Potentiometric and Spectrophotometric Studies of Copper(II) Complexes of Some Ligands in Aqueous and Nonaqueous Solution

Transcription

1 Journal of the Chinese Chemical Society, 2007, 54, Potentiometric and Spectrophotometric Studies of Copper(II) Complexes of Some Ligands in Aqueous and Nonaqueous Solution Ardeshir Shokrollahi, Mehrorang Ghaedi* and Hamed Ghaedi Chemistry Department, University of Yasouj, Yasouj , Iran Stoichiometry and equilibrium study of copper-ligands including mercaptobenzoxazole (MBO), 4-propyl 2-thiouracyl (PTU), methyl-2-pyridylketone oxime (MPKO), phenyl-2-pyridylketone oxime (PPKO), 4,6-dihydroxy-2-mercaptopyrimidine (DHMP), N,N -phenylene bis(salicylaldimine) (PBS) and 1,2-bis(2-hydroxyphenyl)naphtaldiimine (BHNPDI) were conducted in aqueous and nonaqueous solution by potentiometry and spectrophotometry. Stability constants of the complexes are determined at 25 1 C and 0.1 or 0.05 M ionic strength in water or acetonitrile solvents. Oximes ligand protonation constants and copper-ligands complexes stability and hydrolysis constants were calculated using the BEST program in aqueous solution. The stability constants of copper-ligands complexes were calculated using the KINFIT program in acetonitrile solution. The results of these two methods are made self-consistent, then rationalized assuming an equilibrium model including the species, ML, MLH, MLOH and ML 2 (where the charges of the species have been ignored for the sake of simplicity) (L MBO, PTU, MPKO, PPKO, DHMP, BHNPDI and PBS). Keywords: Copper ion; Ligands; Potentiometric study; Spectrophotometric study; Stability constant; KINFIT program; Best program. INTRODUCTION The determining of thermodynamic parameters of complexes using potentiometric or spectrophotometric data to develop and propose new methods for selective and sensitized determination of trace amounts of ions is a challenging problem. For example, a high stability constant with fast complex formation which could be obtained from the stability constant led to development of a new analytical method. Their stability constants can be of significance in order to predict different chemical processes such as isolation, extraction, or preconcentration methods, 1,2 since many elements present in trace amounts can be isolated by complexing reagents. The magnitude of the stability constant indicates the particular level of tolerance to the interference by other species. One can see that the complex formed between the ligands and copper with high stability constants is slightly more stable as compared to the one with a lower stability constant. The stability constant is dependent on several parameters such as electronegativity, hardness or softness of the donor atoms in the ligand structure, topology of the ligand and the ionic radii, charge, hardness or softness of the metal ion and its atomic number. 3 Nowadays different programs such as the KINFIT 4 and BEST 5 programs have been used for evaluating the stability constant of complexes or dissociation constants of ligands, using spectrophotometric or potentiometric data Of particular interest have been those involving copper(ii) since they reveal surprising molecular diversity not only in coordination geometry but in more subtle changes in the ligands. In the present work the authors decided to investigate the effect of the ligand structure on the stability constants of the complexation of the copper ion; using this data we can focus on synthesizing new ligands with higher stability constants toward copper ions for its selective and sensitive determination. EXPERIMENTAL Reagents and Apparatus Acids and bases (all from Merck) were of the highest * Corresponding author. Tel: (+98) ; Fax: (+98) ; m_ghaedi@mail.yu.ac.ir

