Resorcin[4]arene Schiff base derivatives: Synthesis,characterization,and extraction studies

Abstract

Novel 2,6-dihydroxyacetophenone[4]arene derivatives with an electron-withdrawing group at the ortho-position are developed. The 2,6-dihydroxyacetophenone[4]arene derivatives are condensed with benzylamine in chloroform to give the Schiff base derivatives of resorcin[4]arene. The acetyl group of the 2,6-dihydroxyacetophenone[4]arenes is condensed with primary amine groups to form imines. The multifunctional target molecules are purified and isolated in good yields. The Schiff base derivatives are characterized by elemental analysis, ¹H and ¹³C nuclear magnetic resonance spectroscopy, infrared spectroscopy, and mass spectrometry. All the Schiff base compounds are successfully used for the extraction of toxic metals including Ni²⁺, Mn²⁺, Hg²⁺, Co²⁺, and Na⁺ using a liquid–liquid solvent extraction process. Among these tested metals, the percentage extraction of Hg²⁺ is the highest. As sodium dichromate has greater oxidative stability to transfer the HCr₂O₇⁻ anion from an aqueous a protonated ligand solution, an anionic extraction study of dichromate is tested at various pH values.

Introduction

The synthesis of calix[4]resorcinarenes by acid-catalyzed cyclocondensation of resorcinol with aliphatic or aromatic aldehydes has received increasing attention in recent times.^1–3 Calix[4]resorcinarenes, having unique sizes and bowl-like cavities, can hold certain ions and neutral molecules by host–guest interactions and have been used in supramolecular chemistry applications in numerous fields. ⁴ The field of tetra-aminoresorcin[4]arenes has been studied extensively and these derivatives can be used to obtain advanced functionalized resorcin[4]arene cavities with specific receptor properties. ⁵ The synthesis and other receptor properties of hetero-functionalized derivatives have been described by Glushko et al. ⁶ The reactions of tetraformyl resorcin[4]arene with amines to obtain imines have been developed. ⁷ Tetraformyl resorcin[4]arene imines, containing free OH groups, can serve as important intermediates for the construction of supramolecular assemblies and have been shown to have great potential in the formation of new types of inherently chiral cavity–containing compounds. ⁷ Schiff bases are very suitable for the formation of metal complexes and in coordination chemistry where they are involved in the formation of chelates.^8–15 New calix[4]arene Schiff bases have been synthesized and their transition temperatures and thermal behavior studies have been described in the literature by polarizing optical microscopy (POM) and differential scanning calorimetry (DSC) techniques. ¹⁶

Heavy metals create many issues worldwide because of their high toxicity effects on humans and animals. Some toxic heavy metals, such as cadmium, lead, mercury, and chromium, are found in industrial wastewaters produced from metal-plating, smelting, battery manufacturing, tanneries, petroleum refining, paints, pesticides, pigment manufacturing, and so on.^17–20 Recently, the molecular recognition of anionic guests through electron-lacking neutral abiotic receptor molecules has been studied. The significance of favorable hydrogen-bonding interactions of amines, amides, or imides (–NH₂/OC–NH/OC=N) for anion binding has been exploited in recent times for the design of calix[4]arene anion receptors. Calix[4]arene-based chelating units can be used for numerous studies on anion coordination. ²¹ In addition, there are a number of articles reporting the use of Schiff base ligands for the detection of cations.^22–28 Benzimidazole-based receptors show anion binding with some anions as detected by UV-Vis and fluorescence techniques.^29,30 Liquid–liquid extraction methods were used to extract Hg²⁺ ions by chromogen ionophore calix[4]arene-aza-crown ether derivatives ³¹ and by the aza-ligating site of p-tert-butylcalix[4]arene aza crown ethers. ³² Studies of dichromate anion extraction by spectrophotometric method with p-tert-butylcalix[4]arene β-ketamine derivatives have been published. ³³ Furthermore, the solid–liquid extraction study of calix[4]arene Schiff bases for chromate ions have been performed at different pH values. ³⁴

In this paper, resorcin[4]arenes with an acetyl group at the ortho-position have been synthesized in excellent yields using sodium methoxide as the base under reflux conditions. Benzylamine was used to generate novel Schiff base derivatives of the resorcin[4]arene. These Schiff bases have been applied for the extraction studies of anionic/cationic toxic metals.

