Bol. Soc. Chil. Quím., 47, 203-211 (2002) ISSN 0366-1644

Tetraazamacrocyclic Complexes of Tin(II) :
Synthesis Spectroscopy and Biological screening

ASHU CHAUDHARY, RATAN SWAROOP AND R. SINGH*

Department of Chemistry, University of Rajasthan, Jaipur - 302 004, India
E-mail : kudiwal@datainfosys.net, Fax : 91 (141) 519221

(Received: June 26, 2001 - Accepted: October 25, 2001)

ABSTRACT

Sixteen to eighteen membered tetraamide macrocyclic ligands N₄L¹ and N₄L² have been prepared by the condensation of 1,2-diaminoethane or 1,3-diaminopropane with phthalic acid in the presence of condensing reagents dicyclohexylcarbodiimide and 4- dimethylaminopyridine. On reduction, these macrocyclic ligands give new tetraazamacrocycles N₄L³ and N₄L⁴ which form complexes with tin(II) chloride. The macrocyclic ligands and their complexes have been characterized by elemental analysis, molecular weight determinations, conductance, IR and ¹H NMR spectral studies. The spectral data suggested hexacoordinated state for tin in these complexes. Conductivity data suggested that they behave as non-electrolytes. An octahedral geometry for these complexes has been proposed as the binding sites are the nitrogen atoms of the macrocycles. The formulation of the complexes of the type [Sn(N₄Lⁿ)Cl₂] has been established on the basis of chemical composition. The ligands and their complexes have been screened in vitro against a number of pathogenic fungi and bacteria to assess their growth inhibiting potential.

Keywords: Teraazamacrocycles, complexes, synthesis, Spectroscopy

RESUMEN

Se han preparado ligandos tetraaza macrociclicos de entre 16 a 18 minbros, por condensación de 1,2-diaminoethane o 1,3-diaminopropano con acido ftalico in presencia de de los reactivos condensantes diciclohexilcarbodimida y 4-dimetilaminopiridina. Reducción de estos macrociclos dio nuevos tetraaza macrociclos N₄L^{3 y} N₄L⁴ los cuales forman complejos con cloruro de Sn(II). Estos macrociclos y sus complejos han sido caracterizados por análisis elemental, determinaciones de pesois moleculares, conductividad y espectros IR y ¹ H NMR. La data espectroscopica sugiere que una hexacoordinación para el estaño en estos complejos. Los datos de conductividad indican que ellos se comportan como no electrolitos. Se propone una geometría octahedrica ya que los sitios de coordinación son los nitrógenos de los macrociclos. La formulación de los complejos como [Sn(N₄Lⁿ)Cl₂] se ha establecido a partir de los datos de la composición química. Los ligandos y sus complejos han sido usados, in vitro, para determinar su potencial inhibidor de crecimiento de hongos y bacterias.

Palabras claves: Teraazamacrociclos, complejos, síntesis, espectroscopia

INTRODUCTION

The tetraazamacrocyclic ligands and their metal complexes have attracted growing interest among the coordination chemists followed by many workers on the metal controlled template and metal free non-template synthesis of macrocyclic species¹. The metal ion directs the reaction preferentially towards cyclic rather than oligomeric or polymeric products². The study of synthetic macrocyclic compounds is a very important area of chemistry in view of their presence in many biologically significant naturally occuring metal complexes. Recently,. Adam and co-workers³ have reported the synthesis and characterization of certain mixed nitrogen, oxygen and sulphur donor macrocycles. The metal template synthesis found to direct the steric course of the condensation reaction resulting in ring closure⁵. The use of metal as template in such reactions has led to the synthesis of many metal complexes of macrocyclic ligands⁶. The successful design and synthesis of such ligands capable forming multinuclear complexes are of great interest for obtaining special effects in magnetic, optical and electrical properties.

Recent interests have been stimulated by the diagnostic and therapeutic medicinal applications of transition metal complexes of macrocyclic ligands. The amide macrocyclic complexes are of special interest since they can function as catalysts in many organic oxidation reactions. Lindoy and co-workers³ have made elegant studies on ligands design and metal ion recognition of tetraaza and mixed polyazamacrocyclic complexes. One of the most interesting aspects of polyaza macrocyclic complexes is that the ligand can be modified. Some transition metal(II) complexes of polyazamacrocyclic ligands containing coordinated secondary amino groups are chemically oxidized to metal(II) complexes containing a higher degree of unsaturation in the ligand⁷. Macrocyclic complexes are thermodynamically more stable and more selective metal ion binders than their open chain analogues. These facts have led to a large amount of research involving such systems⁸. Very recently, dramatic progress in the chemistry of tetraazamacrocyclic complexes in particular has been evident since these structural units are involved in a variety of catalytic, biochemical and industrial processes^9,10.