2 934 J. Chin. Chem. Soc., Vol. 54, No. 4, 2007 Shokrollahi et al. purity available and were used as received. Doubly distilled deionized water was used throughout. Analytical grade nitrate salt of copper ion with the highest purity available was purchased from Merck Company and used without any further purification. 2-Mercaptobenzoxazole (MBO), 6-dihydroxy-2-mercaptopyrimidine (DHMP) and 4-propyl 2-thiouracyl (PTU) were purchased from Merck Company. The ligand phenyl-2-pyridilketon oxime (PPKO) and methyl 2-pyridylketone oxime (MPKO) was synthesized according to the literature. 15 The N,N -phenylene bis(salicylaldimine) (PBS) was synthesized according to the literature. 16 Absorbance measurements were carried out with a Perkin-Elmer UV-Vis spectrophotometer EZ201. All potentiometric ph measurements were made using a Model 686 Metrohm Titoprocessor equipped with a combined glasscalomel electrode. Synthesis of 1,2-Bis(2-hydroxyphenyl)naphtaldiimine (BHNPDI) To 25 ml ethanolic solution of 5 mmol (0.545 g) 2-aminophenol, 2.5 mmol (0.335 g) phthaldehyde in 20 ml ethanol was gradually added. The progress of the reaction was monitored by TLC. After 6 h, 30 ml cooled water was added to the reaction mixture and then an orangish yellow precipitate was filtered and washed with water twice and 1,2-bis(2-hydroxyphenyl)naphtaldiimine was obtained. M.p. ( C) Characteristic IR (KBr, cm -1 ): 3332 (bs, OH), 3066 (w, CH-Aromatic), 2976 (w, CHimine), 1595 (vs, C N-imine) 1495, 1450 (s, C C, aromatic ring), 1273, 1242, 1200 (m, C-N), 1110, 1036 (m, C-O), 851 (m), 747 (m), 697 (m), 586 (m). 1 H NMR (CDCl 3, ppm): (8.31, s, 2H), (7.41, d, 2H), (7.22, m, 4H), (7.12, t, 2H), (6.82, m, 2H), (6.71, d, 2H), (5.1, s, 2H). Potentiometric ph Titrations All potentiometric ph measurements were made on solutions in a 75 ml double-walled glass vessel using a Model 686 Metrohm Titroprocessor equipped with a combined glass-calomel electrode. The temperature was controlled at C by circulating water through the jacket from a constant-temperature bath (MLW thermostat). The cell was equipped with a magnetic stirrer and a tightly fitting cap, through which the electrode system and a 10 ml capacity Metrohm piston burette were inserted and sealed with clamps and O-rings. Atmospheric CO 2 was excluded from the titration cell with a purging steam of purified nitrogen gas. The concentrations of PPKO and MPKO were about M for the potentiometric ph titrations of PPKO and MPKO in the absence and presence of copper ions (in the order of 10-3 M). A standard carbonate-free KOH solution (0.099 M) was used in all titrations. The ionic strength was adjusted to 0.1 M with KNO 3. Before an experimental point (ph) was measured, sufficient time was allowed for the establishment of equilibrium. Ligands protonation constants and their copper complexes protonation, stability and hydrolysis constants were evaluated using the program BEST described by Martell and Motekaitis. 5 The value of K w =[H + ][OH ] used in the calculations was Spectrophotometric Titrations Standard stock solutions of ligands ( M) and copper ions ( M) were prepared by dissolving appropriate and exactly weighed (with an accuracy of g) pure solid compounds in pre-calibrated 25.0 ml volumetric flasks and diluted to the mark with acetonitrile. Working solutions were prepared by appropriate dilution of the stock solutions. According to the spectra reported in Figs. 3-7a, titration of the ligand solution ( M, 2.6 ml) was carried out by the addition of microliter amounts of a concentrated standard solution of the metal ion in acetonitrile ( M) using a pre-calibrated micropipette, followed by absorbance intensity reading at 25.0 C at the related max. Since the volume of titrant added during titration was negligible (at the most 0.05 ml) as compared with the initial volume of the ligands (2.6 ml), no volume correction was carried out. RESULTS AND DISCUSSION Potentiometric results Protonation constant The protonation constants of PPKO and MPKO were obtained under the same conditions of ionic strength and temperature which are applied for the study of binary systems. The titration data for two ligands were obtained and ph-volume plots are shown in Figs. 1-a and 2-a. The maximum number of protons attached to PPKO and MPKO is two. One oxime proton and the other are attached to the nitrogen of the pyridine ring. The overall protonation constants of these two ligands were calculated from computer refinement of the ph-volume data. The obtained values are

