Organotitanium compound
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Organotitanium chemistry is the science of organotitanium compounds describing their physical properties, synthesis, and reactions. Organotitanium compounds in
organometallic chemistry Organometallic chemistry is the study of organometallic compounds, chemical compounds containing at least one chemical bond between a carbon atom of an organic molecule and a metal, including alkali, alkaline earth, and transition metals, and so ...
contain
carbon Carbon () is a chemical element with the symbol C and atomic number 6. It is nonmetallic and tetravalent In chemistry, the valence (US spelling) or valency (British spelling) of an element is the measure of its combining capacity with o ...
-
titanium Titanium is a chemical element with the symbol Ti and atomic number 22. Found in nature only as an oxide, it can be reduced to produce a lustrous transition metal with a silver color, low density, and high strength, resistant to corrosion in ...
chemical bond A chemical bond is a lasting attraction between atoms or ions that enables the formation of molecules and crystals. The bond may result from the electrostatic force between oppositely charged ions as in ionic bonds, or through the sharing of ...
s. They are reagents in
organic chemistry Organic chemistry is a subdiscipline within chemistry involving the scientific study of the structure, properties, and reactions of organic compounds and organic materials, i.e., matter in its various forms that contain carbon atoms.Clayden, J.; ...
and are involved in major industrial processes.


Brief history

Although the first attempt to prepare an organotitanium compound dates back to 1861, the first example was not reported until 1954. In that year
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 slowl ...
was described by Wilkinson and Birmingham. Independently, titanium-based
Ziegler–Natta catalyst A Ziegler–Natta catalyst, named after Karl Ziegler and Giulio Natta, is a catalyst used in the synthesis of polymers of 1-alkenes (alpha-olefins). Two broad classes of Ziegler–Natta catalysts are employed, distinguished by their solubility: * He ...
s were described leading to major commercial applications, for which the 1963
Nobel Prize in Chemistry ) , image = Nobel Prize.png , alt = A golden medallion with an embossed image of a bearded man facing left in profile. To the left of the man is the text "ALFR•" then "NOBEL", and on the right, the text (smaller) "NAT•" then "M ...
was awarded. This technology underscored the technical significance of organotitanium chemistry.


Properties

The titanium
electron configuration In atomic physics and quantum chemistry, the electron configuration is the distribution of electrons of an atom or molecule (or other physical structure) in atomic or molecular orbitals. For example, the electron configuration of the neon atom ...
( rd24s2) vaguely resembles that of
carbon Carbon () is a chemical element with the symbol C and atomic number 6. It is nonmetallic and tetravalent In chemistry, the valence (US spelling) or valency (British spelling) of an element is the measure of its combining capacity with o ...
and like carbon, the +4
oxidation state In chemistry, the oxidation state, or oxidation number, is the hypothetical charge of an atom if all of its bonds to different atoms were fully ionic. It describes the degree of oxidation (loss of electrons) of an atom in a chemical compound. C ...
dominates. Titanium is however a much larger element than carbon, reflected by the Ti-C
bond length In molecular geometry, bond length or bond distance is defined as the average distance between nuclei of two bonded atoms in a molecule. It is a transferable property of a bond between atoms of fixed types, relatively independent of the rest of ...
s being about 30% longer, e.g. 210 pm in tetrabenzyltitanium vs a typical C-C bond of 155 pm. Simple tetraalkyltitanium compounds however are not typically isolable, owing to the large size of titanium and the electron-deficient nature of its tetrahedral complexes. More abundant and more useful than the simple tetraalkyl compounds are mixed ligand complexes with alkoxide and cyclopentadienyl coligands. Titanium is capable of forming complexes with high
coordination number In chemistry, crystallography, and materials science, the coordination number, also called ligancy, of a central atom in a molecule or crystal is the number of atoms, molecules or ions bonded to it. The ion/molecule/atom surrounding the central i ...
s. In terms of oxidation states, most organotitanium chemistry, in solution at least, focuses on derivatives of titanium in the oxidation states of +3 and +4. Compounds of titanium in the +2 oxidation state are rarer, examples being
titanocene dicarbonyl Dicarbonylbis(cyclopentadienyl)titanium is the chemical compound with the formula (''η''5-C5H5)2Ti(CO)2, abbreviated Cp2Ti(CO)2. This maroon-coloured, air-sensitive species is soluble in aliphatic and aromatic solvents. It has been used for the ...
and . is formally a complex of titanium in the oxidation state of −2. Although Ti(III) is involved in Ziegler–Natta catalysis, the organic derivatives of Ti(III) are uncommon. One example is the dimer . Due to the low
electronegativity Electronegativity, symbolized as , is the tendency for an atom of a given chemical element to attract shared electrons (or electron density) when forming a chemical bond. An atom's electronegativity is affected by both its atomic number and the d ...
of titanium, Ti-C bonds are polarized toward carbon. Consequently, alkyl ligands in many titanium compounds are
nucleophilic In chemistry, a nucleophile is a chemical species that forms bonds by donating an electron pair. All molecules and ions with a free pair of electrons or at least one pi bond can act as nucleophiles. Because nucleophiles donate electrons, they are ...
. Titanium is characteristically
oxophilic Oxophilicity is the tendency of certain chemical compounds to form oxides by hydrolysis or abstraction of an oxygen atom from another molecule, often from organic compounds. The term is often used to describe metal centers, commonly the early trans ...
, which recommends the use of
air-free technique Air-free techniques refer to a range of manipulations in the chemistry laboratory for the handling of compounds that are air-sensitive. These techniques prevent the compounds from reacting with components of air, usually water and oxygen; less comm ...
s. On the other hand, high oxophilicity means that titanium alkyls are effective for abstracting or exchanging organyl ligands for oxo groups, as discussed below.


