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UDC 663
DETERMINATION OF ETHER COMPLEX WITH METAL IONS
Usenov Azamat Bakir o'g'li Tashkent State Technical University PhD, [email protected]
Abstract. In this study, the ion-dipole interaction between benzo-thio and benzo-oxo crown ethers and metal ions was determined by adapting the Job's Plot method (Solvent system: 50% Acetonitrile/water). The complexation rate and complexation constant (Ke) of the crown ether-cation complex were calculated. According to the results of this study, crown ethers can be used for activating metal ions for enzymes in bioorganisms, and for removing metal ions in industry and environmental protection.
Annotasiya. Ushbu tadqiqot doirasida benzo-tio va benzo-okso toj efirlari va metall ionlari o'rtasidagi ion-dipol o'zaro ta'siri Job's Plot usulini moslashtirish orqali aniqlandi (Erituvchi tizim: 50% Asetonitril/suv). Toj efir-kation kompleksining komplekslanish tezligi, komplekslanish konstantasi (Ke) hisoblab chiqilgan. Ushbu tadqiqot natijalariga ko'ra, toj efirlari bioorganizmlardagi fermentlar uchun metall ionlarni faollashtirishda, sanoat va atrof-muhitni muhofaza qilishda metall ionlarini olib tashlash uchun qo'llash mumkin.
Аннотация. В этом исследовании ионно-дипольное взаимодействие между бензо-тио и бензо-оксо краун-эфирами и ионами металлов определялось путем адаптации метода Job's Plot (система растворителей: 50% ацетонитрила/воды). Были рассчитаны скорость комплексообразования и константа комплексообразования (Ke) комплекса краун-эфир-катион. Согласно результатам этого исследования, краун-эфиры могут быть использованы для активации ионов металлов для ферментов в биоорганизмах, а также для удаления ионов металлов в промышленности и защите окружающей среды.
Keywords: benzo-thio, benzo-oxo, crown ether, metal ion, sulfur atoms, cation, complex formation constant, complexation constant, Job's Plot method.
Kalit so'zlar: benzo-tio, benzo-okso, toj efiri, metall ioni, oltingugurt atomlari, kation, komplekslanish konstantasi, Job's Plot usuli.
Ключевые слова: бензо-тио, бензо-оксо, краун-эфир, ион металла, атомы серы, константа комплексообразования катиона, константа комплексообразования, метод схемы Job's Plot.
1. Introduction
Crown ethers, as a member of macrocyclic compounds, have been known since the 1960s [1-4]. While they show apolar character with a hydrophobic outer cavity consisting of hydrogen and carbon atoms, they have a hydrophilic polar inner cavity due to the fact that they contain one or more elements such as oxygen, sulfur, nitrogen [5]. Thanks to this polar inner cavity, they form stable complexes with cations. The oxygen, nitrogen and sulfur atoms they contain provide the ability to bind different cations to the ring [6,7]. For example, while their oxygen-containing analogues bind mostly cations of hard metals, sulfur and nitrogen direct this interest to cations that are considered soft. At the same time, crown ethers can selectively complex with some metals from environments containing multiple cations thanks to this inner cavity [7,8]. This selective complexation is defined as metal affinity, and systems that selectively bind soft/hard cations at the same time can be designed with the combination of S, O and N atoms on the ring [3]. Crown ethers, which are stable complexes of metals, have gained a different dimension with the fact that they can be used as metal removal agents [1]. While conductometry offers opportunities such as working at very low concentrations, fast response time and being practical,
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it does not allow the determination of complexation selectivity in multi-element systems compared to potentiometric studies [2,3]. It allows evaluations to be made over the total conductivity in the medium. Binary aqueous-solvent systems have quite a lot of usage areas in the chemical sector [2]. These systems have many uses from biochemistry to biophysics. For this purpose, many water-miscible solvents (acetonitrile, dioxane, ethanol, methanol, DMSO etc.) can be used [3]. The method of continuous variations was introduced by Job in 1928 and is also known as the Job's Plot method [4]. In solutions containing two species such as M and L, M species can bind with B. In some cases, more than one M binds to a single L. One of the methods to determine the amount of binding to L (binding stoichiometry) is to use the Job's Plot graph. It is frequently used in analytical chemistry and biochemistry today [7]. The Job's Plot method is generally determined by plotting the UV absorbance against the mole fraction of one of the complexing compounds [7]. In this study, the complexation rate and constants were determined by applying the conductometry method to Job's Plot using compounds synthesized in our previous study [6] by our group. It was aimed to bind cations with different radii and hardnesses by selecting different heteroatom combinations of crown ethers with approximately the same ring size.
