The Synthesis, Characterization and Comparative Anticorrosion Study of Some Organotin(IV) 4-Chlorobenzoates

Introduction

Corrosion is an electrochemical reaction occured naturally and  can proceed by itself, therefore the corrosion cannot totally be stopped¹. Its reaction can normally only be controlled or slow down its reaction rate to decrease the corrosion process². One of the methods to slow down the corrosion process is by applying the inhibitor. Corrosion inhibitor is a chemical compounds which can protect or slow down the corrosion process^2,3.

The organotin compound are interesting mostly due to their significant effect on the biological activity^4,5.  Their biological activities are fundamentally determined by the number and the nature of organic groups bound to the central Sn atom⁶. The nature of the anionic groups seems acting only as a secondary factor^6,7. The current investigations on the coordinating properties of carboxylates toward organotin compounds have led to the isolation of some new organotin(IV) carboxylates and carboxylate derivatives which have shown some interesting biological activities such as antimicrobial^8-11, antitumor and anticancer^6,10-14, antifungal activity^7,9,15,16, antiplasmodial¹⁷ and the latest development of these compounds has led the new finding as new anticorrosion inhibitor^18-21, therefore the investigation of organotin(IV)  as possible anticorrosion is very challenging, has been and is still attracting much attention^18-21.

In the present work, we reported the anticorrosion inhibition study of some dibutyl, diphenyl- and triphenyltin(IV) 4-chlorobenzoates toward mild steel in DMSO-HCl solution.

Experimental

Materials and Characterization

All reagents used were AR grade. Dibutyltin(IV) dichlorides ([(n-C₄H₉)₂SnCl₂]), diphenyltin(IV) dichloride ([(C₆H₅)₂SnCl₂]), triphenyltin(IV) chloride ([(C₆H₅)₃SnCl]), 4-chlorobenzoic acid were obtained from Sigma, water HPLC grade, sodium hydroxide  (NaOH) and methanol (CH₃OH) were JT Baker products, and were used without further purification.

¹H and ¹³C NMR spectra were recorded on a Bruker AV 600 MHz NMR (600 MHz for ¹H and 150 MHz for ¹³C). All experiments were run in DMSO-D₆  at 298K. The number of runs used for ¹H experiments were 32 with reference at DMSO signal at 2.5 ppm, while the ¹³C were 1000-4000 scans with the reference  DMSO signal at 39.5 ppm.  IR spectra in the range of 4000-400cm^-1 were recorded on a Bruker VERTEX 70 FT-IR spectrophotometer with KBr discs. Elemental analyses (CHNS) were performed on Fision EA 1108 series elemental analyser. The UV spectra were recorded in the UV region and were measured using a UV- Shimadzu UV-245 Spectrophotometer. Measurements were performed in 1 mL quartz-cells. Solutions were prepared using methanol as the solvent with concentration of 1.0×10^-4M.

Preparation of Organotin(IV) Chlorobenzoates

The organotin(IV) chlorobenzoates used in this work were prepared based on the procedure previously reported^13-16,21,and was adapted from the work by Szorcsik et al.⁹. An example procedure in the preparation of dibutyltin(IV) dichlorobenzoate was as follows:

To 3.0383 g (0.01 mol) [(n-C₄H₉)₂SnCl₂] in 50 mL methanol was added 0.8 g (0.02 mol) NaOH. The reaction mixtures were stirred for about 45 minutes. Compound 2 was precipitated out as white solid, filtered off and dried in vacuo till they are ready for analysis and further reaction. The average yield was 2.3508 g (95 %).

To 0.37338 g (1.5 mmol) compound 2 in 50 mL of methanol was added with 2 mole equivalents of 4-chlorobenzoic acid (0.2505 g) and was refluxed for 4 hours at 60 – 70°C. After removal of the solvent by rotary evaporator, the produced compounds [(n-C₄H₉)₂Sn(4-OOCC₆H₄Cl)₂] were dried in vacuo until they are ready for analysis and further use for corrosion inhibition test. The average yields were more than ~ 90 %.

A similar procedure was also adapted in the preparation of diphenyltin(IV) and triphenyltin(IV) derivatives, [(C₆H₅)₂Sn(OOCC₆H₄Cl)₂] and [(C₆H₅)₃Sn(OOCC₆H₄Cl)], respectively. For triphenyltin(IV) only one mole equivalent of the 4-chlorobenzoic acid was used.

