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Solid-state NMR studies of alkali metal ion complexes of p-tertbutyl-calixarenes
16 July 1999
Chemical Physics Letters 308 Ž1999. 65–70 www.elsevier.nlrlocatercplett
Solid-state NMR studies of alkali metal ion complexes of p-tertbutyl-calixarenes Francesca Benevelli a , Waclaw Kolodziejski b, Krzysztof Wozniak c , Jacek Klinowski a,) b
a Department of Chemistry, UniÕersity of Cambridge, Lensfield Road, Cambridge CB2 1EW, UK Department of Inorganic and Analytical Chemistry, Medical UniÕersity of Warsaw, ul. Banacha 1, 02-097 Warszawa, Poland c Department of Chemistry, UniÕersity of Warsaw, ul. Pasteura 1, 02-093 Warszawa, Poland
Received 20 January 1999; in final form 23 April 1999
Abstract Calixarenes of different molecular sizes form complexes with alkali metal cations but have different affinities for small cations ŽLiq and Naq. and large cations ŽRbq and Csq.. p-tert-Butyl-calixw4xarene easily forms complexes with the small cations by virtue of steric recognition. In p-tert-butyl-calixw8xarene, the sites next to the oxygens are more accessible to large cations because of the more extensive hydrogen-bonding network. The characteristic behaviour of p-tert-butyl-calixw6xarene in the complexation of Rbq and Csq is caused by its ‘pinched’ conformation. q 1999 Elsevier Science B.V. All rights reserved.
1. Introduction Calixarenes w1,2x are macrocyclic compounds built of n phenol units linked via alkylidene groups in such a way that large cavities of molecular dimensions are formed Žin Fig. 1, n s 4.. Calixarenes are easily synthesized by condensing phenol and formaldehyde. Their bowl-like structure ŽLatin calix s bowl. allows them to form complexes with a variety of species. Fine control of the size of the molecule Žby changing the value of n. and the introduction of
) Corresponding author. Fax: q44 1223 33 63 62; e-mail: [email protected]
various functional groups makes it possible to ‘tailor’ calixarenes to a variety of applications, such as catalysts, ligands and molecular hosts. The sequestering properties of calixarenes for neutral and charged inorganic and organic species are exceptional, while appropriate substitution renders the cavities shapeselective and suitable for molecular recognition w3x. Chiral calixarenes have also been prepared w4x. Calixarenes are of interest to purification, chromatography, storage, slow release of drugs, transport across membranes w5,6x, ion channels w7x and self-assembling monolayers w8x. NMR is well suited for the study of molecular interactions and dynamics in the solid state. However, there are only a handful of solid-state NMR measurements of calixarenes, all of them recent w9,10x. We have examined several simple calixarenes and their complexes with alkali metal
0009-2614r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved. PII: S 0 0 0 9 - 2 6 1 4 Ž 9 9 . 0 0 5 3 9 - 4
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ever, there is no systematic study of the interactions between alkali metal ions and calixarenes in the solid state. The aim of this work is to examine the properties of such inclusion compounds by NMR, a technique capable of providing quantitative information on complex formation, and on the interaction between the cation and the host molecule.
