Introduction

Copper is an element which represents redox-active properties. Among the transition metals Cu(II) is particularly attractive because, it is both an environmental pollutant and an essential trace element in biological systems. Its selective coordination is therefore a matter of practical importance for normal metabolic processes [1, 2]. Calixarenes are an important class of macrocyclic molecules in supramolecular and coordination chemistry, due to their preorganized structure, and to the ease of introduction of different types of donor groups onto both the lower and upper rim of the Calixarene unit [3–12]. The family of calix[4]crowns incorporated with amide unit (so called calix[4]azacrown) involves molecules combining calix[4]arene element and azacrown unit in their framework, so they exhibit superior recognition ability toward metal ions by the cooperative effects of calixarene and azacrown moieties [13–15]. It is well known that the complexation and extraction properties of these compounds appear to be highly dependent upon the nature and number of donor groups of moieties have been attached to the calixarene framework to achieve cyclic ligands as manifested in their shape, size, and conformation [1]. There are few reports in the literature regarding metal ion complexes and extraction properties of macrocyclic ligands incorporated with azacrown moieties formed on a calix[4]arene platform [16–19]. Thus the present paper deals with the synthesis and characterization of a calix[4]crown with amine units and its complexes with Cu2+ salts and extraction properties with Cu2+, Pb2+, Cd2+ and Ni2+ cations.

Experimental

Materials

5,11,17,23-Tetra-4-tert-butyl-25,27-di(aminoethoxy)-26,28-dihydroxycalix[4]arene [20] and 2-[3-(2-Formylphenoxy)-2-hydroxypropoxy]benzaldehyde [21] were prepared according to literature methods. Starting materials were all purchased commercially and used without further purification. Methanol solvent was distilled and dried before use and other solvents were used without further treatments.

Physical measurements

1H NMR spectra were recorded on a Bruker Avance 400 in CDCl3 solvent and chemical shifts are relative to residual solvent protons as internal standard at room temperature. Elemental analyses were performed on Elementar Vario EL III. FTIR spectra were recorded on a FTIR Spectrometer Bruker Tensor 27 in the region 400–4000 cm−1 using KBr pellets. Electronic absorption spectra were recorded with an Analytick Jena Specord 250 in the region 200–1100 nm. Melting points were obtained with an Electrothermal 9100 and are uncorrected. Cyclic voltammograms were measured using a glassy carbon as working electrode, a platinum wire auxiliary electrode, and a Hg/Hg2 2+ reference electrode. The ferrocenium–ferrocene couple as a standard shows a peak separation of 130 mV under the same experimental conditions. All scans were recorded in deoxygenated DMF containing 10−3 M complexes and 0.1 M LiClO4 as supporting electrolyte.

Synthesis of ligand H 3 L

5,11,17,23-Tetra-4-tert-butyl-25,27-di(aminoethoxy)-26,28-dihydroxycalix[4]arene (0.734 g, 1 mmol) was dissolved in refluxing dry methanol (200 cm3). 2-[3-(2-Formyl phenoxy)-2-hydroxy propoxy]benzaldehyde (0.3 g, 1 mmol) was dissolved in dry methanol (150 cm3) and then it was added dropwise over 2 h to the refluxing solution of the diamine. The reaction mixture was refluxed for a further 20 h. Sodium borohydride (0.19 g, 5 mmol) was carefully added and the reaction mixture was heated at the reflux for a further 2 h. The mixture was cooled to room temperature and the solvent removed under reduced pressure. To the residue was added chloroform (10 cm3) and 1 M sodium hydroxide solution (25 cm3). The chloroform layer was separated and then the solvent removed under reduced pressure. The crude product was taken up in methanol and acidified to pH < 2 with concentrated hydrochloric acid. The solvent was removed under vacuum and the product was slurry in chloroform and was shaken and neutralized with 1 M sodium hydroxide solution where upon the white solid dissolved in the organic layer. The chloroform phase was separated and dried over anhydrous sodium sulfate, and the solvent removed under reduced pressure [15, 22]. Yield: 0.65 g (65%). IR (KBr, cm−1) 3387 m, 3300 m, 3251 m, 2956 s, 1598 w, 1482 s, 1364 w, 1295 w, 1245 s, 1202 w, 1116 m, 1033 m, 872 w, 810 w, 752 s. 1H NMR (400 MHz, CDCl3): 1.07(s, 18H, C(CH3)3), 1.24 (s, 18H, C(CH3)3), 2.85–2.98 (m, 4H, NHCH 2CH2), 3.27–3.32 (dd, 4H, J = 5.55, ArCH 2NH), 3.33 (s, 2H, NH or OH), 3.88 (d, 2H, J = 13, ArCH2Ar), 3.97 (d, 2H, J = 13, ArCH2Ar), 4.06–4.15 (m, 6H, ArcalixOCH2 and ArCH2Ar), 4.23–4.30 (m, 6H, ArOCH2 and ArCH2Ar), 4.34 (m, 1H, CHOH), 6.89–6.92 (m, 8H, ArH), 7.01 (d, 4H, J = 4.22, ArH), 7.24 (m, 4H, ArH), 8.05(s, 1H, OH). m.p. 120 °C.

