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Spectroscopic study on the complex formation of chromogenic bridged calixarenes with aliphatic amines
Journal of Molecular Structure 650 (2003) 39–44 www.elsevier.com/locate/molstruc
Spectroscopic study on the complex formation of chromogenic bridged calixarenes with aliphatic amines I. Mohammed-Zieglera,b,*, B. Poo´ra, M. Kubinyia,b, A. Grofcsika,b, A. Gru¨nc, I. Bitterc a
Chemical Research Center, Hungarian Academy of Sciences, P.O. Box 17, H-1525 Budapest, Hungary Department of Physical Chemistry, Budapest University of Technology and Economics, H-1521 Budapest, Hungary c Department of Organic Chemical Technology, Budapest University of Technology and Economics, H-1521 Budapest, Hungary b
Received 19 April 2002; revised 30 October 2002; accepted 22 November 2002
Abstract The complex formation between chromogenic capped calix 4 arene derivatives comprising indophenol indicator group(s), and aliphatic amines has been studied by UV/Vis spectroscopy. The equilibrium constants have been determined in ethanol, and—for one ligand—also in dimethylsulfoxide. The results have been interpreted in terms of various types of host– guest interactions and of steric effects. q 2003 Elsevier Science B.V. All rights reserved. Keywords: Calix[4]arene; Optical recognition; Aliphatic amine
1. Introduction Calixarenes, obtained in the condensation reaction of formaldehyde and p-substituted phenols, are often referred to, as the third generation of supramolecular receptors (after cyclodextrins and crown ethers). Calixarene derivatives supplied with ligating functions have been the subject of numerous studies, due to their potential applicability as active ligands in electrochemical and/or optical sensors (optodes) showing affinity to bind alkali/alkali earth metal ions or amines selectively [1 – 3]. Recently, calixarenes aroused the interest especially in the field of * Corresponding author. Address: Chemical Research Center, Hungarian Academy of Sciences, P.O. Box 17, 1525 Budapest, Hungary. Tel.: þ36-1438-4141; fax: þ36-1325-7554. E-mail address: [email protected] (I. Mohammed-Ziegler).
mimicking biological systems, such as enzymatic functions including enantioselective reactions and sensing [4 –6]. In our former works various types of chromogenic calixarene molecules have been synthesized and tested as potential ligands for optical sensors by spectroscopic measurements [7 – 11]. In the present paper two new chromogenic calix[4]arene derivatives 1a,b (Fig. 1.) prepared recently in our laboratory [17] and for comparison a calix[4]binaphto(crown) ether 1c [2] have been studied to elucidate their recognition properties toward aliphatic amines. The interaction between the chromogenic hosts and amine guests was associated with coloration due to the presence of indophenol indicator moiety inserted into the calixarene core. Thus, the process could be efficiently monitored by UV/Vis spectrophotometry and the equilibrium constants of the complex formation
0022-2860/03/$ - see front matter q 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0022-2860(03)00079-6
40
I. Mohammed-Ziegler et al. / Journal of Molecular Structure 650 (2003) 39–44
Fig. 1. Structure of the studied calix[4]arenes.
reactions could be calculated from the spectroscopic data obtained in different solvents in the presence of various aliphatic amines. Ligand 1c was already reported to recognize butylamines selectively [2], the preliminary results obtained with our ligands 1a and 1b [10,11] indicated a similar coloration process with amines. Besides UV/Vis spectroscopy, IR [12 – 14] and NMR [15,16] are the most widely used spectroscopic methods for studying phenol-amine type complexes in solution. In our laboratory the interaction of 1a and 1b with t-butylamine in MeOH-d4 and DMSO-d6 was investigated by 1H NMR spectroscopy [17] and unexpectedly, the NMR spectra did not indicate the formation of internal complexes. Instead, an endo/exo quinoide tautomerism of the indophenol moieties was established, and only a simple acid –base equilibrium accompanied by the coloration of solution was detected. It has to be noted, however, that monitoring the interaction of 1c with t-butylamine (reported complex formation constant 1090 dm3 mol21) [2] in an NMR tube, no observable sign of complexation was found except for signal broadening possibly due to the rapid tautomerisation of the indophenol moieties [17]. It is important to underline that for
NMR spectroscopic experiments much higher ligand and substrate concentrations are required than for UV/Vis spectroscopic measurements, since the latter method has significantly higher sensitivity. NMR spectroscopy with our substrates seemed not to provide unambiguous evidences for processes governed by weak intermolecular forces taking place in dilute solutions. The UV/Vis measurements were expected to be much more informative in this respect.
