Tin is one of the earliest metals known and was used as a component of bronze from antiquity. Because of its hardening effect on copper, tin was used in bronze implements as early as 3,500 BC. Tin mining is believed to have started in Cornwall and Devon (esp. Dartmoor) in Classical times, and a thriving tin trade developed with the civilizations of the Mediterranean. However the lone metal was not used until about 600 BC. The last Cornish Tin Mine, at South Crofty near Camborne closed in 1998 bringing 4,000 years of mining in Cornwall to an end. The word "tin" has cognates in many Germanic and Celtic languages. The American Heritage Dictionary speculates that the word was borrowed from a pre-Indo-European language. The later name "stannum" and its Romance derivatures come from the lead-silver alloy of the same name for the finding of the latter in ores; the former "stagnum" was the word for a stale pool or puddle. Tin is metallic element which belongs to group IVA of the periodic table (Z = 50, A = 118.71) with electronic configuration of [Kr] 5s4d105p2 (Taddese and Shemelis, 2002).
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The element has a melting point of 231.9 Â°C and a boiling point of 2507 Â°C. Below the melting point it exists in three allotropic forms: grey tin (stable below 13 Â°C); white tin (the malleable and ductile metallic form); and, above 161 Â°C, the brittle form of Sn. Inorganic tin compounds, in which the element may be present in the oxidation states of +2 or +4 are used in a variety of industrial processes for the strengthening of glass, as a base for colors, as catalysts, as stabilizers in perfumes and soaps, and as dental anticariogenic agents. The oxides of tin are amphoteric, commonly forming stannous and stannite salts (oxidation state +2) and stannic and stannate compounds (oxidation state +4); the oxidation state +3 is reported to be unstable. From these compounds organotin compounds can be synthesized (EGVMS, 2002).
1.2 GENERAL METHOD FOR THE SYNTHESIS of ORGANOTIN COMPLEXES
The first organotin compound was prepared over 150 years ago. In 1849, in a paper devoted largely to the reaction which occurred when ethyl iodide and zinc were heated together in a sealed tube. Tin also effected the decomposition of iodide of ethyl at about the same temperature (150 Â°C to 200 Â°C); the iodide became gradually replaced by a yellowish oily fluid, which solidified to a crystalline mass on cooling: no gas was evolved either on opening the tube or subsequently treating the residue with water. This paper is often held to mark the first systematic study in organometallic chemistry (Rochow, 1966).
Frankland subsequently showed that the crystals were diethyltin diiodide (equation 1.1). In independent work, Löwig established that ethyl iodide reacted with a tin/sodium alloy to give what is now recognised to be oligomeric diethyltin, which reacted with air to give diethyltin oxide, and with halogens to give diethyltin dihalides (though using incorrect atomic weights, the compositions that he ascribed to these compounds are wrong). As an alternative to this so-called indirect route was devised by Buckton in 1859, who obtained tetraethyltin by treating tin tetrachloride with Frankland's diethylzinc.
This direct route was developed by Letts and Collie, who were attempting to prepare diethylzinc by reaction 1.2, and instead isolated tetraethyltin which was formed from tin which was present as an impurity in the zinc. They then showed that tetraethyltin could be prepared by heating ethyl iodide with a mixture of zinc and tin powder.
The indirect route was improved by Frankland who showed that the tin(IV) tetrachloride could be replaced by tin(II) dichloride which is easier to handle and reacts in a more controllable fashion
In 1900, Grignard published his synthesis of organomagnesium halides in ether solution. These reagents were much less sensitive to air than Frankland's solvent-free organozinc compounds, and they rapidly replaced and extended the scope of the zinc reagents as a source of nucleophilic alkyl and aryl groups. In 1903, Pope and Peachey described the preparation of a number of simple and mixed tetraalkylstannanes, and of tetraphenyltin, from Grignard reagents and tin tetrachloride or alkyltin halides, and reactions of this type soon became the standard route to alkyl- and aryl-tin compounds.
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This early work is summarised in Krause and von Grosse's Organometallische Chemie which was published first in 1937 and which described examples of tetraalkyl and tetraaryl-stannanes, and of the organotin halides, hydrides, carboxylates, hydroxides, oxides, alkoxides, phenoxides, R2Sn(II) compounds (incorrectly), distannanes (R3SnSnR3), and oligostannanes (R2Sn)n. Tin played a full part in the great increase of activity in organometallic chemistry which began in about 1949, and this was stimulated by the discovery of a variety of applications. Structural studies have always been prominent in organotin chemistry, and particularly the structural changes which occur between the solution and solid states. Mössbauer spectroscopy was extensively used during the 1960s and 1970s for investigating structures in the solid state, but it has now largely given place to X-ray crystallography and high resolution solid state tin NMR spectroscopy.
