научная статья по теме COMPLEXATION OF 4-NITROBENZO-15-CROWN-5 WITH MG2+, CA2+, SR2+ AND BA2+ METAL CATIONS IN ACETONITRILE-METHANOL BINARY SOLUTIONS Химия

Текст научной статьи на тему «COMPLEXATION OF 4-NITROBENZO-15-CROWN-5 WITH MG2+, CA2+, SR2+ AND BA2+ METAL CATIONS IN ACETONITRILE-METHANOL BINARY SOLUTIONS»

ЖУРНАЛ НЕОРГАНИЧЕСКОЙ ХИМИИ, 2009, том 54, № 12, с. 2003-2008

КООРДИНАЦИОННЫЕ СОЕДИНЕНИЯ

УДК 541.49

COMPLEXATION OF 4'-NITROBENZO-15-CROWN-5 WITH Mg2+, Ca2+, Sr2+ AND Ba2+ METAL CATIONS IN ACETONITRILE-METHANOL

BINARY SOLUTIONS

© 2009 Gholam Hossein Rounaghi*, Arezoo Ghaemi

Department of Chemistry, Faculty of Sciences, Ferdowsi University of Mashhad, Iran Поступила в редакцию 20.05.2008 г.

The complex formation reactions between Mg2+, Ca2+, Sr24" and Ba2+ metal cations with macrocyclic ligand, 4'-ni-trobenzo-15C5, were studied in acetonitrile (AN)-methanol (MeOH) binary mixtures at different temperatures using conductometric method. The results show that 4'-nitrobenzo-15C5 forms 1 : 1 [ML] complexes with Mg2+, Ca2+ and Sr24" metal cations in solutions. But in the case of Ba2+ cation a 1 : 2 [ML2] complex is formed in these solvent systems. The stability of the complexes is sensitive to the solvent composition and a non-linear behavior was observed for variation of log Kf of the complexes versus the composition of the binary mixed solvents. The stability constants of complexes decrease suddenly with increasing the concentration of methanol in this binary system. The values of thermodynamic parameters (AH° and AS°) for formation of (4'-nitrobenzo-15C5.Mg)2+, (4'-nitrobenzo-15C5.Ca)2+ and

(4'-nitrobenzo-15C5.Sr)2+ complexes were obtained from temperature dependence of the stability constants and the results show that these parameters are affected by the nature and composition of the mixed solvents. A non-linear behavior is observed between the AS° and the composition of the mixed solvents.

Key words: Mg2+, Ca2+, Sr2+ and Ba2+ metal cations, 4'-nitrobenzo-15-crown-5, acetonitrile-methanol binary mixtures, conductometry.

In 1967 Pedersen discovered crown ethers and described their capability to form complexes with metal cations [1]. The ability of crown ethers to form stable complexes with cations, mainly with alkali and alkaline earth metal cations, has spurred interest in these compounds [2]. These compounds have aroused a great interest in recent years because of their characteristics which are of importance in preparation chemistry and in biology.

New fields of application in research and potential uses in industry have been found for these macrocyclic compounds [3, 4]. One of the unique features of crown ethers is that they can chelate metal ions. Different sized macrocyclic polyethers chelate different sized metal ions. In these particular complexes, the crown ether is referred to as the host, while the metal ion is called the guest [5]. Crown ethers function as phase transfer catalysts by complexing a cation then bringing the complexed-cation-anion pair into the non-polar phase [6, 7]. By complexing the cation in the cavity of the crown ether, these compounds act as solubi-lizing agents. Macrocyclic compounds play an important role in chemistry such as their application in separation of metal cations [8, 9], in construction of ion selective electrodes [10, 11], chemical analysis [12] and chiral separation [13].

* The corresponding author (E-mail address: ghrounaghi@ya-hoo.com).

