Mechanism of Reversible Alkyne Coupling at Zirconocene: Ancillary Ligand Effects

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Abstract
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.

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