In 1962, Kuivila showed that the reaction of trialkyltin hydrides with alkyl halides
(hydrostannolysis) (equation 1.6) was a radical chain reaction involving short-lived trialkyltin radicals, R3Sn., in 1964, Neumann showed that the reaction with non-polar alkenes and alkynes (hydrostannation) (equation 1.7) followed a similar mechanism, and these reactions now provide the basis of a number of important organic synthetic methods.
Salts of the free R3Snâˆ’ anion and R3Sn+ cation have been examined by X-ray crystallography.The formation of short-lived stannylenes, R2Sn:, has been established, and by building extreme steric hindrance into the organic groups, long-lived stannylenes have been isolated, and stable compounds with double bonds to tin, e.g. R2Sn=CR"2, R2Sn=SiR"2, R2Sn=SnR"2, and R2Sn=NR" have been prepared. It is convenient to denote the number of valence electrons m, and the number of ligands n, by the notation m-Sn-n. For example the radical R3Snâ€¢ would be a 7-Sn-3 compound. A major development in recent years has been the increasing use of organotin reagents and intermediates in organic synthesis, exploiting both their homolytic and heterolytic reactivity. In parallel with these developments, organotin compounds have found a variety of applications in industry, agriculture, and medicine, though in recent years these have been circumscribed by environmental considerations. In industry they are used for the stabilization of poly(vinyl chloride), the catalysis of the formation of the polyurethanes, and the cold vulcanization of silicone polymers, and also as transesterification catalysts.
An overview of the principal groups of organotin compounds and their interconversions is given in Scheme 1.1, which deals mainly with tin(IV) compounds, and Schemes 1.2 and 1.3 which cover compounds related to tin(III) and tin(II) species, respectively. It should be emphasised that, particularly with respect to Scheme 1.3, some of the reactions shown are as yet known only for specific organotin compounds, and are not necessarily general reactions.
Products which result from the formation of a new tin-carbon bond are boxed in the Schemes. The four principal ways in which this can be accomplished are the reaction of metallic tin or a tin(II) compound with an organic halide, of an organometallic reagent RM (M = lithium, magnesium, or aluminium) with a tin(II) or tin(IV) halide, of a trialkyltin hydride with an alkene or alkyne, or of a triorganotin-lithium reagent (R3SnLi) with an alkyl halide.
The reaction which is most commonly used is that of a Grignard reagent with tin
tetrachloride; complete reaction usually occurs to give the tetraorganotin compound (Scheme 1.1). This is then heated with tin tetrachloride when redistribution of the groups R and Cl occurs to give the organotin chlorides, RnSnCl4-n (n = 3, 2, or 1) (the Kocheshkov comproportionation). Replacement of the groups Cl with the appropriate nucleophile X (HO-, RCO2 -, RO- etc.) then occurs readily to give the derivatives RnSnX4-n.
The Kocheshkov redistribution reaction between tetraorganotin compounds and tin tetrahalideis a special case of alkylation of tin by organometallic compounds but for each carbon -tin bond which is formed, one is lost.
Scheme 1.1 Organotin synthesis based on the Grignard and Kocheshkov reactions.
Scheme 1.2 Organotin synthesis based on reactions of SnH and SnM compounds.
Scheme 1.3 Routes to lower valence state organotin compounds.
1.6 Palladium metal
Palladium was discovered by William Hyde Wollaston in 1803. This element was named by Wollaston in 1804 after the asteroid Pallas, which was discovered two years earlier. Wollaston found palladium in crude platinum ore from South America by dissolving the ore in aqua regia, neutralizing the solution with sodium hydroxide, and precipitating platinum as ammonium chloroplatinate with ammonium chloride. He added mercuric cyanide to form the compound palladium cyanide, which was heated to extract palladium metal (Griffith, 2003).
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Palladium chloride was at one time prescribed as a tuberculosis treatment at the rate of 0.065g per day (approximately one milligram per kilogram of body weight). This treatment did not have many negative side effects, but was later replaced by more effective drugs (Kapa, 2004).