3 Potentiometric and Spectrophotometric Studies of Copper(II) Complexes J. Chin. Chem. Soc., Vol. 54, No. 4, shown in Table 1 for the first time. Binary complex formation equilibria Oximes are analytical reagents widely used in preconcentration, extraction and spectrophotometry for metal ion determination 2,19 but little attention has been paid to complexation and distribution equilibria related to these systems. 20,21 In this part we investigated potentiometric study of complexation of new synthesis oximes including methyl-2-pyridylketone oxime (MPKO) and phenyl-2-pyridylketone oxime (PPKO) with copper ions. Figs. 1-a and 2-a display a representative set of potentiometric titration curves obtained for Cu 2 -PPKO/MPKO systems. The formation constants of copper ion with PPKO and MPKO binary complexes were determined under stable conditions of 0.1 M ionic strength and 25 C; models and values are shown in Table 1. The concentration distribution diagrams of binary systems are obtained in terms of percent metal ion as a function of ph, and are shown in Fig. 1-b and 2-b. Because of their smaller molecular weight and larger Table 1. Logarithm of cumulative stability constants for the interac-tion of H + and Cu 2+ with the PPKO and MPKO at 25 C and ionic strength of 0.1 M System Cu 2+ H + L Log Cu-PPKO Cu-MPKO contents of acidic functional groups, PPKO and MPKO (Scheme I) can form metal complexes that are more soluble, bio-available and mobile than those formed by other ligands. Due to the presence of the oxime group and incor- Fig. 1. Potentiometric titration curves for PPKO in the absence and presence of Cu 2+ ion with M NaOH at 25 C and I 0.1 M NaNO 3 (a) and the corresponding distribution diagram (b). Fig. 2. Potentiometric titration curves for MPKO in the absence and presence of Cu 2+ ion with M NaOH at 25 C and I 0.1 M NaNO 3 (a) and the corresponding distribution diagram (b).

4 936 J. Chin. Chem. Soc., Vol. 54, No. 4, 2007 Shokrollahi et al. Scheme I Structure of investigated ligands poration of electrons, they act as a soft base and bind selectively to soft acid as copper ion. The ML and ML 2 for MPKO in comparison to PPKO could be formed in higher ph, while the abundance and stability constant of respective complexes for PPKO is higher. The high stability constant of ML 2 for copper: PPKO complexes makes it superior as a suitable ligand for developing an analytical method for selective determination of copper ion. 22 In the literature, it has been pointed out which similar ions, including cobalt and nickel, could be strongly complexed with oximes in alkaline ph. Therefore, the copper determination based on complexation with MPKO with the ability for complexation with copper ion as ML 2 in alkaline media are expected to suffer seriously from interference of Co(II) and Ni(II) ions. It occurred to us that incorporation of a phenyl group instead of a methyl group in PPKO in comparison to MPKO decreases the solubility of complex, which may be attributed to the higher lipophobicity of PPKO. 23,24,25 In detail, the possible complexation forms and their stability between copper ions and PPKO or MPKO have been investigated using potentiometry as a general accurate and powerful method. Results of Figs. 1-b and 2-b show that type and percent of the major form of complexes has strong dependency on ph. Using the Best program following the ph measurement, the nature of different possible forms of complexes between ligands and copper ions and the protonation constants of ligands and the stability constants of its complexes with copper ions has been evaluated and the results are given in Table 1. Figs. 1-b and 2-b indicate that in the ph range only Cu (PPKO) 2 and in ph higher than 9 only Cu (MPKO) 2 with an abundance of about 100 would be formed; decreasing ph leads to protonation of ligands and extremely reduces their ability for complexation with copper ions. In the case of PPKO, at higher ph, formation of a precipitate of a related hydroxide (Fig. 1-b) occurs. In potentiometric investigation of PPKO and MPKO with copper ions due to known interference of Co(II) and Ni(II) ion with oximes in alkaline ph, it seems analytical methods based on complexation with PPKO have more selectivity toward MPKO ligands. Spectrophotometric results Complexation of transition and heavy metal cations is favored by the presence of soft donor atoms such as nitrogen or sulfur, but also ligands containing the harder oxygen atoms can bind these cations. These ligands play a key role in the speciation of Cu(II) in various media and, therefore, in controlling its