Chemistry
Research Paper
E-ISSN No : 2454-9916 | Volume : 8 | Issue : 7 | Jul 2022
CHEMICAL SPECIATION OF TERNARY COMPLEXES OF LASPARTIC ACID AND ETHYLENEDIAMINE WITH Co(II), Ni(II) AND Cu(II) IN LOW DIELECTRIC MEDIA 1
2
*R. Swaroopa Rani , G. Nageswara Rao 1 2
Department of Chemistry, ANITS Engg. College, Sangivalasa Visakhapatnam, India. (*Corresponding Author) Department of Inorganic & Analytical Chemistry, School of Chemistry, Andhra University, Visakhapatnam, India.
ABSTRACT Chemical speciation of ternary complexes of Co(II), Ni(II) and Cu(II) ions with L-aspartic acid and ethylenediamine was studied pH metrically in 0.0–60.0% v/v Dioxan-water mixtures maintaining an ionic strength of 0.16 mol L-1 at 303.0±0.1 K. Alkalimetric titrations were carried out in different relative concentrations (M:L:X = 1:2.5:2.5, 1:2.5:5.0, 1:5.0:2.5) of metal (M) to aspartic acid (L) to ethylenediamine (X). Stability constants of ternary complexes were calculated and various models were refined with MINIQUAD75. The best fit chemical models were selected based on statistical parameters and residual analysis. The species detected were MLX, ML2X and MLXH for Co(II), Ni(II) and Cu(II) in Dioxan-water mixtures. The stabilities of the complexes followed the Irving-Williams order, i.e., Co(II) < Ni(II) < < Cu(II). KEYWORDS: Chemical speciation, ternary complexes, aspartic acid, ethylenediamine, Dioxan, MINIQUAD75. INTRODUCTION: Chemical speciation of transition metals is important to study their mobility, distribution, bioavailability, toxicity and setting environmental standards. Bioavailability of a metal ion depends on whether it is in free state or complexed with various constituents during biological reactions. Controlling factors such as pH, temperature and ionic strength change the complexation behaviour of metals and binding sites. Thus complexation can signify the bioavailability of the metal ions in various biosystems. Most metabolic reactions are catalyzed by metalcontaining enzymes, the activity of which is due to metal-enzyme-substrate complexes. The active site of the enzymes has a lower polarity than biofluids. The specificity and selectivity of enzyme-substrate reactions can be achieved by manipulating the equivalent solution dielectric constants (ESDC) at the active site [1]. Acidity and basicity of a molecule is governed by its structure and solvent effects [2, 3]. L-aspartic acid (Asp), a non-essential amino acid, plays an important role in maintaining the solubility and ionic character of proteins [4]. It acts as a tridentate ligand and also as a neurotransmitter [5]. Ethylenediamine (en) is used as monodentate, bidentate or a bridging ligand [6]. It is used in the manufacture of EDTA, carbamate fungicides, surfactants and dyes. It is also useful in manufacturing accelerator or curing agent in epoxy industry. It is involved in the synthesis of seven-membered ring components with β-ketoesters resulting secondary amines and β-enaminoesters [7]. The en plays an important role in the synthesis of Schiff bases [8]. The protonation constants of ethylenediamine were reported earlier by theoretical calculations [9, 10]. Cobalt is essential for the production of the red blood cells, and its salts are widely used in industrial materials, paint products, fertilizers, feeds and disinfectants [11-13]. Special cobalt-chromium-molybdenum alloys are used for prosthetic parts such as hip and knee replacements [14]. Nickel is found in urease, which accounts for 6% of the soluble cellular proteins [15] and catalyses the hydrolysis of urea to yield ammonia and carbamate. Copper containing enzymes and proteins constitute an important class of biologically active compounds, whose biological functions include electron transfer, dioxygen transport, oxidation, reduction and disproportionation [16]. The aim of the present study is to understand the role of metal ions at active site cavities in bioactive molecules like enzymes and proteins and to know the effect of dielectric constant of the medium on the chemical speciation of the ternary complexes of Co(II), Ni(II) and Cu(II) with Asp and en. Since the dielectric constant at the active site cavities is very small compared to that in biofluids, low dielectric constant is mimicked by using a water soluble organic solvent like Dioxan (Dox) which is a non-polar solvent capable of acting as hydrogen bond acceptor with random structure. Protonation constants [17, 18] and binary stability constants of Asp [19] and en [20] with Co(II), Ni(II) and Cu(II) have been reported earlier. Hence, chemical speciation of their ternary complexes is reported in the present communication. Experimental: Aqueous solutions (0.1 mol L-1) of Co(II), Ni(II) and Cu(II) chlorides (GR Grade, E-Merck, Germany) were prepared by dissolving them in triple distilled water. 