The Basic Chemistry of Organopalladium Compounds
The following six fundamental reactions of Pd complexes are briefly explained in order to understand how reactions either promoted or catalyzed by Pd proceed (Amatore, 1999) First, a brief explanation of the chemical terms specificto organopalladium chemistry is given:
1. Oxidative addition (OA)
2. Insertion (IS)
3. Transmetallation (TM)
4. Reductive elimination (RE)
5. Î²-H elimination
6. Elimination of Î²-heteroatom groups and Î²-carbon
It should be noted that sometimes different terms are used for the same process.
This situation arises from the fact that chemical terms specific to organometallic chemistry originate from inorganic chemistry, and these terms differ from the ones originating from organic chemistry.
1.6.1 'Oxidative' Addition
The 'oxidative' addition is the addition of a molecule X-Y to Pd(0) with cleavage of its covalent bond, forming two new bonds. Since the two previously nonbonding electrons of Pd are involved in bonding, the Pd increases its formal oxidation state by two units, namely, Pd(0) is oxidized to Pd(II). This process is similar to the formation of Grignard reagents from alkyl halides and Mg(0). In the preparation of Grignard reagents, Mg(0) is oxidized to Mg(II) by the 'oxidative' addition of alkyl halides to form two covalent bonds. Another example, which clearly shows the difference between 'oxidation' in organic chemistry and 'oxidative' addition in organometallic chemistry, is the 'oxidative' addition of H2 to Pd(0) to form Pd(II) hydride. In other words, Pd(0) is 'oxidized' to Pd(II) by H2.
According to the 18-electron rule, a stable Pd(0) complex with an electron configuration of the next highest noble gas is obtained when the sum of d electrons of Pd and electrons donated from ligands equals eighteen. Complexes which obey the 18-electron rule are said to be coordinatively saturated. Pd(0) forms complexes using d10 electrons (4d8 and 5s2). Coordinatively saturated complexes are formed by donation of electrons from the ligands until the total number of electrons reaches eighteen. This means that four ligands which donate two electrons each can coordinate Pd(0) to form a coordinatively saturated Pd(0) complex. In other words, the coordination number of Pd(0) is four. The oxidative addition occurs with coordinatively unsaturated complexes. As a typical example, the saturated Pd(0) complex, Pd(PPh3)4 (four-coordinate, 18 electrons) undergoes reversible dissociation in situ in a solution to give the unsaturated 14-electron species Pd(PPh3)2, which is capable of undergoing oxidative addition. Various Ïƒ-bonded Pd complexes are formed by oxidative addition. In many cases, dissociation of ligands to supply vacant coordination sites is the first step of catalytic reactions.
Oxidative addition is facilitated by higher electron density of Pd, and in general,
Ïƒ-donor ligands such as R3P attached to Pd facilitate oxidative addition. On the other hand, Ï€-acceptor ligands such as CO and alkenes tend to suppress oxidative addition. A number of different polar and nonpolar covalent bonds are capable of undergoing the oxidative addition to Pd(0). The widely known substrates are C-X (X = halogen and pseudohalogen). Most frequently observed is the oxidative addition of organic halides of sp2 carbons, and the rate of the addition decreases in the following order; C-I > C-Br >>> C-Cl >>> F. Aryl fluorides are almost inert (GRUSHIN, 2002).
Recently a breakthrough has occurred in the discovery of facile oxidative addition of sp2 C-C bonds by using electron-rich ligands such as P(t-Bu)3 or N-heterocyclic carbine ligands. Alkenyl, aryl halides, acyl halides and sulfonyl halides undergo oxidative addition. Diazonium salts and triflates, which undergo facile oxidative addition, are treated as pseudohalides. It should be pointed out that some Pd-catalyzed reactions of alkyl halides, and even alkyl chlorides are emerging, indicating that facile oxidative addition of alkyl halides is occurring.
The following compounds with H-C and H-M bonds undergo oxidative addition to form Pd hydrides. Reactions of terminal alkynes and aldehydes are known to start by the oxidative addition of their C-H bonds. The reaction, called 'orthopalladation', occurs on the aromatic C-H bond in 1.8 at an ortho position of such donor atoms as N, S, O and P to form a Pd-H bond and palladacycles. Formation of aromatic palladacycles is key in the C-H bond activation in a number of Pd catalyzed reactions of aromatic compounds. Some reactions of carboxylic acids and active methylene compounds are described as starting by oxidative addition of their acidic O-H and C-H bonds. Hydrogen ligands on transition metals, formed by oxidative additions, are traditionally, and exclusively, called 'hydrides', whether they display any hydridic behavior or not. Thus Pd(0) is oxidized to H-Pd(II)-H by the oxidative addition of H2 (KITAMURA, 2001).
