Interaction of Copper(II) Ions with Certain Oxyacids and Azoles

Abstract

Compounds composed of [Cu(HIm, metIm)_2–3L] ∙ nH₂O (n = 0, 2) were obtained during the interaction of slightly soluble tartrate and copper(II) salicylate composed of CuL ∙ nH₂O (n = 1–2, L²⁻—Tar²⁻, Sal²⁻) with imidazole (HIm) and 2-methylimidazole (metIm). Mono- and bi-ligand salts were analyzed; the process of their thermal decomposition was studied. The solubility constants K_S of the CuC₄H₄O₆ ∙ 2H₂O tartrate and CuC₇H₄O₃ ∙ H₂O salicylate of copper(II) with at ionic strength of 0.3 were determined. The IR spectroscopy method showed the participation in complexation of the nitrogen atom N(3) of imidazole and the oxygen atoms of the carboxyl groups of oxyacids, as well as the hydroxyl group of salicylic acid in the mixed-ligand salts of copper(II). The compositions and stability of the imidazole-tartrate(salicylate) copper(II) complexes in an aqueous solution were determined by performing photometry and spectrophotometry and of the monoligand complexes [CuTar] and [CuSal] were determined by solubility and isomolar series methods.

1. Introduction

Tartaric C₄H₆O₆ (H₂Tar), salicylic C₇H₆O₃ (H₂Sal) acids, and some of their salts with cations of biometals (such as Fe, Zn, Cu, Mn, and Co) are used for the development of new drugs (e.g., the acidic salicylate of copper(II) Cu(C₆H₄(OH)COO)₂ is used in dermatology), biologically active substances, and fungicidal compositions [1]. They can be interesting for researchers working on the synthesis of new materials with predictable properties [2].

References [3,4,5] described the methods of extracting copper(II) salts with tartaric acid of different compositions from aqueous solutions, for example, salts CuC₄H₄O₆ ∙ 3H₂O and CuC₄H₄O₆ ∙ 2H₂O [3]; MTart compounds (where M²⁺ is the ion of Mn, Fe, Co, Ni, Cu, and Zn) [4]; and the coordinated polymer {[Cu₂(Tart)₂(H₂O)₂] · 4H₂O}_n [5].

Both phenolic and carboxylic groups are present in salicylic acid, so salicylic acid can form acidic and medium salts. When the copper ion is coordinated by one or two functional groups (−COOH and −OH), homo- and hetero-nuclear compounds are formed. For example, there is the low-soluble copper salicylate CuC₇H₄O₃ · H₂O [1]; compounds M(HSal)₂ ∙ nH₂O (n = 3 for cobalt and copper salts, and n = 2 for the lead salt) [6]; and heteronuclear mixed-ligand salicylates [CuSr(Ba)(HSal)₄(DMAA)₄H₂O] and [CuCu(HSal)₄(H₂O)₂] · 2DMAA (DMAA is dimethylacetamide) [7], which are used as precursors for the synthesis of new compounds.

The complexing of copper(II) with tartaric and salicylic acids in an aqueous solution is investigated. The formation of complexes between ions Cu²⁺ and Tart²⁻ (L²⁻) at 25 °C in a 1 M solution of NaClO₄ has been studied [8]. The presence of complexes with different compositions in the pH range of 1–4, namely, CuL, CuHL, CuL₂, CuHL₂, CuH₂L₂, Cu₂L₂, Cu₂L₃, and Cu₂L₄ (no charges listed), has been shown. The logarithms of the stability constants of these complexes have been determined. The data on the stability constants of the tartrate and salicylate copper(II) complexes are presented in [8,9,10].

In biological systems, metal ions typically interact with several ligands. The anions of carboxylic acids, oxyacids, amino acids, vitamins, and azoles containing donor atoms of oxygen and nitrogen can participate as ligands in such mixed-ligand metal compounds. Azoles are used to produce anti-infective drugs.

