Organotin chemistry is the scientific study of the synthesis and properties oforganotin compounds orstannanes, which areorganometallic compounds containingtin–carbon bonds. The first organotin compound was diethyltin diiodide ((CH3CH2)2SnI2), discovered byEdward Frankland in 1849.[1] The area grew rapidly in the 1900s, especially after the discovery of theGrignard 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 formulaR4−nSnCln for values ofn 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 aspyridine. 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 ofhydrogen cyanide.[4]
Organotin hydrides have the formulaR4−nSnHn for values ofn 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.[5]
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 groupSnIV−O−SnIV is called astannoxane (which is a tin analogue ofethers), and the groupSnIV−O−H is also called a stannanol (which is a tin analogue ofalcohols).[6] Structurally simplest of the oxides and hydroxides are the triorganotin derivatives. A commercially important triorganotin hydroxide is theacaricidecyhexatin (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.[7] The simple tin geminal diols (R2Sn(OH)2, the tin analogues ofgeminal diolsR2C(OH)2) and monomeric stannanones (R2Sn=O, the tin analogues ofketonesR2C=O) are unknown. Diorganotin oxides (R2SnO) are polymers except when the organic substituents are very bulky, in which case cyclictrimers or, in the case where R isCH(Si(CH3)3)2dimers, withSn3O3 andSn2O2 rings. The distannoxanes exist as dimers with the formula[R2SnX]2O2 wherein the X groups (e.g.,chloride –Cl,hydroxide –OH,carboxylateRCO2−) 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 becoordinated to five and even six atoms instead of the regular four. These hypercoordinated compounds usually haveelectronegative substituents. Numerous examples of hypercoordinated compounds are provided by the organotin oxides and associated carboxylates and related pseudohalide derivatives.[7] The organotin halides for adducts, e.g.(CH3)2SnCl2(bipyridine).
The all-organic penta- and hexaorganostannates(IV) have even been characterized,[8] while in the subsequent year a six-coordinated tetraorganotin compound was reported.[9] A crystal structure of room-temperature stable (inargon) all-carbon pentaorganostannate(IV) was reported as thelithium salt with this structure:[10]
In this distortedtrigonal bipyramidal structure the carbon to tinbond lengths (2.26 Åapical, 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 forR3Sn+ intermediates. Such cations are analogous tocarbocations. They have been characterized crystallographically when the organic substituents are large, such as 2,4,6-triisopropylphenyl.[11]
Tin radicals, with the formulaR3Sn, are calledstannyl radicals.[2] They are a type oftetrel 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.[12]
Organotin(II) compounds are somewhat rare. Compounds with the empirical formulaSnR2 are somewhat fragile and exist as rings or polymers when R is not bulky. The polymers, calledpolystannanes, have the formula(SnR2)n.
In principle, compounds of tin(II) might be expected to form a tin analogues ofalkenes with a formaldouble bond between two tin atoms (R2Sn=SnR2) or between a tin atom and acarbon group atom (e.g.R2Sn=CR2 andR2Sn=SiR2). Indeed, compounds with the formulaR2Sn=SnR2, calleddistannenes ordistannylenes, which are tin analogues ofethylenesR2C=CR2, are known for certain organic substituents. The Sn centres in stannenes are trigonal. But, contrary to theC centres in alkenes which aretrigonal planar, the Sn centres in stannenes tend to be highlypyramidal.Monomeric compounds with the formulaSnR2, tin analogues ofcarbenesCR2 are also known in a few cases. One example isSn(SiR3)2, where R is the very bulkyCH(Si(CH3)3)2. Such species reversiblydimerize to the distannylene upon crystallization:[13]
Stannenes, compounds with tin-carbon double bonds, are exemplified by derivatives ofstannabenzene.Stannoles,structural analogs ofcyclopentadiene, 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 thecubane-type cluster and aprismane. These cages contain Sn(I) and have the formula [Sn(C6H3-2,6-Et2)]n wheren = 8, 10 and Et stands forethyl group.[14] Astannyne contains a tin atom to carbon group atomtriple bond (e.g.R−Sn≡C−R andR−Sn≡Si−R), and adistannyne a triple bond between two tin atoms (R−Sn≡Sn−R). Distannynes only exist for extremely bulky substituents. Unlikealkynes, theC−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.[15]
Organotin compounds can be synthesised by numerous methods.[16] Classic is the reaction of aGrignard reagent with tin halides for exampletin tetrachloride. An example is provided by the synthesis of tetraethyltin:[17]
The symmetrical tetraorganotin compounds, especially tetraalkyl derivatives, can then be converted to various mixed chlorides byredistribution reactions (also known as the "Kocheshkov comproportionation" in the case of organotin compounds):
A related method involves redistribution of tin halides withorganoaluminium 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 alkyliodide or anallyl halide) orcrown-complexed alkali metal salt catalyst.Lewis acids or anionic 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:[18]
The organotin hydrides are generated by reduction of the mixed alkyl chlorides. For example, treatment ofdibutyltin dichloride withlithium aluminium hydride gives thedibutyltin dihydride, a colourless distillable oil:[19]
TheWurtz-like coupling ofalkyl sodium compounds with tin halides yields tetraorganotin compounds.