Reaction of Grignard reagents with carbonyl groups can be understood as an insertion of an unsaturated C=O bond of the carbonyl groups into the Mg-carbon bond to form Mg alkoxide. Similarly, various unsaturated ligands such as alkenes, alkynes and CO formally insert into an adjacent Pd-ligand bond in Pd. The term 'insertion' is somewhat misleading. The insertion should be understood as the migration of the adjacent ligand from the Pd to the Pd-bound unsaturated ligand. The reaction below is called 'insertion' of an alkene to a Ar-Pd-X bond mainly by inorganic chemists. Some organic chemists prefer to use the term 'carbopalladation' of alkenes.
Organometallic compounds M-R and hydrides M-H of main group metals (M= Mg, Zn, B, Al, Sn, Si, Hg) react with Pd complexes (A-Pd-X) formed by oxidative addition, and the organic group or hydride is transferred to Pd by substituting X with R or H. In other words, alkylation of Pd or hydride formation takes place and this process is called transmetallation. The driving force of transmetallation is ascribed to the difference in electronegativity of two metals, and the main group metal M must be more electropositive than Pd for transmetallation to occur. The oxidative addition-transmetallation sequence is widely known. Reaction of benzoyl chloride with Pd(0) gives benzoylpalladium chloride and subsequent transmetallation with methyl tributyltin generates benzoylmethyl palladium (TIETZE,1996).
1.6.4 Reductive Elimination
Similar to 'oxidative', the term 'reductive' used in organometallic chemistry has a different meaning from reduction in organic chemistry. Reductive elimination is a unimolecular decomposition pathway, and the reverse of oxidative addition. Reductive elimination (or reductive coupling) involves loss of two ligands of cis configuration from the Pd center, and their combination gives rise to a single elimination product. In other words, coupling of two groups coordinated to Pd liberates the product in the last step of a catalytic cycle. By reductive elimination, both the coordination number and the formal oxidation state of Pd(II) are reduced by two units to generate Pd(0), and hence the reaction is named 'reductive' elimination. The regenerated Pd(0) species undergo oxidative addition again. In this way, a catalytic cycle is completed by a reductive elimination step. No reductive elimination occurs in Grignard reactions. Without reductive elimination, the reaction ends as a stoichiometric one. This is a decisive difference between the reactions of Pd complexes and main group metal compounds (Ozawa, 1980).
1.6.5 Î²-H Elimination (Î²-Elimination, Dehydropalladation)
Another termination step in a catalytic cycle is syn elimination of hydrogen from carbon at Î²-position to Pd in alkylpalladium complexes to give rise to Pd hydride (H-Pd-X) and an alkene. This process is called either 'Î²-hydride elimination' or 'Î²- hydrogen elimination'. Most frequently used is 'Î²-hydride elimination', because the Î²-H is eliminated as the Pd-hydride (H-Pd-X). The proper and unambiguous term of this process is 'dehydropalladation' in a cis manner. This is somewhat similar to a E1 or E2 reaction in organic chemistry, althought it is anti elimination.
Insertion of alkene to a Pd hydride to form alkyl palladium and elimination of Î²-H from the alkyl palladium are reversible steps. The Î²-H elimination generates an alkene. Both the hydride and the alkene coordinate to Pd, increasing the coordination number of Pd by one. Therefore, the Î²-H elimination requires coordinative unsaturation of Pd complexes. The Î²-H to be eliminated should be cis to Pd (KANEDA, 1979).
Alcohols are oxidized by Pd(II) species. In this case, carbonyl compounds are formed by the Î²-H elimination from the Pd alkoxides.
1.6.6 Elimination of Î²-Heteroatom Groups and Î²-Carbon
In addition to Î²-H, Î²-heteroatoms and even Î²-carbon are eliminated, although they are observed less extensively. Elimination of Î²-heteroatoms seems to be specific to Pd(II) complexes. When heteroatom groups (Cl, Br, OAc, OH, etc.) are present on Î²-carbon to Pd, their elimination with PdX takes place. Most importantly the Pd(II) species is generated by the elimination of the heteroatom groups. Thus Pd(II)- catalyzed oxidative reactions become possible. For example, HO-Pd-X, which is a Pd(II) species, is formed by the elimination of Î²-OH. No reductive elimination to give Pd(0) and HO-X occurs. Usually elimination of Î²-heteroatoms is faster than that of Î²-H (Larsen, 1994).
