Organotin (IV) Compounds: Chemistry, Properties and Uses

Introduction

1.1 Tin Element

It is undeniably that many research and studies had been done in the field of organotin chemistry for the past half century. Sn which is symbol bared by tin which is called as stannum in Latin. It is placed on group 14 and period 5 in the periodic table with atomic number of 50. It has an electronic configuration of [Kr] 4d¹⁰ 5s² 5p². The table 1.1 below show properties of tin (Davies, et al., 2008).The chemical properties was shared similarity with germanium and lead. Moreover, tin form compounds which has +2 and +4 as its oxidation number with +4 has slightly higher stability. Hence, formation of tetrahedral tin atoms occur when four valence electrons involved in the sp³ hybridization.

Table 1.1: Tin Properties (Davies, et al., 2008)

Based on Tin Chemistry: Fundamentals, Frontiers and Application, it was stated that tin has the stable isotopes of ten and also with the highest number of any element in the periodic table of isotopes. Thus, tin exhibit vary characteristic of the mass spectra. From on the table 1.2 shown below, it appear that both isotopes of 117 and 119 with spin of ½ are used in the Nuclear Magnetic Resonance (NMR) spectroscopy (Davies, et al., 2008).

Table 1.2: Stable isotopes of Tin (Davies, et al., 2008. p. 3)

It is found that metallic tin occur in two states which is α-tin and β-tin. The β-tin exists as distorted cubic structure and function as electric conductor. Whereas, α-tin is formed when β-tin is converted slowly at temperature below 10°C. Besides that, α-tin exits as a diamond structure which have the properties as a semiconductor. The physical appearances of tin is white and possess inertness at ambient temperature. However, tin will only undergo oxidation to SnO₂ at 200°C since it is a malleable post transition metal (Davies, et al., 2008).

1.2 History of Tin

Tin has been discovered around 3500 BC and it is known as an element that strengthen copper by forming copper-tin alloy which actually begin as early as Bronze Age civilization. The tin present in surface of Earth is only approximately 2 ppm which is far lesser compared to zinc, copper and lead. The production of tin mainly from mining and smelting (Davies, 2004).

In 1849, gives the birth of the first organotin(IV) compound, diethyltin diiodide synthesized by Edward Frankland which ultimately set as a new era in the field of tin chemistry. Frankland basically studies the behavior of the reaction between ethyl iodide and zinc heated in a sealed tube. It was that time found out that the decomposition of iodide of ethyl is affected by tin at the range of temperature between 150°C to 200°C (Davies, 2004). Threfore, his research practically set as a seed for further studies and was further stimulated around 1949 when various application of tin was discovered.

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Truly structural changes of compounds between the solution and solid states plays a crucial role in the organotin(IV) chemistry and its applications. In early studies of the 60s and 70s, Mössbauer spectrosocopy was used to determine the sturcture of the complexes. Nevertheless, it is now been dominantly used with X-ray crstallography and ¹¹⁹Sn NMR due to better technology and resolution (Davies, 2004).

1.3 Preparation of Organotin(IV) Carboxylates

Organotin(IV) carboxylates’s formula is R_nSn(O₂CRËŠ)_4-n . The complexes can be produced by reacting organotin(IV) oxide or hyrdroxide with comparable carboxylic acid. (equation 1.1 to1.3)

R₃SnOH or (R₃Sn)₂O + RËŠCOOH R₃SnOOCRËŠ + H₂O (1.1)

R₂SnO + 2RËŠCOOR  R₂Sn(OOCRËŠ)₂ + H₂O (1.2)

RSn(O)OH + 3RËŠCOOH  RSN(OOCRËŠ)₃ (1.3)

Esterification reaction is accomplished between organotin(IV) oxides or hydroxides and carboxylic acids by azeotrophic dehydration. The above equations’ reactions will depend on the mixing of different mole ratio of acid and base with appropriate solvent. The side products, water was removed by Dean-Stark apparatus and molecular sieve.