0.05 mol L-1 solutions of Ethylenediamine (AR, Qualigen, India) and LAspartic acid (AR, Qualigen, India) were also prepared. To increase the solubility of the ligands and metal salts, 0.05 mol L-1 hydrochloric acid was maintained
in the solutions. 1, 4- Dioxan (Finar, India) was used as received. The strength of alkali was determined using the Gran plot method [21, 22]. Errors in the concentrations of ligands, metal ions and alkali were subjected to analysis of variance (ANOVA) [23]. Titrations were carried out in the medium containing varying concentrations of Dox maintaining an ionic strength of 0.16 mol L-1 with sodium chloride at 303.0±0.1 K. The measurements were recorded with an ELICO (Model LI-120) pH meter of 0.01 readability in conjunction with a glass and calomel electrode. The pH meter was calibrated with 0.05 mol L-1 potassium hydrogen phthalate in acidic region and 0.01 mol L-1 borax solution in basic region. The glass electrode was equilibrated in a well stirred Dox-water mixtures containing inert electrolyte. The effect of variations in asymmetry potential, liquid junction potential, activity coefficient, sodium ion error and dissolved carbon dioxide on the response of glass electrode were accounted for in the form of correction factor (log F) [24] which was computed from the experimental and simulated acid-base titration data calculated by SCPHD program [25]. It also accounts for the solvent effect on pH. Titration of strong acid with alkali was carried out at regular intervals to check whether complete equilibration was achieved. The calomel electrode was refilled with Dioxan-water mixtures of equivalent composition as that of the titrand. In each of the titrations, the titrand consisted of 1 mmol of hydrochloric acid in a total volume of 50 mL. Titrations were carried out in the presence of different relative concentrations of the metal (M) to Asp (L) to en (X) (M:L:X = 1:2.5:2.5, 1:2.5:5.0, 1:5.0:2.5) with 0.4 mol L-1 NaOH (Table 1). The best-fit chemical model for each system investigated was arrived at using a nonlinear least squares analysis program MINIQUAD75 [26]. Results and discussion. Modeling of chemical speciation: A preliminary investigation of alkalimetric titrations of mixtures containing different mole ratios of Asp and en in the presence of hydrochloric acid and inert electrolyte indicates that no condensed species were formed. The protonation constants and the stability constants of the binary metal complexes of these ligands were fixed in refining ternary complexes and in testing various chemical models using MINIQUAD75. The best fit model was chosen based on the statistical parameters like χ2, R-factor, skewness and kurtosis given in Table 2. The ternary complex species detected are MLX, ML2X and MLXH for Co(II), Ni(II) and Cu(II). A very low standard deviation (SD) in the overall stability constants (log β) indicates the precision of these parameters. The small values of Ucorr (sum of squares of deviations in the concentrations of the metal, the ligands and the hydrogen ion at all experimental points corrected for degrees of freedom) indicate that the models represent the experimental data. Small values of mean, standard deviation and mean deviation for the systems corroborate that the residuals are around a zero mean with little dispersion. For an ideal normal distribution, the values of kurtosis and skewness should be three and zero, respectively. The kurtosis values in the present study indicate that most of the residuals are very nearer to leptokurtic and a few form mesokurtic patterns. The values of skewness recorded in Table 2 are between -2.34 and 3.07. These data evince that the residuals form a part of normal distribution hence, least–squares method can be applied to the present data. The sufficiency of the model is further evident from the low crystallographic R-values recorded.
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