Hydrostannylation involves the metal-catalyzed addition of tin hydrides across unsaturated substrates.[20]
Alternatively,stannides attack organic electrophiles to give organostannanes, e.g.:[2]: 49
Important reactions, discussed above, usually combine organotinhalides andpseudohalides withnucleophiles. All-alkyl organotin compounds generally do nothydrolyze except in concentratedacid; the major exception being tinacetylides.[21] Anorganostannane addition isnucleophilic addition of anallyl-,allenyl-, orpropargylstannanes toaldehydes andimines,[citation needed] whereashydrostannylation conveniently reduces only unpolarized multiple bonds.[22]
Organotin hydrides are unstable to strong base, disproportionating tohydrogen 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, organotin reactions are unpopular, because organotin wastes are difficult to separate from the desired product and toxic even in extremely low concentrations. Strategies to remove the wastes include forming insolubleiodides orfluorides or covalently affixing the tin compounds to a solid polymer surface.[23]
Nevertheless, theStille reaction is considered is a keycoupling technique. In the Stille reaction,sp2-hybridizedorganic halides (e.g.vinyl chlorideCH2=CHCl) catalyzed bypalladium:
Organotin compounds are also used extensively inradical chemistry (e.g.radical cyclizations,Barton–McCombie deoxygenation,Barton decarboxylation, etc.).
![Tetraethyltin, boiling point 63–65° at 12 mmHg is a catalyst.[24] The "Et" symbol stands for ethyl group.](/mt/?noimg=&dark=on&url=https%3a%2f%2fen.wikipedia.org%2fwiki%2fFile%3aTetraethyltin.svg)
Organotin compounds, mainly diorganotin dithiolates (formulaR2Sn(SR')2), stabilizepolyvinyl chloride during commercial fabrication. The plastic dehydrochlorinates and exhibits undesirable brittleness if heated unstabilized. The stabilizers work by reducingallylic chlorides to allylicmercaptans and absorbing catalytichydrogen chloride. This application consumes about 20,000 tons of tin each year.[2]: 384–385
Diorganotin carboxylates, e.g.,dibutyltin dilaurate, catalyze the formation ofpolyurethanes,vulcanization ofsilicones, and certaintransesterifications in industry.[2]
n-Butyltin trichloride feedschemical vapor deposition oftin dioxide layers duringglass bottle manufacture.
Trialkyltin compounds are strongbiocides. Tributyltin and triphenyltin derivatives are comparably toxic tohydrogen cyanide. Depending on the organic groups, trialkyltins can be powerfulbactericides andfungicides, but they arephytotoxic and therefore cannot be used in agriculture.
Tributyltins are e.g. 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 aremiticides andacaricides.Tributyltin oxide has been extensively used as awood preservative.[2]
Reflecting their high bioactivity, "tributyltins" were once used in marineanti-fouling paint.[2] Concerns[25] over off-target toxicity andbioaccumulation (some reports describe biological effects to marine life at a concentration of 1nanogram per liter) led to a worldwide ban by theInternational Maritime Organization. As anti-fouling compounds, organotin compounds have been replaced bydichlorooctylisothiazolinone.[26]
Monoorgano, diorgano- and tetraorganotin compounds are far less dangerous than triorganotin compounds,[4] althoughDBT may be immunotoxic.[27]
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