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{CMeCMe(4-F-o-tolyl)}+ (D). Analysis of this system by pre-equilibrium kinetics (−85 °C) gives Keq = 1.4(1) M-1 for 2-butyne coordination to A and kinsert = 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.