Organotin compounds or stannanes are chemical compounds based on tin with hydrocarbon substituents. Organotin chemistry is part of the wider field of organometallic chemistry. The first organotin compound was diethyltin diiodide ((C2H5)2SnI2), 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.
- 1 Structure of organotin compounds
- 2 Preparation of organotin compounds
- 3 Reactions of organotin compounds
- 4 Use and toxicity
- 5 Applications
- 6 See also
- 7 References
- 8 External links
Structure of organotin compounds
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 entire series R4−nSnCln are known for many R groups and values of n up to 4. The tetraorgano derivatives are invariably tetrahedral.
Organotin halides and hydrides
The mixed organic-chloro compounds are also tetrahedral, although they form adducts with good Lewis bases such as pyridine. The fluoride tend to associate such that dimethyltin difluoride forms sheet-like polymers. The mixed organic tin hydrides, e.g. dialkyltin dihydride, are also generally monomeric. The parent member of this series, stannane (SnH4), is an unstable colourless gas.
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 hexacoordinated tin centres, especially for the diorgano- and monoorgano derivatives. Structurally simplest are the triorganotin oxides/hydroxides.
- 2 R3SnOH R3SnOSnR3 + H2O
With only two organic substituents on each Sn centre, the diorganotin oxides and hydroxides form families of complex structures. The simple geminal diols (R2Sn(OH)2) and monomeric stannanones (R2Sn=O) are unknown. Diorganotin oxides are called (R2SnO) are polymers except with bulky R groups, which are cyclic trimers or, in the case of R = CH(SiMe3)2 dimers, i.e., Sn3O3 and Sn2O2. The distannoxanes exist as dimers of dimers with the formula [R2SnX]2O2 wherein the X groups (e.g., chloride, hydroxide, carboxylate) can be terminal or bridging (see Table). The hydrolysis of the monoorganotin trihalides has the potential to generate stannanoic acids, RSnO2H. 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 [(BuSn)12O14(OH)6]2+.
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 hypervalency are provided by the organotin oxides and associated carboxylates and related pseudohalide derivatives. The organotin halides for adducts, e.g. Me2SnCl2(bipyridine.
The all-organic penta- and hexaorganostannates 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) all-carbon pentaorganostannane was reported as the lithium salt with this structure:
Tin radicals (organic derivatives of tin(III))
Tin radicals, with the formula R3Sn, are called stannyl radicals. They are invoked as intermediates in certain atom-transfer reactions. For example, tributyltin hydride (tri-n-butylstannane) 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 SnR2 are somewhat fragile and exist as rings or polymers when R is not bulky. The polymers, called polystannanes, have the formula (SnR2)n.
In principle divalent tin compounds might be expected to form analogues of alkenes with a formal double bond. Indeed compounds with the formula Sn2R4, called distannenes, are known for certain organic substituents. The Sn centres tend to be highly pyramidal. Monomeric compounds with the formula SnR2, analogues of carbenes are also known in a few cases. One example is Sn(SiR3)2, where R is the very bulky CH(SiMe3)2 (Me = methyl). Such species reversibly dimerize to the distannylene upon crystallization:
- 2 R2Sn (R2Sn)2
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 [Sn(C6H3-2,6-Et2)2]3, which affords the cubane and a prismane. These cages contain Sn(I) and have the formula [Sn(C6H3-2,6-Et2)]n where n = 8, 10. A stannyne contains a carbon to tin triple bond and a distannyne a triple bond between two tin atoms (RSnSnR). Distannynes only exist for extremely bulky substituents. Unlike alkynes, the C-Sn-Sn-C 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 of organotin compounds
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:
- 4 EtMgBr + SnCl4 → Et4Sn + 4 MgClBr
The symmetrical tetraorganotin compounds can then be converted to various mixed chlorides by redistribution reactions (also known as the "Kocheshkov comproportionation"):
- 3 R4Sn + SnCl4 → 4 R3SnCl
- R4Sn + SnCl4 → 2 R2SnCl2
- R4Sn + 3 SnCl4 → 4 RSnCl3
A related method involves redistribution of tin halides with organoaluminium compounds.
The mixed organo-halo tin compounds can be converted to the mixed organic derivatives, as illustrated by the synthesis of dibutyldivinyltin:
- Bu2SnCl2 + 2 C2H3MgBr → Bu2Sn(C2H3)2 + 2 MgBrCl
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:
- Bu2SnCl2 + 1/2 LiAlH4 → Bu2SnH2 + 1/2 LiAlCl4"
Reactions of organotin compounds
Important reactions involving organotin compounds are the Stille reaction (coupling reaction with sp2-hybridized organic halides catalyzed by palladium):
and organostannane additions (nucleophilic addition of an allyl-, allenyl-, or propargylstannanes to an aldehydes and imines). Organotin compounds are also used extensively in radical chemistry (e.g. radical cyclizations, Barton–McCombie deoxygenation, Barton decarboxylation, etc.).
Use and toxicity
- Tetraorganotins are very stable molecules with low toxicity and low biological activity. They are unusable as biocides, but they can be metabolized to toxic triorganotin compounds. They are used as starting materials for catalysts.
- Triorganotins can be very toxic. Tri-n-alkyltins are phytotoxic and therefore cannot be used in agriculture. Depending on the organic groups, they can be powerful bactericides and fungicides. Tributyltins are used as industrial biocides, e.g. as antifungal agents in textiles and paper, wood pulp and paper mill systems, breweries, and industrial cooling systems. Tributyltins are also used in marine anti-fouling paint. Triphenyltins are used as active components of antifungal paints and agricultural fungicides. Other triorganotins are used as miticides and acaricides.
- Diorganotins have no antifungal activity, low toxicity, and low antibacterial activity, except for diphenyltins. They are used in polymer manufacturing, as PVC heat stabilizers, catalysts, in the manufacturing of polyurethane and silicone curing. DBT is however immunotoxic, and a recent paper suggests a link to auto-immune related diseases.
- Monoorganotins have no biocidal activity and their toxicity to mammals is very low. Methyltin, butyltin, octyltin and monoestertins are used as PVC heat stabilizers.
- Many different organotin complexes are being studied in anticancer therapy, observing that their cytotoxicity and selectivity towards cancer cell is higher than that of cisplatin.
An organotin compound is commercially applied as a hydrochloric acid scavenger (or heat stabilizer) in polyvinyl chloride and as a biocide. Tributyltin oxide has been extensively used as a wood preservative. Tributyltin compounds are used as marine anti-biofouling agents. Concerns over toxicity of these compounds (some reports describe biological effects to marine life at a concentration of 1 nanogram per liter) have led to a worldwide ban by the International Maritime Organization. n-Butyltin trichloride is used in the production of tin dioxide layers on glass bottles by chemical vapor deposition.
Organotin compounds are used commercially in a wide range of applications such as biocides, insecticides, chemical intermediates and as catalysts.
Tetrabutyltin starting material for the di- and tributyl compounds
Triphenyltin chloride, a white crystalline solid, used as a biocide and an intermediate in chemical synthesis
Trimethyltin chloride also a biocide
Hexamethylditin used as an intermediate in chemical synthesis
|Core organic chemistry||Many uses in chemistry|
|Academic research, but no widespread use||Bond unknown|
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- Organic Syntheses, Coll. Vol. 4, p.881 (1963); Vol. 36, p.86 (1956). Link
- National Pollutant Inventory Fact Sheet for organotins
- Industry information site
- Organotin chemistry in synthesis
- EU bans certain organotin compounds in consumer products