Vladimir V. Burlakov

Five-membered metallacycles of titanium and zirconium – attractive compounds for organometallic chemistry and catalysis

In these days a renaissance of metallacycles as an increasingly important class of organometallic compounds for synthetic and catalytic applications is evident, making such very attractive for a plethora of investigations. Titanocene and zirconocene bis(trimethylsilyl)acetylene complexes, regarded as three-membered metallacycles (1-metallacyclopropenes), present a rich chemistry towards unsaturated molecules. By elimination of the alkyne these complexes form by reaction with unsaturated compounds five-membered titana- and zirconacycles, all of which are relevant to stoichiometric and catalytic C-C coupling and cleavage reactions of unsaturated molecules.

Titanocene and zirconocene σ-alkynyl complexes in CC single bond coupling and cleavage reactions

Group 4 metallocene mono- and bis-σ-alkynyl complexes of the type L2M(σ-CCR) and L2M(σ-CCR)2 with M=titanium and zirconium in the oxidation states +3 and +4 and L=Cp (η5-cyclopentadienyl) and Cp* (η5-pentamethylcyclopentadienyl) are important compounds for stoichiometric and catalytic CC single bond coupling and cleavage reactions. Detailed investigations show five-membered metallacyclocumulenes L2M(η4-1,2,3,4-RC4R) as the key intermediates in both reactions of a CC single bond cleavage of different 1,4-substituted 1,3-butadiynes RCCCCR to alkynyl groups and the opposite reaction of CC single bond formation starting from alkynyl groups under the formation of 1,4-substituted 1,3-butadiynes. Depending on different metals M and ligands L, coupling or cleavage is favoured. Combination of both reactions offered the first CC single bond metathesis in homogeneous solution, which is photocatalyzed and titanocene-mediated. It proceeds via titanocene–mono-alkynyl complexes, which are interesting species also for other stoichiometric and catalytic CC coupling reactions. Some similarities regarding the σ-to-π conversion exist between the coupling of the alkynyl groups at titano- and zirconocenes to complexed 1,3-butadiynes on one side and the coupling of phenyl groups at chromium to complexed diphenyl on the other side.

Si–H Activation in Titanocene and Zirconocene Complexes of Alkynylsilanes RC≡CSiMe2H (R=tBu, Ph, SiMe3, SiMe2H): A Model To Understand Catalytic Reactions of Hydrosilanes

An agostic interaction between the Si–H bond and the metal center (depicted on the right) is the characteristic feature of the title complexes, which could be prepared by acetylene exchange reactions. IR, NMR, and X-ray structural investigations reveal that the effect of the Si-H-metal interaction is considerably stronger at low temperatures and in the solid state. This mode of bond activation is important in active catalysts for hydrosilylation and dehydrogenative polysilane reactions.

Katalysatordesaktivierungen bei der Acetylen‐Polymerisation mit Titanocen‐ bzw. Zirconocen‐Komplexen des Bis(trimethylsilyl)acetylens

