ЖУРНАЛ НЕОРГАНИЧЕСКОЙ ХИМИИ, 2010, том 55, № 3, с. 398-403
КООРДИНАЦИОННЫЕ ^^^^^^^^^^^^^^ СОЕДИНЕНИЯ
УДК 541.49
EFFECT OF NON-AQUEOUS SOLVENTS ON STOICHIOMETRY AND SELECTIVITY OF COMPLEXES FORMED BETWEEN 4-NITROBENZO-15-CROWN-5 WITH Fe3+, Y3+, Cd2+, Sn4+, Ce3+ AND Au3+ METAL CATIONS © 2010 G. H. Rounaghi*, M. H. Soorgi, M. S. Kazemi
Department of Chemistry, Faculty of Sciences, Ferdowsi University of Mashhad, Iran E-mail: ghrounaghi@yahoo.com Поступила в редакцию 02.12.2008 г.
The complexation processes between Fe3+, Y3+, Cd2+, Sn4+, Ce3+ and Au3+ metal cations with macrocyclic ligand, 4'-nitrobenzo-15-crown-5 (4'NB15C5), were studied in acetonitrile (AN), methanol (MeOH) and nitromethane (NM) solvents at different temperatures using conductometric method. The conductance data show that the stoichiometry of the complexes formed between this macrocyclic ligand and Cd2+, Au3+ cations is 1 : 1 (ML), but in the case of Fe3+, Y3+ and Ce3+ metal cations, 2 : 1 (M2 : L) and 2 : 2 [M2 : L2 complexes are formed in nitromethane solutions. The results show, that the selectivity of 4'NB15C5 for the studied metal cations in methanol solutions at 15°C is: Sn4+ > Cd2+ > Y3+ > Fe3+ « Ce3+ > Au3+, but in the case of acetonitrile, the stability order was found to be: Y3+ > Au3+ > Fe3+ > Cd2+. The values of stability constants of the 1 : 1 [M : L] complexes were determined from conductometric data using a GENPLOT computer program. The values of thermodynamic parameter (AH° and AS°) for formation of the complexes were obtained from temperature dependence of the stability constants, using the van't Hoff plots. The results show that the values of standard enthalpy (AH° ) and standard entropy (AS °) change with the nature of the non aqueous solvents.
Key words: 4'-nitrobenzo-15-crown-5; Fe3+, Y3+, Cd2+, Sn4+, Ce3+ and Au3+ cations; Acetonitrile; Methanol; Nitromethane; Conductometry.
1. INTRODUCTION
Conductance measurements of an electrolyte solution in the presence of a crown compound provide two valuable pieces of information. The first, is detection of complexation between the crown compound and the cation. Furthermore, the stability constant of the crown compound- cation complex can be determined from the conductance data. Macrocyle design parameters, such as cavity size, the type and the number of donor atoms, the type and the number of proton-ionizable groups within and without the macrocycle cavity, chirality's, the substituent groups, and steric hindrance and also the solvent parameters can all be used to obtain the desired selectiv-ities [1, 2].
Today, the chemistry of the crown compounds forms an important part of the literature [3, 4]. Some of the papers deal with their applications such as ionopheres in the construction of ion selective electrode [5—7], in ion exchange membrane [8, 9], in the preparation and precon-centration of the metal cations [10] and in the recovery of rare earth metal elements [11, 12].
The goal of the present investigation is to study the effect of nature of the cations and especially the solvent properties on the stoichiometry, stability and also the se-
*To whom correspondence should be addressed.
lectivity ofthe complexes formed between 4'-nitrobenzo-15-crown-5 and Fe3+, Y3+, Cd2+, Sn4+, Ce3+ and Au3+ metal cations in methanol, acetonitrile and ni-tromethane using the conductometric technique.
2. EXPERIMENTAL
2.1. Material
4'-Nitrobenzo- 15C5 (Merck), thin chloride (SnCl4 5H2O), cadmium nitrate (Cd(NO3)2 ■ 4H2O), iron nitrate (Fe(NO3)3 ■ 6H2O), gold chloride (AuCl3 ■ H2O), yttrium nitrate (Y(NO3)3 ■ 6H2O), cerium nitrate (Ce(NO3)3 6H2O) methanol (CH3OH), acetonitrile (CH3CN) and nitromethane (CH3NO2) (all from Merck) were used with the highest purity.
2.2. Procedure
The experimental procedure used to obtain the stability constants of the complexes is as follows: a solution of metal salt (10-4 M) was placed in a titration cell and the conductance of solution was measured, then a step-by-step increase in the crown ether concentration was carried out by a rapid transfer from crown ether solution prepared in the same solvent (2 x 10-3 M) to the titration cell using a microburette and the conductance of the solution
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Fig. 1. Molar conductance-mole ratio plots for (4'NB15C5 ■ Cd2+) complex in NM at 15°C (*), 25°C (x), 35°C (a), 45°C (■) and 55°C (♦).
