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.
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 R4−nSnCln 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 Organotin hydrides have the formula R4−nSnHn 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
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 SnIV−O−SnIV is called a stannoxane (which is a tin analogue of ethers), and the group SnIV−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:
2 R3SnOH ⇌ R3SnOSnR3 + H2O 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+.
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. (CH3)2SnCl2(bipyridine). 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) all-carbon pentaorganostannate(IV) was reported as the lithium salt with this structure:
In this distorted trigonal bipyramidal structure the carbon to tin bond lengths (2.26 Å apical, 2.17 Å equatorial) are longer than regular C-Sn bonds (2.14 Å) reflecting its hypercoordinated nature.
Triorganotin cations Some reactions of triorganotin halides implicate a role for R3Sn+ 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 R3Sn, are called stannyl radicals. 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 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, compounds of tin(II) might be expected to form a tin analogues of alkenes with a formal double bond between two tin atoms (R2Sn=SnR2) or between a tin atom and a carbon group atom (e.g. R2Sn=CR2 and R2Sn=SiR2). Indeed, compounds with the formula R2Sn=SnR2, called distannenes or distannylenes, which are tin analogues of ethylenes R2C=CR2, 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 SnR2, tin analogues of carbenes CR2 are also known in a few cases. One example is Sn(SiR3)2, where R is the very bulky CH(Si(CH3)3)2. Such species reversibly dimerize to the distannylene upon crystallization:
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