Titanocene dichloride
Encyclopedia
Titanocene dichloride is the organotitanium compound
Organotitanium compound
Organotitanium compounds in organometallic chemistry contain carbon to titanium chemical bonds. Organotitanium chemistry is the science of organotitanium compounds describing their physical properties, synthesis and reactions...

 with the formula (η5
Hapticity
The term hapticity is used to describe how a group of contiguous atoms of a ligand are coordinated to a central atom. Hapticity of a ligand is indicated by the Greek character 'eta', η. A superscripted number following the η denotes the number of contiguous atoms of the ligand that are bound to...

-C5H5)2TiCl2, commonly abbreviated as Cp2TiCl2. This metallocene
Metallocene
A metallocene is a compound typically consisting of two cyclopentadienyl anions bound to a metal center in the oxidation state II, with the resulting general formula 2M. Closely related to the metallocenes are the metallocene derivatives, e.g. titanocene dichloride, vanadocene dichloride...

 is a common reagent in organometallic and organic synthesis
Organic synthesis
Organic synthesis is a special branch of chemical synthesis and is concerned with the construction of organic compounds via organic reactions. Organic molecules can often contain a higher level of complexity compared to purely inorganic compounds, so the synthesis of organic compounds has...

. It exists as a bright red solid that slowly hydrolyzes in air. Cp2TiCl2 does not adopt the typical "sandwich" structure like ferrocene
Ferrocene
Ferrocene is an organometallic compound with the formula Fe2. It is the prototypical metallocene, a type of organometallic chemical compound consisting of two cyclopentadienyl rings bound on opposite sides of a central metal atom. Such organometallic compounds are also known as sandwich compounds...

 due to the 4 ligand
Ligand
In coordination chemistry, a ligand is an ion or molecule that binds to a central metal atom to form a coordination complex. The bonding between metal and ligand generally involves formal donation of one or more of the ligand's electron pairs. The nature of metal-ligand bonding can range from...

s around the metal centre, but rather takes on a distorted tetrahedral shape.

Preparation

Cp2TiCl2 continues to be prepared from titanium tetrachloride
Titanium tetrachloride
Titanium tetrachloride is the inorganic compound with the formula TiCl4. It is an important intermediate in the production of titanium metal and the pigment titanium dioxide. TiCl4 is an unusual example of a metal halide that is highly volatile...

, in the same way as its original synthesis by Wilkinson
Geoffrey Wilkinson
Sir Geoffrey Wilkinson FRS was a Nobel laureate English chemist who pioneered inorganic chemistry and homogeneous transition metal catalysis.-Biography:...

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


The reaction is conducted in THF. Work-up sometimes washing with hydrochloric acid
Hydrochloric acid
Hydrochloric acid is a solution of hydrogen chloride in water, that is a highly corrosive, strong mineral acid with many industrial uses. It is found naturally in gastric acid....

 to convert hydrolysis derivatives to the dichloride. Recrystallization from toluene
Toluene
Toluene, formerly known as toluol, is a clear, water-insoluble liquid with the typical smell of paint thinners. It is a mono-substituted benzene derivative, i.e., one in which a single hydrogen atom from the benzene molecule has been replaced by a univalent group, in this case CH3.It is an aromatic...

 forms acicular
Acicular (crystal habit)
Acicular, in mineralogy, refers to a crystal habit composed of a radiating mass of slender, needle-like crystals. Minerals with this habit tend to be fragile and complete, undamaged specimens can be uncommon.-Examples:...

 crystals.
Cp2TiCl2 can also be prepared by using freshly distilled cyclopentadiene
Cyclopentadiene
Cyclopentadiene is an organic compound with the formula C5H6. This colorless liquid has a strong and unpleasant odor. At room temperature, this cyclic diene dimerizes over the course of hours to give dicyclopentadiene via a Diels–Alder reaction...

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


This reaction is conducted under a nitrogen atmosphere and by using THF as solvent. The product is purified by soxhlet extraction using toluene
Toluene
Toluene, formerly known as toluol, is a clear, water-insoluble liquid with the typical smell of paint thinners. It is a mono-substituted benzene derivative, i.e., one in which a single hydrogen atom from the benzene molecule has been replaced by a univalent group, in this case CH3.It is an aromatic...

 as solvent.

The complex is pseudotetrahedral. Each of the two Cp rings are attached as η5
Hapticity
The term hapticity is used to describe how a group of contiguous atoms of a ligand are coordinated to a central atom. Hapticity of a ligand is indicated by the Greek character 'eta', η. A superscripted number following the η denotes the number of contiguous atoms of the ligand that are bound to...

 ligands. Viewing the Cp ligands as tridentate, the complex has a coordination number
Coordination number
In chemistry and crystallography, the coordination number of a central atom in a molecule or crystal is the number of its nearest neighbours. This number is determined somewhat differently for molecules and for crystals....

 of 8.