Fig. 2. Molar conductance-mole ratio plots for complex-ation of 4NB15C5 with Fe3+ cation in NM at 15°C (*), 25°C (x), 35°C (■), 45°C (a) and 55°C (♦).
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in the cell was measured after each transfer at the desired temperature. The conductance measurements were performed using a digital Amel conductivity apparatus, Model 160, in a water-bath termostated at a constant temperature maintained within ±0.1°C.
3. RESULTS AND DISCUSSION
The changes of molar conductance (Am) versus the ligand to the cation molar ratios ([L]t/[M]t) for complex-ation of 4'NB15C5 with Fe3+, Y3*, Cd2+, Sn4+, Ce3+ and Au3+ metal cations in methanol, acetonitrile and ni-tromethane were studied at different temperatures. Four typical series of molar conductance values as a function of ligand/metal cation mole ratios in NM and MeOH solvents are shown in Figures 1—4.
The stability constants of the 4'NB15C5 crown ether complexes at each temperature were calculated from variation of molar conductance as a function of ligand/ metal cation mole ratios using a GENPLOT computer program. The values of the stability constants (logX^) for the 4'NB15C5 • MB+(MB+= Fe3+, Y3*, Cd2+, Sn4+, Ce3+ and Au3+) complexes in various solvent systems at different temperatures are listed in Table 1.
As is obvious from Figure 1, addition of 4'NB15C5 crown ether to Cd2+ ion in nitromethane solutions at different temperatures results in an increase in molar conductivity. This indicates that the (4'NB15C5 • Cd2+)
complex in NM solvent is more mobile than free solvated Cd2+ ion. The slope of the corresponding molar conductivity versus ligand/cation mole ratio plots changes sharply at the point where the ligand to cation mole ratio is about 1, which is an evidence for formation of a relatively stable 1:1 complex between Cd2+ ion and 4NB15C5 ligand in nitromethane solutions. But a gradual increase in the molar conductance was observed for some of the studied metal cations upon addition of the ligand to their solutions, which does not show a considerable change in the curvature ofthe plots at mole ratio 1, indicating that a weaker 1 : 1 [M : L] complex is formed in solutions. In most cases, as the temperature increases, the curvature of the mole ratio plots for complexation of 4NB15C5 with the the studied metal cations decreases which is an evidence for formation of stronger complexes at lower temperatures, therefore, the complexation processes are exothermic in these solvent systems and due to a decrease in viscosity ofsolvents with increasing temperature results in an increase the solvation of the dissolved species in solutions [13].
An interesting behaviour was observed for complexation of 4NB15C5 with Fe3+, Y3+, Sn4+ and Ce3+ cations in nitromethane solutions. As is obvious from Figures 2 and 3, addition of 4NB15C5 to Fe3+ and Y3+ cations solutions at different temperatures causes the molar conductivity to initially decrease until the mole ratio reaches to 1/2 and then to increase. These graphical results indi-
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Fig. 4. Molar conductance-mole ratio plots for complexation of 4'NB15C5 with Sn4+ cation in MeOH at 15°C (*), 25°C (x), 35°C (a), 45°C (■) and 55°C (♦).
cate that 2 : 1 [M2 : L] complexes are formed between 4'NB15C5 and Fe3+ and Y3+ cations in nitromethane solutions and these complexes are less mobile than free sol-vated Fe3+ and Y3+ cations. It seems that further addition of the ligand to [M2 : L] complexes, causes formation of [M2 : L2] complexes with a club sandwich structure which are less solvated than [M2 : L] complexes in nitromethane solutions and, therefore, the molar conductivity increases. Similar behaviour was observed for Sn4+ and Ce3+ cations in nitromethane solutions. Since the cavity size of 4'NB15C5 is not big enough to fit two M"+ (M"+ = Fe3+, Y3+, Sn4+ and Ce3+) cations, but it may suggest that the second M"+ cation probably interacts with the ligand via oxygen donor of —NO2 group and also with the n electron system of benzo group and, therefore, they form a 2: 1 [M2 : L] complex with the metal cations in nitromethane solutions and further addition of 4'NB15C5 results in formation a [M2 : L2] complex with a club sandwich structure.
Since the donor number of methanol (DN = 20.0) is bigger than those of acetonitrile (DN =14.1) and nitromethane (DN = 2.7), therefore, the metal cations are much more solvated in this solvent in compared to the other two organic solvents. Therefore, as is obvious for Table 2, all of the metal cations form only 1 : 1 [M : L] complexes with 4'-nitrobenzo-15-crown-5 in methanol solutions.
As is obvious from Figure 4, addition of 4'NB15C5 to Sn4+ cation in acetonitrile solutions at different temperatures causes the molar conductivity to initially increase very slowly until the mole ratio reaches 1 : 1 and then to increase rapidly. Such behaviour may be described according to the following equilibria:
4'NB15C5 + Sn4+— (4'NB15C5 • Sn4+), (I)
(4'NB15C5 • Sn4+) + 4'NB15C5
[(4'NB15C5)2 • Sn
4+ -.
(II)
It seems that addition of the
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