2. Experimental The UV/Vis measurements were performed on a Perkin – Elmer Lambda 2S spectrophotometer. The stoichiometry of the complexes were determined by Job’s method [18], 5 £ 1024 mol dm23 ethanol solutions of the reactants were used. Preliminary measurements with 1a,b,c showed that their complex formation with some amines is slow (takes 30 – 40 min), thus, the spectra were taken following this period. The complex formation constants for the individual calixarene –amine pairs, Kc ; were determined by the method of Benesi and Hildebrand [19],
I. Mohammed-Ziegler et al. / Journal of Molecular Structure 650 (2003) 39–44
from the absorption spectra of a series of solutions containing the calixarene ligand in the same (5 £ 1025 M) concentration and the amine in different concentrations. The concentration of ethylamine in the stock solution was determined by HPLC technique, adopting the method of Forlay-Frick et al. [20], developed originally for the determination of some amino acid derivatives (such as selenomethionin) and of some antibiotics. The HPLC measurements were carried out on a Millipore Waters 3000 equipment connected with a Supelco LC-18-08 column (with the length of 25 cm and the diameter of 5 mm) and a Millipore Waters 470 scanning fluorescent detector. The eluent contained 90% (v/v) acetonitrile and 10% (v/v) water, and 2% (v/v) of phosphoric acid was added to acidify the solvent mixture. The system was calibrated with a series of propyl-amine ethanol stock solutions of known concentration, assuming that the sensitivities
41
of the system for ethylamine and propyl-amine are identical.
3. Results and discussion The common feature of the spectra of ligands 1 ðlmax ¼ 520 – 534 nmÞ in the presence of amines is the appearance of a new band in the region of 652 – 667 nm (see Fig. 2). The quantitative evaluation of the spectral changes by the Job’s method applying various molar ratios indicated the formation of an adduct with 1:1 stoichiometry. The application of the method of Benesi and Hildebrand is shown in Fig. 2. The upper part (A) shows a series of spectra with the same initial concentration of calixarene 1a and with various initial concentrations of di-n-propyl-amine. The lower part (B) shows the Benesi – Hildebrand plot. Similar
Fig. 2. (A) Absorption spectra of 1a-di-n-propyl-amine systems in ethanol. Concentrations: ðc1a Þ0 ¼ 5 £ 1025 M in each solution, (camine)0 ¼ (a) 0 M, (b) 2.7 £ 1023 M, (c) 5.5 £ 1023 M, (d) 8.2 £ 1023 M, (e) 2.7 £ 1022 M, (f) 7.2 £ 1022 M. (B) Benesi–Hildebrand plot of the absorbance values at 664 nm.
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Table 1 The equilibrium constants of the complex formation of the ligands 1a,b,c with different aliphatic amines in ethanol and DMSO Ligand
Ethyl-amine
n-Propyl-amine
1a 1b 1c 1b in DMSO
436.3 234.7 951.8 –
46.4 78.7 103.9 87.7
n-Butyl-amine 18.7 62.6 155 (95.9 [2]) 71.3
t-Butyl-amine 138.7 109.8 1090 [2] 107.9
diagrams were obtained for other calixarenes and amines. The complex formation constants obtained this way are presented in Table 1. As will be shown, they suggest the formation of internal supramolecular complexes in the concentration range of these dilute solutions, and do not seem to be related to the acid– base equilibria, governing the composition of systems with higher calixarene and amine concentrations, studied by NMR measurements in Ref. [17]. As can be seen, the equilibrium constants for primary amines are generally higher, than those obtained for dipropylamine and for triethylamine. This supports the formation of supramolecular complexes in these dilute solutions, since the basicity of the three types of amines varies in the opposite order, therefore one would expect an opposite trend for the equilibrium constants in simple acid –base reactions. The most significant effects that seem to control the stability of the amine –calixarene complexes, are: (i) the numbers of hydrogen atoms in the amino group and (ii) steric effects. It is exactly true for calixcrown 1c, where the order of Kc values is in accordance with the reaction model suggested by Kubo et al. [2]. According to this model the complexation of 1c starts with the protonation of the amine guest by one of the hydroxyl groups in the calixarene host, resulting in the formation of an ammonium ion, which is simultaneously bound by the neighboring crown ring through hydrogen bonds. The complex, therefore, is stabilized by the ionic indophenolate– ammonium cation interaction and by the hydrogen bonds with the oxygen donor atoms in the crown ether moiety. Consequently, primary amines can form the most stable complexes (three hydrogen bonds), while tertiary amines are hardly bound. In addition, the complex stability is also affected by steric factors. In the series of n-primary amines the complex stability
3-Amino-propanol
Di-n-propyl-amine
Triethyl-amine
250.6 258.2 123.4 67.7
77.2 27.2 71.2 –
39.8 16.2 18.4 24.5
