Thermal Decomposition and IR Spectra of Mg(II) Compounds with Caffeine

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1 Thermal and IR Spectra of Mg(II) Compounds with Caffeine a S. C. MOJUMDAR, b M. MĚLNÍK, and C E. JONA a Institute of Inorganic Chemistry, Slovak Academy of Sciences, SK Bratislava uachmoju@savba.sk b Department of Inorganic Chemistry, Faculty of Chemical Technology, Slovak University of Technology, SK82 37 Bratislava c Department of Chemistry, University of Trenčín, SK2 32 Puchov Received 28 September 998 Thermogravimetry (TG), differential thermal analysis (DTA), and other analytical methods have been applied to the investigation of the thermal behaviour and structure of the compounds Mg(ac)223H 2 (/) Mg(Cl 2H 2 (II), Mg(Cl 2 H 2 (III) and 5H 2 (IV) (ac = СНзСОО", Clac = ССН 2 СОО", Cl 2 ac = ChCHCOO, Cl 3 ac = Cl 3 CCOO~, and caf = caffeine). Thermal decomposition of these compounds is a multistage process. The compositions of the complexes and the solid state intermediate and resultant products of thermolysis have been identified by means of elemental analysis and complmetric titration. The possible scheme of destruction of the complexes is suggested. Heating of the compounds first results in a release of water molecules. In complexes /, II, and III the loss of the volatile ligand occurs (on the TG curves) in one step ( 2caf or caf) and in complex IV in two steps ( caf, caf). The final product of the thermal decomposition was. The thermal stability of the complexes increases in the sequence: /, IV, II, III. Caffeine was coordinated to Mg(II) through the N(9) atom of its heterocyclic ring. IR data suggest a unidentate coordination of carboxylates to magnesium in complexes / IV. It is welldocumented that heterocyclic compounds play a significant role in many biological systems, especially ivdonor ligand systems being a component of several vitamins and drugs [, 2]. It is not surprising, therefore, that many authors have investigated heterocyclic compounds and also examined them as ligands in coordination compounds of several central atoms [3 3]. In order to enhance understanding of drug metal ion interactions, we have been studying the thermal properties of magnesium(ii) complexes with caffeine, which is known as an important component of biological systems. The revealment of the relationship between the structure and thermolysis of metal carboxylate complexes, the study of the influence of metal and ligand nature on the process of thermal decomposition are of a certain interest. This work is a continuation of previously reported studies [4 9] on the thermal and spectral properties of magnesium(ii) complexes with pyridine and substituted pyridines. This paper describes the preparation of complexes formed by the acetate, mono, di, and trichloroacetates with caffeine, along with thermal analyses and IR spectral investigation of prepared complexes. "Ух"Г I 3 EXPERIMENTAL Caffeine The complexes were prepared by treating caffeine (. mol, complexes I, IV and.5 mol, complexes II, III) with appropriate magnesium(ii) acetate or halogenoacetate (.5 mol) in hot ethanol solution. The solution was left to stand at room temperature. The fine microcrystals that precipitated were filtered off, washed with cold ethanol and dried at room temperature. The infrared spectra were obtained on Philips analytical PU98 FTIR spectrometer by using Nujol mulls in the v range 2 4 cm". The thermal decomposition was carried out on Paulik Paulik Erdey Derivatograph (Type OD 2, MOM Budapest) in air atmosphere by using a platinum crucible with Chem. Papers 53 (5) (999) 39