Furthermore, organotin(IV) carboxylates can be prepared by reacting organotin(IV) chloride with metal carboxylates. (equation 1.4). Meanwhile, when a tin-carbon cleaved with carboxylic acid, it can also produce organotin(IV) carboxylates or cleaved with mercury(I) or mercury (II), or lead (IV) carboxylate as shown below (Davies, 2004). (equation 1.5-1.6)

R_nSnCI_4-n + 4-nRˊCOOM  R_nSn(OOCRˊ)_4-n + 4-nMCI (1.4)

R₄Sn + RËŠCOOH R₃SnOOCRËŠ +RH (1.5)

R₄Sn + RËŠCOOM  R₃SnOOCRËŠ +RM (1.6)

1.4Structures of Triorganotin(IV) Complexes

Basically the general formula for triorganotin(IV) complexes would be R₃SnX and it is widely studied. This is due to the higher biological activity of triorganotin(IV) complexes compared to diorganotin(IV) complexes. The R group highly influence the biocidal activities as it contains three Sn-C whereas their volatility and solubility is affected by the X group (Kizlink, 2001). The toxicity also decreases with a decrease of organic groups bind to the tin atom. However, reviews indicated that X group itself is active biologically and an increased of aqueous solubility will lead to increased of activity (Davies, et al., 2008).

Meanwhile, chelation of triorganotin monomer or polymer with a five coordination leads to a decreased of activities (Davies, et al., 2008). 1:1 molar ratio of carboxylic acid and triorganoitn(IV) base are used to prepare triorganotin(IV) complexes (IMTIAZ-UD-DIN and BADSHAH, 2010; Win, 2012).

There are two main types of structures possess by triorganotin(IV) complexes which is chain and discrete structures. Figure 1.1 shows frequent occuring stuctures of triorganotin(IV) complexes with different coordination geometries (Hadjikakou and Hadjiliadis, 2009).

Figure 1.1: Frequent occurring structures in triorganotin(IV) complexes (A-C) (Hadjikakou and Hadjiliadis, 2009. p. 236)

At figure 1.1, complexes A to C falls on the category of discrete structures. It is commonly found that Ar₃SnO₂CRËŠ exhibit discrete structures such as triphenyltin carboxylates. Complexes A shows possible dicarboxylate ligand that can form a linear polymer by bridging triorganotin(IV) groups. Complexes B is a structure that closely resemble to trigonal bipyramid containing two equivalent CO bonds with facial alkyl groups whereas complexes C shows tetrahedral geometry with two non-equivalent CO bonds. The axial sites for complexes A to C are all occupied by O-Sn-O electronegative substituents (Davies, et al., 2008).

1.5Tin Application

It is undeniably that tin compounds have contributed and played a crucial role in various fields such as its property as an anticancer agent, in vitro anti-bacterial, wood preservatives, pesticides etc (Davies, et al., 2008). The main focus of tin application would be its organotin complexes due to its biologically active compounds in potentially lowering cancerous cells. Therefore, tremendous focus have actually diverted to anti-cancer field though it’s still have a wide applications.

Table 1.3Industrial uses of organotin(IV) compounds (Omae, 2002)

1.5.1Agriculture

Triorganotin(IV) compounds are proven to be extremely useful in agricultural and industry as they act as fungicides, molluscides, acaricides ,biocides and pesticides based on the research and reviews conducted (Kizlink, 2001; Nath, et al., 2013). For examples, toxicity towards insects and mammals are contributed by trimethyltins whereas Gram-negative bacteria are affected by tri-n-propyltins. In addition, tri-n-butyltin and triphenyltin compounds are effective against fungi. Presence of triorganotins pose as a lethal to mosquitoes and their larvae.

Moreover,tributytin chloride act as an strong repellent for rodents in crops. Besides that, snails control is affected by both triphenyltin acetate and triphenyltin chloride as molluscicides. This will help to prevent schistosome infections in human (Piver, 1973). According to Kizlink studies, the presence of n-butyl, phenyl and cyclohexyl groups will greatly increase the biocidal activity (Davies, et al., 2008).

Tricyclohexyltin hydroxide and trineophenyltin oxide acts as a acaricides which are used on citrus and vegetable crops though they are not conditioned to resistance environment (Batt, 2006). Whereas triphenyltin(IV) hydroxide and triphenytltin(IV) acetate are used in high value crops when there is potential for the crops to be infected such as early blight, Alternaria solani (GUENTHNER, et al., 2000). Therefore, the common crops used for fungicides are potatoes, pecans etc.

With all the advantageous as biocides, ,little did people know that when triorganotin(IV) compunds are capable of adsorbed into the soil. This will eventually lead to contamination of surface water due to runoff (Okoro, et al., 2011).

1.5.2 Anti-cancer Activity

In 1965, platinum complexes which is known as cis-diamminedichloroplatinum or cisplatin that characterized anti-proliferative activity has been discovered by Rosenberg (Alama, et al., 2009). Since then, platinum (II) complexes has been used as anti-cancer agent (Lippard and Jamieson, 1999). Regardless of its success, conducted studies has shown that it possess side effects (Langer, et al., 2013). Therefore, a non-platinum metal complexes field has been been prompted to studied with continual investigation of new complexes as antitumor drugs.