decreases with the increasing chain length of the alkyl group. At the same time, the highest Kc was measured for t-butylamine, which might be due to an additional binding factor via Me – p interaction between the t-Bu group and the naphthalene ring [2]. The considerations discussed above are not completely valid for capped calixarenes 1a,b. As can be seen in Table 1, the complex formation constants for primary amine complexes (except for ethylamine complexes) are substantially lower than those for 1c, indicating a much weaker binding. Besides, the order of the Kc values is not strictly controlled by the order of the amines. These differences may be attributed to the structural differences between 1c and 1a,b, in respect of the indicator group and the binding site (see Fig. 1). The indophenol moieties of compounds 1a,b exist in a stable endo-quinoide tautomeric form [17], which means that the phenolic OH group is far from the binding site, whereas both OH groups of 1c are close to the cavity. Consequently, the model of binding in case of 1a,b should be modified. Two consecutive processes are assumed (Fig. 3): (i) first the protonation of amine by one of the exophenolic OH affording exo-calix ion-pair (acid – base equilibrium, K1 ), (ii) rapid tautomerisation of the indophenolate resulting in the formation of an endo/ exo-quinoide structure followed by stabilizing the ammonium cation via H-bonds (complexation, K2 ). In this respect a question emerges: which electron donating groups in the carboxamide cap can accept the ammonium protons? Since the carbonyl groups are oriented out of the cavity [17], there remain the phenolether oxygens (and the quinone carbonyl of the other indophenol unit in 1a) to accept protons. Considering the tetrahedral geometry of the ammonium cation, in principle, primary amines can form both internal (A) and external (B) supramolecular complexes. Structure A comprising
I. Mohammed-Ziegler et al. / Journal of Molecular Structure 650 (2003) 39–44
43
Fig. 3. Scheme of the equilibria assumed.
three H-bonds should be more stable than complex B where only two H-bonds are utilized for the stabilization. Obviously, the two kinds of complexes can be in equilibrium with each other in which the proportion of the individual forms strongly depends on the bulkiness of the substituents. In the internal complexes (A), a severe steric repulsion should be accounted between the apical substituent of the amine and the upper part of the bridging unit. Of the amine series, the smallest ethylamine gave by far the highest Kc values with 1a (even with the sterically more crowded 1b), which may be attributed to the formation of such an internal complex, although we have no structural evidences to support it. From steric reasons the other amines with ligands 1a and 1b can form at most external complexes (B) of much lower stability (two hydrogen bonds). Since the two coloration processes cannot be separated spectroscopically (the different indophenolate– ammonium ion-pairs can not be distinguished), they appear as
a single equilibrium where K1 (acid – base equilibrium) and K2 (complexation) may be involved in the measured constants with unknown proportions. Due to the weak interactions, their contribution to the K values can be comparable and so not allowing deeper conclusion on the structural feature of complexes. It should be pointed out, however, that the presence of hydroxyl group in 3-aminopropanol increases the equilibrium constant of complex formation with carboxamide bridged ligands 1a,b compared to that of propylamine, but not in the case of the crown bridged calix[4]arene 1c. Additional interaction between the carboxamide oxygen and the hydroxyl group (in the amine) may occur resulting in a significant enhancement of complex stability. In contrast, the hydroxyl group does not seem to interact with the etheric oxygens of 1c. In order to compare the complex formation in a protic and in an aprotic solvent, analogous measurements have been carried out with 1b– amine systems
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in dimethylsulfoxide. As apparent from Table 1, in the majority of cases, the equilibrium constants obtained with the same amine in the two solvents do not differ significantly. However, much lower stabilities were obtained with 3-aminopropanol in DMSO than in ethanol. This observation is in accord with the more polar and nucleophilic character of DMSO, which much easily disrupt the intramolecular OH· · ·OC bond than the less polar and self-solvated ethanol. In conclusion, the results indicate that calix[4]arenes capped by diamide bridges form strongly polar supramolecular complexes with various types of amines. Of the calixarenes studied, 1b may be worth testing in an analytical sensor for the detection of aminoalcohols. Acknowledgements The authors are grateful to the Hungarian Scientific Research Found for financial support (contract number T 025561). I. M.-Z. expresses her gratitude to the Varga Jo´zsef Foundation for financial support. Special thanks to Mr Zolta´n Bala´zs Nagy and Mr Pe´ter Forlay-Frick for the technical help in performing HPLC measurements.
