Organotin

Organotin compounds or stannanes are chemical compounds based on tin with hydrocarbon substituents. Organotin chemistry is part of the wider field of organometallic chemistry ^[1]. The first organotin compound was diethyltin diiodide, discovered by Edward Frankland in 1849. An organotin compound is commercially applied as a hydrochloric acid scavenger (or heat stabilizer) in polyvinyl chloride and as a biocide. Tributyltin oxide (or tributyltin for short) 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 world-wide ban by the International Maritime Organization. n-Butyltin trichloride is used in the production of tin oxide layers on glass bottles by chemical vapor deposition.

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Preparation of organotin compounds

• reaction of a grignard reagent with tin halides for example tin tetrachloride
• Wurtz reaction like coupling of alkyl sodium compounds with tin halides yields tetraorgano tin compounds
• by an exchange reaction of tin halides with organo aluminium compounds (AlR₃)
• triorganotin halides are prepared in the Kocheshkov redistribution reaction

As an example the organic synthesis of tributyl-[(Z)-5-phenyl-2-penten-2-yl]stannane ^[2]:

A Grignard reagent is prepared from magnesium turnings and (Z)-2-bromo-5-phenyl-2-pentene in dry tetrahydrofuran and titrated with tributyltin chloride until the solution decolourises. The resulting solution is stirred at room temperature for 1 hour and the solvent is removed in a rotavapor. Diethyl ether is added and the ether extract is washed with brine and filtered and the ether evaporates in a rotavapor. The crude product is kugelrohr distilled to yield tributyl-[(Z)-5-phenyl-2-penten-2-yl]stannane as a colourless oil.

Reactions of organotin compounds

• Stille reaction

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 are 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, eg. 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.
• Monoorganotins have no biocidal activity and their toxicity to mammals is very low. Methyltin, butyltin, octyltin and monoestertins are used as PVC heat stabilizers.

Important compounds

• Tetrabutyltin starting material for the di- and tributyl compounds
• Dialkyl- and monoalkyltin octylthiogylates used as heat stabilizers for polyvinyl chloride
• Tributyltin oxide, a colorless to pale yellow liquid used in wood preservation
• Triphenyltin acetate, an off-white crystalline solid, used as an insecticide and a fungicide
• Triphenyltin chloride, a white crystalline solid, used as a biocide and an intermediate in chemical synthesis
• Trimethyltin chloride also a biocide
• Triphenyltin hydroxide, an off-white powder, used as a fungicide and to sterilize insect
• Fenbutatin oxide, a very stable white crystalline solid, for control of mites
• Azocyclotin, a colorless crystalline solid, used as a long-acting acaricide for control of spider mites on plants
• Cyhexatin, a white crystalline solid, used as an acaricide and miticide
• Hexamethylditin used as an intermediate in chemical synthesis
• Tetraethyltin, boiling point 63–65° /12 mm is a catalyst ^[3]

Hypercoordinated stannanes

Unlike their carbon analogues, tin compounds can also be coordinated to five atoms instead of the regular four. These hypercoordinated compounds usually have electronegative substituents for stabilization. In 2007 a room-temperature stable (in argon) all-carbon pentaorganostannane was reported ^[4] as the lithium salt with this structure:

In this distorted trigonal bipyramidal structure the carbon to tin bond lengths (2.26Å apical, 2.17Å equatorial) are larger than regular C-Sn bonds (2.14Å) reflecting its hypervalent nature.

See also

References