Corrosion inhibitor measurement

Mild steel hard roll plate types were cut in 2 x 1 cm² and were successively polished with abrasive paper starting from grid 240, 400, 600 and 800. After the surface of the mild steel were homogenous, the surface were thoroughly washed with distilled water, dilute HCl and finally with acetone. The surface area of each mild steel was then measured, weight out and stored in a vacuum desiccator until they were used.

The potentiostat used was ER644 Integrated Potentiostat eDAQ type. It has three electrode cells which is assembly consisted of a working electrode of mild steel (2 cm² exposed are), a saturated AgCl as a reference electrode and a platinum counter electrode. After being assembled, they were immersed in corrosive medium as the electrolyte and connected to potentiostat. The first step was the measurement of rate inhibition without inhibitor, all three electrodes were immersed for 10 minutes in the electrolyte. The potential was set up with scanning rate of 0.5 mV/s. The current change occurred were measured and recorded, the data obtained were then processed to determine the potentiodynamic graph (η against ln |j|) in which the current corrosion density (I_corr0) and corrosion potential (I_corri) were obtained using Tafel extrapolation method²¹. The similar way was used in the measurement of inhibition rate with inhibitor of organotin(IV) chlorobenzoate. The concentrations of inhibitor used were 0, 10, 20, 40, 60, 80 and 100 mg/L. The data in each measurement were recorded and then the percentages of inhibition efficiency (the corrosion rate) were calculated based on Equation 1.

where %IE: the percentage of inhibition efficiency; I_corr0: current before the addition of inhibitor; I_corri:  current after the addition of inhibitor

Results and Discussion

The synthesis of compounds of dibutyltin(IV) di-4-chlorobenzoate, [(n-C₄H₉)₂Sn(4-OOCC₆H₄Cl)₂] (3), diphenyltin(IV) di-4-chlorobenzoate [(C₆H₅)₂Sn(OOCC₆H₄Cl)₂] (6) and triphenyltin(IV) 4-chlorobenzoate, [(C₆H₅)₃Sn(4-OOCC₆H₄Cl)] (9), were successfully performed from their chlorides [(n-C₄H₉)₂SnCl₂] (1), [(C₆H₅)₂SnCl₂] (4) and  [(C₆H₅)₃SnCl] (7), respectively, where all of the reactions in all cases were performed via [(n-C₄H₉)₂SnO] (2), [(C₆H₅)₂Sn(OH)₂] (5) and [(C₆H₅)₃SnOH] (8) respectively similar to those previously reported^{13-16, 21}. The scheme 1 is an example of reaction step that occurred in the preparation of dibutyltin(IV) di-4-chorobenzate. The microanalytical data of all compounds synthesized are very good and all values obtained are in agreement to those of calculated values as shown in Table 1.

Figure 1: The scheme of preparative route of the dibutyltin(IV) di-4-chlorobenzoate  Click here to View figure

 

Table 1: The microanalytical data of the organotin(IV) compounds synthesized

Compound	Elemental analysis found (calculated)
	C	H
[(n-C4H9)2Sn(4–C6H4(Cl)COO)2] (3)	48.2 (48.5)	4.8 (4.8)
[(C6H5)2Sn(4–C6H4(Cl)COO)2] (6)	53.2 (53.4)	3.3 (3.1)
[(C6H5)3Sn(4–C6H4(Cl)COO)] (9)	57.5 (57.6)	3.8 (3.7)

Some spectroscopy techniques were used in the characterizations of the compounds synthesized. The ¹H and ¹³C chemical shifts of the compounds prepared are shown Table 2. A number of signals in the spectra recorded have been characterized carefully. The chemical shift (δ) of butyl protons attached to the tin metal appeared in the range of 0.92 ppm for Hδ up to 1.37-1.62  ppm for Hα and Hβ, and the carbons of butyl ligands are observed at position comparable with other similar compounds reported previously^14,21,23.  The chemical shift of phenyl protons attached to tin metal appeared in the range of 7.34 – 7.62 ppm, while the carbon of carboxyl group of all compounds as expected appeared in the region of 176 ppm^14,21,23. The carbon atoms of the phenyl ligand as also expected appeared in δ of 131 – 126 ppm, while the carbons in the chlorobenzoate derivatives appeared in δ range of 140 – 130 ppm close to the reported values of similar compounds^{14, 21}.