Although the complexation reactions of crown ethers with metal cations have been studied in both aqueous and non-aqueous solvents, the data about stability of crown ether complexes in mixed solvents have been carried out to a limited extent and little attention has been paied to these complexes in mixed solvent systems. Studies of crown ether complexes in various solvents, show that the thermo-dynamic and kinetic parameters for complexation processes and even the stoicheiometry of crown ether complexes are affected by the nature and composition of the solvent system [14, 15]. The goal of the present investigation is to study the effect of the nature and composition of the mixed solvents on selectivity and thermodynamics of complex-ation reactions of 4'-nitrobenzo-15C5 with Mg2+, Ca2+, Sr2+ and Ba2+ metal cations in AN-MeOH binary systems using the conductometric technique.

EXPERIMENTAL

4'-Nitrobenzo-15C5 (Fluka), magnesium nitrate (Merck), calcium nitrate (Merck), strontium nitrate (Merck) and barium perchlorate (Merck) were used without further purification. The solvents: acetonitrile and methanol (both from Merck company) were used with highest purity.

The experimental procedure to obtain the stability constants of complexes is as follows: a solution of metal salt (1.0 x 10-4 M) was placed in a titration cell and the conductance of the solution was measured, then a step-by-step in-

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Fig. 1. Molar conductance-mole ratio plots for (4'-nitroben-zo-15C5.Mg)2+ complex in pure acetonitrile at different temperatures: 1 - 15, 2 - 25, 3 - 35, 4 - 45, 5 - 55°C.

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Fig. 2. Molar conductance-mole ratio plots for (4'-nitroben-zo-15C5.Ba)2+ complex in pure acetonitrile at different temperatures: 1 - 15, 2 - 25, 3 - 35, 4 - 45, 5 - 55°C.

crease of the crown ether solution prepared in the same solvent (2.0 x 10-3 M) was carried out by a rapid transfer to the titration cell using a microburette and the conductance of the solution in the cell was measured after each transfer at the desired temperature.

The conductance measurements were performed on a digital AMEL conductivity apparatus, model 160, in a water bath thermostated at a constant temperature which maintained within ±0.03°C. The electrolytic conductance was measured using a cell consisting of two platinum electrodes to which an alternating potential was applied. A con-ductometric cell with a cell constant of 0.73 Cm-1 was used throughout the studies.

RESULTS AND DISCUSSION

The changes of molar conductance (Am) versus the ligand to cation mole ratio for the complexation of 4'-ni-trobenzo-15C5 with Mg2+, Ca2+, Sr2+ and Ba2+ cations in acetonitrile-methanol binary systems were studied at different temperatures. The changes of Am versus ligand/cat-ion mole ratio ([L]t/[M]t) for (4'-nitrobenzo-15C5.Mg)2+ and (4'-nitrobenzo-15C5.Ba)2+ complexes in pure acetonitrile and also for (4'-nitrobenzo-15C5.Ba)2+ complex in a binary mixture of acetonitrile-nitromethane (mol % ni-tromethane = 90) are shown in Figures 1-3, respectively.

The stability constants of the crown ether complexes at each temperature were calculated from variation of molar conductance as a function of ligand/cation mole ratios using a GENPLOT computer program [16]. The details of calculation of the stability constants of complexes by con-ductometric method have been described elsewhere [17].

The stability constants (logKf) for (4'-nitrobenzo-15C5.M)2+ (M2+ = Mg2+, Ca2+ and Sr2+) complexes in various solvent systems are listed in Table 1. Assuming that the activity coefficients of cation and complex have the same value, Kf, is a thermodynamic equilibrium constant on the molar concentration scale. Plots of lnKf versus 1/T

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2 3 4 [L]t/[M]t

Fig. 3. Molar conductance-mole ratio plots for (4'-nitroben-zo-15C5.Ba)2+ complex in acetonitrile-nitromethane (mol % NM = 90.0) binary system at two different temperatures: 1 - 15, 2 - 25°C.

