Introduction

Calixarenes have been of great interest in the field of supramolecular chemistry for over 20 years [1, 2]. They can be readily functionalized by numerous selective procedures on the phenolic or phenyl units and are available not only in the calix[4]arene series [3, 4], but also calix[6]arenes [5, 6], calix[5]arenes [7, 8] and calix[8]arenes [9, 10]. Calixarenes can take a variety of conformational forms. For example, calixarene can be locked into one of four kinds of conformers [11,12,13], which are cone, partial-cone, 1,2-alternate, and 1,3-alternate conformations. 8-Hydroxyquinoline as a coordinating substituent has been previously introduced onto a calix[4]arene via ether linkages to give a bis(8-hydroxyquinoline)calix[4]arene derivative, exhibiting luminescence and energy transfer properties [14]. In this study, we report the synthesis of p-tert-calix[4]arene bearing 8-hydroxyquinoline, 3, and its complexation with lanthanide ions.

Results and discussion

As shown in Scheme 1, ligand 3 was synthesized in three steps from p-tert-butylcalix[4]arene. Treatment of the calizarene with Cl(CH2)3CN in the presence of K2CO3 [15] gave the dicyanopropoxy-p-tert-butylcalix[4]arene (1) as a white powder in 94% yield. The reduction of the obtained dicyanopropoxy derivative using borane gave calixarene (2) bearing 4-aminobutoxy substituents in 90% yield. Coupling calixarene (2) with 7-carboxy-8-hydroxyquinoline using DMAP and HBTU in CH2Cl2 provided 3 in a yield of 67%. The calix[4]arene unit having the cone conformation was deducted from its 1H NMR spectrum in CDCl3, as shown by the presence of characteristic AB systems at 3.30 and 4.24 ppm (J = 13 Hz) for the ArCH2Ar protons of the macrocycle. The formation of the amido function was deduced from the presence of a triplet corresponding to the CONH protons. Two singlets (0.94 and 1.30 ppm, 6.78 and 7.07 ppm) in an integration ratio 18:4 were attributed to the –C(CH3)3 and ArH and triplets at 3.98 ppm to the ArOCH2 groups of the calix[4]arene unit.

Scheme 1
scheme 1

Synthesis of 3: a K2CO3, Cl(CH2)3CN, acetonitrile, b BH3, THF, c DMAP, HBTU, 7-carboxy-8-hydroxyquinoline

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The stability constants (βxy) for lanthanoids (La3+, Nd3+, Eu3+, Gd3+, and Yb3+) with calixarene (3) being the concentration ratio [MxL xn+y ]/[Mn+]x[L]y (where Mn+ = metal ion and L = ligand 3) were determined in acetonitrile by UV/vis absorption spectrophotometry at 25 °C.

The spectra of the different concentrations of lanthanum perchlorate with the ligand 3 (at 2.5 × 10−5 mol L−1) in acetonitrile were recorded between 220 and 320 nm, with the ionic strength maintained by Et4NClO4 (0.01 mol L−1) (Fig. 1).The formed species in solution and the logarithms of stability constants were determined and calculated by using the calculation program Letagrop [18]. The best values for the formation constants (βxy) of the various complex species and their molar absorption coefficients in various wavelengths were deduced from the fit between the experimental and calculated spectra. The UV/vis spectra of ligand 3 between 220 and 320 nm corresponded with the π → π* transitions of the aromatic moieties. Upon the addition of lanthanoid perchlorates (La3+, Nd3+, Eu3+, Gd3+, and Yb3+), the spectra in most cases showed distinct changes. The maximum absorption of the ligand 3 was observed at 256 nm and its complexation with lanthanoids were manifested by a hypochromic displacement.

Fig. 1
figure 1

UV/vis absorption spectra on complexation of La (ClO4)3 with ligand 3 in acetonitrile (0 ≤  RM/L ≤ 1.6) at 25 °C (CL = 2.5 × 10−5 mol L−1)

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Table 1 lists the stability constant (logβxy) data for the lanthanoid complexation with ligand 3. The data are in accord with the formation of two mononuclear species and biligand stoichiometry with ligand 3. Since the lanthanoids have high electropositive and hard acid character, they interact with hard bases such as oxygen, and the amido derivative of calix[4]arene, 3, is a good candidate for complexation with them [19,20,21,22,23]. Mononuclear lanthanoid species formed by the ligand gave the logarithmic values of the stability constants between 4.71 and 6.60, which significantly increased for Eu3+, where the selectivity S 3+Eu 3+/Nd is close to 77. It seems that the particular stability of Eu3+ complex is due to the coherence between the ion size and the cavity size of the calixarenes amide. The biligand complexes (ML2) formed by the ligand 3 with the cations Eu3+, Gd3+ and Yb3+ have similar stability level (from 12.31 to 12.74 logarithmic units). These stability changes indicate the significant complexation selectivity (\({\text{S}}_{{{\text{Gd}}^{3 + } /{\text{Nd}}^{3 + } }} =\) 9 × 103) in favor of the gadolinium complex compared with the neodymium complex, and suggests that two units of the ligand are organized more effectively around the Gd3+ ion through favorable bond distances. Taking into account the logarithm of stepwise constants calculated from logβ, logK12 is smaller than logK11 except for Gd3+ (Table 2).