Besides that, there has been many reviews and studies on organotin carboxylates due to its antitumor potential. Generally, triorganotin(IV) complexes had been widely known to possess superior activity than diorganotin deriavatives (Baul, et al., 2005; Ali, et al., 2011; Yip, et al., 2012). This rules apply to some of the activties and had been recognized as R₃SnL > R₂SnL₂ > RSnL₃. Literature had indicated that triphenyltin(IV) complexes have remarkable activty in in vitro antitumor against human mamary tumour (MCF-7) and colon carcinoma (WIDR) (Baul, et al., 2009).

It is also reported that the actvity of triphenyltin(IV) complexes are higher than other complexes with p-hydroxybenzoic acids. This is because of the high values of half inhibitory concentration (IC₅₀) lipoxygenase inhibition compared to organitins(IV) and also reference cisplastin. Besides that, the inhibition of lipoxygenase and the activity of anti-proliferative against smooth muscle tumor, a leiomyosarcoma cells are higher for triorganitin(IV) compounds compared to diorganotin(IV) compounds (Nu, Li and Li, 2014).

High lipophilic behaviour ,ability to penetrate cell membrane and promotion of binding to biological molecules due to its phenyls group of triorganotin(IV) carboxylates showed high cytotoxic activity further confirms its impressive cytotoxicity in vitro against human lung cell line and human hepatocellular carcinoma cell line (Ma, et al., 2014).

Furthermore, spontaneous disproportionation reactions in solution may undergone by triorganotin(IV) derivatives into di- and tetraorganotin(IV) derivatives. In the mean time, lost of alkyl or aryl group may happened in in vivo during interception of aromatase enzyme. Therefore, there is possibility to recognize diorganotin(IV) complexes might be the ultimatum of cytotoxic agent and pharmacokinetic considerations in relation to commonly observed triorganotin(IV) compounds’ high activity (Alama, et al., 2009).

The organotin carboxylates can be further studied by exploring the ligand of the carboxylic acid, coordination number of the central tin atoms which play a crucial role in determine factor of the anti-tumor activity or cytotoxicity properties (Hadjikakou and Hadjiliadis, 2009; Ding, et al., 2012; Thorpe, et al., 2013). In conclusion, simultaneous in researching leads to potential in discovery in designing new anticancer drugs which will greatly help in our advancement in the medical field.

1.5.3 Antifouling

Exploit of trioganotin(IV) compounds can be used as a biocidal agent in anti-fouling paints for ships. In fact, tri-n-butyltin oxide (TBTO) is the first organotin compound to be discover for its properties. The function of anti-fouling systems is to coat and paint ships that acts as protective layer in order to inhibit attachment of Chlamydomas sp. or acorn barnacles. Shipping industry will face a serious problem if marine fouling occur due to increased surface roughness and resistance in water. Therefore, a consumption of 40% of fuel is needed to maintain the normal speed (Omae, 2003).

Furthermore, triorganotin(IV) compounds are resistance towards corrosion on aluminum hulls which is why it is favoured. Besides that the ability of tributyltin oxide to mix with pain solvents as a colorless liquid can be used in many biocidal applications. However, tributyltin oxide is highly soluble in seawater and thus giving a short term protection. Whereas, triphenyltin(IV) fluoride has become a common antifungal paint due to its long life protection from algae and shells (Omae, 2003).

Even though both tributyltin oxide and triphenyltin(IV) fluoride made a good antifouling paint, studies had shown that they contribute contamination to the aquatic environment (Hartl, 2012). It was suggested that biocides release is caused by high pressure hosing activities in which the paint particles become attached with the sediments and also leaching (Konstantinou and Albanis, 2004)

1.5.4 Wood preservatives

It is undeniable that wood is a precious gift from the mother nature and serves as as a wide applicant in the world. Specific wood species are needed for construction due its superior physical, mechanical and aesthetically pleasing performance (Jusoh, 2012). However, not all wood species are durable for outdoor activities. Therefore, organotin(IV) compounds are set as a wood preservatives such as tributyltin(IV) oxide and tributyltin(IV) naphthenate.

Based on Kizlink research in year 2000, triorganotin(IV) compounds are found to inhibit the growth of mycelium from wood-destroying fungi Coniophora puteana, Serpula lacrymans and mould suspensions. It is reported that butyl group in n-alkyi chains contained by triorganotin(IV) compounds such as bis(tributyltin) oxide and tributyltin N, N-diethyldithiocarbamate is highly effective against fungi.