2. Experimental studies
2.1. Devices and chemical materials used. The salts used in this study were obtained commercially (Sigma Aldrich, Carlo Erba, Merck, Roth) with high purity (> 98.5%). In the preparation of aqueous solutions, ultrapure water with a resistance of 18.3 MQ/cm produced from the Human Corporation brand New Human Power I S-UV model ultrapure water device was used. In the preparation of the solvent system (50% Acetonitrile/water), HPLC purity (Sigma Aldrich, gradient grade > 99.9%) Acetonitrile was used. In the experiments, precision balance (KERN ABJ (d=0.1 mg)), automatic pipette, dispenser (Brand brand 0.5-5 mL and 1-10 mL), ultrasonic bath (Bandolin), magnetic stirrer (IKA C-MAG HS-7), pH meter, conductometer were used.
2.2. Synthesis of benzo-thio and benzo-oxo crown ethers. The crown ethers used in the study were synthesized by microwave synthesis method according to the procedures given in our previous studies. The synthesized U1 (bis(1,2-dibenzo) octathio tetracarbonyl-29-crown-6) benzo-thio crown ether and U2 (bis(1,2-dibenzo) tetrathio tetracarbonyl-29-crown-6) benzo-oxo crown ether were used in the study [5].
2.3. Conductometric method. All experiments carried out within the scope of this study were carried out in a temperature cell at 25oC. The solution mixture to be determined for complexation was prepared in a 10 mL volumetric flask and taken to the conductivity cell. In order to ensure homogeneous distribution, magnetic fish was added and the solution was stirred at a constant speed for 3 minutes to reach a constant temperature. The electrode was immersed and 2 minutes were waited for the temperature to reach equilibrium. At the end of the fifth minute, 10 consecutive measurements were taken. The study was carried out repeatedly and the average of the results was used in the calculations [1-7].
Results and discussion
In this study, ion-dipole interactions between benzo-thio and benzo-oxo crown ethers and metal ions were determined by adapting Job's Plot method to conductometry (Solvent system: 50% Acetonitrile/water). Conductivity parameters such as complexation rate, complexation constant, free Gibbs energy were calculated. The complexation of compounds, which were previously synthesized by classical and microwave synthesis methods and whose metal selectivities were determined by liquid-liquid ion pair metal extraction, with metal salts such as U1 (bis(1,2-dibenzo) octathio tetracarbonyl29-crown-6), U2 (bis(1,2-dibenzo) tetrathio tetracarbonyl-29-crown-6), NaCl, KCl, MgCl2.6H2O, CaCl2.2H2O, ZnCl2, FeSO4.7H2O, AgNO3, CoCl2.8H2O was investigated by conductometric method (Fig. 1) [7].
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Using the data obtained from the study carried out conductometrically, the complexation rate, complexation constant and free Gibbs energy given in Table 1 were calculated using equations 1-3 [3]. Accordingly, the complexation equation (1) is as follows;
aMm++ bL ^ MaL'^+ (1)
According to this equation, ¿Vf"4, L symbolize cation and ligand, respectively. As a result, the equilibrium constant Ke for complex formation at different rates can be written as (2).
