Aminopolycarboxylate zinc(II) complexes with 1,10-phenanthroline and 2,2′-bipyridyl: kinetic studies in the surfactants solutions systems

Aminopolycarboxylate zinc(II) complexes with 1,10-phenanthroline and 2,2′-bipyridyl {[Zn(IDA)(H2O)2], [Zn(IDA)(bipy)(H2O)]·2H2O and [Zn(IDA)(phen)(H2O)]·2H2O} were synthesized. In order to confirm the composition and purity of the synthesized complex compounds, elemental analysis was used. Next, the kinetics of the substitution reaction of two water molecules in the zinc(II) iminodiacetate complex for 1,10-phenanthroline and 2,2′-bipyridyl in two surfactant solvents: CTAB and Triton X-100 were investigated. The kinetic studies were carried out using the stopped flow method. The kinetic research were carried out at 3 different temperatures: 288.15, 293.15 and 298.15 K and at different molar concentrations of the complex compound [Zn(IDA)(H2O)2]: 1 mM; 0.75 mM, 0.5 mM and 0.25 mM and at a constant molar concentration of ligands: i.e. 1,10-phenanthroline and 2,2′-bipyridyl, were 0.05 mM. Changes in absorbance during the kinetic run of the tested reactions were measured at a wavelength of 260 nm. Thanks to the conducted kinetic studies, the order of the reaction was determined, and the observable rate constants of the reaction rates of the substitution of two aqua molecules into the N-donor ligand were determined by the stopped—flow method using the Glint program. In the next step the thermodynamic parameters of complexes: {[Zn(IDA)(H2O)2], [Zn(IDA)(bipy)(H2O)]·2H2O and [Zn(IDA)(phen)(H2O)]·2H2O} in aqueous solutions by use potentiometric titrations were determined. The Hyperquad2018 program was used for determining of stability constants. In addition, the stoichiometry of complexes of zinc(II) with N-heterocyclic ligands in aqueous solutions was determined using the conductometric titrations.


Introduction
Complex compounds exhibit a number of unusual properties and are important for several reasons. Firstly, many industrial catalysts are complexes containing metal ions, e.g. Grubbs catalysts containing ruthenium(II/III) ions as central atoms in complex compounds, characterized by high catalytic activity in the olefin metathesis reaction [1][2][3][4]. Secondly, transition metal complexes are essential in biochemistry, e.g. complexes containing iron(II/III) ion [5,6], copper(II) ion [7,8] and zinc ions [9,10] are key components of some enzymes or catalysts in biological reactions. It is worth paying special attention to complex compounds containing zinc ions in the coordination sphere. Zinc ions play an important role in many metalloenzymes because they have a d 10 configuration of valence electrons, thanks to which they do not undergo the influence of the field of ligands, which may determine the geometry of the resulting complex [11][12][13][14]. Zinc plays an important role in the human body. It is the active center of many enzymes, for example carbonic anhydrase. The effect of carbonic anhydrase causes that this enzyme can release up to 106 molecules per second of carbon monoxide(IV) [14][15][16][17]. It is one of the fastest enzymatic reactions. Polycarboxylate coordination compounds of zinc(II) are a very interesting group of chemical compounds for physicochemical and biological studies. They are used in many fields: medicine, photonics, biological research [18,19]. They are also used as semiconductors [20]. Polycarboxylate coordination compounds containing transition metals as the central ions exhibit interesting magnetic, biological and structural properties [21][22][23][24]. In addition to the metal ion, it is also worth paying attention to the ligands that are located in the coordination sphere of the complex compound. The main interest in chelate ligands stems from their ability to form stable complex compounds. 1,10-Phenanthroline [25][26][27][28], 2,2′-bipyridyl [29][30][31] are N-heterocyclic chelate ligands that form stable complex compounds, e.g. with zinc, copper(II) or iron(III) ions. 1,10-phenanthroline occurs in the form of nine constitutional isomers that differ in the arrangement of chemical bonds and the location of nitrogen atoms. In turn, 2,2′-bipyridyl can occur in the form of six constitutional isomers, differing in the position of nitrogen atoms and the arrangement of chemical bonds. 1,10-phenanthroline as a chelate ligand also willingly forms coordination joints, where the central ion is also mainly ions: zinc, copper(II) or iron(III). Surfactants [32], i.e. surface active compounds are used as solvents in the substitution reaction of aqua ligand to N-heterocyclic ligand (1,10-phenanthroline or 2,2′-bipyridyl), and they affect the value of the observable rate constant of the kinetic reactions studied [33,34]. This is due to the properties of surfactants that serve to mimic the physical interactions between micelles and membrane cells [35,36]. In recent years, there has been a significant increase in interest in kinetic studies on the substitution of one ligand in complex compounds with other ligands. First of all, single-site ligands are substituted with chelated ligands in complex compounds because complex compounds containing chelated ligands in the coordination sphere are much more stable than complex compounds containing single-site ligands. The central axiom of chemical kinetics is the belief that every existing chemical reaction is the resultant of coexisting elementary reactions, creating a certain network of parallel and successive processes [37][38][39]. One of the best methods for studying fast chemical reactions is the stopped flow method. The stopped flow method makes it possible to study the kinetics of relatively fast chemical reactions (with half-lives in the order of milliseconds) in the liquid phase. The advantage of the stopped-flow technique is primarily the fact that small volumes of solutions are used (it is economical), and the tests are carried out under anaerobic conditions and at a constant temperature [40][41][42][43].
In this article we described the rate of substitution reaction of two water molecules N-heterocyclic ligand (1,10-phenanthroline and 2,2′-bipyridyl) in [Zn(IDA) (H 2 O) 2 ]. The surface-active compounds CTAB and Triton X-100 were used as the reaction medium for the kinetic research. The values of the rate constants of the tested reactions were determined. On the basis of the obtained test results, the reaction order was determined using the graphical method. Thermodynamic stability of the chemical species in aqueous solutions was characterized by the potentiometric titration (PT) and conductometric titration (CT) methods. In addition, an association mechanism for the substitution reactions studied was proposed.