Results and discussion

The 2,6-dihydroxyacetophenone[4]arene derivatives (1a–h) were synthesized by following the previously published procedure described by our group. ³⁵ Compounds 1a–h were the starting materials, which were condensed with benzylamine in chloroform to generate the Schiff base derivatives 2a–h (Scheme 1). The main purpose for preparing these resorcin[4]arene Schiff bases was for the recognition of anions and cations of toxic metals present in aqueous media. This will be useful for various applications in environmental, clinical, industrial, and many other research areas. ²¹ The Schiff base derivatives were obtained in excellent yields. The physical properties of the Schiff base derivatives (2a–h) are listed in Table 1.

Scheme 1.

Synthesis of the resorcin[4]arene Schiff bases.

Table 1.

Physical properties of compounds 2a–h.

Compound	Molecular weight	Molecular formula	Melting point (°C)	Color	Yield (%)
2a	1125	C72H76N4O8	194	Orange	68
2b	1181	C76H84N4O8	187	Light orange	70
2c	1238	C80H92N4O8	216	Dark yellow	71
2d	1294	C84H100N4O8	130	Yellowish orange	72
2e	1350	C88H108N4O8	190	Dark orange	74
2f	1406	C92H116N4O8	165	Orange	74
2g	1462	C96H124N4O8	205	Light red	75
2h	1518	C100H132N4O8	168	Yellow	76

In the ¹H nuclear magnetic resonance (NMR) spectra of the imines, the eight hydroxyl groups of the parent ring are observed at about δ 12 ppm. The protons of the mono-substituted benzene rings are obtained at around δ 7.28 ppm and the signals of the –CH₂– protons of benzyl groups are found at about δ 4.7 ppm. The presence of these two peaks confirms the formation of the Schiff bases with benzylamine. The characteristic signals of the bridging protons of the parent ring occurred between 4 and 4.5 ppm as a triplet. All the Schiff base compounds were effective for dichromate anion and cation extraction by liquid–liquid solvent extraction. Highly toxic chromate and dichromate anions are present in soil and water. Chromium (VI) is a human and animal carcinogen. ²¹ Cationic toxic metals also affected on human health. ¹⁷ Therefore, we focused on liquid–liquid extraction studies to remove the toxic anions and cations by Schiff base derivatives. Solvent extraction experiments have been performed to remove alkali metal and transition metal cations such as Co²⁺, Hg²⁺, Mn²⁺, Ni²⁺, and Na⁺ and dichromate anions from the aqueous phase into the organic phase using compound 2. Extraction of the cations/anions of the toxic metal from the aqueous phase into the organic phase occurs with these compounds. The concentration of cations remaining in the aqueous phase has been measured spectrophotometrically and the results are given in Figures 1–3. The resulting data are given in Table 2 and Figure 1.

Figure 1.

Plot of the percentage extraction of metal cations with ligand 2h.

Figure 2.

Plot of the percentage extraction of Hg²⁺ cations with ligands 2a–h.

Figure 3.

Plot of the percentage extraction of dichromate anion versus ligands (2a–h) at different pH values.

Table 2.

Extraction of metal cations with ligand 2h.

Metal cation	Co2+	Hg2+	Mn2+	Ni2+	Na+
Percentage extraction	24.49	37.69	14.87	9.27	29.52

Concentration of metal cation = 1 × 10⁻⁴ M. Concentration of ligands in CH₂Cl₂ = 1 × 10⁻³ M.

We found that compound 2h is particularly successfully useful for the extraction of metal cations from the aqueous phase because of the imine group on the ligand. Furthermore, from these extraction data, we found that the maximum extraction achieved for mercury occurred with ligand 2h. The extraction of Hg²⁺ was also tried with ligands 2a–g and the obtained data are given in Table 3 and Figure 2.

Table 3.

Percentage extraction of Hg²⁺ with ligands 2a–h.