2. Experimental Fig. 1. The structure of p-tert-butyl-calixw4xarene.
2.1. Sample preparation cations using X-ray diffraction and multinuclear high-resolution solid-state NMR. While several calixarene–metal cation complexes have been reported, most of them require that the hydroxyl groups of the calixarene be derivatized to esters or ethers w11x because of the poor solubility of the p-tertbutyl-calixarenes. The Csq–p-tertbutylcalixw8xarene complex is used for the recovery of caesium from nuclear waste solutions w12,13x, and as a lanthanide complexing agent under basic conditions w14x. Hexacid calixarene, also known as superuranophile, is a metal complex of derivatized calixarene used for the recovery of uranium w15x. The stability constants of these complexes have been measured potentiometrically w16,17x. Derivatives other than ethers and esters have been examined: calixacrowns show a high NaqrCsq selectivity, and are used as carriers through supported liquid membranes w13x. The conformation of the calixarene in these complexes is normally a cone, except when derivatization forces other conformations, as in the case of 1,3-calixw4xbis-crown, which shows an alternate 1,3 conformation w13x. There is evidence of some conformational freedom for the metal complexes in solution w18x. Alkyl ketone residues have been introduced in the lower rim of calixw5xarene and calixw6xarene w19x, which show affinity for complexation of alkali metal cations. Phosphoryl and amide ligands, both at the lower and the upper rim, facilitate extraction of lanthanides and actinides w20,21x. The selectivity of different sized calixarenes to bind and transport alkali metal cations has been extensively studied in solution w22x, revealing a remarkable selectivity of calixw4xarenes for Csq. How-
Parent calixarenes were obtained from Aldrich and used without further purification. Calixarene complexes were prepared by dissolving the appropriate alkali metal hydroxide in the minimum amount of water at room temperature and then adding 700 mg of p-tertbutyl-calixw4xarene, p-tertbutyl-calixw6xarene or p-tertbutyl-calixw8xarene Žabbreviated as c4, c6 and c8, respectively.. The amount of hydroxide was chosen so as to obtain a 20:1 cationr calixarene molar ratio. We selected alkali metals with favourable NMR properties and which are known to give calixarene complexes in solution. The products, which we denote c4M, c6M or c8M ŽM s Li, Na, Rb, Cs., are not soluble and form a white suspension. After 48 h under vigorous stirring at room temperature, the suspension was filtered and washed with water until neutral pH was reached. The solid was afterwards dried at 608C overnight. 2.2. X-ray diffraction XRD patterns were recorded using a Philips 1710 powder diffractometer with Cu K a radiation Ž40 kV, 40 mA. at 0.025 steps at the rate of 1 s per step over the range 4 - 2 u - 60. 2.3. Solid-state NMR NMR spectra were recorded at room temperature using a Chemagnetics CMX-400 spectrometer operating at 155.41, 105.77, 130.84 and 52.4 MHz for 7 Li, 23 Na, 87 Rb and 133 Cs, respectively, and a commercial MAS probehead with zirconia rotors 7.5 mm
F. BeneÕelli et al.r Chemical Physics Letters 308 (1999) 65–70
in diameter driven by nitrogen gas. For 133 Cs rotors 4 mm in diameter has been used. The pr2 pulse lengths were 4.5, 7.0, 6.7 and 8.5 ms for 7 Li, 23 Na, 87 Rb and 133 Cs, respectively. To obtain quantitatively reliable spectra of quadrupolar nuclei, it is essential to use strong radiofrequency pulses with small flip angles w23,24x, and we applied 0.35 ms pulses with recycle delays of 0.3 s. 23 Na, 7 Li, 87 Rb and 133 Cs line positions are given in ppm from solid NaCl, LiCl, RbNO 3 and 1 M CsCl water solution. Spectral intensities were compared by integrating spectra of the same nucleus acquired for a constant weight of the sample and the same number of scans.
3. Results and discussion 3.1. X-ray diffraction Powder XRD shows that the calixarenes and their complexes are highly crystalline ŽFig. 2.. The technique is useful for identifying complexes of different types of calixarenes, since in most cases complex formation does not affect the general appearance of the XRD pattern. An exception is c4Cs, for which the absolute intensity of the peaks is only half of that in pure calixarene and in the other samples. XRD patterns of the other cationic complexes of c4 are
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very similar. Except for c4Rb, the peaks at lower 2 u Ž6.78 and 6.98. are less intense, and the peak intensity decreases with increasing cation size. The intensity of the other peaks, such as that at 18.88, is reduced upon complexation. The maximum intensity is at 208 for the pure c4 and its Csq complex, at 19.68 for the Liq and Naq complexes, and at 21.38 for the Rbq complex. The differences between c6 and its complexes are less pronounced than for c4, and the XRD peaks are broader. The complexes lack some peaks at low 2u Ž5.88 and 68. and others have lower intensity, such as the peaks at 208 and 21.48. The maximum intensity is at 18.68 for all samples. XRD peaks from c8 are better resolved than those from c6 and its complexes but, as with c6, there is little difference between the pure calixarene and its complexes. Maximum intensity is observed at very low angles in all the samples Ž68.. Most of the peaks from pure c8 are still present in the complexes. We conclude that powder XRD does not efficiently distinguish different cationic complexes of the same calixarene, probably due to the relative width of calix in which the cations are contained and the relative low percentage of inclusion. When the cation is large and the calix is small, as in the case of c4Cs, the differences between XRD patterns are more significant. 3.2. Solid-state NMR
Fig. 2. Powder X-ray diffraction patterns of Ža. c4Cs, Žb. c4, Žc. c6, and Žd. c8.