physicochemical behavior, biological availability, accumulation and mobility. Depending on the ph value, presence of salts (ionic strength effect) and degree of saturation of binding sites, ligand substances can form either soluble or insoluble complexes with Cu(II), and therefore play a double role in various matrixes. In detail we examined the complexation of copper ion with ligands including MBO, PTU, DHMP, PBS and BHNPDI and their stability constants and complex structures have been calculated using the KINFIT program. MBO Information 2-Mercaptobenzoxazole (Scheme I) has sulfur, nitrogen and oxygen as donor atoms, allowing them to act as monodentate sulfur donors or oxygen and nitrogen donors. The existence of a donating nitrogen atom as well as SH group in 2-mercaptobenzoxazole was expected to increase both the stability and selectivity of its copper complex over other metal ions, especially alkali and alkaline earth cations The spectrum of MBO (L) in acetonitrile shows an absorption band at 296 nm. As can be seen from Fig. 3-a, a decrease in absorbance is observed upon addition of increasing quantities of copper ions to L solution, whereas the absorption intensity changes as a function of the [Cu 2+ ]/[L]

5 Potentiometric and Spectrophotometric Studies of Copper(II) Complexes J. Chin. Chem. Soc., Vol. 54, No. 4, Table 2. Logarithm of stability constants assembling for the interaction of Cu 2+ with the MBO, PTU, PBS, BHNPDI and DHMP in acetonitrile Log System MBO DHMP PTU PBS BHNPDI CuL CuL Fig. 3. UV-visible spectra for titration of MBO ( M) with Cu 2+ ( M) in acetonitrile (T 25 C and I 0.05 M, max 296 nm) (a) and the corresponding molar ratio plot (b). molar ratio (Fig. 3-b). These changes could be attributed to the complexation between the ligand L and Cu 2+ ion. From the inflection point in the absorbance/mole ratio plot at [Cu 2+ ]/[L] values between 0.5 and 1, it can be inferred that both 1:1 ([Cu(L)]) and 1:2 ([Cu(L) 2 ]) complexes are formed in acetonitrile solution. When the ligand molecule reacts with the metal ion, it may form both 1:1 and 1:2, metal toligand, complexes. The resulting log K1 and K2 values (Table 2), obtained from computer fitting of the absorbance/ mole ratio data to a theoretical model including both 1:1 and 1:2 forms, were calculated. It should be noted that fitting on other possible models such as 1:1 or 1:2 single model showed no acceptable results. According to the observed stability trend as well as the high lipophobicity of the MBO and its low solubility in neutral and acidic media, construction of a copper ion selective electrode 26 and developing a sensitive adsorptive stripping voltammetry method for copper determination has been carried out. 27 In MBO due to incorporation and involvement of oxygen atoms in the ring, the probability for complexation of copper ions with this ligand through an oxygen atom will be intensively reduced and due to softsoft interaction of copper ions with sulfur and nitrogen, higher stability constants and more stable complexes could be obtained. DHMP and PUT Information As shown in Scheme I, the existence of a donating nitrogen atom as well as =S (SH) group and =O (OH) group in DHMP and PTU was expected to increase both the stability and selectivity of its complex toward ions including copper ion over other metal ions, especially alkali and alkaline earth cations. It occurred to us that because of the presence of some constituents with non-cyclic sulfur-containing ligands in addition to nitrogen and oxygen, the -electrons results in selective interaction with these ions while oxygen atoms of the ligands mostly interact with this ion as a soft acid through ion-dipole interactions and soft atoms such sulfur and nitrogen through soft-soft interaction; all these points result in fast and reversible complexation and higher sensitivity and selectivity. Their spectra with copper ion and mole ratio plot have been depicted in Figs. 4-a and 5-a and their respective complexes stability constants are presented in Table 2. For both ligands, the decrease in absorbance can be seen in 280 nm which could be attributed to deep complexation of these ligands with copper ions. Results in Table 2, from the computer fitting of the