Ligand	2a	2b	2c	2d	2e	2f	2g	2h
Percentage extraction	20.83	40.04	41.91	25.50	25.59	42.02	22.63	37.69

Concentration of metal cation = 1 × 10⁻⁴ M. Concentration of ligands in CH₂Cl₂ = 1 × 10⁻³ M.

Schiff bases show greater extraction capacity than 2,6-dihydroxyacetophenone[4]arene for cations because of the imine group. The imine group binds easily and provides good extraction ability toward transition metals like Hg²⁺. The percentage extraction of Hg²⁺ found in the reported literature was nearly about 50%.^34,36,37 The best results are obtained with mercury due to its high electronegativity compared to the other tested metals.

The dichromate ion, which includes the oxide moiety of HCr₂O₇⁻/Cr₂O₇²⁻, has potential sites to create hydrogen bonds with ligands. ³⁴ The extraction of dichromate ions with ligands 2a–h has been studied at various pH values of the dichromate solution such as 5, 4, 3, 2, and 1. The percentage extraction of dichromate anions at different pH values shows reasonable extraction by the ligands. By decreasing the pH values of the dichromate solution, the percentage extraction is increased. ³⁴ The data for the extraction studies are listed in Table 4 and a graphical presentation is shown in Figure 3.

Table 4.

Percentage extraction of dichromate anions with ligands 2a–h at different pH values.

Ligand	pH 5 (%)	pH 4 (%)	pH 3 (%)	pH 2 (%)	pH 1 (%)
2a	6.86	10.78	20.58	27.45	32.35
2b	5.88	9.86	17.91	21.93	28.17
2c	5.12	9.17	11.45	18.50	22.65
2d	2.47	7.07	9.44	11.76	18.16
2e	0.98	4.60	7.73	11.76	15.93
2f	2.54	5.30	10.81	16.65	20.29
2g	1.66	3.88	7.02	11.76	14.63
2h	6.17	12.65	19.02	26.23	32.00

Concentration of metal anion = 1 × 10⁻⁵ M. Concentration of ligand in CH₂Cl₂ = 1 × 10⁻³ M.

At the lower pH values of aqueous solutions of dichromate, the anion will exist in the protonated form HCr₂O₇⁻. This mono-anion of HCr₂O₇⁻ will have less free energy of hydration than the dianionic system Cr₂O₇²⁻. ³³ When HCr₂O₇⁻ is transferred from the aqueous phase into the dichloromethane phase, there is a slight loss in hydration energy. ³³ At lower pH values, the percentage extraction increases because of protonation of the imine nitrogen. ²¹ Another reason for improved extraction of dichromate anions at lower pH values is that both the development of NaHCr₂O₇ and the protonation of the imine nitrogen support the extraction into dichloromethane. ²¹ In the aqueous solution at the low pH, the free energy of hydration for alkylammonium ions is less than that of the sodium ion, hence chromate ions are extracted effectively by the ligands. ³³ The maximum percentage extraction of dichromate anions occurred at pH 1.00. At more acidic pH values, the dichromate anion exists as HCr₂O₇⁻/Cr₂O₇²⁻, and due to the highly acidic conditions, the HCr₂O₇⁻ and Cr₂O₇²⁻ dimerize and become the dominate Cr(IV) form. ³³ Upon addition of the NaOH solution into aqueous solution, these ligands do not extract dichromate ions because there is no protonation of the Schiff base in dichloromethane solution. ³⁴ Another factor is that hydrophilic nature of OH improves the extraction ability at the water–dichloromethane interface, which might deliver the proton interchange for transferring Na⁺ ions. Under highly acidic conditions, Na₂Cr₂O₇ is converted into HCr₂O₇⁻ and then ionization in aqueous solution exists in Cr₂O₇²⁻ by the given ionization process ³⁴

$$\begin{array}{l}{\text{H}}_2{\mathrm{CrO}}_7\rightleftarrows {\mathrm{H}}^{+}+{\mathrm{HCr}}_2{\mathrm{O}}_7^{-}\\ {\mathrm{HCr}}_2{\mathrm{O}}_7^{-}\rightleftarrows {\mathrm{H}}^{+}+{\mathrm{Cr}}_2{\mathrm{O}}_7^2\end{array}$$

