Organotin Compounds
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Organotin chemistry is the scientific study of the synthesis and properties of organotin compounds or stannanes, which are organometallic compounds containing tin–
carbon Carbon () is a chemical element; it has chemical symbol, symbol C and atomic number 6. It is nonmetallic and tetravalence, tetravalent—meaning that its atoms are able to form up to four covalent bonds due to its valence shell exhibiting 4 ...
bonds. The first organotin compound was diethyltin diiodide (), discovered by Edward Frankland in 1849. The area grew rapidly in the 1900s, especially after the discovery of the Grignard reagents, which are useful for producing Sn–C bonds. The area remains rich with many applications in industry and continuing activity in the research laboratory.


Structure

Organotin compounds are generally classified according to their oxidation states. Tin(IV) compounds are much more common and more useful.


Organic derivatives of tin(IV)

The tetraorgano derivatives are invariably tetrahedral. Compounds of the type SnRR'R''R have been resolved into individual enantiomers.


Organotin halides

Organotin chlorides have the formula for values of ''n'' up to 3. Bromides, iodides, and fluorides are also known, but are less important. These compounds are known for many R groups. They are always tetrahedral. The tri- and dihalides form adducts with good Lewis bases such as pyridine. The fluorides tend to associate such that dimethyltin difluoride forms sheet-like polymers. Di- and especially tri-organotin halides, e.g. tributyltin chloride, exhibit toxicities approaching that of
hydrogen cyanide Hydrogen cyanide (formerly known as prussic acid) is a chemical compound with the chemical formula, formula HCN and structural formula . It is a highly toxic and flammable liquid that boiling, boils slightly above room temperature, at . HCN is ...
.


Organotin hydrides

Organotin hydrides have the formula for values of ''n'' up to 3. The parent member of this series, stannane (), is an unstable colourless gas. Stability is correlated with the number of organic substituents. Tributyltin hydride is used as a source of hydride radical in some organic reactions.


Organotin oxides and hydroxides

Organotin oxides and hydroxides are common products from the hydrolysis of organotin halides. Unlike the corresponding derivatives of silicon and germanium, tin oxides and hydroxides often adopt structures with penta- and even hexacoordinated tin centres, especially for the diorgano- and monoorgano derivatives. The group is called a stannoxane (which is a tin analogue of ethers), and the group is also called a stannanol (which is a tin analogue of alcohols). Structurally simplest of the oxides and hydroxides are the triorganotin derivatives. A commercially important triorganotin hydroxide is the
acaricide Acaricides are pesticides that kill members of the arachnid subclass '' Acari'', which includes ticks and mites. Acaricides are used both in medicine and agriculture, although the desired selective toxicity differs between the two fields. Termi ...
cyhexatin (also called Plictran, tricyclohexyltin hydroxide and tricyclohexylstannanol), (. Such triorganotin hydroxides exist in equilibrium with the distannoxanes: : With only two organic substituents on each Sn centre, the diorganotin oxides and hydroxides are structurally more complex than the triorgano derivatives. The simple tin geminal diols (, the tin analogues of geminal diols ) and monomeric stannanones (, the tin analogues of ketones ) are unknown. Diorganotin oxides () are polymers except when the organic substituents are very bulky, in which case cyclic trimers or, in the case where R is dimers, with and rings. The distannoxanes exist as dimers with the formula wherein the X groups (e.g.,
chloride The term chloride refers to a compound or molecule that contains either a chlorine anion (), which is a negatively charged chlorine atom, or a non-charged chlorine atom covalently bonded to the rest of the molecule by a single bond (). The pr ...
–Cl,
hydroxide Hydroxide is a diatomic anion with chemical formula OH−. It consists of an oxygen and hydrogen atom held together by a single covalent bond, and carries a negative electric charge. It is an important but usually minor constituent of water. It ...
–OH, carboxylate ) can be terminal or bridging (see Table). The hydrolysis of the monoorganotin trihalides has the potential to generate stannanoic acids, . As for the diorganotin oxides/hydroxides, the monoorganotin species form structurally complex because of the occurrence of dehydration/hydration, aggregation. Illustrative is the hydrolysis of butyltin trichloride to give . File:R2SnO-cyclic-trimer-2D.png, Idealized structure of trimeric diorganotin oxide. File:TBu2SnO-cyclic-trimer-from-xtal-1984-Mercury-3D-balls.png, Ball-and-stick model for (. File:R2SnO-cross-linked-network-Harris-and-Sebald-1987-2D.png, Structure of diorganotin oxide, highlighting the extensive intermolecular bonding.