Mostly, the polyamide macrocyclic complexes formed on reduction with B₂H₆ or LiAlH₄ lead to the formation of their corresponding polyazamacrocycles, which may be used to construct various macropolycyclic systems. Keeping these facts in mind, we have synthesized and characterized the tetraazamacrocyclic complexes of tin(II) in which dicyclohexylcarbodiimide and 4-dimethylaminopyridine act as good condensing reagents for the condensation of 1,2-diaminoethane, 1,3-diaminopropane and phthalic acid. The present paper deals with the striking structural features, synthesis and appreciable biological applications of these complexes.

EXPERIMENTAL

All the glass apparatus used throughout the experimental work was fitted with quickfit interchangeable standard ground joints. It was dried in an electric oven at 130-140° C after cleaning with acid, alkali and water and then rinsed with acetone or rectified spirit. Prior to each experiment, it was cooled in a desiccator or assembled while hot using guard tubes packed with anhydrous calcium chloride as to protect it from the atmospheric moisture. Fractions were carried out on columns of varying lengths and fitted to a ratio head having a water condenser and a guard tube for mixing the reactants and sampling the compounds for analytical purposes. The experiments were carried out in weighing tubes and weighing pipettes for the solid and liquid, respectively. Sealed capillary tubes were used for the determination of melting points.

The chemicals including dicyclohexylcarbodiimide and 4-dimethylaminopyridine and phthalic acid (Fluka) were used as such. 1,2-Diaminoethane, 1,3-diaminopropane and LiAlH₄ were used as obtained from E. Merck. SnCl₂ (BDH) was used without further purification.

Synthesis of the Ligands
N₄L¹(3,4:11,12-Dibenzo-2,5,10,13-tetraoxo-1,6,9,14-tetraazacyclohexadecane)
The reaction is carried out in 2 : 2 molar ratios. Catalytic amount of 4-dimethylaminopyridine and appropriate amount of dicyclohexylcarbodiimide (1.3274g) in minimum amount of dichloromethane at 0° C were kept magnetically stirred in two nacked round bottom 100 ml flask. The reaction is followed by the addition of 1,2-diaminoethane (0.3866g) and phthalic acid (1.0681g) in dichloromethane. The resulting reaction mixture was stirred for about 10-12 hours at 0° C. The solid product was obtained by filtration and washing with dichloromethane. The white product thus obtained was recrystallised from benzene and dried in vacuo. Similarly N₄L² was prepared by reacting (1.0765g), dicyclohexylcarbodiimide, (0.3868g), 1,3-diaminopropane and (0.8662g), phthalic acid.

N₄L³ (1,6,9,14-Tetraazacyclohexadecane)
The reaction is carried out in 1 : 2 molar ratio. The ligand N₄L¹ (0.8765g) was dissolved in tetrahydrofuran and cooled at 0° C. Lithium aluminium hydride (corresponding to N₄L¹) in tetrahydrofuran was stirred for about 10 hours in an ice bath. The reaction mixture was stirred under reflux for 75 hours. After cooling it, 10 ml of water and 10 ml of 15% aqueous sodium hydroxide were added to the reaction mixture at 0° C. The solid material was filtered and the residue washed with tetrahydrofuran. The filtrate and tetrahydrofuran washings were concentrated under reduced pressure to give N₄L³ as liquid. Ligand N₄L⁴ was prepared analogous to the above ligand.

Synthesis of the Complexes
[Sn(N₄L³)Cl ₂] : The reaction is carried out in 1 : 1 molar ratio. To a methanolic solution of tin(II) chloride (0.7891g) was added a solution of N₄L³ (corresponding to SnCl₂) in methanol while stirring. The contents were stirred for about 10 hours. The resultant solid product was filtered off and washed with methanol and dried in vacuo.

In the same manner compound [Sn(N₄L⁴)Cl₂] was synthesized by using the ligand N₄L⁴ with SnCl₂.