2 S. C. MOJUMDAR, M. MĚLNÍK, E. JONA Table. Analytical Data of the Complexes / IV Complexes í II III IV С tui(calc.)/% Wi(found)/% H N Mg The TG and DTA curves for Mg( 3H 2 are shown in Fig.. The TG curve for that complex indicates that it is stable at temperature up to 5 C, when there begins the slow decomposition to, as to the final product formed at 53 C. The TG curve shows two bendings at 29 C and 475 XI. They correspond to the presence of two intermediate decomposition products: Mg( and Mg(. The most probable thermal decomposition scheme is Mg( 3H C > Mg(ac)2 Mg( Mg( C > Mg(ac) C > a sample mass of mg in the temperature range 2 C. The rate of temperature increase of C min was chosen for all measurements. Elemental analyses (C, H, N) were carried out bymeans of an Erba 6 analyzer and the content of Mg was established by complmetric titration. The analytical data of the compounds / IV reported in Table show a good agreement between the experimental and calculated data. RESULTS AND DISCUSSION The thermal decomposition data of the compounds I IV are collected in Table 2. The complexes I IV are thermally relatively stable. Thermal decomposition of the compounds is the multistage process. The subsequent detachment of the ligands was observed. The final product was. The DTA curve for the complex (Fig. ) presents two thermic peaks at 6 C and 37 C corresponding to the loss of 3H 2 and 2caf, respectively, and a broad thermic maximum with the centre about 5 C corresponding to decomposition reactions of 2ac with simultaneous formation of. The TG and DTA curves for Mg(Cl 2H 2 are shown in Fig. 2. The TG curve for that complex indicates that it is stable at temperature up to 3 C, when there begins the slow decomposition to, as to the final product formed at 75 C. The TG curve shows two bendings at 2542 and 5 C. They correspond to the presence of two intermediate decomposition products: Mg(Cl and Mg(Cl. The most probable thermal decomposition scheme is Mg(Cl 2H C >Mg(Clac)2 Table 2. Thermal Data of the Complexes I IV Complex max C DTA Process Дга г % found (calc.) TG Released component Residue Mg( 3H 2 Mg(Cl 2H 2 Mg(Cl 2.H 2 5H (9.24) 66.4 (66.42) 8.2 (8.5) 44. (43.99) 3.6 (3.66) 39.4 (39.4) 4.4 (4.35) 6.5 (6.53) 23.5 (23.46) 23.5 (23.46) 3H 2 2caf 2H 2 2caf H 2 caf 2H 2 зн 2 caf caf 3 Chem. Papers 53 (5) (999)

3 COMPLEXES OF Mg(ll) WITH CAFFEINE ^v I ' \ 4^Ä:^Mg( ' x Q) 2 АО y JG / \ /DTA 6 =. 8 \ /Mg( \ / i. i. i. i erc 8 Fig.. TG and DTA curves of Mg( 3H 2. вгс Fig. 2. TG and DTA curves of Mg(Cl 2H 2. Mg(Cl Mg(Cl 25 5 C 5 75 C * Mg(Cl * The DTA curve for the complex (Fig. 2) presents two thermic peaks at 8 C and 3 ^C corresponding to the loss of 2H 2 and caf, respectively, and a broad thermic maximum with the centre about 65 C corresponding to decomposition reaction of 2Clac with simultaneous formation of. The TG and DTA curves for Mg(Cl 2 H 2 are shown in Fig. 3. The TG curve for that complex indicates that it is stable at temperature up to 6 C, when there begins the decomposition to, as to the final product formed at 65 C. The TG curve shows two bendings at 27 C and 44 C. They correspond to the presence of two intermediate decomposi Chem. Papers 53(5)39 34 (999) 3

