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 degrees 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 degrees C gives K (eq) = 1.4(1) M-1 for 2-butyne coordination to A and k (insert =) 6.3(8) x 10(-4) s(-1) and Delta(G not equal)insert = 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.