Hypercoordinated stannanes

Unlike carbon(IV) analogues but somewhat like silicon compounds, tin(IV) can also be coordinated to five and even six atoms instead of the regular four. These hypercoordinated compounds usually have electronegative substituents. Numerous examples of hypercoordinated compounds are provided by the organotin oxides and associated carboxylates and related pseudohalide derivatives. The organotin halides for adducts, e.g. ). The all-organic penta- and hexaorganostannates(IV) have even been characterized, while in the subsequent year a six-coordinated tetraorganotin compound was reported. A crystal structure of room-temperature stable (in
argon Argon is a chemical element; it has symbol Ar and atomic number 18. It is in group 18 of the periodic table and is a noble gas. Argon is the third most abundant gas in Earth's atmosphere, at 0.934% (9340 ppmv). It is more than twice as abu ...
) all-carbon pentaorganostannate(IV) was reported as the
lithium Lithium (from , , ) is a chemical element; it has chemical symbol, symbol Li and atomic number 3. It is a soft, silvery-white alkali metal. Under standard temperature and pressure, standard conditions, it is the least dense metal and the ...
salt with this structure: : In this distorted trigonal bipyramidal structure the carbon to tin
bond length In molecular geometry, bond length or bond distance is defined as the average distance between Atomic nucleus, nuclei of two chemical bond, bonded atoms in a molecule. It is a Transferability (chemistry), transferable property of a bond between at ...
s (2.26  Ã… apical, 2.17 Ã… equatorial) are longer than regular C-Sn bonds (2.14 Ã…) reflecting its hypercoordinated nature.


Triorganotin cations

Some reactions of triorganotin halides implicate a role for intermediates. Such cations are analogous to carbocations. They have been characterized crystallographically when the organic substituents are large, such as 2,4,6-triisopropylphenyl.


Tin radicals (organic derivatives of tin(III))

Tin radicals, with the formula , are called stannyl radicals.Davies, Alwyn George. (2004) Organotin Chemistry, 2nd Edition Weinheim: Wiley-VCH. They are a type of tetrel radical, and are invoked as intermediates in certain atom-transfer reactions. For example, tributyltin hydride (tris(''n''-butyl)stannane) serves as a useful source of "hydrogen atoms" because of the stability of the tributytin radical.


Organic derivatives of tin(II)

Organotin(II) compounds are somewhat rare. Compounds with the empirical formula are somewhat fragile and exist as rings or polymers when R is not bulky. The polymers, called polystannanes, have the formula . : In principle, compounds of tin(II) might be expected to form a tin analogues of
alkenes In organic chemistry, an alkene, or olefin, is a hydrocarbon containing a carbon–carbon double bond. The double bond may be internal or at the terminal position. Terminal alkenes are also known as Alpha-olefin, α-olefins. The Internationa ...
with a formal
double bond In chemistry, a double bond is a covalent bond between two atoms involving four bonding electrons as opposed to two in a single bond. Double bonds occur most commonly between two carbon atoms, for example in alkenes. Many double bonds exist betw ...
between two tin atoms () or between a tin atom and a
carbon group The carbon group is a group (periodic table), periodic table group consisting of carbon (C), silicon (Si), germanium (Ge), tin (Sn), lead (Pb), and flerovium (Fl). It lies within the p-block. In modern International Union of Pure and Applied Ch ...
atom (e.g. and ). Indeed, compounds with the formula , called distannenes or distannylenes, which are tin analogues of
ethylene 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 bond, carbon–carbon doub ...
s , are known for certain organic substituents. The Sn centres in stannenes are trigonal. But, contrary to the C centres in alkenes which are trigonal planar, the Sn centres in stannenes tend to be highly pyramidal. Monomeric compounds with the formula , tin analogues of carbenes are also known in a few cases. One example is , where R is the very bulky . Such species reversibly dimerize to the distannylene upon crystallization: : Stannenes, compounds with tin-carbon double bonds, are exemplified by derivatives of stannabenzene.
Stannole Stannole is an organotin compound with the chemical formula, formula (carbon, Chydrogen, H)4tin, SnH2. It is classified as a metallole, i.e. an unsaturated five-membered ring containing a heteroatom. It is a structural analog of cyclopentadiene, ...
s,
structural analog A structural analog, also known as a chemical analog or simply an analog, is a chemical compound, compound having a chemical structure, structure similar to that of another compound, but differing from it in respect to a certain component. It can ...
s of cyclopentadiene, exhibit little C-Sn double bond character.