Table 1 The stability constants (LogβML) for lanthanoids cation complexes with ligand 3 in acetonitrile at 25 °C, I = 10−2 M
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Table 2 Logarithms of stepwise stability constants formed by the ligand 3 and the lanthanoid cations in acetonitrile
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The stoichiometry of M:L and M:L2 for the La3+ complex with ligand 3 were confirmed by conductometric titration (Fig. 2).

Fig. 2
figure 2

Conductometric titration of 3 with La3+

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A preliminary evaluation of the binding efficiency ligand 3 base derivatives has been carried out by solvent extraction of metal picrates into dichloromethane at 25 °C under neutral conditions. The extraction percentages (%E) of some lanthanoid metal picrates by ligand 3 from water into dichloromethane are given in Table 3.

Table 3 Extraction percentages (%E) of lanthanoid picrates from water into dichloromethane by ligand 3, at 25 °C (CL = CM = 2.5 × 10−4 mol L−1)
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The extraction efficiency of each lanthanoid cation with ligand 3 was low at 12–14%, but Yb3+ was the most pronounced (14%). These data do not correspond to the stability of complexation with ligand 3 (see Table 1) and show that the complexation with the ligand is not the major driving force for the extraction of these cations.

Experimental

General

UV/vis spectra were obtained by a Perkin Elmer Lambda 25 spectrophotometer. All chemicals were analytical pure and purchased from Aldrich. They were used without further purification. The supporting electrolyte, Et4NClO4, for the stability constant determination was purchased from ACROS Organics.1H NMR spectra were recorded at 300 K using a 300 MHz Bruker SY200 spectrometer. Melting points were determined using an Electrothermal apparatus. The extraction experiments of metal picrates from water into dichloromethane were performed according to the literature procedure [16, 17]. Equal volumes (5 mL) of neutral aqueous solutions of metal picrate (2.5 × 10−4 mol L−1) and dichloromethane solution of calixarene derivatives (2.5 × 10−4 mol L−1) were mixed under magnetic agitation for 24 h, then left standing for 1 h in order to obtain the complete separation of two phases. The concentration of metal picrate remaining in the aqueous phase was determined at 355 nm by UV/vis spectrometer. The percentage extraction (%E) was derived from the following expression, where A0 is the absorbance of the blank solution without calixarene:

$$\% {\text{E}}\, = \,100\left( {{\text{A}}_{0} {-}{\text{A}}} \right)/{\text{A}}_{0} .$$

Preparation of 1

p-tert-Butylcalix[4]arene (5 g, 7.70 mmol), K2CO3 (21 g, 154 mmol) and 4-chlorobutyronitrile (14 mL, 154 mmol) were heated at reflux in acetonitrile (50 mL) for 5 days. The solvent was evaporated and the residue was taken up in dichloromethane (100 mL), washed with 1 M HCl (100 mL), H2O (50 mL) and brine (50 mL), then dried with MgSO4. Dichloromethane was evaporated and the residue was recrystallized from chloroform and methanol to give as white solid (94%) yield. Mp > 300 °C. IR (KBr, ν/cm−1) = 2250 (CN), 3416 (OH). 1H NMR (CDCl3): δ (ppm) = 7.41 (s, 2H, ArOH), 7.05 (s, 4H, ArH), 6.86 (s, 4H, ArH), 4.14 (d, 4H, J = 13.0 Hz, system AB, ArCH2Ar), 4.09 (t, 4H, J = 5.0 Hz, ArOCH2), 3.35 (d, 4H, J = 13.0 Hz, AB system, ArCH2Ar), 3.04 (t, 4H,, J = 5.0 Hz, CH2CN), 2.64 (q, 4H, J = 5.0 Hz, CH2CH2CN), 1.27 (s, 18H, tert-butyl), 0.99 (s,18H, tert-butyl). 13C NMR (CDCl3): δ (ppm) = 150.3, 148.8, 147.6, 142.1, 132.5, 127.5, 125.8 and 125.3 (Ar), 119.4 (–CN), 73.3 (–OCH2), 34.0, 33.9, 31.8, 31.7, 31.0, 26.6 and 14.2 (ArCH2Ar, –CH2CH2– and ––C(CH3)3). MW = 783.09 calcd for C52H66O4N2. MALDI-TOF: m/z = 783.2. Anal. calcd for C52H66O4N2 = C 79.76, H 8.50, N 3.58. Found: C 79.29, H 8.77, N 3.60%.