All the cations have been studied by solid-state NMR. It is clear that the calixarenes are capable of including the cation in a way similar to that observed in other macrocyclic compounds, such as crown ethers. Liq is included in all the calixarenes and the static 7 Li spectrum ŽFig. 3a. shows an intense symmetric peak at y0.3 ppm, which narrows under MAS. The only difference in behaviour between the three calixarenes is the amount of Liq included, as shown by the decreasing spectral intensity with increasing size of the calix ŽFig. 3b–d.. The relative complexation affinity decreases in the series c4 ) c6 ) c8 Žspectral intensity ratio 1:0.46:0.15.. The 23 Na NMR spectra are shown in Fig. 4. The binding ability for Naq decreases with increasing size of the calix. The spectral intensity ratio was
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Fig. 3. Ža. 7 Li spectrum of static c4Li and MAS spectra of samples Žb. c4Li, Žc. c6Li, Žd. c8Li acquired with MAS at 3.2 kHz, 4096 scans and 0.2 s pulse delay.
1:0.74:0.08 for c4rc6rc8, respectively, and the complexation ability can be summarized by the sequence c4 ) c6 ) c8, as observed for Liq. The sodium cation is found in two different environments Žpeaks at y6.5 for c4Na and at y8.5 ppm for c6Na and c8Na., depending on the size of the calixarene. A comparison of the static and proton-decoupled spectra in all the Naq complexes suggests that the interaction between the protons and the cation is
Fig. 4. 23 Na spectra of Ža. static sample c4Na with 3072 scans, Žb. sample c4Na with 1600 scans with MAS at 3.2 kHz, Žc. static sample c6Na with 4096 scans, Žd. sample c6Na, 4096 scans with MAS at 3.2 kHz.
weak in both the environments. Thus the difference between the two complexation sites probably lies in the distance from the oxygens. This is in agreement with the well known preference of sodium cation to form exo complexes, where the phenolic oxygens are mainly involved in the complexation through electrostatic interactions w2x. Calixarenes have much lower affinity for Rbq than for Naq and Liq. p-tert-Butylcalixw4xarene accepts Rbq giving a single line very low intensity at 800 ppm Žspectrum not shown.. In sample c6Rb, the signal is not detectable, while the sample c8Rb gives a very weak line at 8.0 ppm. The intensity in both c4Rb and c8Rb is too low for a detailed study. The relative complexation ability for Rbq is c4 ) c8 ) c6. 133 Cs MAS spectrum ŽFig. 5a. of c4Cs contains two lines at q218 and y200 ppm, corresponding to two different environments for the ceasium cation. A comparison of the spectrum acquired with proton decoupling and the single pulse static spectrum suggests a strong interaction between the protons and the cesium resonating at y200 ppm. This frequency is in good agreement with the reported value for cesium endo complexes in solution w25x, where the phenyl p electrons contribute to the bond through dispersion and induction interactions. The endo complex constitutes the 92% of the overall complex with c4. The remaining 8% is probably in an exo complex. The signals from c6Cs are very weak and similar to that observed for c4Cs.
Fig. 5. 133 Cs spectra of Ža. spinning sample c4Cs, 30720 scans, MAS at 8 kHz; Žb. spinning sample c8Cs, 102400 scans, MAS at 8 kHz.
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The spectrum of c8Cs is completely different and appears to contain a single broad line at 24 ppm ŽFig. 5b.. The spectra measured with static conditions or with MAS are very similar. However, the spectrum acquired with proton decoupling reveals the signal to be a composite, with components at 23.5 and 33.4 ppm. These frequencies are intermediate between the two found in c4Cs. An endo complex is suggested in which the oxygens as well as the phenyl rings can bond to the cation. The relative binding ability of the calixarenes for samples containing Csq is c4 ) c8 ) c6, the same as for Rbq. Calixarenes have two different sites of complexation: endo and exo w26x. In exo complexes phenolic oxygens are capable of ‘hard’ electrostatic interactions with small ‘hard’ cations, such as Liq w27x and Naq w26,28,29x, while the phenyl p electrons participate in endo complexes with ‘soft’ dispersion and induction w30x interactions with a large ‘soft’ cations, such as Csq w25x. Under these conditions the rubidium cation is too large to form stable exo complexes and too small to have enough polarizabilty to form endo complexes. The solid-state NMR analysis of alkali metal cation series confirms this behaviour. The very low ability of p-tert-butylcaliw6xarene to form complexes when the calix p electrons are involved has already been observed w31x. It has been suggested that the pinched conformation of p-tertbutylcaliw6xarene prohibits easy access of the cation to the calix w32x.