There is information on the syntheses of copper(II) biligand salts, including imidazole(HIm) and its derivatives: Cu(1,2-dimethylIm)₂(HSal)₂ and Cu(2-methylIm)₃Sal (1,2-dimethylIm, 2-methylIm, 1,2-dimethylimidazole, and 2-methylimidazole) [11]; Cu(HIm)_n(Hsal)₂, where n = 2, 5, 6 was obtained owing to the reaction of imidazole with the Cu₂(HSal)₄ salt [12]; hexakis-(N-methylimidazole) copper(II) salicylate and [Cu(N-methylIm)₆](Hsal)₂ [13]; and [CuLHSal] (L is deprotonated 4-phenyltyosemycarbazide) [14]. [Cu(HIm)₂(cinn)₂(H₂O)], [Cu(HIm)₂(paba)₂], and [Cu(HIm)₂(clba)₂] have the cinn⁻ — C₉H₇O₂⁻, paba⁻ — C₇H₆NO₂⁻, and clba⁻ — C₇H₄ClO₂⁻ — anions of cinnamon, para-aminobenzoic, and 2-chlorobenzoic acids, respectively [15]. [Cu(HIm)₆]Cl₂ · 4H₂O and [Cu(HIm)₆]Cl₂ · 2H₂O have been synthesized using the hydrothermal method [16].

Both the production of solid mixed-ligand salts (MLSs) and the simultaneous study of mixed-ligand complexes (MLCs) in a solution based on nitrogen and oxygen ligands are of particular interest [17,18,19,20,21,22,23]. Therefore, the purpose of this study was to develop a method for synthesizing the mixed-ligand compounds of copper(II) with tartaric and salicylic acid anions and azoles using previously synthesized slightly soluble tartrate and salicylate of copper(II). Another purpose of the research was to establish the physicochemical properties and structure (the mode of coordinating organic ligands using a complexant) of the synthesized compounds, as well as to determine the composition and stability of the biligand imidazole-tartrate(salicylate) complexes [Cu(HIm)_XL] formed in the solution.

2. Materials and Methods

The objects of the research are copper(II) compounds with biologically active oxycarboxylate ligands—tartaric and salicylic acids—and those with azoles—imidazole and 2-methylimidazole—in the form of solid salts and complex compounds in a solution. Imidazole C₃H₄N₂ has a five-membered cycle with two heteroatoms of nitrogen and has amphoteric properties.

The nitrogen N₍₃₎ atom with an unshared electron pair is capable of protonation (lgB₁ has values in the range of 7–7.7; B₁ is the constant of imidazole protonization) and the coordination of metal ions. The compound 2-methylimidazole C₄H₆N₂ forms less stable complexes than imidazole because of emerging steric hindrances owing to a substituent [24].

To analyze and study the properties of the synthesized salts to determine the composition and stability of the mixed-ligand complexes in the solution, the following were used: thermal, thermogravimetric, and elemental analyses; pH potentiometry, photometry, and spectrophotometry; and IR spectroscopy. The spectra of the solutions were recorded using the spectrophotometer LEKI SS2107UV. The optical density of the solutions was also measured using the photocolorimeter KFK–2–UHL 4.2, with an absorbing layer thickness of l = 10 mm. The pH of the solutions was measured using the pH-meter-673. Its glass electrode was calibrated using buffer solutions with a pH from 3.56 to 6.86. Thermograms of the synthesized salts were recorded using the Netzsch STA 449 F1 device under the following conditions: the crucible material was Al₂O₃, the heating rate was 10 °C per min, and the atmosphere was air (80 mL/min). The IR spectra of the ligands and mono- and bi-ligand salts in tablets from KBr were obtained on the spectrometer Agilent Cary 630 FTIR at a frequency of 400–4000 cm⁻¹. The synthesized salts were analyzed by means of an automatic elemental CHNS analysis on the analyzer EURO EA 3000 using the microbalance Sartorius MSE 3.6P-000-DM. The used reagents were labeled as ch.p. or p.a. and were not additionally recrystallized.

2.1. Synthesis and Solubility of Tartrate and Salicylate Copper(II)

Original monoligand salts of copper(II) were synthesized using aqueous solutions via the reaction between chloride or the nitrate of metal (CuX₂) and tartaric or salicylic acid (H₂L), partially neutralized by sodium hydroxide, with a mole ratio of CuX₂:H₂L = 1:1

CuX₂ + H₂L + (1.8–1.9)NaOH → CuL↓.

The precipitates were released at a pH of 4–5 of the mixture. After being rinsed with water, they were dried in the air. By heating the salts at 125 and 900 °C, the content of water and the oxide of copper(II), respectively, was determined. In addition, the metal ion content was found trilonometrically (it satisfactorily coincided with the results of the metal content in oxide). The elemental analysis confirmed the composition of the monoligand salts.