Syntheses
The syntheses of all complex compounds were performed based on the procedures described in the literature [44].

[Zn(IDA)(H 2 O) 2 ] n
The synthesis was started since weighing 3.19 g of iminodiacetic acid and placing it in a 50 mL round bottom flask. After dissolving the acid in a small amount of water, the flask was placed on a magnetic stirrer and heated to 50 ºC. While heating, the portions 2.54 g Na 2 CO 3 ·10H 2 O were slowly added to the flask. After complete evolution of carbon dioxide, a solution prepared by dissolving 5.54 g of ZnCl 2 ·6H 2 O in water was added in small portions. The crystallized product-[Zn(IDA)(H 2 O) 2 ] n was filtered under reduced pressure and allowed to dry.

[Zn(IDA)(phen)(H 2 O)]·2H 2 O
In a round bottom flask, 3.19 g H 2 IDA was dissolved in a small volume of water. Then the solution was placed on a magnetic stirrer and heated to 50 °C, then 2.54 g Na 2 CO 3 ·10H 2 O was added in portions. When CO 2 completely evaporated as a gas, the solution prepared by dissolving 5.54 g of ZnCl 2 ·6H 2 O in water was added slowly from the solution. At the end of the synthesis of the complex, 4.75 g of 1,10-phenanthroline was added. The precipitated crystals of [Zn(IDA)(phen)(H 2 O)]·2H 2 O were filtered by means of a vacuum filtration kit.

[Zn(IDA)(bipy)(H 2 O)]·2H 2 O
0.80 g H 2 IDA was dissolved in a small volume of water in a round bottom flask. Then the solution was placed on a magnetic stirrer and heated to 50 °C, then 0.64 g Na 2 CO 3 ·10H 2 O was added in portions. When the CO 2 gas bubbles ceased to be released, 3.03 g of Zn(NO 3

Kinetic measurements
The reaction kinetics were tested by the spectrophotometric stopped-flow method using the SX 18 MV-R measuring set from Applied Photophysics. Measurements

Potentiometric titrations (PT)
Potentiometric titrations were performed using CerkoLab microtitration system with a measuring cell equipped with a magnetic stirrer, the Schott-Blue Line 16 pH combination electrode and Hamilton 5 mL syringe. The measurements were carried out at 298.15 K (a Lauda E100 circulation thermostat). The titrated solutions contained: (1) H 2 IDA (1.5 mM) + Zn 2+ (1.5 mM), (2) H 2 IDA (1.5 mM) + Zn 2+ (1.5 mM) + bipy (1.5 mM), (3) H 2 IDA (1.5 mM) + Zn 2+ (1.5 mM) + phen (1.5 mM). A standardized NaOH solution at 50 mM was used as the titrant. A solution of NaOH was added every 60 s in an amount of 0.002 mL. Calibration of pH electrode in the potentiometric measurements were performed according to IUPAC (International Union of Pure and Applied Chemistry) requirements. Standard solutions of precisely known pH value (buffer solutions) were prepared. Ensured that the reference electrode (electrode of standard potential) is properly filled with electrolyte. Made sure that the electrode is clean and free of any debris. Calibrated the potentiometric electrode at three different pH points. The pH of the electrode was measured in each of the calibrated standard solutions. Made sure the electrode was stable and pH readings were repeatable. Calibration and measurements were repeated several times to confirm the repeatability of the results and to make sure the electrode was properly calibrated.