Applications in organic synthesis

Cp2TiCl2 is a generally useful reagent that effectively behaves as a source of Cp2Ti2+. A large range of nucleophiles will displace chloride. Examples:
  • The Petasis reagent
    Petasis reagent
    The Petasis reagent is dimethyl titanocene, Cp2TiMe2, readily prepared by the reaction of methylmagnesium chloride or methyllithium with titanocene dichloride:...

    , Cp2Ti(CH3)2, is prepared from the action of CH3MgCl or MeLi on Cp2TiCl2. This reagent is useful for the conversion of esters into vinyl ethers.
  • The Tebbe's reagent
    Tebbe's reagent
    The Tebbe reagent is the organometallic compound with the formula 2TiCH2ClAl2. It used in the methylenation of carbonyl compounds, that is it converts organic compounds containing the R2C=O group into the related R2C=CH2 derivative...

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

Reactions

Cp2TiCl2 undergoes anion exchange reactions, e.g. to give the pseudohalides. With NaSH
Sodium hydrosulfide
Sodium hydrosulfide is the chemical compound with the formula NaHS. This compound is the product of the half neutralization of hydrogen sulfide with sodium hydroxide. NaHS is a useful reagent for the synthesis of organic and inorganic sulfur compounds. It is a colorless solid that typically smells...

 and with polysulfide
Polysulfide
Polysulfides are a class of chemical compounds containing chains of sulfur atoms. There are two main classes of polysulfides: anions and organic polysulfides. Anions have the general formula Sn2−. These anions are the conjugate bases of the hydrogen polysulfides H2nSn...

 salts, one obtains the sulfido derivatives Cp2Ti(SH)2 and Cp2TiS5.

One Cp ligand can be removed from Cp2TiCl2 to give tetrahedral CpTiCl3. This conversion can be effected by with TiCl4 or by reaction with SOCl2
Thionyl chloride
Thionyl chloride is an inorganic compound with the formula SOCl2. It is a reactive chemical reagent used in chlorination reactions. It is a colorless, distillable liquid at room temperature and pressure that decomposes above 140 °C. Thionyl chloride is sometimes confused with sulfuryl...

.

Ti(II) derivatives

Cp2TiCl2 is a precursor to many Ti(II) derivatives. Titanocene, TiCp2, is itself so highly reactive that it is not known but it can be trapped by conducting the reduction in the presence of ligands. Reduction of titanocene dichloride results in the fulvalene complex shown in the figure.

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
Titanocene dicarbonyl
Dicarbonylbistitanium is the chemical compound with the formula 2Ti2, abbreviated Cp2Ti2. This maroon-coloured, air-sensitive species is soluble in aliphatic and aromatic solvents...

 + MgCl2
Cp2TiCl2 + 2 PR3 + Mg → Cp2Ti(PR3)2 + MgCl2
Cp2TiCl2 + 2 Me3SiCCSiMe3 + Mg → Cp2TiMe3SiCCSiMe3 + MgCl2


With only one equivalent of reducing agent, Ti(III) species result, i.e. Cp2TiCl.

Alkyne and benzyne derivatives of titanocene are well known. One family of derivatives are the titanocyclopentadienes.

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 diene
Diene
In organic chemistry a diene or diolefin is a hydrocarbon that contains two carbon double bonds.Conjugated dienes are functional groups, with a general formula of CnH2n-2. Dienes and alkynes are functional isomers...

s such as 1,3-butadiene
1,3-Butadiene
1,3-Butadiene is a simple conjugated diene with the formula C4H6. It is an important industrial chemical used as a monomer in the production of synthetic rubber. When the word butadiene is used, most of the time it refers to 1,3-butadiene....

 gives η3-allyltitanium complexes. Related reactions occur with diynes. Furthermore, titanocene can catalyze C-C bond metathesis to form asymmetric diynes.

Derivatives of (C5Me5)2TiCl2

The closest relative to titanocene-ethylene complex is that derived by Na reduction of (C5Me5)2TiCl2 in the presence of ethylene. The Cp compound cannot be made. This pentamethylcyclopentadienyl (Cp*) species undergoes many reactions such as cycloadditions of alkynes.

Medicinal uses

Titanocene dichloride was investigated as an anticancer drug. The mechanism by which it acts is not understood, but some conjecture that it might be due to its interactions with the protein transferrin
Transferrin
Transferrins are iron-binding blood plasma glycoproteins that control the level of free iron in biological fluids. In humans, it is encoded by the TF gene.Transferrin is a glycoprotein that binds iron very tightly but reversibly...

.

Further reading

  • Payack, J. F.; Hughes, D. L.; Cai, D.; Cottrell, I. F.; Verhoeven, T. R. "Dimethyltitanocene Titanium, bis(η5-2,4-cyclopentadien-1-yl)dimethyl-" Organic Syntheses, Coll. Vol. 10, p. 355 (2004); Vol. 79, p. 19 (2002).
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