4. Conclusions Calixarenes form complexes with alkali metal cations, but show different affinities for small cations ŽLiq and Naq . and large cations ŽRbq and Csq .. The smallest calixarene, p-tert-butyl-calixw4xarene, easily forms complexes with small cations due to electrostatic interactions between the cation and the phenolic oxygens. With increasing cationic radius, dispersion and induction interactions become more efficient and the cation is admitted into the calix, as is the case for Csq. The Rbq seems to be in an intermediate unfavourable situation. The calix is more accessible to a cation next to the oxygens in p-tert-
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butyl-calixw8xarene, because of the larger and more flexible hydrogen bonding network. Due to its ‘pinched’ conformation, p-tert-butyl-calixw6xarene shows less affinity for the large cations.
References w1x J. Vicens, V. Bohmer ŽEds.., Calixarenes: a Versatile Class ¨ of Macrocyclic Compounds, vol. 3, Kluwer Academic, Dordrecht, 1991. w2x C.D. Gutsche, in: J.F., Stoddart ŽEd.., Monographs in Supramolecular Chemistry, vol. 1, The Royal Society of Chemistry, Cambridge, 1989. w3x H.-J. Schneider, U. Schneider, J. Incl. Phenom. Mol. Recogn. Chem. 19 Ž1994. 67. w4x S. Felix, J.R. Ascenso, R. Lamartine, J.L.C. Pereira, Synth. Commun. 28 Ž1998. 1793. w5x Y. Koide, H. Sato, H. Shosenji, K. Yamada, Bull. Chem. Soc. Jpn 69 Ž1996. 315. w6x T. Jin, M. Kinjo, T. Koyama, Y. Kobayashi, H. Hirata, Langmuir 12 Ž1996. 2684. w7x B. Nilius, J. Prenen, J. Eggermont, G. Droogmans, Pflugers ¨ Arch. Eur. J. Physiol. 435 Ž1998. R243. w8x W.C. Moreira, P.J. Dutton, R. Aroca, Langmuir 11 Ž1995. 3137. w9x A.M.A. Van Wageningen, W. Verboom, X. Zhu, J.A. Ripmeester, D.N. Reinhoudt, Supramol. Chem. 9 Ž1998. 31. w10x E.B. Brouwer, G.D. Enright, J.A. Ripmeester, J. Am. Chem. Soc. 119 Ž1997. 5404. w11x R. Perrin, R. Lamartine, M. Perrin, Pure Appl. Chem. 65 Ž1993. 1549. w12x C. Hill, J.F. Dozol, V. Lamare, H. Rouquette, S. Eymard, B. Tournois, J. Vicens, Z. Asfari, C. Bressot, R. Ungaro, A. Casnati, J. Incl. Phenom. Mol. Recogn. Chem. 19 Ž1994. 399. w13x Z. Asfari, C. Bressot, J. Vicens, C. Hill, J.F. Dozol, H. Rouquette, S. Eymard, V. Lamare, B. Tournois, ACS Symp. Ser. 642 Ž1996. 376. w14x J.M. Harrowfield, M.I. Ogden, A.H. White, F.R. Wilner, Aust. J. Chem. 42 Ž1989. 949. w15x Y.K. Agrawal, M. Sanyal, J. Radioanal. Nucl. Chem. 198 Ž1995. 349. w16x A.F.D. De Namor, E. Gil, M.A.L. Tanco, D.A.P. Tanaka, L.E.P. Salazar, R.A. Schulz, J.J. Wang, J. Phys. Chem. 99 Ž1995. 16776. w17x F. Arnaud-Neu, G. Barrett, S. Cremin, M. Deasy, G. Ferguson, S.J. Harris, A.J. Lough, L. Guerra, M.A. McKervey, M.J. Schwing-Weill, P. Schwinte, J. Chem. Soc., Perkin Trans. 2 Ž1992. 1119. w18x U.C. Meier, C. Detellier, J. Phys. Chem. A 102 Ž1998. 1888. w19x S.E.J. Bell, J.K. Browne, V. McKee, M.A. McKervey, J.F. Malone, M. O’Leary, A. Walker, F. Arnaud-Neu, O. Boulangeot, O. Mauprivez, M.J. Schwing-Weill, J. Org. Chem. 63 Ž1998. 489.
