Mechanism of Reversible Alkyne Coupling at Zirconocene: Ancillary Ligand Effects

The mechanism of reversible alkyne coupling at zirconium was investigated by examination of
the kinetics of zirconacyclopentadiene cleavage to produce free alkynes. The zirconacyclopentadiene rings
studied possess trimethylsilyl substituents in the α-positions, and the ancillary Cp2, Me2C(η5-C5H4)2, and
CpCp* (Cp* = η5-C5Me5) bis(cyclopentadienyl) ligand sets were employed. Fragmentation of the
zirconacyclopentadiene ring in Cp2Zr[2,5-(Me3Si)2-3,4-Ph2C4] with PMe3, to produce Cp2Zr(η2-PhC⋮CSiMe3)(PMe3) and free PhC⋮CSiMe3, is first-order in initial zirconacycle concentration and zero-order in incoming
phosphine (kobs = 1.4(2) × 10-5 s-1 at 22 °C), and the activation parameters determined by an Eyring
analysis (ΔH⧧ = 28(2) kcal mol-1 and ΔS⧧ = 14(4) eu) are consistent with a dissociative mechanism. The
analogous reaction of the ansa-bridged complex Me2C(η5-C5H4)2Zr[2,5-(Me3Si)2-3,4-Ph2C4] is 100 times
faster than that for the corresponding Cp2 complex, while the corresponding CpCp* complex reacts 20
times slower than the Cp2 derivative. These rates appear to be largely influenced by the steric properties
of the ancillary ligands.