Extraction of an anion Aⁿ⁻ by the receptor LHⁿ⁺ is assumed by the following equilibrium ³³

$$n{\left({\mathrm{LH}}^{n+}\right)}_{\mathrm{org}}+n{\mathrm{A}}_{\mathrm{aq}}^{n-}\rightleftharpoons {\left[\left({\mathrm{LH}}^{n+}\right)n,{\mathrm{A}}_n^{n-}\right]}_{\mathrm{org}}$$

(1)

Thus, K_ex is defined by equation (2) and the calculated value of K_ex leads to log K_ex of 5.95 for a 10⁻³ M concentration of ligand 2h

$$K_{\mathrm{ex}}=\frac{{\left[\left({\mathrm{LH}}^{n+}\right)n,{\mathrm{A}}_n^{n-}\right]}_{\mathrm{org}}}{{[{\mathrm{A}}^{n-}]}_{\mathrm{aq}}^n{[{\mathrm{LH}}^{n+}]}_{\mathrm{org}}^n}$$

(2)

Conclusion

The 2,6-dihydroxyacetophenone[4]arene derivatives (1a–h) have been synthesized with quantitative yields and excellent purity. The formation of the resorcin[4]arene Schiff base derivatives has been completed by condensation of 2,6-dihydroxyacetophenone[4]arene with benzylamine, and all the products were obtained in excellent yield and characterized by ¹H and ¹³C NMR spectroscopy, IR spectroscopy, and mass spectrometry. The presence of imine nitrogen atoms in the target molecules makes them suitable for the extraction of cations/anions. Metal extraction studies have shown that the Schiff base compounds are useful for extracting certain transition metal cations and alkali metal cations. Dichromate anion extraction studies using ligand 2 at different pH values showed them to be good extractants of ${\mathrm{HCr}}_2{\mathrm{O}}_7{-}^{/}{\mathrm{Cr}}_2{\mathrm{O}}_7{2}^{-}$ at lower pH values. Compound 2h proved to be the best for extracting Hg²⁺ cation. These results may provide scope for the extraction of other toxic metals. The synthesized macrocycles may serves as an important intermediates for the construction of novel supramolecular architectures.

Materials and methods

Materials

All reagents and solvents used were of L.R. grade without further purification, and the aldehydes were purchased from Sigma-Aldrich.

Instruments

Thin-layer chromatography (TLC) was run on aluminum pre-coated TLC silica gel 60 F254 plates (Merck, Germany) and visualization was performed using iodine or UV light. Melting points were recorded on a programmable Veego melting point apparatus. ¹H NMR (CDCl₃) and ¹³C NMR (CDCl₃) spectra were recorded using a Bruker Avance spectrometer (400 MHz). IR spectroscopic data were recorded using a Perkin Elmer FTIR 377 spectrometer using the KBr powder pellet technique. Mass spectra were obtained using an XEVO-G2S QTOF instrument. Elemental analysis were recorded on a EURO VECTOR EA3000 CHNS-O Analyzer. The UV absorption spectra of all compounds were measured on a Jasco V-630 UV-Vis spectrophotometer.

Preparation of 2,6-dihydroxyacetophenone[4]arene derivatives (1a–h)

The derivatives of 2,6-dihydroxyacetophenone[4]arene were synthesized by following our group’s published procedure. ³⁵ 2,6-Dihydroxyacetophenone (0.500 g, 3.289 mmol) and the aliphatic aldehyde (3.289 mmol) in tetrahydrofuran (THF; 20 mL) was treated with NaOMe (0.1 g, 1.85 mmol) at room temperature. The reaction mixture was then heated at reflux with constant stirring for 72 h. After completion of the reaction, the mixture was cooled to room temperature and the solvent was evaporated under vacuum. The crude product was dissolved in MeOH (25 mL) and a few drops of AcOH were added until pH 7. A yellow precipitate was formed in the solution and it was stirred at 0–5 °C for 30 min. The product was then filtered, washed with cold methanol, and purified by column chromatography using ethyl acetate and hexane and then dried at 40–45 °C. All the synthesized compounds have been characterized by IR, ¹H and ¹³C NMR, and mass spectrometry.

