The First Titanacyclic Five‐Membered Cumulene. Synthesis, Structure, and Reactivity

Abstract

The first five-membered titanacyclic cumulene C p , T i -c (tBu)=C=C=k(tBu) (5) was prepared by treatment of the titanocene generator Cp2Ti(Me3SiC2SiMe3) with the di-tertbutylbutadiyne in a molecular one-to-one ratio. The reaction of 5 with "CpzZr" or of the analogous complex Cp2-krC(tBu)=C=C=k(tBu) with "Cp,Ti" leads to cleavage of the central C-C bond in the metallacyclocumulene complexes to afford the same product, the heterodimetallic u,n-alkynyl-bridged titanocene-zirconocene complex CpzZr(p-q' :q2-In cyclizations of unsaturated compounds using the metallocene fragments "Cp2M" (M = Ti or Zr) ['], which are frequently employed preparatively, little is known about reactions with 1 ,Cdisubstituted 1 ,3-butadiyne~[~I.Recently, we have found that two equivalents of the titanocene complex Cp2Ti(Me3SiC2SiMe3)[31, an excellent titanocene source, react with one equivalent of disubstituted butadiynes to give dinuclear complexes of type 114] or Z5]. The obtained types of complexes are well-known[6], but we have found a new way of preparation in which, depending on different substituents, the central C-C single bond of the starting butadiyne is cleaved. R" R"With the complex Cp2Zr(THF)(Me3SiC2SiMe3)[7a] as zirconocene synthon the cleavage reaction to complexes of type l[7bl is more favoured, but when Cp2Zr(Py)(Me3SiC2-SiMe3) (Py = pyridine)[sal was used five-(3) and sevenmembered (4) zirconacyclic cumulene complexes were formed, depending as well on the substituentsLsb1. However, C=CtBu)Cp,Ti(p-q' :q2-C=CtBu) (6), in which each alkynyl group is a-bonded to one and rr-bonded to the other metal as shown by X-ray diffraction studies, In solution the two alkynyl groups become equivalent. The coupling reaction of "Cp2Ti" with bis(trimethylsily1)butadiyne in a one-to-two molecular ratio leads to the unsymmetrically substituted tita-analogous cumulene complexes of titanocene have been unknown so far.In the present paper we report on the synthesis, spectral characteristics, and some chemical properties of the first five-membered metallacyclic cumulene complex of titanocene which can act as an intermediate in the cleavage reaction of diynes. Results and DiscussionThe reaction of Cp2Ti(Me3SiC2SiMe3) with equimolar amounts of tBuC=C-C=CtBu in hexane at room temperature affords green solutions from which the metallacyclic cumulene 5 can be isolated by cooling to -40 "C. Also small amounts of the dinuclear complex 2a are formed after a reaction time of I to 2 days.

Dibenzometallacyclopentadienes, boroles and selected transition metal and main group heterocyclopentadienes: Synthesis, catalytic and optical properties

“…Titanium analogues have also been studied [79][80][81]. Reaction of [TiCl 2 Cp 2 ] with two equivalents of Me 3 Si-C C-C C-SiMe 3 afforded the 2,4-diethynylmetallacycle 66 in 65% yield, the structure of which has also been determined by X-ray crystallography (Figs.…”

Ferrocenylsubstituierte Metallacyclen des Titanocens – Oligocyclopentadienylkomplexe mit vielversprechenden Eigenschaften

“…[15] Daher kann der Metallacyclus 2 am besten als Metallacyclocumulen mit drei C=C-Bindungen beschrieben werden. [19] Die Ti-C-Abstände entsprechen typischen [23] Entgegen der Befunde von CV-und In-situ-UV/ Vis-NIR-spektroskopischen Messungen hat die berechnete energetisch niedrigste Struktur von 2 + C C 2 -Symmetrie, wodurch das ungepaarte Elektron in einem gemischtvalenten Zustand über das ganze System delokalisiert ist. Die atomaren Spindichten wurden hauptsächlich an den Eisenzentren gefunden (0.420/Fe), während die Dichte am Titanatom sehr gering ist (0.03).…”

Peculiarities of Vibrational Spectra and Electronic Structure of the Five‐Membered Metallacyclocumulenes of the Group 4 Metals

“…Synthesis: Cp 2 M(η 4 -tBuC 4 tBu) [1][2][3] and Cp* 2 M(η 2 -Me 3 SiC 2 -SiMe 3 ) [23][24][25] were synthesized according to published methods.…”

“…From the geometry data of 1-3, the cumulene structure of the C 4 moiety was inferred. [1][2][3] All four ring C atoms possess p orbitals normal to the plane of the cumulene cycle. In addition, the sp-hybridized C2 and C3 atoms possess p orbitals that lie in this plane.…”

“…[1][2][3] These complexes are of great interest, not only theoretically as aromatic organometallic compounds, but also practically, because some of their derivatives are active catalysts in the polymerization of olefins and ε-caprolactone. [4] The three complexes are isostructural and display bent metallocene structures that contain a planar five-membered metallacycle (Figure 1).…”