INTRODUCTION:

Crowns and crypts have the unusual property to form stable complexes with metal ions.^1-2 These macrocycles which contain varying combinations of ligating atoms can be tailored to accommodate specific metal ions by the fine tuning of the ligand design features^3-5, such as the macrocyclic hole size⁶, nature of the ligand donors, donor set, donor array, ligand conjugation, ligand substitution, number and sizes of the chelate rings, ligand flexibility, and nature of the ligand backbone^7-10. Increasing the number of benzo substituents in a crown ether framework can significantly alter the properties of the ligand. Generally, adding benzo substituents will increase the rigidity of the crown ether framework and enhance lipophilicity, but decrease the basicity of the oxygens attached to the aromatic rings.

The stability of crown complexes depends on the number of ethereal ‘oxygen’ atoms, geometric disposition of ethereal ‘oxygen’ atoms, solvent effect, size and shape of the macrocycle and size of the metal ion. The complexation properties are controlled by the ring size, the structure of the crown ether, nature of the substituents and the stability of the ion-pairs. Numerous theoretical studies employing abinitio¹¹, Monte Carlo^12-13and molecular mechanics methods¹⁴ have been focused on the structure and selectivity of crown ethers towards metal ions. These can be very selective extracting agents for metal ions and are often used for the determination of alkali, alkaline earth and transition elements. Their chemistry and analytical use including extraction have been comprehensively reviewed.^15-17 Coordination chemistry of macrocyclic ligands with silver ion has been a fascinating area of current research interest to the modern chemists all over the world.^18-19The continued interest and quest in designing new macrocyclic ligands stem mainly from their use as models for protein-metal binding sites in a substantial array of metalloproteins in biological systems, as synthetic ionophores, as models to study the magnetic exchange phenomena, as therapeutic reagents in chelate therapy for the treatment of metal intoxication, as cyclic antibiotics that owe their antibiotic actions to specific metal complexation, to study the guest-host interactions, in solvent extraction and separation of various ionsand in catalysis.^19-26

MATERIALS AND METHODS:

The nitrophenols and crown ethers used were of E. Merck and S. Aldrich respectively. The commercially available regents of AR grade were used without further purification. Results of elemental analysis of synthesized compounds agreed with required value within experimental error. IR spectra were recorded by Perkin Elmer RX1 (4000-450 cm^-1). UV-visible spectral data were recorded through Systronic double beam spectrophotometer-2203 (600-200 nm). The ¹H−NMR spectra of ligands and synthesized complexes were recorded in CDCl₃ by Bruker DRX-300 at.

EXPERIMENTAL:

Preparation of silver salt of nitrophenols; Ag (ONP), Ag (DNP) and Ag (TNP):

About 4 mmol (0.644 gm) of sodium salt of 2-nitrophenol was taken in a conical flask and 25 ml of 95% (^V/_V) ethanol was added to it. The ethanolic solution of sodium salt of 2-nitrophenol was heated on a water bath and continuously shaken till gets completely dissolved and the solution becomes homogeneous. Then, 4 mmol of AgNO₃ (0.68 gm)was dissolved in 25 ml of 95% (^V/_V) ethanol and continuously shaken till the silver nitrate gets completely dissolved. The freshly prepared ethanolic solution of silver nitrate was slowly added to the ethanolic solution of sodium salt of 2-nitrophenol and continuously shaken. On adding the ethanolic solution of silver nitrate, brownish orange coloured silver salt of 2-nitrophenol (AgONP) precipitated out.¹⁷ The mixture was continuously stirred on hot plate equipped with magnetic stirrer for 45 minutes to ensure complete precipitation.