4 S. C. MOJUMDAR, M. MĚLNÍK, E. JONA \ I г Mg(CI 2 ac)^caf) I 2 Д : 4l6 i v.,mg(ci 2 ^DTA / \ /TG с ) 8 / i i 2 4 в/ с 6 8 Fig. 3. TG and DTA curves of Mg(Cl 2 H 2. tion products: Mg(Cl 2 and Mg(Cl 2. The most probable thermal decomposition scheme is C > Mg(Cl3 Mg(Cl 2 H C >Mg(Cl C * Mg(Cl3 Mg(Cl 2 27 ~ 44Q C > Mg(Cl 2 Mg(Cl л/r^ni \ X), ~ л 2 y The DTA curve for the complex (Fig. 3) presents two thermic peaks at 65 C and 33 TJ corresponding to the loss of H 2 and caf, respectively, and an thermic maximum at 54 C corresponding to decomposition reaction of 2Cl 2 ac with simultaneous formation of. The TG and DTA curves for 5H 2 are shown in Fig. 4. The TG curve for that complex indicates that it is stable at temperature up to HO^C, when there begins the decomposition to, as to the final product formed at 75 TJ. The TG curve shows four bendings at 6 C, 225 C, 385 C, and 55 C. They correspond to the presence of four intermediate decomposition products: 3H 2,,, and. The most probable thermal decomposition scheme is 5H 2 Q ~ 6 Q q C > 6 C > Mg(Cl3 3H 2 3H C + 6 ~ Q C > > The DTA curve for the complex (Fig. 4) presents two thermic peaks at 2 C and 75^ corresponding to the loss of 2H 2 and 3H 2, respectively, and an thermic maximum with the centre about 25 C corresponding to decomposition reaction of caf. Further thermic and thermic peaks occur at 4 C and 7 C corresponding to the loss of remaining caf and 2Cl 3 ac, respectively, with simultaneous formation of. According to the anion ligands, the thermal stability of the complexes is increased as follows: /(ac), JV(Cl 3 ac), tf(clac), JJJ(Cl 2 ac). Heating of the compounds first results in a release of water molecules in one step in complexes I III and in two steps in complex IV. Caffeine was eliminated in complexes I III in one step and in complex IV in two steps ( caf, caf). The results of thermal decomposition of the complexes were compared with literature results of metal(ii) complexes with carboxylates and halogenocarboxylates [2 23]. The stoichiometry of thermal decomposition can also be influenced by the changes of experimental conditions [24 26]. The modes of the coordinated ligands in the complexes have been investigated by means of infrared absorption spectra. The most important infrared frequencies attributed to the vibrations of the complexes / IV are reported in Table 3. The absorption bands 32 Chem. Paoers 53 (S) П999

5 COMPLEXES OF Mg(ll) WITH CAFFEINE вгс 8 Fig. 4. TG and DTA curves of 5H 2. Table 3. Infrared Spectral Data ( > = 2 4 cm ) of Complexes I IV Assignment Caffeine / v\ II III IV v(co) v(cn) 7(CCC) Mg N 26, 24, , , , 228, 237 l^as(coo) Vs(COO) Acoo i/(c C) V(C H) ac v(ch) ring i/(oh) J(HOH) P(H 2 ) v() , , 758, , 8, , 37, , , , 76, , ,, , , , 943, , , 76, , 84, 94 32, 393 as = asymmetric, s = symmetric. v (OH) and i(hoh) which occur in the v range cm and cm"", respectively, confirm the presence of water of crystallization. The absorption bands which occur in the v range 6 cm (rocking and wagging stretching) and cm v (Mg O) confirm the presence of water as coordinated in the complexes [27]. The presence of water as water of crystallization and as coordinated water in the compounds is further supported by the thermal decomposition data. Carboxylate ions can coordinate to metal ions in a number of ways such as unidentate, bidentate (chelating) or bridging and there is an evidence of that fact in the IR spectrum. The analysis of COO~ group bands frequencies allowed on determination of parameter Дс = v {у as (COO )) P(v s (COO~)). The magnitude of Acoo has been used by Nakamoto [28] as a criterion of the way of carboxylate binding with metal ions. Calculated from the examined spectra values of Acoo are in the v range cm". These values and three bands (COO deformation) at cm of complexes I IV are in good accord with the literature data for Chem. Papers 53(5)39 34 (999) 33