Analytical Methods and Physical Measurements
The molecular weights were determined by the Rast Camphor Method¹². Conductivity measurements in dry dimethylformamide were performed with a conductivity Bridge type 305. Nitrogen and chlorine were estimated by Kjeldahl's¹³ and Volhard's¹³ method, respectively. Tin was estimated as tin-oxide gravimetrically¹⁴. Infrared spectra of the ligands and corresponding complexes were recorded in the range 4000-200 cm^-1 with the help of Nicolet - Magna FTIR-500 spectrophotometer as KBr pellets. ¹H NMR spectra were recorded in DMSO-d₆ using TMS as standard on a JEOL FX-90Q spectrometer.

RESULTS AND DISCUSSION

New series of sixteen to eighteen membered tetraazamacrocyclic ligands and their tin(II) complexes were derived by the condensation of phthalic acid with 1,2-diaminoethane or 1,3-diaminopropane in the presence of condensing reagents dicyclohexylcarbodiimide and 4-dimethylaminopyridine as shown in Fig. 1.

N₄L¹_, X=(CH₂)₂
N₄L²_, X=(CH₂)₃

Fig. 1

All the macrocyclic ligands and their complexes are coloured solids. These are freely soluble in dimethylformamide, dimethylsulphoxide and tetrahydrofuran. The complexes have sharp melting points. The ligands and their metal derivatives are stable at room temperature and are non-hygroscopic. Conductance value 12-24 ohm^-1 cm² mol^-1 in anhydrous dimethylformamide at 10^-3 M concentration show them to be non-electrolytes.(See Table I)

Table I. Physical properties and analytical data of ligands and their tin(II) complexes

Infrared Spectra

A comparative study of the IR spectra of the tetraamides, tetraazamacrocyclic ligands and their corresponding tin(II) complexes confirmed the formation of the macrocyclic complexes with the proposed coordination pattern. The ligands N₄L¹ and N₄L² show the absence of band corresponding to primary amino and hydroxy groups, which confirms their involvement in the formation of tetraamide macrocycles. The presence of four bands in the regions 1645-1710, 1488-1574, 1235-1287 and 624-676 cm^-1 is due to the amide I (n C=O), amide II (n C-N + d NH), amide III (d NH) and amide IV (n C=O) bands, respectively¹⁵. It provides a strong evidence for the presence of a closed cyclic product. A single sharp band observed for amide ligands N₄L¹ and N₄L² in the region 3170-3231 cm^-1 is due n (NH) of amide group¹⁶. Strong and sharp absorption bands appear in the regions 2800-3049 and 1402-1466 cm^-1 in all the complexes may be ascribed to the C-H stretching and bending vibrations, respectively¹⁷.

In the spectra of macrocyclic complexes [Sn(N₄L³)Cl₂] and [Sn(N₄L⁴)Cl₂] as compared to their analogous metal free tetraaza ligands the slight negative shift in the n (N-H) band was observed. It is ascribed to the coordinated N-H stretching vibration. The shift to lower frequency of n (N-H) mode, along with the appearance of a new band in the region 440-478 cm^-1 assignable to the n (Sn-N) vibrations suggested that the amide nitrogen is coordinated to the tin atom¹⁸. The Sn-Cl stretching vibrations of compounds have been assigned at 482-496 cm^-1 as reported earlier also¹⁹. The infrared spectral data of the tetraazamacrocyclic ligands and their complexes are recorded in Table II.

Table II. IR Spectral data (in cm^-1) of tetraamides, tetraazamacrocyclic ligands and their tin(II) complexes

¹H NMR Spectra

The bonding pattern in the resulting complexes has been further substantiated by the proton magnetic resonance spectra of the ligands and their respective tin derivatives. The chemical shift values have been recorded in Table III. The ¹H NMR spectra of the amide ligands, tetraazamacrocycles and their macrocyclic complexes reveal the signals expected for Fig. 1. In the spectra of all the complexes no band could be assigned for hydroxyl and amino groups, suggesting that the proposed macrocyclic complexes have been formed by the condensation. Broad signals exhibited by the ligands N₄L¹ and N₄L² at d 7.89 and 8.02 ppm, respectively, are due to the amide proton²⁰ (CO-NH). The signals attributed to the methylene protons d 3.45 and 3.52 ppm for N₄L¹ and N₄L² were also assigned. The multiplet of aromatic protons were observed at d 7.27-8.10 ppm in the spectra of the ligands and their complexes.

