Titanocene bis(trimethylsilyl)acetylene

Titanocene bis(trimethylsilyl)acetylene is a formally titanium(II) organometallic compound with the formula Ti(C5H5)2C2(Si(CH3)3)2. This complex and its zirconium analogue are often referred to as Rosenthal's reagent, after the first chemist to synthesize it, Uwe Rosenthal. This article will discuss its history, synthesis, structure, reactivity, and applications.

History
Titanocene bis(trimethylsilyl)acetylene complexes were first mentioned by the group of Vol’pin in Moscow in 1961. Using the isolobal analogy, the group argued that silacyclopropanes would be a stable group of compounds, due to their similarities to the cyclopropenyl cation. However, true three-membered rings containing a silicon atom and a carbon-carbon double bond, silirenes, were not reported until 1971. Seyferth and coworkers were the first to synthesize these molecules. Later, Vol’pin again utilized the isolobal analogy to react diphenylacetylene with titanocene (Cp2Ti, where Cp = cyclopentadienyl, rather than dialkylsilene) in an attempt to synthesize unsaturated 1-heterocyclopropanes. Although this was unsuccessful, titanocyclopropane (Cp2Ti(η2-PhC2Ph)) was isolated. In 1988, Vol’pin selected the alkyne bis(trimethylsilyl)acetylene as the most likely reactant for the synthesis of a stable titanocene-alkyl complex. The group, led by the postdoctoral associate Rosenthal, successfully obtained Cp2Ti(η2-Me3SiC2SiMe3) in high yield, as a yellow-orange substance.

Crystal Structure
For much of the history of titanocene bis(trimethylsilyl)acetylene, there has been no X-ray crystal structure. Many attempts to obtain crystals failed, due to the complex’s extremely high solubility in all suitable solvents. However, researchers obtained many crystal structures of similar compounds of the type Cp2Ti(η2R3SiC2SiR3), such as Cp=Cp*, R=tBu, and R=Ph. The crystal structure of the parent complex was not obtained until suitable crystals were serendipitously recovered from reaction mixtures. Once successfully obtained, the crystal structure displayed a bent titanocene with the coordinated alkyne ligand located between the Cp ligand planes. The angles between the titanium-coordinated alkyne ligand and each Cp ligand plane are 21.5° and 25.2°, respectively. To themselves, the Cp ligands form an angle of 46.6°. The Si atoms bonded to the alkyne carbons are almost perfectly in plane, with a torsion angle of 6.5°.

The triple bond of the alkyne has a length of 1.283(6) Å. This value is longer than that of the free alkyne (1.208 Å), and closer to that of a double bond (1.331 Å). Furthermore, the distances between the titanium center and the carbon atoms of the coordinated alkyne are 2.136(5) Å and 2.139(4) Å. These values fall within the range of reported endocyclic Ti-C(sp2) σ-bonds.

Computation
Researchers have calculated the bonding nature of various metallocene acetylene complexes. Cp2Ti(η2-Me3SiC2SiMe3) was modeled using a B3LYP density functional theory (DFT) computation. This revealed the metallocyclopropane group is composed of two in-plane σ-bonds from the carbons to the metal, and one out-of-plane π-bond that also interacts with the metal. This type of interaction is a 3-center, 2-electron bond. Although the aromatic stabilization is the lowest for titanium of the Group 4 metals, the complex is aromatic. These computational results were in agreement with the X-ray structural data.

IBO Analysis
Further DFT calculations were carried out using the PBE0 D3BJ/ def2-TZVP functional and visualization in IBOview. These illustrate the nature of the frontier orbitals in titanocene bis(trimethylsilyl)acetylene. The blue and purple orbitals display the highest occupied molecular orbital (HOMO) found on the complex. These are located between the coordinated alkyne and the metal, in the 2-electron, 3-center system. The green and yellow orbitals display the lowest occupied molecular orbital (LUMO), found on titanium.

Original Synthesis
The first successful synthesis of titanocene bis(trimethylsilyl)acetylene was accomplished by Uwe Rosenthal in 1988, via the reduction of Cp2TiCl2 with magnesium and the alkyne Me3SiC2SiMe3, in THF.

This synthesis was immediately used to make other similar titanocene and zirconocene alkyne complexes.

Variations
Under the same conditions, various zirconium complexes were synthesized, most utilizing other stabilizing ligands, including pyridine and THF.

Notably, this synthesis also enabled the subsequent synthesis and characterization of the first zirconocene-alkyne complex without addition stabilizing ligands. This was accomplished with the reduction of racemic (EBTHI)ZrCl2 [EBTHI = 1,2-ethylene-1,1‘-bis(η5-tetrahydroindenyl)].

The metal-alkyne interaction and general reactivity
There exist 2 resonance forms of this complex, the acetylenic pi-complex, and the metallocyclopropene complex. The major type of interaction dictates the reaction pathway the complex will follow. The insertion pathway involves insertion of the substrate to form a metallocycloprane ring, followed by loss of the alkyne. The dissociation pathway involves dissociation of the alkyne to generate the reactive Cp2Ti intermediate, which is then trapped by reaction with the substrate. The interaction between the metal and alkyne can be controlled by changing the metal (Ti or Zr) and the ligands, including the type of Cp ligand and the substitution on the alkyne. The Cp2Ti species is an unstable Ti(II), d2 complex with 14 total electrons. Because it contains a lone electron pair held in 2 valence orbitals, its reactivity can be compared to carbenes. This form often undergoes reactions with a variety of olefins to yield metallacycles.

A special feature of titanocene bis(trimethylsilyl) and its zirconium analogues is the ability it derives from coordination of the alkyne to stabilize the metallocene fragment. This alkyne can be released under relatively mild conditions to yield the reactive and unstable Cp2Ti intermediate. This reactivity manifests in a variety of reactions, some of which are detailed below. For a comprehensive review, visit "Recent Synthetic and Catalytic Applications of Group 4 Metallocene Bis(trimethylsilyl)acetylene Complexes".

Reactions with carbonyl compounds
Titanocene bis(trimethylsilyl)acetylene reacts with carbonyl compounds to generate metallacyclic titanium-dihydrofuran complexes. The constitution of the products depends on the steric bulk of the groups on the carbonyl compound, with the metallocyclopropane product only being obtained with sufficiently sterically bulky groups, such as R/R' = phenyl.

Ring Enlargement
Heterocyclic systems containing C=N bonds undergo a ring enlargement via a coupling reaction.

Polymerization of Acetylene
Polymerization of acetylene was achieved at 20-60 °C when titanocene bis(trimethylsilyl)acetylene was utilized as a precatalyst. Yield and properties of the resulting polyacetylene could be modulated by the solvent used. 100% trans-polyacetylene could be obtained in pyridine.

Oligimerization of 1-Alkenes
Titanocene bis(trimethylsilyl)acetylene afforded the linear polymerization of 1-alkenes with a selectivity over 98%. This reaction also accomplished a turnover number of 1200-1500.

Recent Developments
Past synthesis, including those mentioned previously, have been straightforward, but require extreme caution in the exclusion of water and air to obtain a pure, catalytically useful complex. The success of the synthesis is also heavily dependent on the quality of Mg(0) used. In 2020, Beckhaus and coworkers reported a more robust synthesis of titanocene bis(trimethylsilyl)acetylene from Cp2TiCl2 and EtMgBr. This synthesis is predicted to have a positive impact on the growth of investigations into applications of the complex. Similarly, the synthesis of other titanocene bis(trimethylsilyl) acetylene complexes have been reported, such as the low-valent ansa-dimethylsilylene, dimethylmethylene–bis(cyclopentadienyl)titanium.