6 S. C. MOJUMDAR, M. MELNIK, E. JÓNA unidentately bonded acetates structures [29]. The absorption bands which occur in the v range cm (v(mg N)) confirm the coordination of caffeine to Mg ion through the N(9) atom of its heterocyclic ring [3]. The crystal structure of metal(ii) complexes with caffeine also proved that the caffeine coordinates to metal ions through the N(9) atom of its heterocyclic ring [3]. Without Xray analysis, no definite structure can be described for the different components. However, the spectroscopic and analytical data available enable to predict the structures which are supported also by the thermal decomposition studies. Acknowledgements. The authors wish to thank the Ministry of Education of the Slovak Republic for financial support. REFERENCES. Jóna, E., Kubranová, M., Simon, P., and Mroziňski, J., J. Therm. Anal. 46, 325 (996). 2. Jóna, E., Sirota, A., Simon, P., and Kubranová, M., Thermochim. Acta 258, 6 (995). 3. Jóna, E., Sirota, A., Simon, P., Horvath, I., and Kubranová, M., Progress in Coordination and Organometallic Chemistry, Proc. Int. Conf. Smolenice, Slovakia. P Slovak University of Technology Press, Bratislava, Papánková, В., Mašlejová, A., and Simon, P., TER MANAL '97, Proc. Int. Conf. Belušské Slatiny, Slovakia. P. 48. CHTFSTU Press, Bratislava, Skoršepa, J. S., Gyoryová, К., and Mělník, M., J. Therm. Anal. 44, 69 (995). 6. Gyoryová, К. and Nour ElDien, F. A., TERM ANAL '97, Proc. Int. Conf. Belušské Slatiny, Slovakia. P. 62. CHTFSTU Press, Bratislava, Jóna, E., Hvastijová, M., and Kohút, J., J. Therm. Anal. 4, 6 (994). 8. Mělník, M., Sirota, A., Ondrejkovičová, L, Jóna, E., and Hudecová, D., Progress in Coordination and Organometallic Chemistry, Proc. Int. Conf. Smolenice, Slovakia. P. 25. Slovak University of Technology Press, Bratislava, Langfelderová, H., Macášková, L., Mělník, M., and Gažo, J., Z. Anorg. Chem. 445, 233 (978).. D'ascenzo, G., Ceipidor, U. В., Cardarelli, E., and Magri, A. D., Thermochim. Acta 3, 449 (975).. Mělník, M., Macášková, L., and Mroziňski, J., Polyhedron 7, 745 (988). 2. Mělník, M., Potočnak, L, Macášková, L., and Mikloš, D., Polyhedron 5, 259 (996). 3. Jóna, E. and Jamnický, M., J. Therm. Anal. 27, 359 (983). 4. Mojumdar, S. C., Jóna, E., and Mělník, M., TER MANAL '97, Proc. Int. Conf. Belušské Slatiny, Slovakia. P. 58. CHTFSTU Press, Bratislava, Mojumdar, S. C, Mělník, M., and Jóna, E., NOVTECH '97, Proc. Int. Conf. Žilina, Slovakia. P. 67. Juraj Matušik Press, Žilina, Mojumdar, S. C, Mělník, M., Jóna, E., and Enamullah, M., J. Bangladesh Acad. Sei., in press. 7. Mojumdar, S. С, Mělník, M., and Jóna, E., J. Anal Appl. Pyrolysis 48, (999). 8. Mojumdar, S. C, Mělník, M., and Valko, M., Pol. J. Chem. 73, 457 (999). 9. Mojumdar, S. C, Valko, M., and Mělník, M., Chem. Papers 52, 65 (998). 2. Allan, J. R. and Gavin, J. J. Therm. Anal. 8, 263 (98). 2. Mroziňski, J. and Heyduk, E., Thermochim. Acta 93, 85 (985). 22. Mělník, M., Mroziňski, J., and Holloway, C. E., J. Coord. Chem. 29, 29 (993). 23. D'ascenzo, G., Ceipidor, U. В., and Angelis, G. D., Thermochim. Acta 58, 75 (972). 24. Sramko, Т., Liptay, G., and Jóna, E., J. Therm. Anal. 2, 27 (977). 25. Masuda, Y., Thermochim. Acta 39, 235 (98). 26. Jóna, E., Sramko, Т., and Gažo, J., J. Therm. Anal. 6, 23 (979). 27. Deveto, G., Ponticelli, G., and Preti, C, J. Inorg. Nucl. Chem. 37, 635 (975). 28. Nakamoto, K., Infrared Spectra of Inorganic and Coordination Compounds. 2nd Edition, p. 8. Wiley Interscience, New York, Stoilova, D., Nikolov, G., and Balarev, K., Izv. Akad. Nauk SSSR, Ser. Khim. 9, 37 (976). 3. Ondrejkovičová, L, Drobuliaková, D., and Mělník, M., Current Trends in Coordination Chemistry, Proc. Int. Conf. Smolenice, Slovakia. P Slovak University of Technology Press, Bratislava, Mělník, M., Koman, M., and Glowiak, Т., Polyhedron 7, 767 (998). 34 Chem. Papers 53 (5) (999)