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w20x F. Arnaud-Neu, G. Barrett, S. Fanni, D. Marrs, W. McGregor, M.A. McKervey, M.J. Schwing-Weill, V. Vetrogon, S. Wechsler, J. Chem. Soc., Perkin Trans. 2 Ž1995. 453. w21x F. Arnaud-Neu, V. Bohmer, J.F. Dozol, C. Gruttner, R.A. ¨ ¨ Jakobi, D. Kraft, O. Mauprivez, H. Rouquette, M.J. Schwing-Weill, N. Simon, W. Vogt, J. Chem. Soc., Perkin Trans. 2 Ž1996. 1175. w22x S.R. Izatt, R.T. Hawkins, J.J. Christensen, R.M. Izatt, J. Am. Chem. Soc. 107 Ž1985. 63. w23x A. Samoson, E. Lippmaa, Chem. Phys. Lett. 100 Ž1983. 205. w24x P.P. Man, J. Klinowski, A. Trokiner, H. Zanni, P. Papon, Chem. Phys. Lett. 151 Ž1988. 143. w25x J.M. Harrowfield, M.I. Ogden, W.R. Richmond, A.H. White, J. Chem. Soc. Chem. Commun. Ž1991. 1159.
w26x R. Abidi, M.V. Baker, J.M. Harrowfield, D.S.C. Ho, W.R. Richmond, B.W. Skelton, A.H. White, A. Varnek, G. Wipff, Inorg. Chim. Acta 246 Ž1996. 275. w27x B. Brzezinski, F. Bartl, G. Zundel, J. Phys. Chem. B 101 Ž1997. 5611. w28x J. Havlicek, M. Tkadlecova, M. Vyhnankova, E. Pinkhassik, I. Stibor, Coll. Czech. Chem. Commun. 61 Ž1996. 1783. w29x Y. Israeli, C. Detellier, J. Phys. Chem. B 101 Ž1997. 1897. w30x S. Ahn, S.K. Chang, T. Kim, J.W. Lee, Chem. Lett. Ž1995. 297. w31x S.J. Shinkai, K.J. Araki, T. Matsuda, O. Manabe, Bull. Chem. Soc. Jpn. 62 Ž1989. 3856. w32x T. Harada, S. Shinkai, J. Chem. Soc., Perkin Trans. 2 Ž1995. 2231.
Chemical Physics Letters 308 Ž1999. 65–70 www.elsevier.nlrlocatercplett
Solid-state NMR studies of alkali metal ion complexes of p-tertbutyl-calixarenes Francesca Benevelli a , Waclaw Kolodziejski b, Krzysztof Wozniak c , Jacek Klinowski a,) b
a Department of Chemistry, UniÕersity of Cambridge, Lensfield Road, Cambridge CB2 1EW, UK Department of Inorganic and Analytical Chemistry, Medical UniÕersity of Warsaw, ul. Banacha 1, 02-097 Warszawa, Poland c Department of Chemistry, UniÕersity of Warsaw, ul. Pasteura 1, 02-093 Warszawa, Poland
Received 20 January 1999; in final form 23 April 1999
Abstract Calixarenes of different molecular sizes form complexes with alkali metal cations but have different affinities for small cations ŽLiq and Naq. and large cations ŽRbq and Csq.. p-tert-Butyl-calixw4xarene easily forms complexes with the small cations by virtue of steric recognition. In p-tert-butyl-calixw8xarene, the sites next to the oxygens are more accessible to large cations because of the more extensive hydrogen-bonding network. The characteristic behaviour of p-tert-butyl-calixw6xarene in the complexation of Rbq and Csq is caused by its ‘pinched’ conformation. q 1999 Elsevier Science B.V. All rights reserved.