15-(1-(Benzylimino)ethyl)-35,55,75-tris(1-(benzylimino)ethyl)-2,4,6,8-tetraethyl-1,3,5,7(1,3)-tetra-benzenacyclooctaphan-14,16,34,36,54,56,74,76-octol (2a); general procedure

A mixture of compound 1a (1.00 g, 1.30 mmol) and benzylamine (2.27 mL, 20.80 mmol) in 20 mL of CHCl₃ was stirred for 24 h at room temperature. After completion of the reaction, water was added to the organic layer and was separated. The aqueous layer was extracted twice with CHCl₃ and combined organic layers were dried over anhydrous MgSO₄ and filtered. The solvent was evaporated on a rotary evaporator under vacuum at 40 °C. The oily crude residue was treated with hexane and methanol to give a solid product. The solid was filtered off and washed with chilled MeOH. The obtained product was dried in an oven at 60 °C. Color: Orange; yield: 68%, m.p. 194 °C; ¹H NMR (400 MHz, CDCl₃): δ 11.98 (s, 8H, ArOH), 7.24 (s, 4H, ArH), 7.33 (m, 20H, ArH benzylamine), 4.69 (s, 8H, –CH₂N), 4.42 (t, 4H, J = 8.0 Hz, –CH–), 2.69 (s, 12H, –CH₃CN), 2.22 (q, 8H, J = 8.0 Hz, –CH₂–), 0.96 (t, 12H, J = 8.0 Hz, –CH₃); ¹³C NMR (400 MHz, CDCl₃): δ 12.39, 19.74, 25.45, 35.15, 47.98, 107.71, 126.50, 126.95, 127.10, 128.07, 130.33, 138.75, 161.62, 177.41; IR (KBr) ν/cm⁻¹: 2956.75 (–CH₂–), 1547.19 (C=N), 1450.25 (ArNH), 730.21 (C=C). Anal. calcd for C₇₂H₇₆N₄O₈: C, 76.84; H, 6.81; N, 4.98; found: C, 76.83; H, 6.80; N, 4.96.

The other derivatives of the Schiff bases were synthesized using a similar pathway from compounds 1b–h with different amounts of benzylamine: 1b (2.10 mL, 19.39 mmol), 1c (1.78 mL, 18.14 mmol), 1d (1.86 mL, 17.00 mmol), 1e (1.76 mL, 16.11 mmol), 1f (1.66 mL, 15.25 mmol), 1g (1.58 mL, 14.47 mmol), and 1h (1.50 mL, 13.73 mmol).

15-(1-(Benzylimino)ethyl)-35,55,75-tris(1-(benzylimino)ethyl)-2,4,6,8-tetrapropyl-1,3,5,7(1,3)-tetra-benzenacyclooctaphan-14,16,34,36,54,56,74,76-octol (2b): Light orange; yield 70%, m.p. 187 °C; ¹H NMR (400 MHz, CDCl₃): δ 11.96 (s, 8H, ArOH), 7.28 (s, 4H, ArH), 7.41 (m, 20H, ArH benzylamine), 4.82 (s, 8H, –CH₂N), 4.54 (t, 4H, J = 8.0 Hz, –CH–), 2.52 (s, 12H, –CH₃CN), 2.18 (q, 8H, J = 7.6 Hz, –CH₂–), 1.38 (m, 8H, –CH₂), 1.00 (t, 12H, J = 7.6 Hz, –CH₃); ¹³C NMR (400 MHz, CDCl₃): δ 13.80, 19.72, 20.86, 32.65, 34.53, 47.98, 107.67, 126.87, 126.97, 127.80, 128.45, 130.32, 135.79, 161.59, 177.39; IR (KBr) ν/cm⁻¹: 3322.69 (O–H str.), 2926.78 (–CH₂–), 1545.16 (C=N), 1450.96 (ArNH), 732.54 (C=C). Anal. calcd for C₇₆H₈₄N₄O₈: C, 77.26; H, 7.17; N, 4.74; found: C, 77.25; H, 7.15; N, 4.73.