6 938 J. Chin. Chem. Soc., Vol. 54, No. 4, 2007 Shokrollahi et al. absorbance/mole ratio data, the Log K1 and K2 values, obtained from a theoretical model including both 1:1 and 1:2 forms, were calculated. In a PTU ligand, it seems that complexation occurs through binding of copper ions to sulfur and nitrogen and in less content through ion-dipole interaction with oxygen and a higher stability constant would be achieved, while in DHMP with a similar structure but more donation oxygen atoms, complexation through ion-dipole interaction will be increased and formation constants will be reduced. Schiff s Base Complexation of PBS and BPNDI Information Schiff bases and transition metal-schiff base complexes are important intermediates in certain biologic processes, such as non-enzymatically controlled trans-amination reactions. 27 During the past two decades, considerable attention has been paid to the chemistry of the metal complexes of Schiff bases containing nitrogen and other donors. 28,29 This may be attributed to their stability, biological activity and potential applications in many fields such as oxidation catalysis and electrochemistry. Schiff s bases are characterized by their capacity to completely co-ordinate a metal ion, forming chelate rings. To our knowledge, no such studies have been done on copper complexes of these polydentate Schiff base ligands. Thus, the complexation of these ligands with this cation was investigated spectrophotometrically in acetonitrile solution at C in order to obtain clues about the stability and selectivity of the resulting complexes. The spectra of their complexes with copper ions are presented in Figs. 6-a and 7-a, and their stability constant Fig. 4. UV-visible spectra for titration of DHMP ( M) with Cu 2+ ( M) in acetonitrile (T 25 C and I 0.05 M, max 280 nm) (a) and the corresponding molar ratio plot (b). Fig. 5. UV-visible spectra for titration of PTU ( M) with Cu 2+ ( M) in acetonitrile (T 25 C and I 0.05 M, max 280 nm) (a) and the corresponding molar ratio plot (b).

7 Potentiometric and Spectrophotometric Studies of Copper(II) Complexes J. Chin. Chem. Soc., Vol. 54, No. 4, and nature of the respective complexes are summarized in Table 2. The ligands, on interaction with Cu(II) perchlorate, yield complexes corresponding to the general formula [ML 2 (H 2 O) 2 ]. 30 The analytical data show that the metal to ligand ratio is 1:2. They are soluble in common organic solvents. The present study aims to investigate the spectrophotometric behavior of two polydentate Schiff base ligands PBS and BPNDI as in (Scheme I) with copper ion in acetonitrile. The Schiff base ligands PBS and BHNPDI were chosen according to their structures by changing both the aldehyde and the amine parts in order to study their influence on the metal coordination sphere. The log K f ML are and and log K f ML 2 are and for PBS and BHNPDI. The differences among these values are probably due to the change in the nature of the metal environment in the four complexes and the interior core of desired complexes. In our view, the higher core of PBS and its greater flexibility make it suitable and superior for complexation with copper ions in comparison to BHNPDI. In fact, the presence of the primary amine nitrogen in the metal coordination sphere may lead to ligand distortion in both complexes, unlike the less rigid BHNPDI complexes that reduce the stability constant. The spectrum of Cu(II) complex of PBS in acetonitrile is shown in Fig. 7-a. The absorption at about 430 nm is assigned to a ligandmetal charge transfer transition. 15 An additional band at 254 nm is inter-ligand ( - *) transition centered on a coordinated Schiff base. The band at about 280 nm was assigned to a phenyl ring n-n transition. 31,32 As can be seen in Fig. 7-a, the bands in the nm range, by analogy, were attributed to a n-n* transition originating in the -CH Nchromophore, while the lower energy bands were due to d-d transitions. The electronic spectra of Cu(II) complexes Fig. 6. UV-visible spectra for titration of PBS ( M) with Cu 2+ ( M) in acetonitrile (T 25 C and I 0.05 M, max 420 nm) (a) and the corresponding molar ratio plot (b). Fig. 7. UV-Vis spectra for titration of BHNPDI ( M) with Cu 2+ ( M) in acetonitrile (T 25 C and I 0.05 M, max 350 nm) (a) and the corresponding molar ratio plot (b).

