Reactions of Substituted Zirconocene−Bis(trimethylsilyl)ethyne Complexes with Terminal Alkynes

The complexes [Zr(η²-Me₃SiC⋮CSiMe₃)(η⁵-C₅Me₄R)₂] (R = Me (5), H (6), SiMe₃ (7)) reacted smoothly with terminal alkynes HC⋮CR‘ (R‘ = CMe₃, SiMe₃, ferrocenyl (Fc), Ph) to give the agostic alkenyl−alkynyl complexes [Zr(η¹-C⋮CR‘){η³-C(SiMe₃)CH(SiMe₃)}(η⁵-C₅Me₄R)₂] (R/R‘ = Me/CMe₃ (5a), Me/SiMe₃ (5b), H/CMe₃ (6a), H/SiMe₃ (6b), H/Fc (6c), H/Ph (6d), SiMe₃/CMe₃ (7a), SiMe₃/Fc (7c)). Complex 7a, formed in situ from 7 and HC⋮CCMe₃, dimerized tert-butylethyne exclusively to give the head-to-tail dimer Me₃CC⋮CC(CMe₃)CH₂, and when all the terminal alkyne was consumed, 7 was restored. Complex 7 reacted similarly with 1-hexyne, phenylethyne, and (trimethylsilyl)ethyne, affording the respective alkenyl−alkynyl complexes, which also changed back to 7 on evaporation of the reaction solutions; none of these terminal alkynes was dimerized with 5−7. Heating of complex 6 with LiC⋮CSiMe₃ in toluene or THF yielded the diamagnetic zirconate complex Li[Zr(η¹-C⋮CSiMe₃)(η²-Me₃SiC⋮CSiMe₃)(η⁵-C₅HMe₄)₂] (6e) and its solvatomorph Li[Zr(η¹-C⋮CSiMe₃)(η²-Me₃SiC⋮CSiMe₃)(η⁵-C₅HMe₄)₂]·C₄H₈O (6f), respectively. The lithium cation in 6e and 6f is placed in a position at the side of the alkynyl triple bond and close to C^β of the η²-Me₃SiC⋮CSiMe₃ ligand, apparently interacting with both moieties. All presented complexes were characterized by the standard spectral methods, and the structures of 5a, 6a, 6c, 6d, and 6e were determined by single-crystal X-ray diffraction.