Synthesis and characterization of [Cp2V(μ-η2:η4-butadiyne)ZrCp′2] heterodimetallic complexes (Cp′ = C5H4t-Bu, C5H4Me).: Formation mechanism and theoretical (ELF) evidence for the existence of planar tetracoordinate carbon (ptC)

Reaction of (C5H4R)(2)Zr(C=CPh)(2) (R = Me, t-Bu) with vanadocene Cp2V yields the heterodimetallic complexes Cp2V(mu-eta(2):eta(4)-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(mu-eta(2):eta(4)-butadiyne)Zr(C(5)H(4)t-Bu)(2) (4), the [Cp2V] and [(C(6)H(4)t-Bu)(2)Zr] 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 [(C(5)H(4)t-Bu)(2)Zr]. 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)(2)Zr(eta(4):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(mu-eta(2):eta(4)-butadiyne)Zr(C5H4R)(2) heterodimetallic compounds is evidenced by NMR studies of either the formation of such zirconacyclocumulene species by addition to (C5H4R)(2)Zr(C=CPh)(2) of a catalytic amount of Cp2V or of the formation of compounds 4 or 5 by addition of Cp2V to (C5H4R)(2)Zr(eta(4):1,2,3,4-PhC=C=C=CPh) (R = t-Bu, Me, respectively; synthesized from dialkynyl zirconocene complexes (C5H4R)(2)Zr(C=CPh)(2) via h nu daylight). The X-ray structure of the (C5H4R)(2)Zr(C=CPh)(2) (R = H (6), t-Bu (7), SiMe3 (8)) dialkynyl zirconocene complexes is also reported.