Compounds


Alkyl titanium chlorides and alkoxides

Simple alkyl complexes of titanium, e.g. , where Ph is
phenyl In organic chemistry, the phenyl group, or phenyl ring, is a cyclic group of atoms with the formula C6 H5, and is often represented by the symbol Ph. Phenyl group is closely related to benzene and can be viewed as a benzene ring, minus a hydrogen ...
, are rare. Several mixed alkyl-titanium-halides and alkyl-titanium-alkoxides are utilized in organic synthesis, even if they are not often well characterized.Hartwig, J. F. Organotransition Metal Chemistry, from Bonding to Catalysis; University Science Books: New York, 2010. At least from the commercial perspective, the most useful organotitanium compounds are generated by combining
titanium(III) chloride Titanium(III) chloride is the inorganic compound with the formula TiCl3. At least four distinct species have this formula; additionally hydrated derivatives are known. TiCl3 is one of the most common halides of titanium and is an important catalys ...
and
diethylaluminium chloride Diethylaluminium chloride, abbreviated DEAC, is an organoaluminium compound. Although usually given the chemical formula (C2H5)2AlCl, it exists as a dimer, C2H5)2AlClsub>2 It is a precursor to Ziegler-Natta catalysts employed for the production ...
. As
Ziegler–Natta catalyst A Ziegler–Natta catalyst, named after Karl Ziegler and Giulio Natta, is a catalyst used in the synthesis of polymers of 1-alkenes (alpha-olefins). Two broad classes of Ziegler–Natta catalysts are employed, distinguished by their solubility: * He ...
s, such species efficiently catalyze the
polymerization In polymer chemistry, polymerization (American English), or polymerisation (British English), is a process of reacting monomer, monomer molecules together in a chemical reaction to form polymer chains or three-dimensional networks. There are ...
of
ethene Ethylene (IUPAC name: ethene) is a hydrocarbon which has the formula or . It is a colourless, flammable gas with a faint "sweet and musky" odour when pure. It is the simplest alkene (a hydrocarbon with carbon-carbon double bonds). Ethylene is ...
. The process is
heterogeneous Homogeneity and heterogeneity are concepts often used in the sciences and statistics relating to the uniformity of a substance or organism. A material or image that is homogeneous is uniform in composition or character (i.e. color, shape, siz ...
and no organotitanium intermediates have been well characterized for this process. Numerous organotitanium reagents are produced by combining titanium tetrachloride, titanium tetraalkoxides, or mixtures thereof with organolithium, organomagnesium, and organozinc compounds. Such compounds find occasional use as stoichiometric reagents in
organic synthesis Organic synthesis is a special branch of chemical synthesis and is concerned with the intentional construction of organic compounds. Organic molecules are often more complex than inorganic compounds, and their synthesis has developed into one o ...
. Methyltitanium trichloride, nominally , can be prepared by treating
titanium(IV) chloride Titanium tetrachloride is the inorganic compound with the formula . It is an important intermediate in the production of titanium metal and the pigment titanium dioxide. is a volatile liquid. Upon contact with humid air, it forms thick clouds ...
with
dimethylzinc Dimethylzinc, also known as Zinc methyl, DMZ, or DMZn is a colorless volatile liquid Zn(CH3)2, formed by the action of methyl iodide on zinc at elevated temperature or on zinc sodium alloy. :2Zn + 2CH3I → Zn(CH3)2 + ZnI2 The sodium assists the ...
in
dichloromethane Dichloromethane (DCM or methylene chloride, methylene bichloride) is an organochlorine compound with the formula . This colorless, volatile liquid with a chloroform-like, sweet odour is widely used as a solvent. Although it is not miscible with ...
at −78 °C. It delivers a
methyl group In organic chemistry, a methyl group is an alkyl derived from methane, containing one carbon atom bonded to three hydrogen atoms, having chemical formula . In formulas, the group is often abbreviated as Me. This hydrocarbon group occurs in many ...
s to carbonyl compounds and
alkyl halide The haloalkanes (also known as halogenoalkanes or alkyl halides) are alkanes containing one or more halogen substituents. They are a subset of the general class of halocarbons, although the distinction is not often made. Haloalkanes are widely ...
s. "Methyltriisopropoxytitanium" is a related reagent. A dialkyltitanium species is implicated for Ti-promoted
cyclopropanation In organic chemistry, cyclopropanation refers to any chemical process which generates cyclopropane () rings. It is an important process in modern chemistry as many useful compounds bear this motif; for example pyrethroids and a number of quinolo ...
s starting from a Grignard reagent and an ester. This reaction is the basis of the
Kulinkovich reaction The Kulinkovich reaction describes the organic synthesis of cyclopropanols via reaction of esters with dialkyldialkoxytitanium reagents, generated in situ from Grignard reagents bearing hydrogen in beta-position and titanium(IV) alkoxides such ...
: "Lombardo's reagent" is used for
methylenation In organic chemistry, methylenation is a chemical reaction that inserts a methylene () group into a chemical compound: :\ce + \longrightarrow \ce\ce Typically, the reaction is used to prepare terminal alkenes from aldehydes and, less frequentl ...
. It is functionally related to the ''Dibromomethane-Zinc-Titanium(IV) Chloride'' reagent. This chemistry addresses a shortcoming of the
Wittig reagent In organic chemistry, Wittig reagents are organophosphorus compounds of the formula R3P=CHR', where R is usually phenyl. They are used to convert ketones and aldehydes to alkenes: : Preparation Because they typically hydrolyze and oxidize readily ...
by methylenating enolisable carbonyl groups without loss of stereochemical integrity (Lombardo Methylenation). It can for example also be applied in a conversion of a
ketene In organic chemistry, a ketene is an organic compound of the form , where R and R' are two arbitrary monovalent chemical groups (or two separate substitution sites in the same molecule). The name may also refer to the specific compound etheno ...
into an
allene In organic chemistry, allenes are organic compounds in which one carbon atom has double bonds with each of its two adjacent carbon centres (). Allenes are classified as diene#Classes, cumulated dienes. The parent compound of this class is propa ...
: :