For the preparation of AgDNP, 4 mmol (0.824 gm) of sodium salt of 2,4-dinitrophenol was taken in a conical flask. 25 ml of 95% (^V/_V) ethanol was added to it. The ethanolic solution of sodium salt of 2,4-dinitrophenol was heated on a water bath and continuously shaken till it gets completely dissolved and the solution becomes homogeneous. Also, 4 mmol of AgNO₃ (0.68 gm)was dissolved in 25 ml of 95% (^V/_V) ethanol and continuously shaken till the silver nitrate gets completely dissolved. The freshly prepared ethanolic solution of silver nitrate was slowly added to the ethanolic solution of sodium salt of 2,4-dinitrophenol and continuously shaken. As soon as the ethanolic solution of silver nitrate was added, yellow coloured silver salt of 2,4-dinitrophenol (AgDNP) precipitated out. The mixture was continuously stirred on hot plate equipped with magnetic stirrer for 45 minutes to ensure complete precipitation.

Similarly, silver salt of 2,4,6-trinitrophenol (AgTNP) was prepared by taking 4 mmol (1.004 gm) of NaTNP and using the above discussed procedure. Some physical properties of synthesized silver salts are given in table-1.

Table 1: Physical properties of silver salts

Compound	Colour	Melting point (⁰C)	% Nitrogen
AgONP	Brownish orange	270^d	5.69
AgDNP	Yellow	265^d	9.62
AgTNP	Yellowish Brown	245^e	12.51

d – decomposition temp, e – explosion temp

Preparation of adducts of 18-crown-6 and dibenzo18-crown-6 ether with silver salts of 2-nitrophenol, 2,4-dinitrophenol and 2,4,6-trinitrophenol:

The dried organic salt (0.002 mol) was suspended in 50 ml dry methanol and heated it with constant stirring to get a clear solution. Stoichiometric proportion of 18-crown-6 ether (0.528 gm /0.002 mol) was added in that and then the reaction mixture was refluxed on a hot plate equipped with magnetic stirrer at 50-55⁰C. A clear solution was formed. It was filtered and concentrated to half of its bulk. On cooling this solution, solid crystalline product began to precipitate. The product was separated and allowed to stand overnight then filtered on a buchner funnel. The compound was washed with few drops of cold dry methanol and dried over KOH desiccator.

18C6.Ag(ONP): C₁₈H₂₈NO₉Ag

18C6.Ag(DNP): C₁₈H₂₇N₂O₁₁Ag

18C6.Ag(TNP): C₁₈H₂₆N₃O₁₃Ag

Similarly, in the preparation of adducts of dibenzo18-crown-6, same amount of appropriate dried silver salt was refluxed with 0.72 gm (0.002 mol) of dibenzo18-crown-6 ether.

DB18C6.Ag(ONP): C₂₆H₂₈NO₉Ag

DB18C6.Ag(DNP): C₂₆H₂₇N₂O₁₁Ag

DB18C6.Ag(TNP): C₂₆H₂₆N₃O₁₃Ag

Table 2: Percentage composition of silver-crown ether complexes

Compound	% C	% N	% O	% Ag
18C6.Ag(ONP)	42.36	2.74	28.22	21.13
18C6.Ag(DNP)	38.93	5.04	31.69	19.42
18C6.Ag(TNP)	36.01	7.00	34.65	17.96
DB18C6.Ag(ONP)	51.49	2.31	23.74	17.78
DB18C6.Ag(DNP)	47.93	4.30	27.02	16.55
DB18C6.Ag(TNP)	44.84	6.03	29.86	15.48

RESULT AND DISCUSSION:

UV-Visible study:

Electronic absorption spectra of synthesized compounds provide small but significant evidence of bonding in complexes. The electronic spectral studies of synthesized complexes shows only some deviation of p - p*, n - p*, s - p* and s - s* transitions due to shifting of electron density from donor atoms to cationic species. There is also good probability of d–d transitions in complexes, which is characteristic of transition and rare earth metal complexes. The slight change in spectral band position is usually taken as either solvent effect or interaction of electron cloud of donor atom of ligand with silver ion. The bond energy in organic compound possess energy equivalent to absorption energy of ultraviolet or visible radiation as they form bonds by ion-dipole interactions and/or involvement of non-bonding electrons with central metal ions.^27-29 The electrons of bonding orbitals can go to higher energy by excitement due to absorption of radiation in UV region.