1. Introduction Calixarenes w1,2x are macrocyclic compounds built of n phenol units linked via alkylidene groups in such a way that large cavities of molecular dimensions are formed Žin Fig. 1, n s 4.. Calixarenes are easily synthesized by condensing phenol and formaldehyde. Their bowl-like structure ŽLatin calix s bowl. allows them to form complexes with a variety of species. Fine control of the size of the molecule Žby changing the value of n. and the introduction of
) Corresponding author. Fax: q44 1223 33 63 62; e-mail: [email protected]
various functional groups makes it possible to ‘tailor’ calixarenes to a variety of applications, such as catalysts, ligands and molecular hosts. The sequestering properties of calixarenes for neutral and charged inorganic and organic species are exceptional, while appropriate substitution renders the cavities shapeselective and suitable for molecular recognition w3x. Chiral calixarenes have also been prepared w4x. Calixarenes are of interest to purification, chromatography, storage, slow release of drugs, transport across membranes w5,6x, ion channels w7x and self-assembling monolayers w8x. NMR is well suited for the study of molecular interactions and dynamics in the solid state. However, there are only a handful of solid-state NMR measurements of calixarenes, all of them recent w9,10x. We have examined several simple calixarenes and their complexes with alkali metal
0009-2614r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved. PII: S 0 0 0 9 - 2 6 1 4 Ž 9 9 . 0 0 5 3 9 - 4
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ever, there is no systematic study of the interactions between alkali metal ions and calixarenes in the solid state. The aim of this work is to examine the properties of such inclusion compounds by NMR, a technique capable of providing quantitative information on complex formation, and on the interaction between the cation and the host molecule.
2. Experimental Fig. 1. The structure of p-tert-butyl-calixw4xarene.
2.1. Sample preparation cations using X-ray diffraction and multinuclear high-resolution solid-state NMR. While several calixarene–metal cation complexes have been reported, most of them require that the hydroxyl groups of the calixarene be derivatized to esters or ethers w11x because of the poor solubility of the p-tertbutyl-calixarenes. The Csq–p-tertbutylcalixw8xarene complex is used for the recovery of caesium from nuclear waste solutions w12,13x, and as a lanthanide complexing agent under basic conditions w14x. Hexacid calixarene, also known as superuranophile, is a metal complex of derivatized calixarene used for the recovery of uranium w15x. The stability constants of these complexes have been measured potentiometrically w16,17x. Derivatives other than ethers and esters have been examined: calixacrowns show a high NaqrCsq selectivity, and are used as carriers through supported liquid membranes w13x. The conformation of the calixarene in these complexes is normally a cone, except when derivatization forces other conformations, as in the case of 1,3-calixw4xbis-crown, which shows an alternate 1,3 conformation w13x. There is evidence of some conformational freedom for the metal complexes in solution w18x. Alkyl ketone residues have been introduced in the lower rim of calixw5xarene and calixw6xarene w19x, which show affinity for complexation of alkali metal cations. Phosphoryl and amide ligands, both at the lower and the upper rim, facilitate extraction of lanthanides and actinides w20,21x. The selectivity of different sized calixarenes to bind and transport alkali metal cations has been extensively studied in solution w22x, revealing a remarkable selectivity of calixw4xarenes for Csq. How-
Parent calixarenes were obtained from Aldrich and used without further purification. Calixarene complexes were prepared by dissolving the appropriate alkali metal hydroxide in the minimum amount of water at room temperature and then adding 700 mg of p-tertbutyl-calixw4xarene, p-tertbutyl-calixw6xarene or p-tertbutyl-calixw8xarene Žabbreviated as c4, c6 and c8, respectively.. The amount of hydroxide was chosen so as to obtain a 20:1 cationr calixarene molar ratio. We selected alkali metals with favourable NMR properties and which are known to give calixarene complexes in solution. The products, which we denote c4M, c6M or c8M ŽM s Li, Na, Rb, Cs., are not soluble and form a white suspension. After 48 h under vigorous stirring at room temperature, the suspension was filtered and washed with water until neutral pH was reached. The solid was afterwards dried at 608C overnight. 