Synthesis and Characterization of [Cp2V(μ-η2:η4-butadiyne)ZrCp‘2] Heterodimetallic Complexes (Cp‘ = C5H4 t-Bu, C5H4Me). Formation Mechanism and Theoretical (ELF) Evidence for the Existence of Planar Tetracoordinate Carbon (ptC)

Reaction of (C5H4R)2Zr(C⋮CPh)2 (R = Me, t-Bu) with vanadocene Cp2V yields the heterodimetallic complexes Cp2V(μ-η2:η4-butadiyne)Zr(C5H4R)2 (4, R = t-Bu; 5, R = Me). Both compounds have been characterized by elemental analysis, Raman spectroscopy, and magnetic susceptibility. According to the X-ray structure of Cp2V(μ-η2:η4-butadiyne)Zr(C5H4 t-Bu)2 (4), the [Cp2V] and [(C5H4 t-Bu)2Zr] metallocene moieties are bonded to a butadiene framework through the two internal carbon atoms for [Cp2V] and through the two internal and the two external carbon atoms for [(C5H4 t-Bu)2Zr]. The distances and angles observed around the internal carbon atoms of the butadiene framework indicate that both carbons are planar and tetracoordinate. These structural considerations are supported by a topological analysis of the electron localization function (ELF). The existence of the (C5H4R)2Zr(η4:1,2,3,4-PhCCCCPh) (R = H, SiMe3, t-Bu, Me) zirconacyclocumulene species as a key intermediate in the formation of the (Cp2V(μ-η2:η4-butadiyne)Zr(C5H4R)2 heterodimetallic compounds is evidenced by NMR studies of either the formation of such zirconacyclocumulene species by addition to (C5H4R)2Zr(C⋮CPh)2 of a catalytic amount of Cp2V or of the formation of compounds 4 or 5 by addition of Cp2V to (C5H4R)2Zr(η4:1,2,3,4-PhCCCCPh) (R = t-Bu, Me, respectively; synthesized from dialkynyl zirconocene complexes (C5H4R)2Zr(C⋮CPh)2 via hν daylight). The X-ray structure of the (C5H4R)2Zr(C⋮CPh)2 (R = H (6), t-Bu (7), SiMe3 (8)) dialkynyl zirconocene complexes is also reported.