Titanocene derivatives

Attempted synthesis of "titanocene", i.e. , produces a
fulvalene Fulvalene (bicyclopentadienylidene) is the member of the fulvalene family with the molecular formula C10H8. It is of theoretical interest as one of the simplest non-benzenoid conjugated hydrocarbons. Fulvalene is an unstable isomer of the mor ...
complex. The titanocene dimer was recognised in the 1970s but not structurally characterised until 1992, and the investigations led to many innovations on cyclopentadienyl complexes of titanium. Only in 1998 was a true titanocene derivative identified, the paramagnetic species . In contrast to titanocene itself, titanocene dichloride and to some extent titanocene monochloride have rich and well defined chemistries.
Tebbe's reagent Tebbe's reagent is the organometallic compound with the formula (C5H5)2TiCH2ClAl(CH3)2. It is used in the methylenation of carbonyl compounds, that is it converts organic compounds containing the R2C=O group into the related R2C=CH2 derivative. It ...
, prepared from titanocene dichloride and
trimethylaluminium Trimethylaluminium is one of the simplest examples of an organoaluminium compound. Despite its name it has the formula Al2( CH3)6 (abbreviated as Al2Me6 or TMA), as it exists as a dimer. This colorless liquid is pyrophoric. It is an industriall ...
, is used as a
methylenation In organic chemistry, methylenation is a chemical reaction that inserts a methylene () group into a chemical compound: :\ce + \longrightarrow \ce\ce Typically, the reaction is used to prepare terminal alkenes from aldehydes and, less frequentl ...
agent (conversion of to ). Tebbe's reagent adds simple alkenes to give titanocyclobutanes, which can be regarded as stable
olefin metathesis Olefin metathesis is an organic reaction that entails the redistribution of fragments of alkenes (olefins) by the scission and regeneration of carbon-carbon double bonds. Because of the relative simplicity of olefin metathesis, it often create ...
intermediates. These compounds are reagents in itself such as 1,1-bis(cyclopentadienyl)-3,3-dimethyltitanocyclobutane, the adduct of Tebbe's reagent with
isobutene Isobutylene (or 2-methylpropene) is a hydrocarbon with the chemical formula . It is a four-carbon branched alkene (olefin), one of the four isomers of butylene. It is a colorless flammable gas, and is of considerable industrial value. Productio ...
catalysed with 4-dimethylaminopyridine. The
Petasis reagent The Petasis reagent, named after Nicos A. Petasis, is an organotitanium compound with the formula Cp2Ti(CH3)2. It is an orange-colored solid. Preparation and use The Petasis reagent is prepared by the salt metathesis reaction of methylmagnesium ...
or dimethyl titanocene (1990) is prepared from titanocene dichloride and
methyllithium Methyllithium is the simplest organolithium reagent with the empirical formula CH3Li. This s-block organometallic compound adopts an oligomeric structure both in solution and in the solid state. This highly reactive compound, invariably used in so ...
in
diethyl ether Diethyl ether, or simply ether, is an organic compound in the ether class with the formula , sometimes abbreviated as (see Pseudoelement symbols). It is a colourless, highly volatile, sweet-smelling ("ethereal odour"), extremely flammable liq ...
. Compared to Tebbe's reagent it is easier to prepare and easier to handle. It is also a methylenation reagent. The
