Organotin chemistry

Organotin chemistry is the scientific study of the synthesis and properties of organotin compounds or stannanes, which are organometallic compounds containing tin–carbon bonds. The first organotin compound was diethyltin diiodide ((CH₃CH₂)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.

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

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

Organotin chlorides have the formula R_4−nSnCl_n 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.

Organotin hydrides have the formula R_4−nSnH_n for values of n up to 3. The parent member of this series, stannane (SnH₄), 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 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 Sn^IV−O−Sn^IV is called a stannoxane (which is a tin analogue of ethers), and the group Sn^IV−O−H 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 cyhexatin (also called Plictran, tricyclohexyltin hydroxide and tricyclohexylstannanol), (C₆H₁₁)₃SnOH. 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 (R₂Sn(OH)₂, the tin analogues of geminal diols R₂C(OH)₂) and monomeric stannanones (R₂Sn=O, the tin analogues of ketones R₂C=O) are unknown. Diorganotin oxides (R₂SnO) are polymers except when the organic substituents are very bulky, in which case cyclic trimers or, in the case where R is CH(Si(CH₃)₃)₂ dimers, with Sn₃O₃ and Sn₂O₂ rings. The distannoxanes exist as dimers with the formula [R₂SnX]₂O₂ wherein the X groups (e.g., chloride –Cl, hydroxide –OH, carboxylate RCO₂−) 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 [(CH₃(CH₂)₃Sn)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 hypercoordinated compounds are provided by the organotin oxides and associated carboxylates and related pseudohalide derivatives. The organotin halides for adducts, e.g. (CH₃)₂SnCl₂(bipyridine).