Bei der katalytischen Acetylen-Polymerisation mit den Metallocen-Alkin-Komplexen Cp2M(L)(η2-Me3SiC2SiMe3), 1: M = Ti, ohne L, 2: M = Zr, L = thf wurden unerwartete, katalytisch inaktive Nebenprodukte beobachtet. Die Reaktion von 1 mit Acetylen wurde NMR-spektroskopisch untersucht und gibt bei –20 °C quantitativ das Titanacyclopentadien Cp2Ti–CH=CH–C(SiMe3)=C(SiMe3) (3). Bei 0 °C lagert sich 3 um, wobei eine Kopplung zwischen einem Cp-Liganden und dem Titanacyclopentadien erfolgt und der Dihydroindenylkomplex 4 gebildet wird. In der entsprechenden Reaktion von 2 fallen unter analogen Bedingungen das Zirconacyclopentadien Cp2Zr–CH=CH–C(SiMe3)=C(SiMe3) (5) und der dimere acetylidverbruckte Komplex [Cp2ZrC(SiMe3)=CH(SiMe3)]2[μ-σ(1,2)-C≡C] (6) an. Wahrend sich 5 zu einer Mischung bisher nicht identifizierter paramagnetischer Species zersetzt, konnte 6 isoliert und mittels NMR-Spektroskopie und Kristallstrukturanalyse charakterisiert werden. Bei der Umsetzung von rac-(ebthi)Zr(η2-Me3SiC2SiMe3) (ebthi = ethylenbistetrahydroindenyl) mit 2-Ethinyl-pyridin wird der Komplex rac-(ebthi)ZrC(SiMe3)=CH(SiMe3)](σ-C≡CPy) 7 gebildet, von dem auch eine Kristallstrukturanalyse angefertigt wurde. Desactivation of Catalysts in the Polymerization of Acetylene by Bis(trimethylsilyl)acetylene Complexes of Titanocene or Zirconocene Unexpected inactive byproducts were observed in the catalytic polymerization of acetylene using metallocene alkyne complexes Cp2M(L)(η2-Me3SiC2SiMe3), 1: M = Ti, without L; 2: M = Zr, L = thf. The reaction of 1 was investigated in detail by NMR to give quantitatively at –20 °C the titanacyclopentadiene Cp2Ti–CH=CH–C(SiMe3)=C(SiMe3) (3). Around 0 °C 3 starts to rearrange to yield the dihydroindenyl complex 4 via coupling of one Cp-ligand with the titanacyclopentadiene. In the reaction of 2 under analogous conditions a zirconacyclopentadiene Cp2Zr–CH=CH–C(SiMe3)=C(SiMe3) (5) and the dimeric complex [Cp2Zr(C(SiMe3)=CH(SiMe3)]2[μ-σ(1,2)-C≡C] (6) were observed. Whereas 5 decomposes to a mixture of unidentified paramagnetic species, 6 was isolated and investigated by NMR spectroscopy and X-ray analysis. In the reaction of rac-(ebthi)Zr(η2-Me3SiC2SiMe3) (ebthi = ethylenbistetrahydroindenyl) with 2-ethynyl-pyridine the complex rac-(ebthi)ZrC(SiMe3)=CH(SiMe3)](σ-C≡CPy) 7 was obtained, which was investigated by an X-ray analysis.

Intramolekulare Insertion eines η5-cyclopentadienyl-ringes in einem bis-η5-cyclopentadienyltitanacyclopentadien

Abstract The reaction of in situ-generated titanocene “Cp 2 Ti” with 2 equiv. of alkynes Me 3 SiCCR yields a mixture of symmetrically ( R = Ph ( 3 ), Py ( 6 ) ) and unsymmetrically ( R = Ph ( 4 ) , Py ( 7 ) substituted titanacyclopentadienes. Complex 7 is unstable and rearranges with an intramolecular insertion of one Cp of the titanocene fragment into the unsymmetrically substituted titanacyclopentadiene to produce the dihydroindenyl complex 8 , which was characterized by an X-ray structure analysis.

Alkinkomplexe des titanocens und permethyltitanocens ohne zusätzliche liganden — erste strukturvergleiche

Abstract The first complex of titanocene with an alkyne, Cp 2 Ti(PhC 2 SiMe 3 ), having no additional ligands, was structurally characterized and compared with the permethyltitanocene analogue Cp 2 ★Ti(PhC 2 SiMe 3 ) to examine the different influences of Cp 2 Ti and Cp 2 ★Ti on alkyne complexation.

Isomerization of olefins by titanocene and zirconocene alkyne complexes

An efficient isomerization of aliphatic and cyclic olefins is achieved by using well-defined metallocene alkyne complexes as catalysts. Titanocene complexes isomerize 1-alkenes to internal alkenes under mild conditions. The titanium complex was recovered quantitatively. Cyclic olefins, e.g. cyclohexadienes, also undergo isomerization, but a competing intermolecular hydrogen transfer reaction takes place as well. This side-reaction is much more favoured for zirconocene than for titanocene complexes.

Dimerization of titanacyclocumulenes to titanium substituted radialenes: synthesis, stability and reactions of five-membered titanacyclocumulenes with a coupling of two 1,4-diphenyl-1,3-butadiyne between two titanocene molecules to radialene-like fused titanacyclopentadiene compounds

Abstract The reactions of the excellent titanocene source Cp2Ti(η2-Me3SiC2SiMe3) with different 1,4-substituted 1,3-butadiynes RCC–CCR give, by substitution of the acetylene in a one to one complexation of titanocene and the diyne, different five-membered titanacyclocumulenes Cp2Ti[η4-(1-2-3-4)–RC4R]. While these complexes are very stable for R=tBu, the metallacyclocumulene with R=Ph is unstable in solution and stabilizes by dimerization to dinuclear isomers: a fused titanacyclopentadiene–titanacyclopentene complex 1 and a compound 2 consisting of two fused titanacyclopentadiene ring systems and thus possessing a titanium substituted radialene structure. With the monomethyl–cyclopentadienyl complex (H4MeC5)2Ti(η2-Me3SiC2SiMe3) in the reaction with PhCC–CCPh the titanium substituted radialene 7 was isolated in a low yield among some other as yet unidentified complexes. Complexes 2 and 7 were investigated by an X-ray crystal structure analysis.