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COMPLEXATION OF 4'-NITROBENZO-15-CROWN-5 WITH Mg2+, Ca2+, Sr2+ AND Ba2+ 2005

Table 1. Log Kf values of (4'-nitrobenzo-15C5.Mg)2+, (4'-nitrobenzo-15C5.Ca)2+ and (4'-nitrobenzo-15C5.Sr)2+ complexes in AN-MeOH binary mixtures at different temperatures

Medium log Kf ± SDa

15°C 25°C 35°C 45°C 55°C

(4'-Nitrobenzo-15C5.Mg)2+ b

Pure AN 3.48 ± 0.04 3.49 ± 0.03 3.47 ± 0.03 3.39 ± 0.03 3.41 ± 0.04

75.0% AN-25.0% MeOHc 2.78 ± 0.11 2.74 ± 0.10 2.73 ± 0.14 2.75 ± 0.12 2.78 ± 0.11

50.0% AN-50.0% MeOH 2.75 ± 0.13 2.74 ± 0.13 2.73 ± 0.14 2.75 ± 0.12 2.77 ± 0.11

25.0% AN-75.0% MeOH 2.75 ± 0.13 2.74 ± 0.14 2.74 ± 0.14 2.79 ± 0.10 2.77 ± 0.11

Pure MeOH 2.69 ± 0.14 2.77 ± 0.12 2.74 ± 0.13 2.74 ± 0.14 2.79 ± 0.10

(4'-Nitrobenzo-15C5.Ca)2+ b

Pure AN 3.33 ± 0.02 3.24 ± 0.03 3.20 ± 0.03 3.11 ± 0.02 2.99 ± 0.03

75.0%AN-25.0%MeOHc 2.73 ± 0.14 2.82 ± 0.09 2.75 ± 0.13 2.75 ± 0.12 2.78 ± 0.11

50.0%AN-50.0%MeOH 2.77 ± 0.12 2.73 ± 0.15 2.77 ± 0.12 2.77 ± 0.11 2.77 ± 0.11

25.0%AN-75.0%MeOH 2.78 ± 0.11 2.74 ± 0.14 2.77 ± 0.12 2.80 ± 0.10 2.75 ± 0.13

Pure MeOH 2.77 ± 0.11 2.76 ± 0.12 2.74 ± 0.13 2.75 ± 0.13 2.80 ± 0.10

(4'-Nitrobenzo- 15C5.Sr)2+ b

Pure AN d d d d d

75.0%AN-25.0%MeOHc d d d d d

50.0%AN-50.0%MeOH 2.77 ± 0.12 2.77 ± 0.12 2.75 ± 0.13 2.76 ± 0.11 2.80 ± 0.10

25.0%AN-75.0%MeOH 2.77 ± 0.12 2.72 ± 0.12 2.76 ± 0.12 2.75 ± 0.13 2.83 ± 0.08

Pure MeOH 2.70 ± 0.06 2.81 ± 0.09 2.73 ± 0.14 2.76 ± 0.12 2.77 ± 0.11

a SD = standard deviation.

b The concentration of each metal cation was 1.0 x 10-4 M. c Composition of binary mixtures is expressed in mol % for each solvent system. d The salt is not dissolved in these solvent systems.

in all cases were linear. A typical example of these plots is shown in Figure 4.

The changes in standard enthalpy (A H for the com-plexation reactions were determined in usual manner from the slopes of the Van't Hoff plots, and the changes in standard entropy (AS°) were calculated from the relationship A G°c 298 15 = A H ° - 298.15 A The results are summarized in Table 2.

The changes of log Kf versus the mole fraction of AN for (4'-nitrobenzo-15C5.Mg)2+ and (4'-nitrobenzo-15C5.Ca)2+ complexes are shown in Figures 5 and 6, respectively. The changes of the standard entropy of com-plexation of Mg2+ and Ca2+ cations by 4'-nitrobenzo-15C5 with the mole fraction of AN is shown in Figure 7.

As is evident in Figure 1, addition of 4'-nitrobenzo-15C5 to Mg2+ cation solutions causes an increase in the molar conductivity which indicates that (4'-nitrobenzo-15C5.Mg)2+ complex is more mobile than free solvated Mg2+ ion. Similar behavior was observed for complexation of Ca2+ and Sr2+ cations with the ligand. It is clear from Figure 1 that the slope of each curve changes at the point where [L]t/[M]t is about 1

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