Table 1. Analysis data of mono- and bi-ligand salts of copper(II) with tartaric and salicylic acids, imidazole, and 2-methylimidazole.

Compound	N, %		C, %		H, %		CuO, %		H2O, %
	f*	c*	f	c	f	c	f	c	f	c
CuC4H4O6 ∙ 2H2O	–	–	19.1	19.38	2.7	3.23	33.1	32.12	14.6	14.54
Cu(C3H4N2)3C4H4O6 ∙ 2H2O	17.9	18.60	34.1	34.52	4.0	4.43	16.6	17.61	8.1	7.97
CuC7H4O3 · H2O	–	–	37.8	38.59	2.7	2.76	35.9	36.54	8.3	8.27
Cu(C3H4N2)3C7H4O3	21.0	20.80	46.9	47.54	3.9	3.96	21.0	19.69	–	–
Cu(C4H6N2)2C7H4O3	17.9	15.40	49.5	49.50	4.6	4.40	20.4	21.87	–	–

f*—found; c*—calculated.

shows the data of the analyses of the original salts and the tartrate and salicylate of copper(II). The solubility of the monoligand salts in the (H, Na)NO₃ solutions with an ionic strength of I = 0.3 at 25 °C was studied.

Table 2. Data on solubility and calculation of solubility constants K_S of salts CuC₄H₄O₆ ∙ 2H₂O and CuC₇H₄O₃ ∙ H₂O in 0.3 mole/L of solutions (H, Na)NO₃ (K_h1 (Cu²⁺) = 3.1 ∙ 10⁻⁸; for H₂Tar: logB₁ = 3.95, logB₂ = 6.76; for H₂Sal: logB₁ = 13.7, logB₂ = 16.53).

№	pH	CCuTar, mole/L	−logKS (CuTar ∙ 2H2O)	pH	CCuSal, mole/L	−logKS (CuSal · H2O)
1	4.62	3.82 ∙ 10−4	7.44	5.43	7.20 · 10−3	13.57
2	3.50	5.03 ∙ 10−4	7.52	5.05	7.10 · 10−3	13.68
3	3.48	5.63 ∙ 10−4	7.46	5.14	7.53 · 10−3	13.61
4	3.46	6.54 ∙ 10−4	7.37	4.93	7.80 · 10−3	13.65
5	3.14	7.85 ∙ 10−4	7.44	4.96	8.90 · 10−3	13.57
6	3.11	8.45 ∙ 10−4	7.41	4.48	1.15 · 10−2	13.58
7	2.97	9.66 ∙ 10−4	7.43	4.25	1.32 · 10−2	13.60

Calculation results: for CuC₄H₄O₆ · 2H₂O logβ₁ (optim.) = 3.74; β₁ = 4.67 ∙ 10³; log$\overline{K}$s = −7.44; standard deviation is s² = 2.68 · 10⁻³; for CuC₇H₄O₃ · H₂O logβ₁ (optim.) = 11.27; β₁ = 1.86 · 10¹¹; log$\overline{K}$s = −13.61; s² = 1.77 · 10⁻³.

shows the data on solubility, the calculation of the solubility constants K_S of the tartrate CuC₄H₄O₆ ∙ 2H₂O and salicylate CuC₇H₄O₃ · H₂O of copper, and the calculation of the stability constants of the complexes of the [CuL] composition, taking into account the hydrolysis of ion Cu²⁺ and the protonization of acid anions. To simultaneously calculate according to the solubility data the solubility constants K_S of the salts composed of 1:1 CuL ∙ nH₂O and the stability constants ꞵ₁ of the complexes composed of 1:1 CuL, formed in the saturated solution, the author program “Solubility” [25] was used. In this program, the following notations were used: f—ligand protonization function, f = 1 + B₁h + B₂h²; B_i—common constants of anion protonization of oxyacids: hydrolysis function ω = 1 + K_h1/h; K_h1—hydrolysis constant of Cu²⁺ ion in the first step, h = [H⁺]. When using the material balance equations for the metal and ligand (in the saturated solution composed C_M = C_L) and the expression for the constant of heterogeneous equilibrium K_S = [Cu][L] (charges are omitted), the equation for K_S, reduced to linearity, can be written as follows:

C_Cu = [CuL] + √K_S∙√fω,

and the stability constant value of the complex is ꞵ₁ = [CuL]/K_S. According to the data in Table 2, the values of K_S and ꞵ₁ were calculated by using the method of minimizing the averaging variance lgK_Si.