Synthesis of complexes

To a refluxing solution of ligand (0.1 mmol) in absolute ethanol was added dropwise a solution of the metal salts of Cu2+ (CuX2, X = ClO4 − and Cl−) (0.1 mmol) dissolved in the same solvent and refluxing was continued for a further 4 h. The solvent was concentrated and products were filtered off, washed with diethyl ether and n-hexane. The complexes were isolated as air-stable, soluble in chloroform, DMF, ethanol and methanol, and insoluble in n-hexane and diethyl ether.

[Cu(H 3 L)](ClO4)2

Dark green precipitates. Yield 0.08 g (64%). m.p. 280 °C (Dec.). Anal. Calc. (%) for [C65H82Cl2CuN2O15] (formula weight = 1265.8): C, 61.68; H, 6.53; N, 2.21. Found (%): C, 61.43; H, 6.47; N, 2.3. FTIR (KBr pellet, cm−1): 3504 s, 2958 s, 1603 m, 1484 s, 1363 w, 1296 w, 1248 m, 1199 m, 1113 s, 876 w, 758 m, 625 m. Λ M (DMF): 160 Ω−1mol−1 cm2.

[Cu(H 3 L)Cl]Cl

Brown precipitates. Yield 0.08 g (70%). m.p. 260 °C (Dec.). Anal. Calc. (%) for [C65H82Cl2CuN2O7] (formula weight = 1137.81): C, 68.61; H, 7.26; N, 2.46. Found (%): C, 68.50; H, 6.98; N, 2.29. FTIR (KBr pellet, cm−1): 3437 s, 2958 s, 1598 m, 1486 s, 1397 w, 1360 w, 1297 w, 1237 s, 1200 s, 1115 s, 1017 s, 873 m, 752 s. Λ M (DMF): 99 Ω−1mol−1 cm2.

Single-crystal X-ray diffraction

Single crystal of H 3 L was obtained from methanol solution at 60 °C. Crystallographic data were collected at 273 K with the Oxford Cryosystem Cobra low temperature attachment. The data were collected using a Bruker Apex2 CCD diffractometer with a graphite monochromated MoKα radiation at a detector distance of 5 cm and with APEX2 software [23]. The collected data were reduced using SAINT program [23], and the empirical absorption corrections were performed using SADABS program [23]. The structure was resolved by direct methods and refined by least-squares using the SHELXTL software package [24]. Materials for publication were prepared using SHELXTL [24] and ORTEP-III [25]. Table 1 summarizes crystal data and structure refinement for H 3 L.

Table 1 Crystal data and structural refinement for H 3 L
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Solvent extraction

A solution of ligand (H 3 L) (10 ml, 10−5 mmol/L) in chloroform and an aqueous solution containing nitrate salt of metal cations Cu2+, Pb2+, Cd2+ and Ni2+ (10 ml, 10−5 mmol/L) were mixed and shaken in a stoppered glass tube with a magnetically stirred in a thermostatic water bath at 25 °C for 1 h and finally left standing for an additional 15 min. Then, the aqueous phase was separated and the concentration of metal ion remaining in the aqueous phase was determined by atomic absorption. The extraction percentage (E%) was calculated by the following expression [26]:

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

where A0 and A are the initial and final absorbance of the metal ion before and after the extraction in aqueous phase, respectively. The extraction was repeated other concentrations of H 3 L, 2 × 10−5 mmol/L and 4 × 10−5 mmol/L, in chloroform (10 ml) and 10 ml aqueous solution containing (1 × 10−5 mmol/L) nitrate salts of metal cations Cu2+, Pb2+, Cd2+ and Ni2+.