The s - s* transitions occur at very high energy (125–200 nm) while n - s* transition in the region (150– 230 nm). Water molecule shows n - s* transition having l_max 167 nm. Transitions to antibonding p* orbitals are associated only with unsaturated centers in the molecule. In all nitrophenols, important electronic transitions are s - s*, n - s*, p - p* transition of nitro group (–NO₂) and p - p* transition of phenyl group. The n - s* transition of phenolic (C–O) group is observed at high energy and phenyl ring p - p*, s - s* transitions between 180–250 nm region. The antibonding orbitals s* and p* are quantized energy level of higher energies. Besides bonding electronic transitions, intermolecular transition known as charge transfers (C–T) and M « L transitions of high intensity are usually observed in dyes, complex molecule and coordination complexes.³⁰

IR study:

The n(O-H) stretching vibration in almost all nitrophenols are observed as very broad and medium band in the region 3150–3320 cm^-1 which almost disappears in their salts. The disappearance of n(O-H) band suggests bonding of phenolic oxygen with metal ions on deprotonation of phenolic -OH group. The n(C–O) band shifts to higher frequency due to acquiring higher (C–O) bond order on deprotonation in silver metal salts. This increase is attributed to bonding of phenolic oxygens (C–O) in all complexes. The stretching bands of -NO₂ in AgONP, AgDNP and AgTNP are at around 1605 cm^-1, 1618±5 cm^-1 and 1642±5cm^-1. These bands shifted to lower frequency in complexes. The n(NO₂) located near 845±10 cm^-1 also shifted to the lower vibrational frequency in the complexes. The IR band observed near 740–780 cm^-1 is attributed to phenyl ring n(C–H) out of plane bending vibration. The crown ethers display n(–CH₂–) stretching vibrations at 2920±10 cm^-1 and these are little affected on bonding with silver ion in complexes. The crown ethers in uncoordinated state display n(C–O–C) stretching vibration band near 1116±22 cm^-1. This n(C–O–C) vibration band shifted to lower frequency by 10-58 cm^-1in almost all complexes suggesting crown ether’s oxygen interaction with silver ion.³¹ Some complexes are hygroscopic in nature and thus their IR spectrum displays a broad band around 3350–3420 cm^-1, with maxima near 3405±10 cm^-1.^32,33 In the far-IR region new bands, absent in the spectrum of the free ligands, are found in the 425–621 cm^-1region, which may be assigned to the n(Ag–O_crown) stretching frequency.^34-36 Some FTIR-spectrum of the synthesized complexes are shown in figure 1.1–1.4. Since nature of FTIR peaks are almost similar thus spectrum of only four compounds are given. Thus IR studies of the complexes also suggest bonding of silver salt of nitrophenols with crown ether oxygen atoms.

Fig. 1.1 – FTIR Spectrum of Ag. TNP18C6

Fig. 1.2 – FTIR Spectrum of Ag. ONPDB18C6

Fig. 1.3 – FTIR Spectrum of Ag. DNPDB18C6

Fig. 1.4 – FTIR Spectrum of Ag. TNPDB18C6

¹H-NMR Study:

