Scientific paper Speciation Study on the Interaction of Some Divalent Transition Metal Ions with Succinic acid Dihydrazide in Aqueous Medium Atreyapurapu Satyanarayana,* Dunkana Negussa Kenie and Pulipaka Shyamala Department of Physical and Nuclear Chemistry and Chemical Oceanography, Andhra University, Visakhapatnam -530003, India * Corresponding author: E-mail: sn_atre@yahoo.co.in Received: 10-06-2013 Abstract The interaction of succinic acid dihydrazide with Mn(II), Co(II), Ni(II), Cu(II) and Zn(II) was investigated in aqueous medium at 303K and ionic strength of 0.1 mol dm-3. The speciation and stability constants of the metal complexes formed were determined by pH-metric method. Mononuclear and homo binuclear species in different protonation and de-protonation states were identified, the corresponding formation constants calculated using MINIQUAD-75 computer program, and species distribution diagrams produced using the HYSS computer program. Selection of the best-fit chemical models was based on statistical parameters. Keywords: Formation constant, speciation, succinic acid dihydrazide, metal complexes 1. Introduction Dihydrazides, which are synthesized by condensation reaction of hydrazine with corresponding diesters,1,2 are widely used for coating of epoxy resins in varnishes and nickel corrosion inhibitors.34 Further they are employed as latent hardeners, curing agents, and cross-linkers in many industrial products including semiconductors.56 Hydrazides display antimicrobial, antifungal, antibacterial and anti-tubercular properties7-10 based on their tendency to form metal chelates with transition metal ions. Majority of the studies reported in the literature on metal complexes of dihydrazides are aimed at preparation and elucidation of their structures by physicochemical met-hods.11-16 Studies on the solution equilibria of their metal complexes are scanty.1718 Dihydrazide contains two hydrazide groups and each hydrazide group is known to exist either in ketonic [NH2 NH C(=O) R C(=O) NH.NH2] or enolic [NH2 N=C (OH) R C(OH)=N NH2] form. Depending on the pH the terminal amino groups of the neutral ligand may be proto-nated. These ligands may also lose protons from enolic form either at higher pH or on interaction with metal ions. Hence there is a chance of formation of a variety of coexisting mono nuclear and binuclear species in protonated, unprotonated and deprotonated forms.19 The formation of binuclear species has special significance. The presence of two metal ions in the same species separated by some distance not too far, leads to several interesting and important applications including unusual magnetic properties.20 Depending on the degree of flexibility of the molecule between the two coordination sites there is a possibility of formation of supramolecular architectures like double helices and coordination polymers.21 They can be used to mimic metalloproteins and to study structure- reactivity relations-hip.22 The metal centers are found to act synergistically in catalyzing reactions.23 24 Speciation study which gives the nature and extent of formation of all the species in solution is an essential pre-requisite to understand their behavior in biological and other systems. Therefore, in this paper we report the results of a speciation study on the interaction of succinic acid dihydrazide (SADH) with Mn(II), Co(II), Ni(II), Cu(II) and Zn(II) in aqueous medium. 2. Experimental 2. 1. Material, Solutions & Apparatus All the chemicals used were of AnalaR or equivalent grade. Succinic acid dihydrazide (SADH) (Fluka) was re- crystallized twice from water, dried at 100 °C and a ~0.05 mol dm-3 solution was prepared freshly in 0.1 mol dm-3 HCl just before the titrations. All other solutions, including metal chlorides, hydrochloric acid, sodium hydroxide, and sodium chloride, were prepared using doubly glass distilled water through which nitrogen gas was purged to expel any dissolved oxygen or carbon dioxide. A Control Dynamics pH-meter model APX 175 E/C in conjunction with a combination electrode (0-14 pH range) was used for pH measurements. The pH correction factor was calculated before each set of experimental titrations by Gran method.25-27 2. 2. Titration Procedure and Data Analysis Calvin-Wilson titration technique modified by Ros-sotti28,29 was employed for the study of protonation and complex equilibria of the ligand (SADH). Titrations were carried out at 1:1, 1:2 and 2:1 ratios of initial concentrations of metal to ligand. In some of the titrations, the upper pH limit of rejecting data was determined by the appearance of opalescence leading to precipitation indicated by a downward drift of the pH-meter dial readings. For a system containing a metal ion and a ligand forming N complexes, the formation constant of a complex can be represented as, Table 1. Protonation constants (Log PClh) of SADH in aqueous medium (Temp. 30 ± 1 °C and ionic strength, I = 0.1 mol dm-3) Log Pmh (SD*) Np* U*X 108 X2 pH-Range Pmih = [MmLlHhli / [M]im.