8 940 J. Chin. Chem. Soc., Vol. 54, No. 4, 2007 Shokrollahi et al. are generally poor indicators of geometry. In the complex formation, all four donating atoms (two nitrogen and two oxygen atoms) participated. In acetonitrile complexation of PBS and BHNPDI as ligands without sulfur atoms, MBO, DHMP and PTU as sulfur donating atoms toward copper ions have been examined. In Schiff base ligands of PBS and BHNPDI the greater resonance and interior core of ligands favor their complexation with copper ions, while more rigidity and the higher interior core of ligands in PBS increased the complexation probability and their stability constants. CONCLUSION According to the results presented in the article, one can be noticed that potentiometric investigation of complexation in water solvent, more information about ph dependency of complexation could be obtained. Using the obtained data, selection of proper choice of parameter to predict selective and sensitive method. Received July 27, REFERENCES 1. Gracia, E. A.; Gomis, D. B. Mikrochem. Acta 1996, 124, Cao, S.; Zhang, M. Trace Microprobe J. Techn. 1999, 17, Dwyer, F. P.; Mellor, D. P. Academic Press Inc.: London, UK, Dye, J. L.; Nicely, V. A. J. Chem. Edu. 1971, 48, Martell, A. E.; Motekaitis, R. J. VCH, Publishers: New York, Seleem, H. S.; El-Shetary, B. A.; Khalil, S. M. E.; Shebl, M. Serb J. Chem. Soc. 2003, 68, Safavi, A.; Rastegarzadeh, S. Anal. Sci. 1999, 15, Moghimi, A.; Shokrollahi, A.; Shamsipur, M.; Aghabozorg, H.; Ranjbar, M. J. Molecular Structure 2004, 701, Moghimi, A.; Shokrollahi, A.; Shamsipur, M.; Aghabozorg, H.; Shockravi, A. Inorg. Chem, 2003, 42, Patel, R. N.; Singh, N.; Shrivastava, R. P.; Shukla, K. K.; Singh, P. K. Proc. Indian Acad. Sci. (Chem. Sci.) 2002, 114, Gao, H. W.; Zhao, J. F. Croat. Chem. Acta 2003, 76, Machado, C. M. M.; Cukrowski, I.; Gameiro, P.; Soares, H. M. V. M. Anal. Chim. Acta, 2003, 493, Bakr, M. F. J. Chin. Chem. Soc. 2003, 50, Wang, Y. M.;.Cheng, T. H.; Liu, G. C. J. Chin. Chem. Soc. 2000, 47, Furniss, B.; Nanford, S.; Rogers, A. J. H.; Smith, V.; Tatchell, P. W. G. A. R. Textbook of Practical Organic Chemistry; Fourth Edition, New York, Longman: London, 1987; p Baran, Y.; Erk, B. J. Tur. of Chemistry, 1996, 20, Schwarzenbach, G.; Flaschka, H. Methuen, London, Singh, R.B.; Garg, B. S.; Singh, R. P. Talanta 1979, 26, Soylak, M.; Tuzen, M. J. Hazard. Mater. 2006, 137, Izquierdo, A.; Compano, R.; Granados, M. Polyhedron 1991, 10, Izquierdo, A.; Granados, M.; Beltran, J. L. Talanta 1992, 39, Ghaedi, M.; Amini, M. K.; Rafi, A.; Gharaghani, S.; Shokrolahi, A. Ann di Chim. 2005, 95, Eskandari, H.; Ghaziaskar, H. S.; Ensafi, A. A. Anal. Sci. 2001, 17, Wang, Y-M., Chung, Ch.-S.; Lo, J.-M.; Wu, Y.-L. Polyhedron 1999, 18, Para, A. Carbohydrate Polymers 2004, 57, Ghaedi, M.; Shokrollahi, A. Feressen. Environm. Bull. 2006, 15, Akhond, M.; Ghaedi, M.; Tashkhourian, J. Bull. Korean Chem. Soc. 2005, 26, Snell, I. E. E.; Metzler, D. E. Interscience: New York, Djebbar, S. S.; Benali, B. O.; Deloume, J. P. Polyhedron 1997, 16, Hattacharyya, P. B.; Parr, J.; Ross, A. T. J. Chem. Soc. Dalton 1998, Raman, N.; Ravichandran, S.; Thangaraja, C. J. Chem. Sci. 2004, 116, Iqbal, M.; Khurshid, S. J.; Iqba, M. Can. J. Chem. 1993, 71, Bosnich, B. J. Chem. Soc. 1986, 90, 627.