Organic derivatives of tin(I)

Compounds of Sn(I) are rare and only observed with very bulky ligands. One prominent family of cages is accessed by pyrolysis of the 2,6-diethylphenyl-substituted tristannylene n(C6H3-2,6-Et2)2sub>3, which affords the cubane-type cluster and a prismane. These cages contain Sn(I) and have the formula n(C6H3-2,6-Et2)sub>''n'' where ''n'' = 8, 10 and Et stands for
ethyl group In organic chemistry, an ethyl group (abbr. Et) is an alkyl substituent with the formula , derived from ethane (). ''Ethyl'' is used in the International Union of Pure and Applied Chemistry The International Union of Pure and Applied ...
. A stannyne contains a tin atom to carbon group atom
triple bond A triple bond in chemistry is a chemical bond between two atoms involving six Electron pair bond, bonding electrons instead of the usual two in a covalent bond, covalent single bond. Triple bonds are stronger than the equivalent covalent bond, sin ...
(e.g. and ), and a distannyne a triple bond between two tin atoms (). Distannynes only exist for extremely bulky substituents. Unlike
alkyne \ce \ce Acetylene \ce \ce \ce Propyne \ce \ce \ce \ce 1-Butyne In organic chemistry, an alkyne is an unsaturated hydrocarbon containing at least one carbon—carbon triple bond. The simplest acyclic alkynes with only one triple bond and n ...
s, the core of these distannynes are nonlinear, although they are planar. The Sn-Sn distance is 3.066(1) Å, and the Sn-Sn-C angles are 99.25(14)°. Such compounds are prepared by reduction of bulky aryltin(II) halides.


Preparation

Organotin compounds can be synthesised by numerous methods. Classic is the reaction of a Grignard reagent with tin halides for example tin tetrachloride. An example is provided by the synthesis of tetraethyltin: : The symmetrical tetraorganotin compounds, especially tetraalkyl derivatives, can then be converted to various mixed chlorides by redistribution reactions (also known as the "Kocheshkov comproportionation" in the case of organotin compounds): : : : A related method involves redistribution of tin halides with
organoaluminium compound Organoaluminium chemistry is the study of compounds containing bonds between carbon and aluminium. It is one of the major themes within organometallic chemistry. Illustrative organoaluminium compounds are the dimer trimethylaluminium, the monomer ...
s. In principle, alkyltin halides can be formed from direct insertion of the metal into the carbon-halogen bond. However, such reactions are temperamental, typically requiring a very weak carbon-halogen bond (e.g. an alkyl iodide or an
allyl In organic chemistry, an allyl group is a substituent with the structural formula . It consists of a methylene bridge () attached to a vinyl group (). The name is derived from the scientific name for garlic, . In 1844, Theodor Wertheim isolated a ...
halide) or crown-complexed alkali metal salt catalyst.
Lewis acid A Lewis acid (named for the American physical chemist Gilbert N. Lewis) is a chemical species that contains an empty orbital which is capable of accepting an electron pair from a Lewis base to form a Lewis adduct. A Lewis base, then, is any ...
s or an ionic solvent may also promote the reaction. The mixed organo-halo tin compounds can be converted to the mixed organic derivatives, as illustrated by the synthesis of dibutyldivinyltin: : The organotin hydrides are generated by reduction of the mixed alkyl chlorides. For example, treatment of dibutyltin dichloride with lithium aluminium hydride gives the dibutyltin dihydride, a colourless distillable oil: : The Wurtz-like coupling of alkyl sodium compounds with tin halides yields tetraorganotin compounds. Hydrostannylation involves the metal-catalyzed addition of tin hydrides across unsaturated substrates. Alternatively, stannides attack organic electrophiles to give organostannanes, e.g.: :LiSnMe3 + CCl4 â†’ C(SnMe3)4 + LiCl.