Results and discussion

calix[4]crown with amine units (H 3 L)

Scheme 1 illustrates the used reactions for obtaining H 3 L ligand. We have found that in the reaction between 5,11,17,23-Tetra-4-tert-butyl-25,27-di(amino ethoxy)-26,28-dihydroxy calix[4]arene and 2-[3-(2-Formylphenoxy)-2-hydroxypropoxy] benzaldehyde the [1 + 1] Schiff-base macrocycle is formed as the majority product. In situ reduction with sodium tetrahydroborate provides the ligand H 3 L in a good yield. This ligand was characterized by IR, 1H NMR spectroscopy and single-crystal X-ray diffraction.

Scheme 1
scheme 1

Preparation steps of H 3 L. 1 Dry methanol, Refluxed for 20 h 2 room temperature, NaBH4, Refluxed for 2 h, acidic extraction

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The IR spectrum of H 3 L clearly shows the aliphatic and aromatic O–H stretching band at 3387 and 3300 cm−1. The IR spectrum of the reduced ligand H 3 L features a secondary amine N–H stretching at 3251 cm−1.

1H NMR spectra

The structure of H 3 L was fully characterized using 1H NMR spectroscopy. The labeling scheme used in the 1H NMR spectrum of H 3 L is given in Scheme 2 and the corresponding spectra is given in Fig. 1. 1H NMR data show that compound H 3 L has a cone conformation. A typical AB pattern [20, 27] was observed for the methylene bridge ArCH2Ar protons as the pair of doublets at 3.88 and 3.97 (JAB = 13 Hz) for the equatorial protons of methylene groups, whereas the axial protons appeared as composite multiplet peaks in the range of 4.06–4.15 and 4.23–4.30 ppm as the overlapped peaks with the ArOCH2- protons Hg and Hh, respectively. So each mentioned composite peaks is integrated for six protons. The Splitting pattern of ArCH2Ar protons shows that four equatorial and four axial protons is appeared in slightly different δ due to the space arrangement of crown moiety. The signals located in the 3.27–3.32 ppm range (J = 5.55) were assigned to the protons ArCH 2NH. The signals situated in the 6.89–7.26 ppm range with a total integration corresponding to the 16 protons were assigned to the protons on aromatic rings. The NHCH 2CH2 (c) protons were seen around δ 2.91 ppm as multiplet peak with integration corresponding to four protons. The tert-butyl protons appeared as two singlets at δ 3.00 and 3.09 ppm with a total integration of thirty-six protons. A singlet peak integrated for two protons at δ 3.33 ppm can be assigned to the OH protons or NH.

Scheme 2
scheme 2

1H assigments for H 3 L

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Fig. 1
figure 1

1H NMR spectrum of H 3 L (400 MHz) in CDCl3 solvent

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Crystal structure of H 3 L

The solid state structure of ligand H 3 L is determined by X-ray crystallography (Fig. 2). In this structure a water molecule can be seen in a far distance to H 3 L molecule as only an oxygen atom and the hydrogen atoms could not be resolved. Two out of four tert‐butyl groups is disordered. The dominating component of the disorder is rotation around C–C axis. The two distinct positions of the tert-butyl groups could be easily identified from the difference Fourier maps. On the other hand, no disorder was indicated for the remaining tert‐butyl groups. The C40–N1 and C51–N2 bond lengths of 1.455 and 1.472 Å, respectively, are within the range of N–C single bonds [28], indicating that the Schiff base bonds is reduced.