Table III. ¹H NMR Spectral data (d ppm) of tetraamides, tetraazamacrocyclic ligands and their tin(II) complexes

The ¹H NMR spectra of N₄L³ and N₄L⁴ do not show any signal assignable to amide protons. It suggests the reduction of carboxyl groups. A broad signal in the spectra of both these ligands and their complexes at d 6.17-6.34 ppm corresponds to the secondary amino protons (C-NH-C). Further, the ligands N₄L³, N₄L⁴ and their complexes gave a multiplet at d 3.13-3.29 corresponding to methylene protons (N-CH₂) of amine moiety. A multiplet in the region d 1.92-2.01 ppm may be attributed to the central methylene protons -C-CH₂-C- . All these facts clearly supporting the structures proposed in Fig. 1.

Biological Study
Antifungal Activity

The antifungal activity of tetraazamacrocyclic ligands and their tin complexes has been evaluated by Radial Growth Method²¹ using Czapek's agar medium having the composition, glucose 20g, starch 20g, agar-agar 20g and distil water 1000 ml. The principle involved in this technique is to 'poison' the nutrient medium with a fungitoxicant and then allowing a test fungus to grow on such a medium.

The test compound is incorporated in the potato-dextrose agar medium in requisite amount to give a certain concentration (50, 100 and 200 ppm) and thoroughly mixed by constant stirring. The medium is then poured into the petriplates and stored in a refrigerator. A disc of 7 mm of fungal culture of a specific age growing on solid medium is then cut with a sterile cork borer. The plates and flask containing media are incubated at specific temperature, favourable for the growth of the test fungus. Suitable checks are kept where the culture discs are grown under the same condition on the agar medium without any fungicide. Three replicates were used in each case. The colony diameter after four days is measured to evaluate the fungitoxicity. The percent inhibition was calculated as : %inhibition = (C-T)x100/C where C and T are the diameters of the fungus colony in control and test discs, respectively.

The pathogenic fungi used for these investigations are Fusarium oxysporum and Alternaria alternata.

Antibacterial Activity
The activity against bacteria was evaluated by the Inhibition Zone Technique²². In this technique sterilized hot nutrient agar having the composition peptone 5g, beef extract 5g, NaCl 5g, agar-agar 20g and distil water 1000 ml and 5 mm diameter paper discs of Whatman No. l were used. The agar medium is poured in the petriplates. After solidified, the petriplates are stored in freeze in inverted position. The solutions of the test compounds in methanol in 500 and 1000 ppm concentrations were prepared in which either the discs are dipped in solution of the test sample and placed on seeded plates or after placing the paper disc on seeded plates, required quantity of the test sample is pipetted on the disc. The petriplates having these discs on the seeded agar should first be placed at low temperature for two hours to allow for the diffusion of a chemical before being incubated at suitable optimum temperature (28±2°C) for 24-30 hours. After the expiry of the incubation period, clear zone of inhibition associated with treated disc was measured in mm.

The compounds were tested against Pseudomonas phaseolicola (-) and Escherichia coli (-).

Mode of Action
The chelation theory²³ accounts for the increased activity of the metal complexes. The chelation reduces the polarity of the metal atom mainly because of partial sharing of its positive charge with the donor groups and possible p electron delocalisation within the whole chelating ring. The chelation increases the lipophilic nature of the central atom which subsequently favours its permeation through the lipid layer of the cell membrane.

The degradative enzymes produced by the microorganism are important in host infection. The enzyme production is here intended to mean both, synthesis of the enzyme by the microorganisms and activity of the enzyme in the medium after it is produced. Since the metal complexes inhibit the growth of microorganism it is assumed that the production of enzyme is being affected and hence the microorganism is unable to utilize the food for it self or the intake of nutrients in suitable forms decreases and consequently the growth of microorganism is arrested, while higher concentration proves fatal. The higher concentration destroys the enzyme mechanism by blocking any of the metabolism pathway and due to the lack of availability of proper food, the organism dies.

The results of biological activity have been compared with the conventional fungicide, Bavistin and the conventional bactericide Streptomycin used as standards. The results achieved out of these studies have been enlisted in Tables IV and V in which the data indicated that the metal chelates are more active than their parent diamines as well as phthalic acid, metal salt, tetraamides and tetraazamacrocycles.