The NMR technique is a powerful tool for studying the complexation of crown ethers and metal cations. The ¹H-NMR spectrum of uncoordinated crown ethers and adduct complexes of silver ion provide useful information regarding structure of complexes. Qualitative and quantitative information on some crown ether interactions leading to supramolecule formation is available from detailed measurements of the chemical shift variations and from the relaxation rates of protons of the crown ethers as a function of the concentration of the silver-metal ion with the crown ether.³⁷ The conformation and the binding of small-ring dibenzo crown ethers with small cations in solution have been previously investigated using NMR techniques, supplemented by quantum chemical calculations.³⁸ The ¹H-NMR study of synthesized crown ether complexes provides useful information regarding structure of complexes.³⁹ Theoretical studies of structural changes on complexation, binding energies and changes in electron distribution can be carried out using ab initio quantum theoretical methods. A complementary study of the NMR chemical shifts and coupling constants can also be used to obtain information regarding complexation and structural changes.⁴⁰ The chemical shift of 1-H in 18C6 and, 1-H as well as 2-H in dibenzo18C6 (fig. 2-3), shows noticeable shifts change on complexation. After formation of the [Ag. L] (Pic^-) complex, the proton chemical shift d(–CH₂–O–) shows significant downfield shifts [∆d(–CH₂–O–) = 0.08-0.24 ppm]. These are small but noticeable changes, showing metal-crown ether bond formation.^41-43The degree of downfield shift shows the relative strength of the complexes.^44-46 In all cases, the exchange between free and complexed crown was fast on the NMR time scale and only a single population average ¹H signal was observed. Formation constants of the resulting 1:1 complexe were determined using quantum mechanical method and reported earlier. The ¹H-NMR spectrum of dibenzo18C6 shows peaks at, d₁(1H)=3.920–3.926 ppm and d_1ˊ(2H)=4.035–4.041 ppm (16H, 8 –CH₂O–), d₂=6.8–7.0 ppm, (8H, aryl –CH–), in CDCl₃.⁴³ Possible reasons for this downfield shift is the conformational change in the macrocyclic skeleton during complexation which could affect the electron density on the hydrogens through the Fermi Contact term.⁴⁷ The shift of –CH₂– signals in complexes from free crown ether suggested the coordination of crown ether oxygen of 18C6 and dibenzo18C6 with silver ion.^48-51 Some ¹H-NMR spectrum of the synthesized complexes are shown in figure 4.1–4.3. Since nature of ¹H-NMR peaks are almost similar thus spectrum of only three compounds are given.

Fig. 2 – 18-Crown-6

Fig. 3 – Dibenzo18-crown-6

Fig. 4.1(a) – ¹H-NMR Spectrum of Ag. TNP18C6

Fig. 4.1(b) – ¹H-NMR Spectrum (extended) of Ag. TNP18C6

Fig. 4.1(c) – ¹H-NMR Spectrum (extended) of Ag. TNP18C6

Fig. 4.2 – ¹H-NMR Spectrum of Ag. TNPDB18C6

Fig. 4.3 – ¹H-NMR Spectrum of Ag. DNPDB18C6

Fig. 4.4 – ¹H-NMR Spectrum of Ag. ONPDB18C6

Table-3: Prominent FTIR bands of silver-crown ether complexes (in cm^-1)

Compound	ν_s(C-O-C)	ν₁(-NO₂), ν₃(-NO₂)	ν_s(C-H) bending, ν_as(-CH₂-) bending
AgONP18	1105.43	1640.08, 836.46	1465.93, 1353.3
AgDNP18	1104.81	1637.70, 836.87	1465.66, 1352.98
AgTNP18	1101.10	1636.58, 835.83	1461.66, 1355.37
AgONPDB18	1126.23	1594.83, 850.93	1455.57, 1360.03
AgDNPDB18	1124.59	1598.55, 840.05	1454.96, 1321.63
AgTNPDB18	1123.38	1595.76, 842.07	1455.09, 1324.86

Table-4: Prominent FTIR bands of silver-crown ether complexes (in cm^-1)

Compound	ν(M-O) / ν(M-O_crown)	ν (N=O) _strin C-NO₂
AgONP18	490, 620	1250.54
AgDNP18	490, 570	1251.96
AgTNP18	475, 500	1251.48
AgONPDB18	510, 601.6	1240.02
AgDNPDB18	480, 610	1216.71
AgTNPDB18	500, 580	1217.05

ACKNOWLEDGEMENT:

We are thankful to the Head, SAIF, CDRI, Lukhnow, for providing IR-spectra, ¹H-NMR spectra and necessary facilities.

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Received on 10.01.2020 Modified on 24.01.2020

Asian J. Research Chem. 2020; 13(2):75-81.

DOI: 10.5958/0974-4150.2020.00016.4