) kcal/mol for the insertion step. Stabilization of C by an agostic interaction and steric crowding in the transition state contribute to the high insertion barrier.