Fig. 2
figure 2

ORTEP representation of the Solid-state molecular structure for H 3 L

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Copper(II) complexes of H 3 L

The infrared spectral bands most useful for determining the mode of coordination of the ligand is the ν(N–H), ν(O–H) and ν(C–O–C) vibrations. The FT-IR spectra of ligand H 3 L display ν(O–Haro, O–Hali) and ν(N–H) at 3387, 3300 and 3251 cm−1. The all three bands is appeared as a strong broad band at 3504 and 3437 cm−1 for the complexes of [Cu(H 3 L)](ClO4)2 and [Cu(H 3 L)Cl]Cl, respectively. These bands is shifted to higher frequencies compared with ligand. Two intense bands at 1245 and 1033 cm−1 were assigned to the C–O–C stretching vibrations. These bands are observed at the lower wave numbers of 1237 and 1017 cm−1 respectively in the complex [Cu(H 3 L)Cl]Cl. This bathochromic shift of 8 and 16 cm−1 with respect to the values in non-coordinated ligand suggests coordination of etheric oxygens to the metal [14]. According to literatures [8, 9], it was found that the presence of intramolecular hydrogen bonds at the phenolic rim of the calixarene cavity prevents the coordination of Cu2+ by oxygen atoms. Thus only oxygen atoms located in the N2O2-macrocyclic crown (the upper part of H 3 L) can participate in the Coordination. Two bands in the regions of 1113.02 and 625.83 cm−1 in IR spectra of [Cu(H 3 L)](ClO4)2, are assigned to the asymmetric Cl–O stretching and the symmetric Cl–O bending vibrations, respectively. Such bands are the main characteristic of uncoordinated perchlorate [29, 30].

The molar conductivities of the complexes in DMF is reported. The conductance value of [Cu(H 3 L)](ClO4)2 complex (160 Ω−1mol−1cm2) lies in the range observed for 2:1 electrolytes (130–170 Ω−1mol−1cm2) and thus they are four-coordinate species as [Cu(H 3 L)]2+ in DMF solution without the coordinated perchlorate counter-ion. The conductance value for [Cu(H 3 L)Cl]Cl complex (99 Ω−1mol−1cm2) shows that this complex behaves as 1:1 electrolytes (65–90 Ω−1mol−1cm2), indicating the presence of five-coordinate species as [Cu(H 3 L)Cl]+ in solution [31].

UV–Vis studies

The UV–Vis spectra of compounds were recorded using 10−3–10−4 M DMF solution. The Vis absorption spectra of copper(II) complexes is shown in Fig. 3, and the spectral data are tabulated in Table 2. The electronic spectra of ligand in the UV region is dominated by two intense intra-ligand bands at 280 and 291 nm assigned to π–π* transitions. After forming the complex, these bands is appeared as a broad band that shifted to lower wavelength at 273 nm, corresponding to the *π → π transition of the benzene rings. In addition to this band, the spectrum has also a broaden band at higher wavelength assigned to ligand-to-metal charge transfer transition [32]. The [Cu(H 3 L)](ClO4)2 complex has a single d → d band at 596 nm with the molar absorptivity of 58.7 M−1 cm−1 consistent with square planar stereochemistry [33]. However, the Vis spectrum of [Cu(H 3 L)Cl]Cl complex shows a single d–d transition occurs at 791 nm (ε = 41.7 M−1 cm−1), with some evidence for a possible low-energy shoulder at 877 nm (ε = 42.3 M−1 cm−1), consistent with near trigonal bipyramidal (TB) stereochemistry [34]. Therefore, the Vis absorption spectra clearly shows that the ligand with the Cu2+ salts form complexes having different coordination geometries.

Fig. 3
figure 3

Vis absorption spectra of [Cu(H 3 L)Cl]Cl (a) and [Cu(H 3 L)](ClO4)2 (b) in DMF

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Table 2 Electronic spectral data of compounds in DMF
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Cyclic voltammetry

The cyclic voltammograms for both complexes was recorded in DMF using the same cell set up to investigate the redox properties of complexes. Results is summarized in Table 3 and shown in Fig. 4. The potential was scanned at scan rate of 100 mv/s and in the range of 1 to −1 V. [Cu(H 3 L)](ClO4)2 and [Cu(H 3 L)Cl]Cl complexes exhibit a reversible one electron reduction process assigned to the metal centered CuII/CuI redox couple at E1/2 = 0.415 and 0.410 V [35] versus a Hg/Hg2 2+ reference electrode in DMF, respectively. The peak separation between the anodic and cathodic peaks for the CuII/CuI redox couples indicates a reversible process for both complexes. Due to the presence of chloride ion on the fifth coordination site, the reductive and oxidative potentials of [Cu(H 3 L)Cl]Cl complex appear to be slightly different from the [Cu(H 3 L)](ClO4)2 complex. Reversibility in the [Cu(H 3 L)](ClO4)2 complex is more than the complex containing chloride ion.