Table IV. Fungicidal screening data of starting materials, tetraamides, tetraazamacrocycles and their tin(II) complexes, percent growth inhibition after 4 days at 30±2°C

Table V. Bacterial screening data of starting materials, tetraamides, tetraazamacrocycles and their tin(II) macrocyclic complexes (percent growth inhibition after 24 hours 28±2° C, conc. in ppm)

ACKNOWLEDGEMENT

The authors are thankful to U.G.C., New Delhi for supporting this work.

REFERENCES

1. M. Shakir, S.P. Varkey and T.A. Khan, Indian, J. Chem., 34A, 72 (1995).         [ Links ]

2. R. Machida, E. Kimura and M. Kodama, Inorg. Chem., 22, 2055 (1983).         [ Links ]

3. K. R. Adam, M. Antalovich, D.S. Baldwin, L.G. Brigden, P.A. Duckworth, L.F. Lindoy, A. Bashall, M. McPartlin and P.A. Tasker, J. Chem. Soc. Dalton Trans, 1869 (1992).         [ Links ]

4. K.R. Adam, M. Antalovich, D.S. Baldwin, P.A. Duckworth, A.J. Leong, L.F. Lindoy, M. McPartlin, J. Chem. Soc. Dalton Trans, 1013 (1993).         [ Links ]

5. D. C. McCollum, L. Hall, C. White, R. Ostrandes, A.L. Rheingold, J. Whelan and B. Bosnich, Inorg. Chem., 33, 924 (1994).         [ Links ]

6. S. Chandra and R. Singh, Indian J. Chem., 34A, 1003 (1995).         [ Links ]

7. H.S. Mountford, D.B. MacQueen, A. Li, J.W. Otvas, M. Calvin, R.B. Frenkel, L.O. Spreer, Inorg. Chem., 33, 1748, (1994).         [ Links ]

8. R.M. Izatt, K. Pawlak, J.S. Bradshaw, R.L. Bruening, Chem. Rev., 91, 1721 (1991).         [ Links ]

9. E. Kimura, S. Wada, M. Shionoya, Y. Okazai, Inorg. Chem., 33, 770 (1994).         [ Links ]

10. E. Kimura, X. Bu, M. Shionoya, S. Wada, S. Maruyama, Inorg. Chem., 31, 4542, (1992).         [ Links ]

11. M. Shakir, S.P. Varkey and O.S.M. Nasman, Indian J. Chem., 35A, 671 (1996).         [ Links ]

12. A.I. Vogel, "A Text Book of Practical Organic Chemistry", 4^th Edition, Longmans, ELBS, London, p. 323 (1978).         [ Links ]

13. A.I. Vogel, "A Text-Book of Quantitative Inorganic Analysis," Longmans Green, ELBS, London, p., 302 (1991).         [ Links ]

14. A.I. Vogel, "A Text-Book of Quantitative Inorganic Analysis," Longmans Green, ELBS, London, p., 355 (1991).         [ Links ]

15. Z.A. Siddiqi and V.J. Mathew, Polyhedron, 13, 779, (1994).         [ Links ]

16. R.W. Hay and I. Praser, Polyhedron, 17, 1931 (1998).         [ Links ]

17. N.B. Colthup, L.H. Dally and S.E. Wiberley, "Introduction of Infrared and Raman Spectroscopy" (Academic Press, New York), 1964.         [ Links ]

18. S. Belwal and R.V. Singh, Bol. Soc. Chil. Quim., 42, 363 (1997).         [ Links ]

19. D.K. Dey, M.K. Saha and L. Dahlenburg. Indian J. Chem., 39A, 1177, (2000).         [ Links ]

20. M. Shakir and S.P. Varkey, Trans. Met. Chem., 19, 606, (1994).         [ Links ]

21. D. Singh, R.B. Goyal and R.V. Singh. Appl. Organomet. Chem., 5, 45 (1991).         [ Links ]

22. N. Fahmi, R.V. Singh, Bol. Soc. Chil. Quim., 41, 65 (1996).         [ Links ]

23. A. Kumari, R.V. Singh and J.P. Tandon, Phosphorus, Sulfur and Silicon, 66, 195 (1992).         [ Links ]