Nugent-RajanBabu reagent Bis(cyclopentadienyl)titanium(III) chloride, also known as the Nugent–RajanBabu reagent, is the organotitanium compound which exists as a dimer with the formula C5H5)2TiClsub>2. It is an air sensitive green solid. The complex finds specialized ...
is a one-electron reductant used in
synthetic organic chemistry Organic chemistry is a subdiscipline within chemistry involving the scientific study of the structure, properties, and reactions of organic compounds and organic materials, i.e., matter in its various forms that contain carbon atoms.Clayden, J.; ...
for the generation of
alcohol Alcohol most commonly refers to: * Alcohol (chemistry), an organic compound in which a hydroxyl group is bound to a carbon atom * Alcohol (drug), an intoxicant found in alcoholic drinks Alcohol may also refer to: Chemicals * Ethanol, one of sev ...
s via anti-Markovnikov ring-opening of
epoxide In organic chemistry, an epoxide is a cyclic ether () with a three-atom ring. This ring approximates an equilateral triangle, which makes it strained, and hence highly reactive, more so than other ethers. They are produced on a large scale for ...
s, and is generated as a
dimer Dimer may refer to: * Dimer (chemistry), a chemical structure formed from two similar sub-units ** Protein dimer, a protein quaternary structure ** d-dimer * Dimer model, an item in statistical mechanics, based on ''domino tiling'' * Julius Dimer ( ...
and used ''
in situ ''In situ'' (; often not italicized in English) is a Latin phrase that translates literally to "on site" or "in position." It can mean "locally", "on site", "on the premises", or "in place" to describe where an event takes place and is used in ...
'' from titanocene dichloride.


Mono-Cp compounds

Less useful in organic chemistry but still prominent are many derivatives of
(cyclopentadienyl)titanium trichloride (Cyclopentadienyl)titanium trichloride is an organotitanium compound with the formula (CH)TiCl. It is a moisture sensitive orange solid. The compound adopts a piano stool geometry. Preparation and reactions (CH)TiCl is prepared by the reaction ...
, . This piano-stool complex is obtained by the
redistribution reaction In chemistry, redistribution usually refers to the exchange of anionic ligands bonded to metal and metalloid centers. The conversion does not involve redox, in contrast to disproportionation reactions. Some useful redistribution reactions are condu ...
of
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 slowl ...
and
titanium tetrachloride Titanium tetrachloride is the inorganic compound with the formula . It is an important intermediate in the production of titanium metal and the pigment titanium dioxide. is a volatile liquid. Upon contact with humid air, it forms thick clouds o ...
. With an electron count of 12, it is far more electrophilic than the titanocene dichloride with an electron count of 16.


Arene complexes

Titanium tetrachloride reacts with
hexamethylbenzene Hexamethylbenzene, also known as mellitene, is a hydrocarbon with the molecular formula C12H18 and the condensed structural formula C6(CH3)6. It is an aromatic compound and a derivative of benzene, where benzene's six hydrogen atoms have each ...
to give salts. Reduced arene complexes include the oxidation states −1, 0, +1.


Carbonyl complexes

Salts of are known.


References

{{ChemicalBondsToCarbon