Rosenthal's reagent is a metallocene bis(trimethylsilyl)acetylene complex with zirconium (Cp2Zr) or titanium (Cp2Ti) used as central atom of the metallocene fragment Cp2M. Additional ligands such as pyridine or THF are commonly used as well. With zirconium as central atom and pyridine as ligand (Zirconocene bis(trimethylsilyl)acetylene pyridine), a dark purple to black solid with a melting point of 125–126 °C is obtained. Synthesizing Rosenthal's reagent of a titanocene source yields golden-yellow crystals of the titanocene bis(trimethylsilyl)acetylene complex with a melting point of 81–82 °C. This reagent enables the generation of the themselves unstable titanocene and zirconocene under mild conditions. The reagent is named after the German chemist Uwe Rosenthal (born 1950) and was first synthesized by him and his co-workers in 1995.
Synthesis Rosenthal's reagent can be prepared by reduction of titanocene or zirconocene dichloride with magnesium in the presence of bis(trimethylsilyl)acetylene in THF. The illustrated product for a titanocene complex can be represented by the resonance structures A and B. If zirconium is used as central atom, additional ligands (e.g. pyridine) are necessary for stabilization.
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. 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)].
Zirconocene bis(trimethylsilyl)acetylene pyridine was originally synthesized by Rosenthal’s group in 1994 after they exchanged the coordinating solvent from tetrahydrofuran (THF) to pyridine. The exchange of the THF for the pyridine ligand provides extra stability in organic solvents preventing dimerization. The original synthesis involved the reduction of zirconocene dichloride and the addition of bis(trimethylsilyl)acetylene in THF before transferring to pyridine as mentioned above. More recently, Tilley and coworkers demonstrated a simpler synthesis with a higher yield bypassing the isolation of the less stable THF adduct. This newer method reacts zirconocene dichloride with 2 equivalents of n-Butyllithium in THF to form a metallacyclopropane which is subsequently substituted by bis(trimethylsilyl)acetylene and pyridine.
Structure and Characterization
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.
Bonding
The two main resonance structures of Zirconocene bis(trimethylsilyl)acetylene pyridine include a variation where the C-C triple bond binds side on to the metal and another with the 1-metallacyclopropene configuration. Density functional theory (DFT) calculations showed metal-carbon sigma bonds in addition to an out of plane pi bond corresponding to the 1-metallacyclopropene depiction being a major resonance form. However, a subsequent series of calculations by Leites and colleagues using a higher level of theory showed molecular orbitals more consistent with the triple bond description.
Reactivity The original creation of zirconocene bis(trimethylsilyl)acetylene pyridine was accompanied by reactivity studies of the complex with common small molecules in the form of carbon dioxide and water. Both reactions involved the loss of the pyridine ligand and creation of bimetallic complexes containing bridging-oxo substituents, with the carbon dioxide inserting to create a series of fused metallacycles and the water’s hydrogen atoms breaking up the metallacyclopropenes.
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