References [1] H.M. Chawla, K. Srinvas, J. Chem. Soc. Chem. Commun. (1994) 2593. [2] Y. Kubo, S. Mruyama, N. Ohhara, M. Nakamura, S. Tokita, J. Chem. Soc., Chem. Commun. (1995) 1727. [3] Q.Y. Zheng, C.F. Chen, Z.T. Huang, Tetrahedron 53 (1997) 10345.
[4] Y. Kubo, S. Maeda, S. Tokita, M. Kubo, Enantiomer 2 (1997) 287. [5] Y. Kubo, N. Hirota, S. Maeda, S. Tokita, Anal. Sci. 14 (1998) 183. [6] L.J. Prins, J. Huskens, F. de Jong, P. Timmerman, D.N. Reinhoudt, Nature 398 (1999) 498. [7] M. Kubinyi, I. Mohammed-Ziegler, A. Grofcsik, I. Bitter, W.J. Jones, J. Mol. Struct. 408/409 (1997) 534. [8] I. Bitter, A. Gru¨n, L. To˝ke, G. To´th, B. Bala´zs, I. MohammedZiegler, A. Grofcsik, M. Kubinyi, Tetrahedron 53 (1997) 16867. [9] I. Mohammed-Ziegler, M. Kubinyi, A. Grofcsik, A. Gru¨n, I. Bitter, J. Mol. Struct. 480/481 (1999) 289. [10] I. Mohammed-Ziegler, M. Kubinyi, A. Grofcsik, A. Gru¨n, I. Bitter, J. Incl. Phenom. Macrocycl. Chem. 35 (1999) 349. [11] I. Mohammed-Ziegler, Investigation of the selective complex forming properties of some calix[4]arene derivatives, PhD Thesis, Budapest University of Technology and Economics, 2000 (in Hungarian, summarized in English in Magy. Ke´m. Foly. 107 (2001) 265–267). [12] Z. Pawełka, T. Zeegers-Huyskens, Bull. Soc. Chim. Belg. 106 (1997) 481. [13] Z. Pawełka, T. Zeegers-Huyskens, Vib. Spectrosc. 18 (1998) 41. [14] T. Kuc, T. Zeegers-Huyskens, Z. Pawełka, J. Mol. Liq. 89 (2000) 147. [15] M. Rospenk, A. Koll, L. Sobczyk, Chem. Phys. Lett. 261 (1996) 283. [16] M. Rospenk, L. Sobczyk, P. Schah-Mohammedi, H.H. Limbach, N.S. Golubev, S.M. Melikova, Magn. Res. Chem. 39 (2001) S81. [17] B. Bala´zs, G. To´th, Gy. Horva´th, A. Gru¨n, L. To˝ke, I. Bitter, Eur. J. Org. Chem. 1 (2001) 61. [18] K.A. Connors, Binding Constants, Wiley, New York, 1987, A Wiley-Interscience Publication. [19] H.A. Benesi, J.H. Hildebrand, J. Am. Chem. Soc. 71 (1949) 2703. [20] P. Forlay-Frick, Z.B. Nagy, J. Fekete, J. Liq. Chromatogr. Rel. Techn. 24 (2001) 827.