Table 3 Cyclic voltammetry data for complexes in DMF solutions at 100 mVs−1 scan rate
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Fig. 4
figure 4

Cyclic voltammogram of [Cu(H 3 L)](ClO4)2 and [Cu(H 3 L)Cl]Cl complexes (1 × 10−3 M in DMF)

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Fig. 5
figure 5

The comparison of the structures of H 3 L and reference compound

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Two phase solvent extraction

In this research, The N2O2-macrocyclic crown is conjugated with calix[4]arene unit to be used the cooperative effects of calixarene in the superior recognition of metal ions [13–15], and the effect of the macrocyclic ring size on the extractability of metal ions [36]. Also, the hydroxyl group at the C-backbone in macrocycle has been chosen because pendant arm enhances the binding strength of the macrocycle toward cations, by cooperative ring-side arm interaction, in comparison with the macrocycle analogues without pendant arms [37].

The extraction data of calix[4]crown with amine units (H 3 L) is investigated for various cations (Fig. 6). The extractability of metal ions increases when ligand changes to the seconder amine compound from the Schiff base macrocycle [38]. Due to the increasing of flexibility, the ligand can easily change its conformational structure during the metal complexation. The percentage of extraction of ligand H 3 L towards various metal cations is investigated and compared with reference compound (N2O2-macrocyclic crown ligand) [38] in Table 4. The similarities between the structure of H 3 L and reference compound is shown in Fig. 5.

Fig. 6
figure 6

Extraction percentage of metal nitrates by H 3 L at 25 °C

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Table 4 Extraction percentage of metal cations by H 3 L and reference compound
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The results of extraction show that this ligand presents a high affinity towards Pb2+ and Cu2+ and no affinity for Ni2+. The increasing of efficiency of H 3 L was in the order Pb2+> Cu2+ > Cd2+ > Ni2+, when the concentration of H 3 L and cation are the same ([ligand]/[cation] = 1). When the concentration of H 3 L to the concentration of metal cation solution increases to two and four times, the same results as mentioned in above is obtained. But, the extraction efficiency of H 3 L increases with increasing of concentration of metal cation. It is mentioning that the results of Ni2+ don’t change. The Comparing of these results with the reference compound (a) indicates that N2O2-macrocyclic crown ligand with the smaller cavity forms more stable mononuclear species with Ni2+ than H 3 L. The 1,3-di-calix[4]arene-N2O2 macrocyclic crown conjugate (H 3 L) shows enhanced selectivity for Cu2+ and Pb2+ with a high extraction level of 74%. Also, the%E of Cd2+ was approximately the same as reference compound.

The results yield an estimation of the extraction abilities of H 3 L ligands towards the above metal cations. It is known that the N2O2-donors macrocyclic ligands with smaller cavity size forms stable complexes toward different nickel(II) salts [36]. However in this research, it is observed the conjugated N2O2 macrocyclic crown with calix[4]arene unit has no affinity toward Ni2+. So it seems that the macrocycle ring size is the major factor to determine the extraction ability. About nickel ion, the cooperative effect of calixarene also hasn’t been able to improve the extraction ability.

Conclusions

A new p-tert-butyl calix[4]crown with amine units (H 3 L) is prepared in a good yield. This ligand acts as N2O2 donor set when reacts with Cu(II) salts. Based on the spectroscopic data, molar conductivity and CHN analysis, it can be predicted the structure of [Cu(H 3 L)](ClO4)2 complex is four coordination with square planar stereochemistry, and the structure of [Cu(H 3 L)Cl]Cl complex is five coordination with near TB coordination geometry. Redox properties of these complexes clearly shows that ligand (H 3 L) can produce a reversible one-electron metal-centered CuII/CuI redox couple. On the basis of liquid–liquid extraction experiments, ligand H 3 L indicates good affinity toward Pb2+ and Cu2+.

Appendix A. Supplementary data

Crystallographic data for the structure have been deposited with the Cambridge Crystallographic Data Centre, CCDC No. 1491921 for H3L. Copies of this information may be obtained free of charge from the Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK (fax: +44 1223-336-033; e-mail for deposition: deposit@ccdc.cam.ac.uk).