15-(1-(Benzylimino)ethyl)-35,55,75-tris(1-(benzylimino)ethyl)-2,4,6,8-tetrabutyl-1,3,5,7(1,3)-tetra-benzenacyclooctaphan-14,16,34,36,54,56,74,76-octol (2c): Dark yellow; yield 71%, m.p. 216 °C; ¹H NMR (400 MHz, CDCl₃): δ 11.97 (s, 8H, ArOH), 7.24 (s, 4H, ArH), 7.28 (m, 20H, ArH benzylamine), 4.53 (s, 8H, –CH₂N), 4.49 (t, 4H, J = 15.6 Hz, –CH–), 2.21 (s, 12H, –CH₃CN), 1.42 (q, 8H, J = 14.4 Hz, –CH₂–), 1.36 (m, 16H, –CH₂–CH₂), 0.91 (t, 12H, J = 7.2 Hz, –CH₃); ¹³C NMR (400 MHz, CDCl₃): δ 13.79, 19.72, 22.47, 30.09, 32.19, 33.03, 47.98, 107.67, 126.49, 126.85, 127.06, 128.33, 128.44, 135.80, 161.64, 177.39. IR (KBr) ν/cm⁻¹: 2923.87 (–CH₂–), 1546.43 (C=N), 1371.92 (ArNH), 731.59 (C=C). Anal. calcd for C₈₀H₉₂N₄O₈: C, 77.64; H, 7.49; N, 4.53; found: C, 77.61; H, 7.46; N, 4.51.

15-(1-(Benzylimino)ethyl)-35,55,75-tris(1-(benzylimino)ethyl)-2,4,6,8-tetrapentyl-1,3,5,7(1,3)-tetra-benzenacyclooctaphan-14,16,34,36,54,56,74,76-octol (2d): Yellowish orange; yield 72%, m.p. 130 °C; ¹H NMR (400 MHz, DMSO-d₆): δ 12.06 (s, 8H, ArOH), 7.34 (s, 4H, ArH), 7.30 (m, 20H, ArH benzylamine), 4.82 (s, 8H, –CH₂N), 4.29 (t, 4H, J = 7.6 Hz, –CH–), 2.21 (s, 12H, –CH₃CN), 2.19 (q, 8H, J = 7.2 Hz, –CH₂–), 1.32 (m, 24H, –CH₂–CH₂), 0.84 (t, 12H, J = 7.2 Hz, –CH₃); ¹³C NMR (400 MHz, CDCl₃): δ 14.26, 20.21, 22.84, 28.17, 32.32, 33.08, 33.69, 48.44, 108.17, 126.76, 126.98, 127.55, 128.01, 130.81, 136.29, 162.07, 177.88. MS (MS ES+): m/z = 1293.74 [M]⁺, 1294.74 [M + 1]⁺; IR (KBr) ν/cm⁻¹: 3392.97 (O–H str.), 2933.23 (–CH₂–), 1597.63 (C=N), 1348.26 (ArNH), 759.14 (C=C). Anal. calcd for C₈₄H₁₀₀N₄O₈: C, 77.98; H, 7.79; N, 4.33; found: C, 77.76; H, 7.77; N, 4.31.