Spectroscopic study on the complex formation of chromogenic bridged calixarenes with aliphatic amines I. Mohammed-Zieglera,b,*, B. Poo´ra, M. Kubinyia,b, A. Grofcsika,b, A. Gru¨nc, I. Bitterc a
Chemical Research Center, Hungarian Academy of Sciences, P.O. Box 17, H-1525 Budapest, Hungary Department of Physical Chemistry, Budapest University of Technology and Economics, H-1521 Budapest, Hungary c Department of Organic Chemical Technology, Budapest University of Technology and Economics, H-1521 Budapest, Hungary b
Received 19 April 2002; revised 30 October 2002; accepted 22 November 2002
Abstract The complex formation between chromogenic capped calix 4 arene derivatives comprising indophenol indicator group(s), and aliphatic amines has been studied by UV/Vis spectroscopy. The equilibrium constants have been determined in ethanol, and—for one ligand—also in dimethylsulfoxide. The results have been interpreted in terms of various types of host– guest interactions and of steric effects. q 2003 Elsevier Science B.V. All rights reserved. Keywords: Calix[4]arene; Optical recognition; Aliphatic amine
1. Introduction Calixarenes, obtained in the condensation reaction of formaldehyde and p-substituted phenols, are often referred to, as the third generation of supramolecular receptors (after cyclodextrins and crown ethers). Calixarene derivatives supplied with ligating functions have been the subject of numerous studies, due to their potential applicability as active ligands in electrochemical and/or optical sensors (optodes) showing affinity to bind alkali/alkali earth metal ions or amines selectively [1 – 3]. Recently, calixarenes aroused the interest especially in the field of * Corresponding author. Address: Chemical Research Center, Hungarian Academy of Sciences, P.O. Box 17, 1525 Budapest, Hungary. Tel.: þ36-1438-4141; fax: þ36-1325-7554. E-mail address: [email protected] (I. Mohammed-Ziegler).
mimicking biological systems, such as enzymatic functions including enantioselective reactions and sensing [4 –6]. In our former works various types of chromogenic calixarene molecules have been synthesized and tested as potential ligands for optical sensors by spectroscopic measurements [7 – 11]. In the present paper two new chromogenic calix[4]arene derivatives 1a,b (Fig. 1.) prepared recently in our laboratory [17] and for comparison a calix[4]binaphto(crown) ether 1c [2] have been studied to elucidate their recognition properties toward aliphatic amines. The interaction between the chromogenic hosts and amine guests was associated with coloration due to the presence of indophenol indicator moiety inserted into the calixarene core. Thus, the process could be efficiently monitored by UV/Vis spectrophotometry and the equilibrium constants of the complex formation
0022-2860/03/$ - see front matter q 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0022-2860(03)00079-6
40
I. Mohammed-Ziegler et al. / Journal of Molecular Structure 650 (2003) 39–44
Fig. 1. Structure of the studied calix[4]arenes.
reactions could be calculated from the spectroscopic data obtained in different solvents in the presence of various aliphatic amines. Ligand 1c was already reported to recognize butylamines selectively [2], the preliminary results obtained with our ligands 1a and 1b [10,11] indicated a similar coloration process with amines. Besides UV/Vis spectroscopy, IR [12 – 14] and NMR [15,16] are the most widely used spectroscopic methods for studying phenol-amine type complexes in solution. In our laboratory the interaction of 1a and 1b with t-butylamine in MeOH-d4 and DMSO-d6 was investigated by 1H NMR spectroscopy [17] and unexpectedly, the NMR spectra did not indicate the formation of internal complexes. Instead, an endo/exo quinoide tautomerism of the indophenol moieties was established, and only a simple acid –base equilibrium accompanied by the coloration of solution was detected. It has to be noted, however, that monitoring the interaction of 1c with t-butylamine (reported complex formation constant 1090 dm3 mol21) [2] in an NMR tube, no observable sign of complexation was found except for signal broadening possibly due to the rapid tautomerisation of the indophenol moieties [17]. It is important to underline that for
NMR spectroscopic experiments much higher ligand and substrate concentrations are required than for UV/Vis spectroscopic measurements, since the latter method has significantly higher sensitivity. NMR spectroscopy with our substrates seemed not to provide unambiguous evidences for processes governed by weak intermolecular forces taking place in dilute solutions. The UV/Vis measurements were expected to be much more informative in this respect.