Reactions

Important reactions, discussed above, usually focus on organotin halides and pseudohalides with nucleophiles. All-alkyl organotin compounds generally do not hydrolyze except in concentrated
acid An acid is a molecule or ion capable of either donating a proton (i.e. Hydron, hydrogen cation, H+), known as a Brønsted–Lowry acid–base theory, Brønsted–Lowry acid, or forming a covalent bond with an electron pair, known as a Lewis ...
; the major exception being tin acetylides. An organostannane addition is nucleophilic addition of an
allyl In organic chemistry, an allyl group is a substituent with the structural formula . It consists of a methylene bridge () attached to a vinyl group (). The name is derived from the scientific name for garlic, . In 1844, Theodor Wertheim isolated a ...
-, allenyl-, or propargylstannanes to aldehydes and imines, whereas hydrostannylation conveniently reduces only unpolarized multiple bonds. Organotin hydrides are unstable to strong base, disproportionating to hydrogen gas and distannanes. The latter equilibrate with the corresponding radicals only in the continued presence of base, or if strongly sterically hindered. Conversely, mineral acids cleave distannanes to the organotin halide and more hydrogen gas. In "pure"
organic synthesis Organic synthesis is a branch of chemical synthesis concerned with the construction of organic compounds. Organic compounds are molecules consisting of combinations of covalently-linked hydrogen, carbon, oxygen, and nitrogen atoms. Within the gen ...
, the Stille reaction is considered is a key coupling technique. In the Stille reaction, sp2-hybridized organic halides (e.g. vinyl chloride ) catalyzed by
palladium Palladium is a chemical element; it has symbol Pd and atomic number 46. It is a rare and lustrous silvery-white metal discovered in 1802 by the English chemist William Hyde Wollaston. He named it after the asteroid Pallas (formally 2 Pallas), ...
: : Organotin compounds are also used extensively in radical chemistry (e.g. radical cyclizations, Barton–McCombie deoxygenation, Barton decarboxylation, etc.).


Applications

An organotin compound is commercially applied as stabilizers in
polyvinyl chloride Polyvinyl chloride (alternatively: poly(vinyl chloride), colloquial: vinyl or polyvinyl; abbreviated: PVC) is the world's third-most widely produced synthetic polymer of plastic (after polyethylene and polypropylene). About 40 million tons of ...
. In this capacity, they suppress degradation by removing allylic chloride groups and by absorbing
hydrogen chloride The Chemical compound, compound hydrogen chloride has the chemical formula and as such is a hydrogen halide. At room temperature, it is a colorless gas, which forms white fumes of hydrochloric acid upon contact with atmospheric water vapor. Hyd ...
. This application consumes about 20,000 tons of tin each year. The main class of organotin compounds are diorganotin dithiolates with the formula . The Sn-S bond is the reactive component. Diorganotin carboxylates, e.g., dibutyltin dilaurate, are used as catalysts for the formation of polyurethanes, for vulcanization of
silicones In Organosilicon chemistry, organosilicon and polymer chemistry, a silicone or polysiloxane is a polymer composed of repeating units of siloxane (, where R = Organyl group, organic group). They are typically colorless oils or elastomer, rubber ...
, and transesterification. ''n''-Butyltin trichloride is used in the production of tin dioxide layers on glass bottles by
chemical vapor deposition Chemical vapor deposition (CVD) is a vacuum deposition method used to produce high-quality, and high-performance, solid materials. The process is often used in the semiconductor industry to produce thin films. In typical CVD, the wafer (electro ...
.