2.2. X-ray diffraction XRD patterns were recorded using a Philips 1710 powder diffractometer with Cu K a radiation Ž40 kV, 40 mA. at 0.025 steps at the rate of 1 s per step over the range 4 - 2 u - 60. 2.3. Solid-state NMR NMR spectra were recorded at room temperature using a Chemagnetics CMX-400 spectrometer operating at 155.41, 105.77, 130.84 and 52.4 MHz for 7 Li, 23 Na, 87 Rb and 133 Cs, respectively, and a commercial MAS probehead with zirconia rotors 7.5 mm
F. BeneÕelli et al.r Chemical Physics Letters 308 (1999) 65–70
in diameter driven by nitrogen gas. For 133 Cs rotors 4 mm in diameter has been used. The pr2 pulse lengths were 4.5, 7.0, 6.7 and 8.5 ms for 7 Li, 23 Na, 87 Rb and 133 Cs, respectively. To obtain quantitatively reliable spectra of quadrupolar nuclei, it is essential to use strong radiofrequency pulses with small flip angles w23,24x, and we applied 0.35 ms pulses with recycle delays of 0.3 s. 23 Na, 7 Li, 87 Rb and 133 Cs line positions are given in ppm from solid NaCl, LiCl, RbNO 3 and 1 M CsCl water solution. Spectral intensities were compared by integrating spectra of the same nucleus acquired for a constant weight of the sample and the same number of scans.
3. Results and discussion 3.1. X-ray diffraction Powder XRD shows that the calixarenes and their complexes are highly crystalline ŽFig. 2.. The technique is useful for identifying complexes of different types of calixarenes, since in most cases complex formation does not affect the general appearance of the XRD pattern. An exception is c4Cs, for which the absolute intensity of the peaks is only half of that in pure calixarene and in the other samples. XRD patterns of the other cationic complexes of c4 are
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very similar. Except for c4Rb, the peaks at lower 2 u Ž6.78 and 6.98. are less intense, and the peak intensity decreases with increasing cation size. The intensity of the other peaks, such as that at 18.88, is reduced upon complexation. The maximum intensity is at 208 for the pure c4 and its Csq complex, at 19.68 for the Liq and Naq complexes, and at 21.38 for the Rbq complex. The differences between c6 and its complexes are less pronounced than for c4, and the XRD peaks are broader. The complexes lack some peaks at low 2u Ž5.88 and 68. and others have lower intensity, such as the peaks at 208 and 21.48. The maximum intensity is at 18.68 for all samples. XRD peaks from c8 are better resolved than those from c6 and its complexes but, as with c6, there is little difference between the pure calixarene and its complexes. Maximum intensity is observed at very low angles in all the samples Ž68.. Most of the peaks from pure c8 are still present in the complexes. We conclude that powder XRD does not efficiently distinguish different cationic complexes of the same calixarene, probably due to the relative width of calix in which the cations are contained and the relative low percentage of inclusion. When the cation is large and the calix is small, as in the case of c4Cs, the differences between XRD patterns are more significant. 3.2. Solid-state NMR
Fig. 2. Powder X-ray diffraction patterns of Ža. c4Cs, Žb. c4, Žc. c6, and Žd. c8.
All the cations have been studied by solid-state NMR. It is clear that the calixarenes are capable of including the cation in a way similar to that observed in other macrocyclic compounds, such as crown ethers. Liq is included in all the calixarenes and the static 7 Li spectrum ŽFig. 3a. shows an intense symmetric peak at y0.3 ppm, which narrows under MAS. The only difference in behaviour between the three calixarenes is the amount of Liq included, as shown by the decreasing spectral intensity with increasing size of the calix ŽFig. 3b–d.. The relative complexation affinity decreases in the series c4 ) c6 ) c8 Žspectral intensity ratio 1:0.46:0.15.. The 23 Na NMR spectra are shown in Fig. 4. The binding ability for Naq decreases with increasing size of the calix. The spectral intensity ratio was
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Fig. 3. Ža. 7 Li spectrum of static c4Li and MAS spectra of samples Žb. c4Li, Žc. c6Li, Žd. c8Li acquired with MAS at 3.2 kHz, 4096 scans and 0.2 s pulse delay.