2. Experimental The UV/Vis measurements were performed on a Perkin – Elmer Lambda 2S spectrophotometer. The stoichiometry of the complexes were determined by Job’s method [18], 5 £ 1024 mol dm23 ethanol solutions of the reactants were used. Preliminary measurements with 1a,b,c showed that their complex formation with some amines is slow (takes 30 – 40 min), thus, the spectra were taken following this period. The complex formation constants for the individual calixarene –amine pairs, Kc ; were determined by the method of Benesi and Hildebrand [19],
I. Mohammed-Ziegler et al. / Journal of Molecular Structure 650 (2003) 39–44
from the absorption spectra of a series of solutions containing the calixarene ligand in the same (5 £ 1025 M) concentration and the amine in different concentrations. The concentration of ethylamine in the stock solution was determined by HPLC technique, adopting the method of Forlay-Frick et al. [20], developed originally for the determination of some amino acid derivatives (such as selenomethionin) and of some antibiotics. The HPLC measurements were carried out on a Millipore Waters 3000 equipment connected with a Supelco LC-18-08 column (with the length of 25 cm and the diameter of 5 mm) and a Millipore Waters 470 scanning fluorescent detector. The eluent contained 90% (v/v) acetonitrile and 10% (v/v) water, and 2% (v/v) of phosphoric acid was added to acidify the solvent mixture. The system was calibrated with a series of propyl-amine ethanol stock solutions of known concentration, assuming that the sensitivities
41
of the system for ethylamine and propyl-amine are identical.
3. Results and discussion The common feature of the spectra of ligands 1 ðlmax ¼ 520 – 534 nmÞ in the presence of amines is the appearance of a new band in the region of 652 – 667 nm (see Fig. 2). The quantitative evaluation of the spectral changes by the Job’s method applying various molar ratios indicated the formation of an adduct with 1:1 stoichiometry. The application of the method of Benesi and Hildebrand is shown in Fig. 2. The upper part (A) shows a series of spectra with the same initial concentration of calixarene 1a and with various initial concentrations of di-n-propyl-amine. The lower part (B) shows the Benesi – Hildebrand plot. Similar
Fig. 2. (A) Absorption spectra of 1a-di-n-propyl-amine systems in ethanol. Concentrations: ðc1a Þ0 ¼ 5 £ 1025 M in each solution, (camine)0 ¼ (a) 0 M, (b) 2.7 £ 1023 M, (c) 5.5 £ 1023 M, (d) 8.2 £ 1023 M, (e) 2.7 £ 1022 M, (f) 7.2 £ 1022 M. (B) Benesi–Hildebrand plot of the absorbance values at 664 nm.
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Table 1 The equilibrium constants of the complex formation of the ligands 1a,b,c with different aliphatic amines in ethanol and DMSO Ligand
Ethyl-amine
n-Propyl-amine
1a 1b 1c 1b in DMSO
436.3 234.7 951.8 –
46.4 78.7 103.9 87.7
n-Butyl-amine 18.7 62.6 155 (95.9 [2]) 71.3
t-Butyl-amine 138.7 109.8 1090 [2] 107.9
diagrams were obtained for other calixarenes and amines. The complex formation constants obtained this way are presented in Table 1. As will be shown, they suggest the formation of internal supramolecular complexes in the concentration range of these dilute solutions, and do not seem to be related to the acid– base equilibria, governing the composition of systems with higher calixarene and amine concentrations, studied by NMR measurements in Ref. [17]. As can be seen, the equilibrium constants for primary amines are generally higher, than those obtained for dipropylamine and for triethylamine. This supports the formation of supramolecular complexes in these dilute solutions, since the basicity of the three types of amines varies in the opposite order, therefore one would expect an opposite trend for the equilibrium constants in simple acid –base reactions. The most significant effects that seem to control the stability of the amine –calixarene complexes, are: (i) the numbers of hydrogen atoms in the amino group and (ii) steric effects. It is exactly true for calixcrown 1c, where the order of Kc values is in accordance with the reaction model suggested by Kubo et al. [2]. According to this model the complexation of 1c starts with the protonation of the amine guest by one of the hydroxyl groups in the calixarene host, resulting in the formation of an ammonium ion, which is simultaneously bound by the neighboring crown ring through hydrogen bonds. The complex, therefore, is stabilized by the ionic indophenolate– ammonium cation interaction and by the hydrogen bonds with the oxygen donor atoms in the crown ether moiety. Consequently, primary amines can form the most stable complexes (three hydrogen bonds), while tertiary amines are hardly bound. In addition, the complex stability is also affected by steric factors. In the series of n-primary amines the complex stability
3-Amino-propanol
Di-n-propyl-amine
Triethyl-amine
250.6 258.2 123.4 67.7
77.2 27.2 71.2 –
39.8 16.2 18.4 24.5