1.3 Classification and Uses of Organotins
Organotin compounds are classified as monosubstituted organotin compounds (RSnX3), disubstituted organotin compounds (R2SnX2), trisubstituted organotin compounds (R3SnX), and tetrasubstituted organotincompounds (R4Sn). In compounds of industrial importance, R is usually a butyl, octyl, or phenyl group and X, a chloride, fluoride, oxide, hydroxide, carboxylate, or thiolate. Organotin compounds have important applications in several areas and hence are made industrially on a large scale. Organotin compounds are now the active components in a number of biocidal formulations, finding applications in such diverse areas as fungicides, miticides, molluscicides, marine antifouling paints, surface disinfectants and wood preservatives. Information on the structures of organotin complexes continues to accumulate, and new applications of organotin compounds are being discovered in industry, ecology and medicine. In recent years, investigations have been carried out to test their anti-tumor activity and it has been observed that indeed several R2SnX2 and R3SnX species show potential as antineoplastic agents, but in many cases there is disappointingly low in vivo toxicity. So far, RSnX3 compounds have had a very limited application, but they are used as stabilizers in PVC films (Craig, 1986).
R2SnX2 compounds are commercially the most important derivatives, and are mainly used in the plastics industry, particularly as heat and light stabilizers in poly vinyl chloride (PVC) plastics to prevent degradation of the polymer during melting and forming of the resin into its final products. They are also used as catalysts in the production of polyurethane foams and in the room-temperature vulcanization of silicone rubbers (Smith, 1998).
Recent tests on their anti-tumour activity have revealed that several R2SnX2 adducts, as well as triorganotin species, show potential as antineoplastic and antituberculosis agents (Galani et al., 2003). R2SnX2 carboxylates are also reported to have antitumor activity (Narayanan, 1990).
R3SnX compounds exhibits a broad spectrum of biological activity which includes, fungicidal (George et al., 1998), antitumor (Davies, 2004), antifouling agents, in wood preservation and miticides (Choudhury et al., 2001), biocidal properties and are used as pesticides or insecticides in agriculture (Van der Kerk, 1975). The most important classes of these compounds are the tributyl-, tricyclohexyl-, and triphenyltin compounds. Tributyltin (TBT) is one of the most efficient and most toxic components added to antifouling paints. Its dispersion in the environment has caused serious deleterious effects on shellfish; even at very low concentrations (Bryan et al., 1991).
The environmental hazards associated with TBT have provoked regulation and restricted use of TBT-containing antifouling paints. Despite of the fact that direct anthropogenic input should have been (in principle) reduced, the decrease in contamination of ecosystems has not been clearly demonstrated in every area. Indeed, due to their weak degradability in anoxic sediments, butyltin compounds remain a potential source of contamination for aquatic environments (George et al., 1998).
Triphenyltin (TPhT) is used in agriculture as acaricide and fungicide (for example, Ph3SnOH and Ph3SnCl are known commercial fungicides) and, to a lesser extent, in antifouling paints. However, its toxic effects are not fully known (Blunden et al., 1986).
It is generally believed that the toxicity of organotin compounds increases with increasing size of the alkyl group of the organotin molecule and with increasing substitution of the tin atom (Craig, 1982). Previous studies on a wide variety of organisms showed that R3SnX compounds were more toxic than their R4Sn and RSnX3 counterparts. Various studies have shown that the replacement of a ligand (X) in such systems (R3SnX, R2SnX2 and R2SnClX) changes the toxicity effect of the organotin moiety. The coordination mode of the carboxylateorganotin group is usually monodentate, bridging bidentate or chelating bidentate. The tridentate coordination mode of the carboxylate group has also been observed in the diorganotin carboxylate compound. On the other hand, the coordination number and environment of the central tin atom can be easily controlled by using different functionalized carboxylic acids with additional oxygen, sulfur or nitrogen donor groups. Organic ligands with sulfur, nitrogen, oxygen and fluorine substituents have long been used to enhance the biological activity of organotin(IV) carboxylates. Also organotin compounds with such ligands have widely been tested for their possible use in cancer chemotherapy.
In general, the biocidical activity of organotin compounds is greatly influenced by the structure of the molecule and the coordination number of the tin atoms. Studies on organotin complexes containing carboxylate ligands with additional donor atom available for coordinating to tin atom have revealed that new structural types may lead to different activities. Various parameters have been tested to estimate relationships between molecular structure and biological activity, especially those that represent the hydrophobicity of the molecules. The toxicity effects of the organotin are strongly influenced by the R-groups (Nguyen et al., 2000).