1:0.74:0.08 for c4rc6rc8, respectively, and the complexation ability can be summarized by the sequence c4 ) c6 ) c8, as observed for Liq. The sodium cation is found in two different environments Žpeaks at y6.5 for c4Na and at y8.5 ppm for c6Na and c8Na., depending on the size of the calixarene. A comparison of the static and proton-decoupled spectra in all the Naq complexes suggests that the interaction between the protons and the cation is
Fig. 4. 23 Na spectra of Ža. static sample c4Na with 3072 scans, Žb. sample c4Na with 1600 scans with MAS at 3.2 kHz, Žc. static sample c6Na with 4096 scans, Žd. sample c6Na, 4096 scans with MAS at 3.2 kHz.
weak in both the environments. Thus the difference between the two complexation sites probably lies in the distance from the oxygens. This is in agreement with the well known preference of sodium cation to form exo complexes, where the phenolic oxygens are mainly involved in the complexation through electrostatic interactions w2x. Calixarenes have much lower affinity for Rbq than for Naq and Liq. p-tert-Butylcalixw4xarene accepts Rbq giving a single line very low intensity at 800 ppm Žspectrum not shown.. In sample c6Rb, the signal is not detectable, while the sample c8Rb gives a very weak line at 8.0 ppm. The intensity in both c4Rb and c8Rb is too low for a detailed study. The relative complexation ability for Rbq is c4 ) c8 ) c6. 133 Cs MAS spectrum ŽFig. 5a. of c4Cs contains two lines at q218 and y200 ppm, corresponding to two different environments for the ceasium cation. A comparison of the spectrum acquired with proton decoupling and the single pulse static spectrum suggests a strong interaction between the protons and the cesium resonating at y200 ppm. This frequency is in good agreement with the reported value for cesium endo complexes in solution w25x, where the phenyl p electrons contribute to the bond through dispersion and induction interactions. The endo complex constitutes the 92% of the overall complex with c4. The remaining 8% is probably in an exo complex. The signals from c6Cs are very weak and similar to that observed for c4Cs.
Fig. 5. 133 Cs spectra of Ža. spinning sample c4Cs, 30720 scans, MAS at 8 kHz; Žb. spinning sample c8Cs, 102400 scans, MAS at 8 kHz.
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The spectrum of c8Cs is completely different and appears to contain a single broad line at 24 ppm ŽFig. 5b.. The spectra measured with static conditions or with MAS are very similar. However, the spectrum acquired with proton decoupling reveals the signal to be a composite, with components at 23.5 and 33.4 ppm. These frequencies are intermediate between the two found in c4Cs. An endo complex is suggested in which the oxygens as well as the phenyl rings can bond to the cation. The relative binding ability of the calixarenes for samples containing Csq is c4 ) c8 ) c6, the same as for Rbq. Calixarenes have two different sites of complexation: endo and exo w26x. In exo complexes phenolic oxygens are capable of ‘hard’ electrostatic interactions with small ‘hard’ cations, such as Liq w27x and Naq w26,28,29x, while the phenyl p electrons participate in endo complexes with ‘soft’ dispersion and induction w30x interactions with a large ‘soft’ cations, such as Csq w25x. Under these conditions the rubidium cation is too large to form stable exo complexes and too small to have enough polarizabilty to form endo complexes. The solid-state NMR analysis of alkali metal cation series confirms this behaviour. The very low ability of p-tert-butylcaliw6xarene to form complexes when the calix p electrons are involved has already been observed w31x. It has been suggested that the pinched conformation of p-tertbutylcaliw6xarene prohibits easy access of the cation to the calix w32x.
4. Conclusions Calixarenes form complexes with alkali metal cations, but show different affinities for small cations ŽLiq and Naq . and large cations ŽRbq and Csq .. The smallest calixarene, p-tert-butyl-calixw4xarene, easily forms complexes with small cations due to electrostatic interactions between the cation and the phenolic oxygens. With increasing cationic radius, dispersion and induction interactions become more efficient and the cation is admitted into the calix, as is the case for Csq. The Rbq seems to be in an intermediate unfavourable situation. The calix is more accessible to a cation next to the oxygens in p-tert-
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butyl-calixw8xarene, because of the larger and more flexible hydrogen bonding network. Due to its ‘pinched’ conformation, p-tert-butyl-calixw6xarene shows less affinity for the large cations.
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