Biological applications

" Tributyltins" are used as industrial
biocide A biocide is defined in the European legislation as a chemical substance or microorganism intended to destroy, deter, render harmless, or exert a controlling effect on any harmful organism. The US Environmental Protection Agency (EPA) uses a sli ...
s, e.g. as antifungal agents in textiles and paper, wood pulp and paper mill systems, breweries, and industrial cooling systems. Triphenyltin derivatives are used as active components of antifungal paints and agricultural fungicides. Other triorganotins are used as miticides and
acaricide Acaricides are pesticides that kill members of the arachnid subclass '' Acari'', which includes ticks and mites. Acaricides are used both in medicine and agriculture, although the desired selective toxicity differs between the two fields. Termi ...
s. Tributyltin oxide has been extensively used as a
wood preservative Wood preservation refers to any method or process, or even technique, used to protect the wood and extend its service life. Most wood species are susceptible to both biological (''biotic'') and non-biological (''abiotic'') factors that cause d ...
. Tributyltin compounds were once widely used as marine anti-
biofouling Biofouling or biological fouling is the accumulation of microorganisms, plants, algae, or small animals where it is not wanted on surfaces such as ship and submarine hulls, devices such as water inlets, pipework, grates, ponds, and rivers that ...
agents to improve the efficiency of ocean-going ships. Concerns over toxicity of these compounds (some reports describe biological effects to marine life at a concentration of 1
nanogram To help compare different ''Order of magnitude, orders of magnitude'', the following lists describe various ''mass'' levels between 10−67 kilogram, kg and 1052 kg. The least massive thing listed here is a graviton, and the most massive thi ...
per liter) led to a worldwide ban by the
International Maritime Organization The International Maritime Organization (IMO; ; ) is a List of specialized agencies of the United Nations, specialized agency of the United Nations responsible for regulating maritime transport. The IMO was established following agreement at a ...
. As anti-fouling compounds, organotin compounds have been replaced by dichlorooctylisothiazolinone. Image:Tetrabutyltin.svg, Tetrabutyltin colorless oil, precursor to the other butyl-tin compounds Image:Tributyltin oxide.png, Tributyltin oxide, a colorless to pale yellow liquid used in wood preservation Image:Fentin acetate.svg, Triphenyltin acetate, an off-white crystalline solid, used as an
insecticide Insecticides are pesticides used to kill insects. They include ovicides and larvicides used against insect eggs and larvae, respectively. The major use of insecticides is in agriculture, but they are also used in home and garden settings, i ...
and a
fungicide Fungicides are pesticides used to kill parasitic fungi or their spores. Fungi can cause serious damage in agriculture, resulting in losses of yield and quality. Fungicides are used both in agriculture and to fight fungal infections in animals, ...
Image:Triphenyltin chloride.png, Triphenyltin chloride, a highly toxic white solid, used as a biocide Image:Trimethyltin chloride.png, Trimethyltin chloride, a toxic white solid, once used as a biocide Image:Triphenyltin hydroxide.svg, Triphenyltin hydroxide, an off-white powder, used as a
fungicide Fungicides are pesticides used to kill parasitic fungi or their spores. Fungi can cause serious damage in agriculture, resulting in losses of yield and quality. Fungicides are used both in agriculture and to fight fungal infections in animals, ...
Image:Azocyclotin.svg, Azocyclotin, a white solid, used as a long-acting
acaricide Acaricides are pesticides that kill members of the arachnid subclass '' Acari'', which includes ticks and mites. Acaricides are used both in medicine and agriculture, although the desired selective toxicity differs between the two fields. Termi ...
for control of spider mites on plants Image:Cyhexatin.svg, Cyhexatin, a white solid, used as an
acaricide Acaricides are pesticides that kill members of the arachnid subclass '' Acari'', which includes ticks and mites. Acaricides are used both in medicine and agriculture, although the desired selective toxicity differs between the two fields. Termi ...
and miticide Image:Hexamethylditin.svg, Hexamethylditin used as an intermediate in chemical synthesis Image:Tetraethyltin.svg, Tetraethyltin,
boiling point The boiling point of a substance is the temperature at which the vapor pressure of a liquid equals the pressure surrounding the liquid and the liquid changes into a vapor. The boiling point of a liquid varies depending upon the surrounding envi ...
63–65° /12 mm is a catalyst. The "Et" symbol stands for
ethyl group In organic chemistry, an ethyl group (abbr. Et) is an alkyl substituent with the formula , derived from ethane (). ''Ethyl'' is used in the International Union of Pure and Applied Chemistry The International Union of Pure and Applied ...
.


Toxicity

The toxicities of tributyltin and triphenyltin derivative compounds are comparable to that of
hydrogen cyanide Hydrogen cyanide (formerly known as prussic acid) is a chemical compound with the chemical formula, formula HCN and structural formula . It is a highly toxic and flammable liquid that boiling, boils slightly above room temperature, at . HCN is ...
. Furthermore, tri-''n''-alkyltins are phytotoxic and therefore cannot be used in agriculture. Depending on the organic groups, they can be powerful bactericides and
fungicide Fungicides are pesticides used to kill parasitic fungi or their spores. Fungi can cause serious damage in agriculture, resulting in losses of yield and quality. Fungicides are used both in agriculture and to fight fungal infections in animals, ...
s. Reflecting their high bioactivity, "tributyltins" were once used in marine
anti-fouling paint Anti-fouling paint is a specialized category of coatings applied as the outer (outboard) layer to the hull of a ship or boat, to slow the growth of and facilitate detachment of subaquatic organisms that attach to the hull and can affect a ve ...
. In contrast to the triorganotin compounds, monoorgano, diorgano- and tetraorganotin compounds are far less dangerous, although DBT may be immunotoxic.


See also

* Organostannane addition * Tributyltin azide * Carbastannatranes


References


External links


National Pollutant Inventory Fact Sheet for organotins

Industry information site


{{ChemicalBondsToCarbon Endocrine disruptors