The kinetics stability of the complexes in various solvent
The reaction kinetics were tested using the stopped-flow method in two surfaceactive solvents: CTAB(aq) and SDS(aq). Initially, the UV spectra of the substrates (Zn(IDA) (Fig. 1), bipy and phen), Zn(IDA) with phen or bipy in the 1:1 volume ratio and complexes Zn(IDA)(phen) and Zn(IDA)(bipy) (Fig. 1) were registered to select the appropriate wave-lengths for kinetic studies (Fig. 2).
Both in CTAB and in Triton X-100, the two aqua ligands were substituted in the zinc coordinating sphere by the 2,2′-bipyridyl or 1,10-phenanthroline molecule. The order of tested reactions was determined using the graphical method. The linear adjustment of the absorption changes as a function of time confirmed that the studied reactions are the pseudo-first order reaction. The values of observable rate constants (k obs ) were using nonlinear least squares methods fitting to the untransformed original equation for the kinetics of the pseudo-first order reaction (Fig. 3). This means that one of the substrates has been used in the large excess (Figs. 4, 5, 6, 7).
Analyzing the obtained results it can be concluded that the values of the observable rate constants kobs increase with the concentration of [Zn(IDA)(H 2 O) 2 ] and the temperature. This relationship is consistent with the theory of the active complex. The reaction of the binary complex with bipy ligand is much faster than the reaction Based on the obtained test results, it was found that the reactions of the complex compound [Zn(IDA)(H 2 O) 2 ]n with 1,10-phenanthroline and 2,2′-bipyridyl are firstorder reactions. Scheme 1 shows the mechanism of the substitution reaction of two water molecules for N,N′-heterocyclic, bidentate nitrogen ligands (phen or bipy) in the complex compound [Zn(IDA)(H 2 O) 2 ] n . The proposed mechanism is based on the assumptions of the associative mechanism. The associative substitution mechanism follows a common pathway of ligand substitution in the metal complex. In the first step, the bipy or phen ligand is attached to the complex compound via two nitrogen atoms coming from the ligand. At this point, an intermediate is formed in which the coordination number is 1 greater than that of the substrate. In the next stage, one water molecule is detached, thanks to which stable coordination compounds are  were performed. In order to determine the thermodynamic parameters determining the stability of the tested systems in the aqueous solutions, the Hyperquad2018 program was used. The theoretical model was applied in such a way that the experimental data were correlated with the theoretical data. In order to determine the constant equilibria in the aqueous solutions, the following model was used: where: p, q, r, s-the stoichiometric coefficients for the reaction, M = Zn 2+ , L = iminodiacetate anion, R = bipy or phen, H = proton (Tables 1, 2, 3).
The  Analyzing kinetic data on the study of the rate of substitution reactions in iminodiacetate complexes of zinc(II) and copper(II) in aqueous surfactant solutions, it can be seen that substitution reactions involving 2,2′-bipyridyl occur faster than those involving 1,10-phenanthroline (Figs. 8,9). Such a relationship is found in stopped-flow kinetic studies in both CTAB and Triton X-100 aqueous solutions [46]. It is noteworthy that despite the similar cationic radius of copper(II) and

Conclusions
The presented paper reports the effect of surface-active compounds (CTAB and Triton X-100) on the kinetics of the substitution reaction of two aqua molecules by 1,10-phenanthroline or 2,2′-bipyridyl. The reaction kinetics have been tested by the stopped-flow spectrophotometrical method. Based on the dependence of the observable rate constants on the concentration of the [Zn(IDA)(H 2 O) 2 ] complex at certain temperatures, it can be concluded that with the increasing temperature and concentration of the zinc complex the value of the observable rate constant kobs increases. This dependence is consistent with the theory of the active complex. Based on the kinetic studies of ligand substitution, the reaction with 2,2′-bipyridyl undergoes faster than with 1,10-phenanthroline. This may be due to the size of the ligand molecule-bipy has a lower molecular weight than phen, and bipy has weaker basic character than phen. Comparing the nature of the ligands, the iminodiacetate has a weakly basic character because it has a donor nitrogen atom in its structure, owing to which the substitution reaction undergoes faster in the case of bipy which is less basic than phen.

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Data availability On request of those interested.

Conflict of interest
The authors declare no conflict of interest.
Institutional review board statement Not applicable.
Informed consent Not applicable.
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