[L]il.[H]ih (1) 011 012 01-1 3.59 (1) 6.16 (1) -11.89 (2) 113 1.527 42.6 1.8-11.5 * (U- sum of the squares of residuals in mass balance equations, NP= number of experimental points, SD=standard deviation in the least significant digit) At lower pH (below ~3.0), SADH (L) mostly exists in its diprotonated form, H2L2+, and with rise of pH, undergoes successive deprotonation to form the mono-proto-nated (HL+) and neutral species (L). P011 and P012 are the formations constants ( Fig. 1) of mono and diprotonated forms of SADH from its neutral form. In basic medium hydrazides are prone to lose a proton32 from the enolic form. As SADH contains two hydrazide groups, there is a chance of losing two enolic protons at higher pH. The formation constant, P01-1 in the best-fit model corresponds to the deprotonation of one of the enolic protons leading to the formation of LH-1 species. The other (LH-2) which corresponds to the formation constant, P01-2 is not observed as its equilibrium may lie well above the pH range of study. However, in the presence of a metal ion the ligand may also lose the second enolic proton forming both de-protonated Mm L; H-1 and Mm L; H-2, type of species. where [M];, [L] and [H] are the free concentrations of metal, ligand and hydrogen ion respectively at ith experimental point. Different species in solution possess different values of stoichiometric coefficients m, I and h. The potentiometric titration data obtained was subjected to analysis by Miniquad-75 program.30 The stoichiometry and stability constants of the species formed in solution were determined by examining various chemically possible composition models for the systems studied. The best-fit models were selected on the basis of U (sum of the squares of residuals in mass balance equations), standard deviations in formation constants and other statistics like X2 test which tests the distribution of errors against a normal one. Species distribution diagrams for all the systems under study were generated using HYSS31 program. 3. Results and Discussion 3. 1. Proton- Ligand Equilibria of Succinic Acid Dihydrazide A perusal of potentiometric titration data of SADH indicates association of protons in the lower pH region and dissociation at higher pH. The best-fit model obtained using Miniquad-75 program (Table 1) contained three formation constants, P011, P012 and P011. Figure 1. Protonation and deprotonation equilibria of SADH Figure 2. Species distribution diagram for proton-ligand equilibria of SADH The species distribution diagram (Fig. 2) indicates that SADH exists in protonated form below pH 6, depro-tonated form above pH 10 and in neutral form between pH 6-10. 3. 2. Metal- Ligand Equilibria of Succinic Acid Dihydrazide The acquired data for metal-ligand equilibria was first subjected to analysis by classical procedures28 to get the formation constants of simple mono-nuclear complexes like ML, ML2 etc. Simulated titration curves were then generated using computer program SOPHD33 developed in our laboratory to see whether these species satisfy the experimental data. The simulated titration curves thus obtained were plotted along with the experimental ones to identify the regions of pH where they differ. Titration curves for some representative systems for all the compositions are shown in Fig.3. The wide difference between the simulated and experimental curves reveals the presence of other major species in addition to simple mono-nuclear complexes. Different chemical models containing chemically plausible species depending on the nature of the ligand, metal and the pH region of difference in the curves were tested using Miniquad-75 program. The required initial estimates of the formation constants were calculated basing on the formation constants of simple complexes and protonation/ deproto-nation constants of the ligand. The best-fit models obtained for all the metal ions and compositions are shown in Table 2. For 1:1 and 1:2 compositions the species converged include protonated MLH, ML2H, ML2H2 and deprotonated MLH-1 MLH2, ML2H_p ML2H 2 in addition to simple ML, ML2 type of complexes. In the case of 2:1 composition, formation of homo binuclear species M2L, M2LH^ and M2LH 2 were observed. 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