Titanocene Dichloride

Titanocene dichloride is the organotitanium compound with the formula ( ''η''⁵-C₅H₅)₂TiCl₂, commonly abbreviated as Cp₂TiCl₂. This metallocene is a common reagent in organometallic and organic synthesis. It exists as a bright red solid that slowly hydrolyzes in air. It shows antitumour activity and was the first non-platinum complex to undergo clinical trials as a chemotherapy drug.

Preparation and structure

The standard preparations of Cp₂TiCl₂ start with titanium tetrachloride. The original synthesis by Wilkinson and Birmingham, using sodium cyclopentadienide, is still commonly used:

:2 NaC₅H₅ + TiCl₄ → (C₅H₅)₂TiCl₂ + 2 NaCl

It can also be prepared by using freshly distilled cyclopentadiene rather than its sodium derivative:

:2 C₅H₆ + TiCl₄ → (C₅H₅)₂TiCl₂ + 2 HCl

Focusing on the geometry of the Ti center, Cp₂TiCl₂ adopts a distorted tetrahedral geometry (counting Cp as a monodentate ligand). The Ti-Cl distance is 2.37 Å and the Cl-Ti-Cl angle is 95°.

Reactions

Halide replacement reactions

Cp₂TiCl₂ serves as a source of Cp₂Ti²⁺. A large range of nucleophiles will displace chloride. With NaSH and with polysulfide salts, one obtains the sulfido derivatives Cp₂Ti(SH)₂ and Cp₂TiS₅.

The Petasis reagent, Cp₂Ti(CH₃)₂, is prepared from the action of methylmagnesium chloride or methyllithium on Cp₂TiCl₂. This reagent is useful for the conversion of esters into vinyl ethers.

The Tebbe reagent Cp₂TiCl(CH₂)Al(CH₃)₂, arises by the action of 2 equivalents Al(CH₃)₃ on Cp₂TiCl₂.

Reactions affecting Cp ligands

One Cp ligand can be removed from Cp₂TiCl₂ to give tetrahedral CpTiCl₃. This conversion can be effected with TiCl₄ or by reaction with SOCl₂.

The sandwich complex (Cycloheptatrienyl)(cyclopentadienyl)titanium is prepared by treatment of titanocene dichloride with lithium cycloheptatrienyl.

Titanocene itself, TiCp₂, is so highly reactive that it rearranges into a Ti^III hydride dimer and has been the subject of much investigation. This dimer can be trapped by conducting the reduction of titanocene dichloride in the presence of ligands; in the presence of benzene, a fulvalene complex, can be prepared and the resulting solvate structurally characterised by X-ray crystallography. The same compound had been reported earlier by a lithium aluminium hydride reduction and sodium amalgam reduction of titanocene dichloride, and studied by ¹H NMR prior to its definitive characterisation.

Reduction

Reduction with zinc gives the dimer of bis(cyclopentadienyl)titanium(III) chloride in a solvent-mediated chemical equilibrium:

Cp₂TiCl₂ is a precursor to Ti^II derivatives.
Reductions have been investigated using Grignard reagent and alkyl lithium compounds. More conveniently handled reductants include Mg, Al, or Zn. The following syntheses demonstrate some of the compounds that can be generated by reduction of titanocene dichloride in the presence of π acceptor ligands:
:Cp₂TiCl₂ + 2 CO + Mg → Cp₂Ti(CO)₂ + MgCl₂
:Cp₂TiCl₂ + 2 PR₃ + Mg → Cp₂Ti(PR₃)₂ + MgCl₂

Alkyne derivatives of titanocene have the formula (C₅H₅)₂Ti(C₂R₂) and the corresponding benzyne complexes are known. One family of derivatives are the titanocyclopentadienes. Rosenthal's reagent, Cp₂Ti(η²-Me₃SiC≡CSiMe₃), can be prepared by this method. Two structures are shown, A and B, which are both resonance contributors to the actual structure of Rosenthal's reagent.

Titanocene equivalents react with alkenyl alkynes followed by carbonylation and hydrolysis to form bicyclic cyclopentadienones, related to the Pauson–Khand reaction). A similar reaction is the reductive cyclization of enones to form the corresponding alcohol in a stereoselective manner.

Reduction of titanocene dichloride in the presence of conjugated dienes such as 1,3-butadiene gives ''η''³-allyltitanium complexes. Related reactions occur with diynes. Furthermore, titanocene can catalyze C–C bond metathesis to form asymmetric diynes.

Derivatives of (C₅Me₅)₂TiCl₂

Many analogues of Cp₂TiCl₂ are known. Prominent examples are the ring-methylated derivatives (C₅H₄Me)₂TiCl₂ and (C₅Me₅)₂TiCl₂.

Catalysis

Titanium catalysts are an attractive from the perspective of green chemistry, i.e. tthe low toxicity and high abundance of titanium.

Medicinal research

Titanocene dichloride was investigated as an anticancer drug. In fact, it was both the first non-platinum coordination complex and the first metallocene to undergo a clinical trial.

References

Further reading

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{{DEFAULTSORT:Titanocene Dichloride
Titanium halides
Titanocenes
Chloro complexes
Titanium(IV) compounds