Interaction of bis(trimethylsilyl)acetylene complex of titanocene with tris(pentafluorophenyl)borane. Synthesis and structure of a new type of zwitterionic metallocene (η5-C5H5){η5-[C5H4B(C6F5)3]}Ti

Abstract In the interaction of the bis(trimethylsilyl)acetylene complex of titanocene Cp 2 Ti(Me 3 SiC 2 SiMe 3 ) with an equimolar amount of B(C 6 F 5 ) 3 in toluene at 20°C, electrophilic substitution of a hydrogen atom in one of the η 5 -C 5 H 5 rings by a B(C 6 F 5 ) 3 group takes place and the paramagnetic zwitterionic titanium complex (η 5 -C 5 H 5 ){η 5 -[C 5 H 4 B(C 6 F 5 ) 3 ]}Ti ( I ) is produced. The formation of I is accompanied by liberation of molecular hydrogen. Bis(trimethylsilyl)acetylene and 1,2-bis(trimethylsilyl)ethane were found in organic products of the reaction. An X-ray diffraction study of complex I revealed the coordination of the ortho -fluorine atoms of two C 6 F 5 groups with the positively charged titanium centre. In contrast to other known titanium, zirconium and hafnium zwitterionic metallocenes, complex I contains a metal in the +3 oxidation state and does not contain any σ-bonded organic radicals at the metal atom.

Terminal Alkynes in the Coordination Sphere of Titanocene Complexes – Elementary Steps in Catalytic Dimerizations and Oligomerizations of Alkynes

The titanocene acetylene complex [Cp*2Ti(η2-Me3SiC≡CSiMe3)] (14) reacts with 1-alkynes such as phenylacetylene (15 a), 1-hexyne (15 b), 1-dodecyne (15 c) and trimethylsilylacetylene (15 d) by ligand exchange and proton shift, to yield exclusively the 1-alkenyltitanocene acetylides [Cp*2Ti(CH=CHR)(C≡CR)] (21) (R = Ph (21 a), CH3(CH2)3 (21 b), CH3(CH2)9 (21 c), SiMe3 (21 d)). The X-ray structure of 21 a is presented. In reaction of acetylene HC≡CH (15 e) with 14 other products are formed. However, no intermediates, like [Cp*2Ti(η2-RC≡CH)] (17), [Cp*2Ti(H)C≡CR] (17) or [Cp*2Ti=C=CHR] (22) in reactions of 14 with 15 are detectable. On the other hand, a stable titanocenehydride [Cp*2Ti(H)OCH3] (23) is obtained by oxidative addition of CH3OH with Cp*2Ti, generated from 14. Terminale Alkine in der Koordinationssphare von Titanocen Komplexen – Elementarschritte in der katalytischen Dimerisierung und Oligomerisierung von Alkinen Der Titanocenacetylenkomplex [Cp*2Ti(η2-Me3SiC≡CSiMe3)] (14) reagiert mit 1-Alkinen wie Phenylacetylen (15 a), 1-Hexin (15 b), 1-Dodecin (15 c) und Trimethylsilylacetylen (15 d) unter Ligandenaustausch und Protonenwanderung ausschlieslich zu 1-Alkenyltitanocenacetyliden [Cp*2Ti(CH=CHR)(C≡CR)] (21) (R = Ph (21 a), CH3(CH2)3 (21 b), CH3(CH2)9 (21 c), SiMe3 (21 d)). Die Einkristallstrukturanalyse von 21 a wird vorgestellt. Reaktionen von Acetylen HC≡CH (15 e) mit 14 ergeben andere Produkte. Erwartete Intermediate der Typen [Cp*2Ti(η2-RC≡CH)] (17), [Cp*2Ti(H)C≡CR] (17) oder [Cp*2Ti=C=CHR] (22) konnten in Reaktionen von 14 mit 15 nicht nachgewiesen werden. Andererseits wird ein stabiles Titanocenhydrid [Cp*2Ti(H)OCH3] (23) durch oxidative Addition von CH3OH mit Cp*2Ti, generiert aus 14, erhalten.