Preparation of 2

A solution of BH3•THF (1 M, 10 mL) was added to the stirred suspension of 1 (1.5 g, 1.91 mmol) in THF (40 mL). The mixture was stirred for 1 h at RT and refluxed for 5 h. BH3•THF (10 mL) was added after cooling the mixture and refluxed for an additional 15 h, then allowed to RT. HCl (6 M, 40 mL) was added and refluxed for 1 h. THF was removed under reduced pressure to give a colorless suspension. The mixture was left for 12 h at 0 °C and filtered, then washed with water and dried under vacuum at room temperature for 24 h to give as white powder (90%) yield. Mp = 168-169 °C. IR (KBr, ν/cm−1) = 3444 (NH), 3385 (OH). 1H NMR (CDCl3): δ (ppm) = 7.65 (s, 2H, ArOH), 7.04 (s, 4H, ArH), 6.82 (s, 4H, ArH), 4.25 (d, 4H, J = 13.0 Hz, AB system, ArCH2Ar), 3.98 (t, 4H, ArOCH2), 3.28 (d, 4H, J = 13.0 Hz, AB system, ArCH2Ar), 2.96 (m, 4H, J = 5.0 Hz, CH2N), 2.87 (t, 4H, NH2), 2.61 (m, 4H, J = 5.0 Hz, CH2CH2N), 2.43 (m, 4H, J = 5.0 Hz, CH2CH2CH2N), 1.27 (s, 18H, tert-butyl), 0.98 (s, 18H, tert-butyl). 13C NMR (CDCl3): δ (ppm) = 150.6, 149.9, 146.8, 141.5, 132.6, 127.8, 125.5 and 125.1(Ar), 76.2 (–OCH2–), 41.9 (–CH2N–), 33.9, 33.8, 31.7, 31.5, 31.0, 30.2 and 27.4 (ArCH2Ar, –CH2CH2– and –C(CH3)3). MW = 791.15 calcd for C52H74O4N2. MALDI-TOF: m/z = 791.59. Anal. calcd for C52H74O4N2 = C 78.94, H 9.43, N 3.54. Found: C 76.68, H 8.86, N 3.70%.

Preparation of 3

HBTU (hexafluorophosphate benzotriazole tetramethyl uronium, 1.27 mg, 3.36 mmol) was added to the mixture of 2 (1.12 g, 1.41 mmol), 7-carboxy-8-hydroxylquinoline (0.63 mg, 3.36 mmol) and DMAP (4-dimethylaminopyridine, 0.41 mg, 3.36 mmol) in dichloromethane (120 mL). After stirring for 3 days at RT, the solvent was evaporated under reduced pressure and the residue washed with methanol (3 × 15 mL), then dried under vacuum to give as a white powder (67%) yield. Mp = 158–160 °C. IR (KBr, ν/cm−1) = 3300 (NH), 3400 (OH), 1700 (CO). 1H NMR (CDCl3): δ (ppm) = 8.85 (dd, 2H, J = 1.3 Hz, Ha), 8.21 (t, 4H, J = 6.0 Hz, NH), 8.06 (dd, 2H, J = 1.3 Hz, Hc), 8.05 (d, 2H, J = 8.8 Hz, He), 7.45 (dd, 2H, J = 1.3 Hz, Hb), 7.44 (s, 2H, ArOH), 7.17 (d, 2H, J = 8.8 Hz, Hd), 7.07 (s, 4H, ArH), 6.78 (s, 4H, ArH), 4.24 (d, 4H, J = 13.0 Hz, AB system, ArCH2Ar), 3.98 (t, 4H, ArOCH2), 3.30 (d, 4H, J = 13.0 Hz, AB system, ArCH2Ar), 3.04 (m, 4H, J = 5.0 Hz, CH2N), 2.65 (m, 4H, J = 5.0 Hz, CH2CH2N), 2.49 (m, 4H, J = 5.0 Hz, CH2CH2CH2N), 1.30 (s, 18H, tert-butyl), 0.94 (s, 18H, tert-butyl). 13C NMR (CDCl3): δ (ppm) = 168.13(C=O), 155.31(C), 150.41(CH), 139.27(CH), 135.90 (C) 123.23(CH), 125.58(CH), 117.33 (C), 112.74(CH) 39.71, 33.93, 33.90, 31.74, 30.99, 27.22 and 27.38 (ArCH2Ar, –CH2CH2– and –C(CH3)3). MW = 1133.46 calcd for C72H84O8N4. MALDI-TOF: m/z = 1133.62. Anal. calcd for C 76.29, H 7.47, N 4.94. Found: C 75.91, H 7.39, N 4.83%.

Conclusion

The binding properties of lanthanoid cations with 3 have been assessed by the determination of stability constants in acetonitrile using UV/vis spectrophotometry. The quantification of the complexation equilibria was used to estimate the stoichiometry of the formed complexes. The results showed the formation of two mononuclear and biligand species with the ligand 3. The interrelationships between the stability constants of lanthanide cations with the ligand 3 and their extraction efficiency were inconsistent.