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.^[1] 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.^[2]

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.^[3]

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.^[4]

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).^[5] 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.^[6] 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⁺.

• 
  Idealized structure of trimeric diorganotin oxide.
• 
  Ball-and-stick model for (((CH₃)₃C)₂SnO)₃.
• 
  Structure of diorganotin oxide, highlighting the extensive intermolecular bonding.

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.^[6] The organotin halides for adducts, e.g. (CH₃)₂SnCl₂(bipyridine).

The all-organic penta- and hexaorganostannates(IV) have even been characterized,^[7] while in the subsequent year a six-coordinated tetraorganotin compound was reported.^[8] A crystal structure of room-temperature stable (in argon) all-carbon pentaorganostannate(IV) was reported as the lithium salt with this structure:^[9]

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.

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.^[10]

Tin radicals, with the formula R₃Sn, are called stannyl radicals.^[2] 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.^[11]

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:^[12]

2 R₂Sn ⇌ R₂Sn=SnR₂

Stannenes, compounds with tin-carbon double bonds, are exemplified by derivatives of stannabenzene. Stannoles, structural analogs of cyclopentadiene, exhibit little C-Sn double bond character.

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-type cluster and a prismane. These cages contain Sn(I) and have the formula [Sn(C₆H₃-2,6-Et₂)]_n where n = 8, 10 and Et stands for ethyl group.^[13] A stannyne contains a tin atom to carbon group atom triple bond (e.g. R−Sn≡C−R and R−Sn≡Si−R), and a distannyne a triple bond between two tin atoms (R−Sn≡Sn−R). 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.^[14]

Organotin compounds can be synthesised by numerous methods.^[15] Classic is the reaction of a Grignard reagent with tin halides for example tin tetrachloride. An example is provided by the synthesis of tetraethyltin:^[16]

4 CH₃CH₂MgBr + SnCl₄ → (CH₃CH₂)₄Sn + 4 MgClBr

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):

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.^{[2]: 45–47}

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 halide) or crown-complexed alkali metal salt catalyst. Lewis acids or an ionic solvent may also promote the reaction.^{[2]: 51–52}

The mixed organo-halo tin compounds can be converted to the mixed organic derivatives, as illustrated by the synthesis of dibutyldivinyltin:^[17]

Bu₂SnCl₂ + 2 CH₂=CHMgBr → Bu₂Sn(CH=CH₂)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:^[18]

2 Bu₂SnCl₂ + Li[AlH₄] → 2 Bu₂SnH₂ + Li[AlCl₄]

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.^[19]

Alternatively, stannides attack organic electrophiles to give organostannanes, e.g.:^[2]: 49

LiSnMe₃ + CCl₄ → C(SnMe₃)₄ + LiCl.

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; the major exception being tin acetylides.^[20] An organostannane addition is nucleophilic addition of an allyl-, allenyl-, or propargylstannanes to aldehydes and imines,^{[citation needed]} whereas hydrostannylation conveniently reduces only unpolarized multiple bonds.^[21]

Organotin hydrides are unstable to strong base, disproportionating to hydrogen gas and distannanes.^[2]: 295 The latter equilibrate with the corresponding radicals only in the continued presence of base, or if strongly sterically hindered.^{[2]: 299, 334–335} Conversely, mineral acids cleave distannanes to the organotin halide and more hydrogen gas.^[2]: 300

In "pure" organic synthesis, the Stille reaction is considered is a key coupling technique. In the Stille reaction, sp²-hybridized organic halides (e.g. vinyl chloride CH₂=CHCl) catalyzed by palladium:

R¹−X + R²−Sn(R³)₃ Pd catalyst———→ R¹−R² + X−Sn(R³)₃

Organotin compounds are also used extensively in radical chemistry (e.g. radical cyclizations, Barton–McCombie deoxygenation, Barton decarboxylation, etc.).

An organotin compound is commercially applied as stabilizers in polyvinyl chloride. In this capacity, they suppress degradation by removing allylic chloride groups^{[clarification needed]} and by absorbing hydrogen chloride. This application consumes about 20,000 tons of tin each year. The main class of organotin compounds are diorganotin dithiolates with the formula R₂Sn(SR')₂. 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, and transesterification.^[2]

n-Butyltin trichloride is used in the production of tin dioxide layers on glass bottles by chemical vapor deposition.

"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. Triphenyltin derivatives are used as active components of antifungal paints and agricultural fungicides. Other triorganotins are used as miticides and acaricides. Tributyltin oxide has been extensively used as a wood preservative.^[2]

Tributyltin compounds were once widely used as marine anti-biofouling agents to improve the efficiency of ocean-going ships. Concerns over toxicity^[22] of these compounds (some reports describe biological effects to marine life at a concentration of 1 nanogram per liter) led to a worldwide ban by the International Maritime Organization. As anti-fouling compounds, organotin compounds have been replaced by dichlorooctylisothiazolinone.^[23]

• Organotin compounds
• 
  Tetrabutyltin colorless oil, precursor to the other butyl-tin compounds
• 
  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 highly toxic white solid, used as a biocide
• 
  Trimethyltin chloride, a toxic white solid, once used as a biocide
• 
  Triphenyltin hydroxide, an off-white powder, used as a fungicide
• 
  Azocyclotin, a white solid, used as a long-acting acaricide for control of spider mites on plants
• 
  Cyhexatin, a white solid, used as an acaricide and miticide
• 
  Hexamethylditin used as an intermediate in chemical synthesis
• 
  Tetraethyltin, boiling point 63–65° /12 mm^{[clarification needed]} is a catalyst.^[24] The "Et" symbol stands for ethyl group.

The toxicities of tributyltin and triphenyltin derivative compounds are comparable to that of hydrogen cyanide. Furthermore, tri-n-alkyltins are phytotoxic and therefore cannot be used in agriculture. Depending on the organic groups, they can be powerful bactericides and fungicides. Reflecting their high bioactivity, "tributyltins" were once used in marine anti-fouling paint.^[2]

In contrast to the triorganotin compounds, monoorgano, diorgano- and tetraorganotin compounds are far less dangerous,^[4] although DBT may be immunotoxic.^[25]

• Organostannane addition
• Tributyltin azide
• Carbastannatranes

• National Pollutant Inventory Fact Sheet for organotins
• Industry information site
• Organotin chemistry in synthesis