[Aj™+]ii[i.]& (2)
In addition, the complexation rate is obtained by calculating the ratio of the amount of salt to the amount of ligand at equilibrium (3) [24]. Here, nM is the number of moles of salt and nL is the number of moles of ligand.
Complex formation rate =
"M
(3)
When equation (1) is applied to the crown ethers U1 and U2, the detailed complexation
mechanism can be drawn as follows (Fig. 1).
Fig. 1. Complexation of metal salts with benzo thio and benzo oxo crown ethers U1 and U2
in 50% acetonitrile/pure water.
Complexation rate and complexation graphs (Fig. 2. a-b-c) were obtained using the conductivity data of U1 and U2 crown ethers and NaCl, KCl, MgCh.6H2O, CaCh.2H2O, ZnCh, FeS04.7H20, AgN03, C0CI2.8H2O metal salts.
Fig. 2. Determination of the rate of complexation of (a) U1 ligand, (b) U2 crown ether with sodium ion in 50% acetonitrile/water at 25 oC, (c) graphs of the observed conductivity (K (^S cm-1)) versus [Na+] (mol L-1) ion change for U1 and U2 complexes with NaCl.
When Fig. 2 is examined, the complexation ratios (salt/ligand) of sodium chloride salt and U1 and U2 crown ethers are determined as 3/2 and 1/1, respectively (Table 1). This situation is due to the fact that U1 thio crown ether contains more sulfur atoms than the other oxo crown ether. Sulfur atoms are oriented towards the outside of the ring due to their bulkiness in the crown ether. While sodium ion, which has a very small ion diameter, fits exactly 1/1 into the inner ring cavity of the oxo crown ether, U2 crown ether forms a sandwich complex. The relatively small ion diameter and small ion charge increased the complexation with sodium. Both crown ethers made almost the most complexation with sodium ion. Finally, when Fig. 2c is examined, it is observed that complexation increases in direct proportion to the increasing concentration.
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Fig. 3. Determination of the rate of complexation of (a) U1 ligand, (b) U2 crown ether with potassium ion in 50% acetonitrile/water at 25 oC, (c) graphs of the observed conductivity (K (^S cm-1)) versus [K+] (mol L-1) ion change for U1 and U2 complexes with KCl.
When the complexes formed with the U1 and U2 crown ethers of potassium chloride salt are examined, it is seen that the complexation rates of the complexes formed are the same (2/3). By using the obtained complexation rates in the calculation of the complexation constant Ke, it was determined that quite high complexation occurred (logKe, U1:14; U2:19). In addition, the complexation graph given in Fig. 3 c allows us to compare the complexation between the two crown ethers. Accordingly, the U2 crown ether, which has a low sulfur content and has the characteristics of an oxo crown ether, has a higher complexation than the other thio crown ether.
Fig. 4. Determination of the complexation rate of (a) U1 ligand, (b) U2 crown ether with magnesium ion in 50% acetonitrile/water at 25 oC, (c) graphs of the observed conductivity (K (^S cm-1)) versus [Mg2+] (mol L-1) ion change for U1 and U2 complexes with MgCl2.
According to the complexation ratios obtained by examining the complexations of decreasing salt concentrations against increasing ligand concentrations (Fig. 4 a-b), the salt/ligand complexation ratios were determined as 1/1 for both U1 and U2 crown ethers. When Table 1 is examined, it is seen that both U1 and U2 crown ethers show less complexation with Mg2+ ion compared to other ions (log Ke values; U1: 6.36; U2: 4.94). However, when their more detailed complexations are examined (Fig. 4c), it is observed that complexation increases
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at high concentrations and U1 crown ether, which is a thio crown ether, has higher complexation than U2 crown ether.
Fig. 5. Determination of the complexation rate of (a) U1 ligand, (b) U2 crown ether with iron ion in 50% acetonitrile/water at 25 oC, (c) graphs of the change in conductivity (K (^S cm-1)) versus [Fe2+] (mol L-1) ion observed for U1 and U2 complexes with FeSO4.