15-(1-(Benzylimino)ethyl)-35,55,75-tris(1-(benzylimino)ethyl)-2,4,6,8-tetrahexyl-1,3,5,7(1,3)-tetra-benzenacyclooctaphan-14,16,34,36,54,56,74,76-octol (2e): Dark orange; yield 74%, m.p. 190 °C; ¹H NMR (400 MHz, DMSO-d₆): δ 12.03 (s, 8H, ArOH), 7.43 (s, 4H, ArH), 7.30 (m, 20H, ArH benzylamine), 4.87 (s, 8H, –CH₂N), 4.29 (t, 4H, J = 7.6 Hz, –CH–), 2.50 (s, 12H, –CH₃CN), 2.19 (q, 8H, J = 6.4 Hz, –CH₂–), 1.23 (m, 32H, –CH₂–CH₂), 0.84 (t, 12H, J = 6.4 Hz, CH₃); ¹³C NMR (400 MHz, CDCl₃): δ 14.14, 20.20, 22.75, 28.40, 29.73, 32.04, 33.10, 33.64, 48.48, 108.17, 126.79, 126.98, 127.55, 128.82, 128.94, 136.32, 162.76, 177.87. MS (MS ES+): m/z = 1349.85 [M]⁺, 1350.85[M + 1]⁺; IR (KBr) ν/cm⁻¹: 3447 (O–H str.), 2928 (–CH₂–), 1597 (C=N), 1394 (ArNH), 701 (C=C). Anal. calcd for C₈₈H₁₀₈N₄O₈: C, 78.30; H, 8.06; N, 4.15; found: C, 78.28; H, 8.04; N, 4.13.

15-(1-(Benzylimino)ethyl)-35,55,75-tris(1-(benzylimino)ethyl)-2,4,6,8-tetraheptyl-1,3,5,7(1,3)-tetra-benzenacyclooctaphan-14,16,34,36,54,56,74,76-octol (2f): Orange; yield 74%, m.p. 165 °C; ¹H NMR (400 MHz, DMSO-d₆): δ 12.07 (s, 8H, ArOH), 7.33 (s, 4H, ArH), 7.30 (m, 20H, ArH benzylamine), 4.88 (s, 8H, –CH₂N), 4.28 (t, 4H, J = 8.0 Hz, –CH–), 2.50 (s, 12H, –CH₃CN), 2.20 (q, 8H, J = 7.2 Hz, –CH₂–), 1.23 (m, 40H, –CH₂–CH₂), 0.84 (t, 12H, J = 6.4 Hz, –CH₃); ¹³C NMR (400 MHz, CDCl₃): δ 14.18, 20.21, 22.73, 28.44, 29.50, 30.01, 31.97, 33.11, 33.64, 48.47, 108.17, 126.99, 127.55, 128.00, 128.63, 130.80, 136.32, 162.14, 177.87. MS (MS ES+): m/z = 1405.96 [M]⁺, 1406.96 [M + 1]⁺; IR (KBr) ν/cm⁻¹: 3460 (O–H str.), 2926 (–CH₂–), 1552 (C=N), 1395 (ArNH), 672 (C=C). Anal. calcd for C₉₂H₁₁₆N₄O₈: C, 78.59; H, 8.32; N, 3.99; found: C, 78.58; H, 8.30; N, 3.97.

15-(1-(Benzylimino)ethyl)-35,55,75-tris(1-(benzylimino)ethyl)-2,4,6,8-tetraoctyl-1,3,5,7(1,3)-tetra-benzenacyclooctaphan-14,16,34,36,54,56,74,76-octol (2g): Light red; yield 75%, m.p. 205 °C; ¹H NMR (400 MHz in CDCl₃): δ 11.95 (s, 8H, ArOH), 7.33 (s, 4H, ArH), 7.23 (m, 20H, ArH benzylamine), 4.71 (s, 8H, –CH₂N), 4.48 (t, 4H, J = 8.0 Hz, –CH–), 2.60 (s, 12H, –CH₃CN), 2.16 (q, 8H, J = 6.4 Hz, –CH₂–), 1.33 (m, 48H, –CH₂–CH₂), 0.86 (t, 12H, J = 13.2 Hz, –CH₃); ¹³C NMR (400 MHz, CDCl₃): δ 14.21, 20.21, 22.78, 28.46, 29.47, 29.82, 30.07, 31.98, 33.12, 33.65, 48.65, 108.17, 126.99, 127.55, 128.01, 128.83, 130.81, 139.31, 162.07, 177.88. MS (MS ES+): m/z = 1461.07 [M]⁺, 1462.08 [M + 1]⁺; IR (KBr) ν/cm⁻¹: 3429 (O–H str.), 2925 (–CH₂–), 1598 (C=N), 1395 (ArNH), 740 (C=C). Anal. calcd for C₉₆H₁₂₄N₄O₈: C, 78.86; H, 8.55; N, 3.83; found: C, 78.83; H, 8.52; N, 3.82.