Table 2: ¹H and ¹³C spectra of the compounds synthesized

Compound	H in butyl or phenyl (ppm)	H in benzoate (ppm)	C in butyl, phenyl and benzoate (ppm)
[(n-C4H9)2Sn(4–C6H4(Cl)COO)2](3)	Hα & Hβ:1.37-1.62 (m); Hγ: 1.29 (m);  Hδ: 0.92 (t)	7.34-7.86 (m)	Cα: 21.3; Cβ: 26.6; Cγ: 25.9; Cδ: 14.2; C1: 174.9; C2: 139.3; C3 & C7: 129.7; C4 & C6: 128.6; C5: 125.1
[(C6H5)2Sn(4–C6H4(Cl)COO)2] (6)	H2 & H6 7.58 (d, 4H); H3 & H5  7.48 (t, 4H); H4 7.36 (t, 2H)	7.89 – 7.99 (m)	C1-6 (phen): 131.7-126.9; C7: 175.9; C8: 139.8; C9: 130.4; C10: 164.9; C11: 129.7; C12: 129.0; C13: 130.2
[(C6H5)3Sn(4–C6H4(Cl)COO)] (9)	H2 & H6 7.57 (d, 6H); H3 & H5  7.45 (t, 6H); H4: 7.31 (t, 3H)	7.81 – 7.89 (d)	C1-6 (phen): 131.1-126.2; C7: 175.4; C8: 139.3; C9 & C13: 130.1; C10 & C12: 128.7; C11: 128.2

Table 3: The characteristic and important IR bands of the organotin(IV) compounds (cm^-1) synthesized.

Compound	3	6	9	References
Sn-O	434.6	591.1	765.50	800-400
Sn-O-C	1029.2	1290.4	1243.3	1050-900
Sn-Bu	674.9	–	–	740-660
CO2 asym	1419.2	1596.7	1558.3	1600-1400
CO2 sym	1558.2	1690.3	1631.6	1700-1550
C-H aliphatic	2954 – 2860	–	–	2960 – 2850
Phenyl	–	1490.5; 725.7	1428.4; 729.4	1450, 730

 

Figure 2: The proposed structure of some compounds synthesized and the suggested numbering of carbons in each compound  Click here to View figure

 

The important FT-IR data and their assignments are presented in Table 3. The characteristic band of the starting materials (1, 4, 7) is the presence of strong stretching band of Sn-Cl bond at 390 – 310 cm^-1. In 1 for example, the Sn-Cl bond appeared at frequency of 334 .2 cm^-1. The other characteristic bands of this compound appear as stretching band from butyl ligands at 1069 cm^-1, and bending vibration of C-H aliphatic stretch of the butyl at frequency of 2956 – 2865 cm^-1. When compound 1 is converted to compound 2, the main stretching band of Sn–Cl disappeared and a new strong band at frequency of 417.4 cm^-1 appeared as one of the main stretching band. This band is characteristic for Sn–O bond in compound [(n-C₄H₉)₂SnO] (2). The stretching band due to the butyls and their bending vibrations are still appearing as expected although the frequencies have little bit shifted. The formation of dibutyltin (IV) dichlorobenzoate compounds, [(n-C₄H₉)₂Sn(OOCC₆H₄Cl)₂], (3) is confirmed by the strong asymmetric stretching bands of the carboxylates which occurred at ca. 1400 cm^-1 and the symmetric stretch at ca. 1600 cm^-1 as well as the present of Sn-O stretching of the acid at 435 cm^-1, and the appearance of these bands is confirming the success of the substitution reaction^13-16,21.

The λ_max of all the compounds obtained from the UV-Vis spectroscopy analyses is summarized in Table 4. It is clear that there was a shifting change in the l_max  for each compound in any steps of the reaction. For example, the compound 1 has l_max of 210.7 nm, while compound 2 has λ_max of  202.9 nm. This information gave an indication that there was a shift to a shorter λ_max value when the conversion of compound 1 to 2 took place. The wave-length shift to a shorter l_max  could occur due to either the solvent used in the measurement or the effect of an auxochrome of the ligand. However in this study, as the solvent used for all measurements was the same (methanol), the change in the λ_max that occured must be due to the auxochrome effect¹³. In the case of compound 1 and 2, there is an oxide group which has electron drawing effect bigger in compound 2 than that of chloride group in 1. As a result, the electron transition in 2 is hard to occur. Thus, the measured λ_max was getting shorter in compound 2 than in compound 1²⁴. Similar results are also observed for other changes as can be seen from Table 4. For example, in compound 3,  the electron drawing effect of 4–C₆H₄(Cl)COOH is less than chloride in 1, so the electron transition in this molecule will be easier (the energy required is less), thus producing longer λ_max, 285.6 nm.