When Fig. 5 and Table 1 are examined, Fe2+ selectively showed high complexation with oxo crown ether (U2), while it showed very low complexation with thio crown ether (U1). The complexation ratios are 1/1 and 3/2, respectively. Increasing the complexation ratio also increases complexation. The compatibility of the cavity diameter of the oxo crown ether with the iron (II) ion provided an increase in complexation.
In this study, the complexation constants for the compounds (U1 and U2) that we synthesized in our previous study [7] were determined by conductometry. For this purpose, the complexation rates were determined by adapting the Job's Plot method to conductometry. In many previous metal/ligand complexation studies, the stoichiometric coefficients were accepted as 1/1 and it was thought that a metal was bound by a ligand. However, in this study, it was observed that sandwich complexes (2/1, 1/2, 2/3, 3/2) were formed in addition to 1:1 (1/1) complexes. Using these complexation rates, the complexation constants Ke, Log Ke ve -AG9 were calculated (Table 1). According to the obtained results, it was observed that the selected U1 and U2 compounds formed complexes with NaCl, KCl, MgCh.6H2O, CaCh.2H2O, ZnCh, FeSO4.7H2O, AgNO3, CoCh.8H2O metal salts.
Table 1
Complexation rates, complexation constants and free Gibbs energies of some cations with
U1 and U2 crown ethers.
Crown Ether Cation Complexation Rate Ke Log Ke -AG0
U1 Na+ 3/2 1.43 x 10+19 19.16 26117.79
K+ 2/3 7.41 x 10+13 13.87 18910.93
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Mg2+ 1/1 2.29 x 10+6 6.36 8672.45
Ca2+ 2/3 6.17 x 10+17 17.79 24256.17
Zn2+ 3/2 5.18 x 10+18 18.71 25515.60
Fe2+ 1/1 5.23 x 10+4 4.71 6432.919
Ag+ 3/2 2.57 x 10+14 14.41 19647.40
Co2+ 1/1 1.03 x 10+6 6.01 8198.47
U2 Na+ 1/1 1.83 x 10+19 19.26 26265.58
K+ 2/3 1.36 x 10+19 19.13 26087.36
Mg2+ 1/1 8.88 x 10+4 4.94 6746.77
Ca2+ 2/3 1.52 x 10+12 12.18 16609.19
Zn2+ 3/2 2.19 x 10+20 20.34 27733.63
Fe2+ 3/2 2.17 x 10+19 19.33 26366.39
Ag+ 2/3 2.05 x 10+14 14.31 19512.58
Co2+ 3/2 1.07 x 10+19 19.03 25946.86
When the o btained complexation data were evaluated, the highest complexation wit
occurred with Na+ ion (log Ke=19.16). The lowest complexation for U1 crown ether was observed with Fe2+ ion (log Ke=4.71). When we sorted from highest complexation to lowest complexation, the following were observed: Na+ > Zn2+ > Ca2+ > Ag+ > K+ > Mg2+ > Co2+ > Fe2+ (logKe = 19.16 > 18.71 > 17.79 > 14.41 > 13.87 > 6.36 > 6.01 > 4.71, respectively). For U2 crown ether, Na+, K+, Zn2+ and Co2+ ions have the highest complexation. The ion that complexes the least is Mg2+ ion. When we sort from highest to lowest complexation, it is as follows: siraladigimizda Zn2+ > Fe2+ > Na+ > K+ > Co2+ > Ag+ > Ca2+ > Mg2+ (log Ke = 20.34 > 19.33 > 19.26 > 19.13 > 19.03 > 14.31 > 12.18 > 4.94, respectively).
Determination of complex formation stoichiometries using Job's Plot method allowed calculation of true complexation constants. Due to the large number of sulfur atoms and conformation, the benzo-thio crown ether (U1) compound has lower affinity for transition metals than the benzo-oxo crown ether (U2) compound.
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