On the other hand, inhibitory activity of fungicides of triphenyltin(IV) compounds appear to be lower than tributyltin(IV) compounds. This studies indicated and proven that smaller molecular volume of R₃Sn have a better establisment to the site of inhibitory action compared to bigger molecular volume.

Whereas in the studies of Jusoh, it was suggested that Alstonia schlaris, Macaranga triloba and Hevea brasiliensis were preserved succesfully by organotin(IV) compounds. This results would be further supported by the binding of tin compounds with the wood cell from the FTIR spectra which serves as a preservations of treatability of the specific wood species.

1.5.5 Poly (vinyl chloride) (PVC) stabilizers

The degradation of poly (vinyl chloride) (PVC) or oxygenolysis occur at lower temperature of its processing temperature. Thus, conjugation of double bonds are established and the decomposition will release hydrogen chloride (HCI) which then be reduced by heat stabilizers through absorption (Arkis and Balkose, 2004; Wang, et al., 2014). The stabilizers are composed of mono or di-disubstituted organotin(IV) compounds which usually added to pipes, films and packing materials due to their thermal stability (Nu, Li and Li, 2014).

Common PVC stabilizers can be differentiated with their alkyl group of organotins which is methyl, butyl and octyl (Okoro, et al., 2011). Since the toxicity of monoorganotin stabilizers towards mammals are extremely low, they are widely used with an added advantage of low raw material cost. On the other hand, the efficiency of octyltins are lesser due to its lower tin content (Batt, 2006).

According to Songwon in 2013 by Tin Intermediates Selection Guide, the examples of organtin(IV) complex used as a heat stabilizers is butyltin(IV) mercaptide, dibutyltin(IV) dilaurate, monobutyltin tris(2-ethylhexanoate) and dioctyltin mercaptide. Tin mercaptides are found to be the most desired due to its high efficiency acting as a weak acid reacting with labile chloride sites on PVC. Besides that, solubility of the stabilizer and lubrication in polymer processing is enhanced by the high molecular weights and ester group in mercaptan ligands (Batt, 2006).

1.6 Other usage of Organotin(IV) compounds

Besides organotin(IV) compounds acts as an antibacterial and antifungal agent, it also has mothproofing properties to treated fabrics such as bis(tributyltin) oxide. This is due the advantage of being lacking of color and staining. Staphylococcus aureus, a bacteria found in hospital also can be controlled by chemical bis(tributyltin) oxide (Piver, 1973).

Diorganotin(IV) compounds are used as catalysts to in process of forming polyurethane and silicon elastomers (Okoro, et al., 2011). Dibutyltin(IV) compounds such as diibutytin(IV) dilaurate is responsible in controlling tapeworms for poultry such as Raillietina cesticillus.

1.7 Effects of Organotin(IV) compounds

Humans are affected by organotins consumption of contaminated food, waters from pipelines of PVC or inhalation from agricultural activities and industry processes. The symptoms includes liver damage, loss of weight and neurological disorder.

Birds are affected by their uptake of food in which their body structure, diet and metabolism of butyltin determines the accumulation in their tissues. Tri-n-butyltin oxide will greatly impact the immunological system and hormones activities of the birds.

Whereas the entrance of organotins to fishes is through uptake of water from the gills and diet factor. The toxic effects would be on the red blood cells, gills and liver in the presence of tributyltin(IV) compounds(Namiesnik, et al., 2013).

Another evidence was proved by studying chronic toxicity of organotin(IV) compounds by observing gastropods as subjects. It was found that gastropods exhibit imposex which can be defined as having male organs such as penis in a female species even with low concentration of tributyltins. This literally means that contamination of organotins greatly affects all biological living species (Omae. 2002)

1.8 Objectives

The main objectives of this research is to synthesis (4-amino-3,5-dichlorobenzoato)triphenyltin(IV) from the reaction between 4-amino-3,5-dichlorobenzoic acid with triphenyltin(IV) oxide.

The second objective of this research is to study the characterization of the synthesized complexes through determination of melting point, carbon, hydrogen, nitrogen elemental analysis, Fourier Transform Infrared Spectroscopy (FTIR) as KBR dics, ¹¹⁹Sn, ¹H and ¹³C Nuclear Magnetic Spectroscopy (NMR) with the means of quantitatively and qualitatively.

The third objective for this study is to determine the structure and coordination geometry of the tin moiety carboxylates. Molar ratio of the reactants, polarity of solvent, temperature of refluxing etc. will contribute to the effects of coordination number and binding mode of the ligand to the tin atom which will be investigated.