Chemistry
Research Paper
E-ISSN No : 2454-9916 | Volume : 8 | Issue : 5 | May 2022
INFLUENCE OF PROPYLENE GLYCOL ON CHEMICAL SPECIATION OF TERNARY COMPLEXES OF BI- AND TRIDENTATE LIGANDS WITH SOME ESSENTIAL METAL IONS 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 Equilibria of complexation of Co(II), Ni(II) and Cu(II) ions with L-aspartic acid and ethylenediamine were investigated pH metrically in 0.0–60.0% v/v propylene glycol-water mixtures maintaining an ionic strength of 0.16 mol L-1 at 303.0±0.1 K. 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 MLXH, MLX and ML2X for Co(II), Ni(II) and Cu(II). The appropriateness of experimental conditions was verified by introducing errors intentionally in the concentrations of ingredients. 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, propylene glycol, MINIQUAD75. 1. INTRODUCTION: Several studies on ternary stability constants of α-amino acids have been reported in different media.1, 2 Metal ions exist in non-exchangeable form, loosely bound to some biological ligands and in equilibrium with a variety of bioligands in different biofluids like blood serum, intestinal fluid, cerebrospinal fluid, gastric juice etc. Thus, simultaneous equilibria involving a variety of metal ions and ligands is possible in bio fluids. Cobalt, nickel and copper have several biological functions.3,4 Therefore, investigation of ternary complexes containing a metal ion and two different ligands drew the attention of several researchers. Mixed ligand complexes can be considered as models for apoenzyme-metal ion-substrate complexes. In peptides, proteins and amino acids, amide group is one of the important binding sites for the coordination of the metal ions.Acidity and basicity of a molecule is governed by its structure and solvent effects.5 Propylene glycol (PG) is chosen as a polar organic solvent to mimic the permittivity of the biological fluids. L- Aspartic acid (Asp) and ethylenediamine (en) are chosen as model compounds to proteins and substrates. Asp, a non-essential amino acid, plays an important role in maintaining the solubility and ionic character of proteins.6 en is used as an important monodentate, bidentate or a bridging ligand.7 It is useful in manufacturing accelerator or curing agent in epoxy industry. It is involved in the synthesis of β-enaminoesters8 and Schiff bases.9 The protonation constants of ethylenediamine were reported earlier by theoretical calculations.10,11 Protonation constants12and binary stability constants of Asp13 and en14 with Co(II), Ni(II) and Cu(II) in PG-water mixtures were reported earlier. Hence, chemical speciation of their ternary complexes is reported in this communication. 2. 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 aqueous solutions of L-aspartic acid (GR Grade E-Merck, Germany) and ethylenediamine (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. Propylene glycol (Finar, India) was used as received. The strength of acid in metal ion solutions was determined using the Gran plot method.15,16 Errors in the concentrations of the ligands, metal ions and alkali were subjected to analysis of variance (ANOVA).17 Titrations were carried out in the medium containing varying concentrations of PG maintaining an ionic strength of 0.16 mol L-1 with sodium chloride at 303.0±0.1 K, with 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 PGwater 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) which was computed from the experimental and simulated acid-base titration data calculated by SCPHD program.18 A correction was applied to the pH meter dial readings to 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 PG-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 MINIQUAD7519, which exploits the advantage of constrained least squares method in the initial refinement and reliable convergence of undamped, unconstrained Marquardt algorithm. The protonation constants and the stability constants of the binary metal complexes of the ligands were fixed in refining ternary complexes. 3. RESULTS AND DISCUSSION: 3.1. Modeling Strategy: 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 existence of the ternary complexes was determined by performing an exhaustive modeling study and the results of a typical system are given in Table 2. The models were evaluated assuming the simultaneous existence of different combinations of species. Models containing various number and combinations of species were generated using an expert system package CEES and these models were refined using MINIQUAD75. As the number of species increased, the models gave better statistics denoting the best fit. The best fit models were chosen based on the statistical parameters like χ2, R-factor, skewness and kurtosis given in Table 3. The ternary complex species detected are MLX, ML2X and MLXH for Co(II), Ni(II) and Cu(II). The present values are compared with those reported earlier studies (Table 4). A very low standard deviation (SD) in the overall stability constants (log β) indicates the precision of the 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 3 are between -0.46 and 3.46. 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. 3.2. Solvent effect on stability of ternary complexes: PG is a protophilic dipolar protic solvent and acts as a structure former. Hence, it removes water from the coordination sphere of metal ions, making them more reactive towards the ligands. As a result, the stability of the complexes is expected to increase. At the same time, it is a coordinating solvent and it competes with the ligands for coordinating the metals. This decreases stability of the complexes. Hence, stability of the complexes may increase or decrease. The variation of overall stability constants with co-solvent content depends upon electrostatic and non-electrostatic factors. Born's classical treatment holds good in accounting for the electrostatic contribution to the free energy change.20 According to this treatment, the energy of electrostatic interaction is related to dielectric
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