Table 4: The λ_max of the UV-Vis Spectra of the Organotin(IV) Compounds

Compound	λmax (nm)
	π– π*	n- π	Benzene ring
[(n-C4H9)2SnCl](1)	–	210.7	–
[(n-C4H9)2SnO](2)	–	202.9	–
[(n-C4H9)2Sn(4–C6H4(Cl)COO)2] (3)	–	285.6	–
[(C6H5)2Sn(4–C6H4(Cl)COO)2] (6)	204.4	296.7	404.3
[(C6H5)3Sn(4–C6H4(Cl)COO)] (9)	207.2	301.5	406.9

In our previous studies on the antifungal, anticancer and anticorrosion activity of the compounds similar to  the compounds reported here^13-16,21, it has been shown that optimal activity of the antifungal, anticancer and anticorrosion has been associated with the number of carbon atoms of the ligand present in the organotin(IV) used^13-16,21,24, where in general, the derivative of triphenyltin(IV) carboxylate which contain 18 carbon atoms has the highest activity^13-16,21. The same phenomena interestingly werre also observed in this study.

As shown in Table 5, the derivatives of triphenyltin(IV) compounds showed the highest inhibition ability in the series, and the diphenyltin(IV) compound was stronger in inhibiting than that of dibutyltin(IV) compound. Thus the number of carbon atoms present as well as the type of the ligands has significant effect on the anticorrosion activity of the organotin(IV) compounds tested.

Table 5: The Percentage Inhibition Efficiency

Compounds	% IE
4-chlorobenzoic acid	-12.1
[(n-C4H9)2SnCl2]	21.9
[(n-C4H9)2SnO]	30.3
[(n-C4H9)2Sn(4–C6H4(Cl)COO)2] (3)	52.3
[(C6H5)2Sn(4–C6H4(Cl)COO)2] (6)	58.1
[(C6H5)3Sn(4–C6H4(Cl)COO)] (9)	64.3

 

It was also observed that the organotin(IV) chlorobenzoate compounds synthesized exhibited much higher corrosion inhibition compared to those of the ligands, starting materials and intermediate products. In this respect, our results are consistent with a well-known fact that many biologically active compounds become more active upon complexation than in their uncomplexed forms²⁶.

Furthermore, the ligand used indeed increased the corrosion activity as shown by their %IE value which was negative. This can be understood that the hydrogen ion of the acid in the medium used increasing the number of hydrogen ion due to ionization occurred to it, as a result the ligand increased the corrosion toward the steel.  According to Crowe²⁷ the actual biological activity of diorganotin compounds of the type RR’SnXY (R and R’ = alkyl or aryl; X and Y= anions) is determined solely by the RR’Sn²⁺ moiety.

Conclusions

The organotin(IV) chlorobenzoate compounds were successfully prepared and it has been shown from the discussion above that they have shown some promising result to be used as anticorrosion inhibitor. The fact that triphenyltin(IV) chlorobenzoate derivative have shown the highest anticorrosion ability was in line with other data relating to the number of carbon atom present in the compound and might also relate to the ability of phenyl ligand to draw electron from the metal center as a result the metal became more positive and attached strongly to the steel and protect the steel from corrosion in the corrosive medium. Further studies are being carried to find the best explanation for this phenomenom.

Acknowledgments

The authors are grateful to Directorate of Research and Community Services, Directorate General of Higher Education, The Ministry of Research, Technology and Higher Education, Republic of Indonesia that provide fund for this project to be undertaken through Hibah Kompetensi (Competitive Research Grant) Scheme 2015 with contract number 162/UN26/8/LPPM/2015, 30 March 2015 . Thanks also go to Prof. Bohari M. Yamin, Universiti Kebangsaan Malaysia for helping in doing microanalysis and Prof. Dr. Hasnah Osman of School of Chemistry, University of Sains Malaysia for NMR experimentation.

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