decreases with the increasing chain length of the alkyl group. At the same time, the highest Kc was measured for t-butylamine, which might be due to an additional binding factor via Me – p interaction between the t-Bu group and the naphthalene ring [2]. The considerations discussed above are not completely valid for capped calixarenes 1a,b. As can be seen in Table 1, the complex formation constants for primary amine complexes (except for ethylamine complexes) are substantially lower than those for 1c, indicating a much weaker binding. Besides, the order of the Kc values is not strictly controlled by the order of the amines. These differences may be attributed to the structural differences between 1c and 1a,b, in respect of the indicator group and the binding site (see Fig. 1). The indophenol moieties of compounds 1a,b exist in a stable endo-quinoide tautomeric form [17], which means that the phenolic OH group is far from the binding site, whereas both OH groups of 1c are close to the cavity. Consequently, the model of binding in case of 1a,b should be modified. Two consecutive processes are assumed (Fig. 3): (i) first the protonation of amine by one of the exophenolic OH affording exo-calix ion-pair (acid – base equilibrium, K1 ), (ii) rapid tautomerisation of the indophenolate resulting in the formation of an endo/ exo-quinoide structure followed by stabilizing the ammonium cation via H-bonds (complexation, K2 ). In this respect a question emerges: which electron donating groups in the carboxamide cap can accept the ammonium protons? Since the carbonyl groups are oriented out of the cavity [17], there remain the phenolether oxygens (and the quinone carbonyl of the other indophenol unit in 1a) to accept protons. Considering the tetrahedral geometry of the ammonium cation, in principle, primary amines can form both internal (A) and external (B) supramolecular complexes. Structure A comprising
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43
Fig. 3. Scheme of the equilibria assumed.
three H-bonds should be more stable than complex B where only two H-bonds are utilized for the stabilization. Obviously, the two kinds of complexes can be in equilibrium with each other in which the proportion of the individual forms strongly depends on the bulkiness of the substituents. In the internal complexes (A), a severe steric repulsion should be accounted between the apical substituent of the amine and the upper part of the bridging unit. Of the amine series, the smallest ethylamine gave by far the highest Kc values with 1a (even with the sterically more crowded 1b), which may be attributed to the formation of such an internal complex, although we have no structural evidences to support it. From steric reasons the other amines with ligands 1a and 1b can form at most external complexes (B) of much lower stability (two hydrogen bonds). Since the two coloration processes cannot be separated spectroscopically (the different indophenolate– ammonium ion-pairs can not be distinguished), they appear as
a single equilibrium where K1 (acid – base equilibrium) and K2 (complexation) may be involved in the measured constants with unknown proportions. Due to the weak interactions, their contribution to the K values can be comparable and so not allowing deeper conclusion on the structural feature of complexes. It should be pointed out, however, that the presence of hydroxyl group in 3-aminopropanol increases the equilibrium constant of complex formation with carboxamide bridged ligands 1a,b compared to that of propylamine, but not in the case of the crown bridged calix[4]arene 1c. Additional interaction between the carboxamide oxygen and the hydroxyl group (in the amine) may occur resulting in a significant enhancement of complex stability. In contrast, the hydroxyl group does not seem to interact with the etheric oxygens of 1c. In order to compare the complex formation in a protic and in an aprotic solvent, analogous measurements have been carried out with 1b– amine systems
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in dimethylsulfoxide. As apparent from Table 1, in the majority of cases, the equilibrium constants obtained with the same amine in the two solvents do not differ significantly. However, much lower stabilities were obtained with 3-aminopropanol in DMSO than in ethanol. This observation is in accord with the more polar and nucleophilic character of DMSO, which much easily disrupt the intramolecular OH· · ·OC bond than the less polar and self-solvated ethanol. In conclusion, the results indicate that calix[4]arenes capped by diamide bridges form strongly polar supramolecular complexes with various types of amines. Of the calixarenes studied, 1b may be worth testing in an analytical sensor for the detection of aminoalcohols. Acknowledgements The authors are grateful to the Hungarian Scientific Research Found for financial support (contract number T 025561). I. M.-Z. expresses her gratitude to the Varga Jo´zsef Foundation for financial support. Special thanks to Mr Zolta´n Bala´zs Nagy and Mr Pe´ter Forlay-Frick for the technical help in performing HPLC measurements.
References [1] H.M. Chawla, K. Srinvas, J. Chem. Soc. Chem. Commun. (1994) 2593. [2] Y. Kubo, S. Mruyama, N. Ohhara, M. Nakamura, S. Tokita, J. Chem. Soc., Chem. Commun. (1995) 1727. [3] Q.Y. Zheng, C.F. Chen, Z.T. Huang, Tetrahedron 53 (1997) 10345.
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