15-(1-(benzylimino)ethyl)-35,55,75-tris(1-(benzylimino)ethyl)-2,4,6,8-tetranonyl-1,3,5,7(1,3)-tetra-benzenacyclooctaphan-14,16,34,36,54,56,74,76-octol (2h): Yellow; yield 76%, m.p. 168 °C; ¹H NMR (400 MHz, CDCl₃): δ 11.95 (s, 8H, ArOH), 7.29 (s, 4H, ArH), 7.23 (m, 20H, ArH benzylamine), 4.68 (s, 8H, –CH₂N), 4.50 (t, 4H, J = 8.0 Hz, –CH–), 2.59 (s, 12H, –CH₃CN), 2.16 (q, 8H, J = 6.0 Hz, –CH₂–), 1.33 (m, 56H, –CH₂–CH₂), 0.8 (t, 12H, J = 6.4 Hz, –CH₃); ¹³C NMR (400 MHz, CDCl₃): δ 14.19, 20.19, 22.76, 28.46, 29.44, 29.77, 29.89, 30.07, 32.02, 33.12, 33.63, 48.47, 108.18, 126.99, 127.53, 128.00, 128.30, 128.94, 136.33, 161.97, 177.86. MS (MS ES+): m/z = 1517.18 [M]⁺, 1518.18 [M + 1]⁺; IR (KBr) ν/cm⁻¹: 3432 (O–H str.), 2925 (–CH₂–), 1597 (C=N), 1390 (ArNH), 742 (C=C). Anal. calcd for C₁₀₀H₁₃₂N₄O₈: C, 79.11; H, 8.76; N, 3.69; found: C, 79.08; H, 8.70; N, 3.68.

Analytical procedure for the extraction study

All the cations of Ni²⁺, Mn²⁺, Hg²⁺, Co²⁺, and Na⁺ were taken directly from the acetate or sulfate salts of the metals. All the cations and sodium dichromate anion (Na₂Cr₂O₇) solutions were prepared in an aqueous medium. Cation and anion salts of the metals were taken at pH 6 in aqueous medium at concentrations of 1 × 10⁻⁴ and 1 × 10⁻⁵ M, respectively. The anion extraction experiment was performed by changing the pH values of the dichromate ion solution using 0.01 M HCl/0.01 M KOH solution. The pH solutions were kept at 6.0, 5.0, 4.0, 3.0, 2.0, and 1.0 for the dichromate solutions.^21,33,34 Ligand solutions of concentration 1 × 10⁻³ M were prepared in CH₂Cl₂ as the solvent. A liquid–liquid extraction study was applied to obtain the host and guest interactions. In the analytical experiment, a solution of the cation or anion (10 mL) and the ligand 2h (10 mL, 1 × 10⁻³ M concentration) was prepared in a stoppered glass tube. The solution was then mechanically shaken for 2 min and then magnetically stirred for 1 h at 25 °C, and finally, the mixture was taken into the separating funnel and left for 30 min.^21,34 The organic layer and aqueous layer were separated and the concentration of the cations remaining in the aqueous phase was analyzed spectrophotometrically. The blank experiment showed no absorbance/extraction of the cations/anions in the absence of ligand. The percentage extraction can be calculated using the following equation^21,34

$$E\%=\frac{\left(A_0-A\right)}{A_0}\times 100$$

(3)

where A₀ is the initial concentration of the cation/anion solution before extraction and A is the final concentration of the cation/anion solution after extraction.

Supplemental Material

CHL915871_Supplemental_Material_CLN – Supplemental material for Resorcin[4]arene Schiff base derivatives: Synthesis, characterization, and extraction studies

Supplemental material, CHL915871_Supplemental_Material_CLN for Resorcin[4]arene Schiff base derivatives: Synthesis, characterization, and extraction studies by Juhi B Upadhyay and Hitesh M Parekh in Journal of Chemical Research