In the saturated solutions of copper(II) tartrate, the copper concentration was determined iodometrically. The concentration of the solution of sodium thiosulfate was specified in the presence of tartaric acid, which exerts little influence on the results of the iodometrical determination of copper(II). In the saturated solutions of copper(II) salicylate, the copper concentration was determined by using the calibration characteristic obtained via the dependence of the optical density of the solutions on the concentration of the synthesized salt CuSal ∙ H₂O (

Table 3. Composition of solutions for building the calibrating characteristic of system CuSal–NaNO₃ (λ = 750 nm; pH 3.6; I = 0.3; C⁰ (CuSal ∙ H₂O) = 8.84 ∙ 10⁻³ mole/L; V_cum = 6 mL).

№	VCuSal, mL	CCuSal, mole/L	D750	Parameters of Straight Line
1	1	1.59 ∙ 10−3	0.043	a = 0.0229, b = 11.222, R2 = 0.993
2	2	3.19 ∙ 10−3	0.060
3	3	4.78 ∙ 10−3	0.073
4	4	6.38 ∙ 10−3	0.091
5	5	7.89 ∙ 10−3	0.112
6	6	9.57 ∙ 10−3	0.133

).

2.2. Synthesis and Thermogravimetry of Mixed-Ligand Salts of Copper(II)

Mixed-ligand compounds were obtained from the synthesized oxycarboxylates of copper(II) CuL ∙ nH₂O and imidazole and 2-methylimidazole according to the reaction

CuL + xHIm(metIm) → Cu(HIm)_X(metIm)_XL↓.

The reaction was carried out in an aqueous solution with a pH of 6.5–8.5, and, subsequently, the precipitation of the mixed-ligand salts from the aqueous solution for the imidazole(methylimidazole)-salicylate salt and from the aqueous–etheric solution for the imidazole tartaric salt was carried out. For this purpose, a weighed portion of the original mono-ligand salt was placed into a small volume of water (8–10 mL). Then, the calculated amount of HIm(metIm) was gradually added to the suspension, creating a molar ratio of CuL:HIm(metIm) equal to 1:1, 1:2, etc. This was carried out until the copper(II) tartrate dissolved completely or the salt of salicylic acid transformed into a new homogeneous, brightly colored bilig and salt. The suspension CuSal–Him(metIm) was held at room temperature for one day. For the mixed-ligand salts, the following procedure was conducted: an elemental analysis; a thermal analysis of the content of water and metal oxide; and a thermogravimetric analysis quantitatively confirming the content of water, ligands, and metal oxide in the salts.

Table 4. Analysis of thermograms of copper(II) biligand salts.

№	Nature of Effect	Temperature Interval, °C	Loss of Mass (from init.), %		Corresponding Process
			f	c
Cu(HIm)3Sal
1	Endo-effects	175–340	49.5	50.57	Loss of imidazole ligand
2	Exo-effects	340–510; 510–900	29.8; 21.0	33.70; 19.69	Destruction of salicylate ion and oxide formation
Cu(metIm)2Sal
1	Endo-effects	170–340	43.3	45.09	Loss of methylimidazole
2	Exo-effects	340–510; 510–900	33.6; 23.3	37.43; 21.87	Destruction of salicylate ion and oxide formation

shows the results of the analysis of the thermograms of the two salts as an example.

The formulas of the synthesized biligand salts were calculated using the results of the analyses (Table 1). The data on the mass content of the copper(II) oxide and water in the salts were averaged using the results of the thermal and thermogravimetric analyses. Since the removal of water, the degradation of the oxyacid anion, azole, and the formation of the corresponding copper(II) oxide occur in different temperature ranges in the synthesized salts, the data of the thermogravimetric analysis of the biligand salts confirm the salt compositions established using other methods and allow us to suggest the mechanism of their thermal decomposition (Table 4).

So, for example, the thermal decomposition of the imidazolesalicylate of copper(II) Cu(HIm)₃Sal in the air proceeds in several stages, which are quantitatively confirmed by the change in the salt mass (f—found; c—calculated). The endothermic process of imidazole loss proceeded at 175–340 °C. Moreover, immediately, with the exothermal effect within 340–900 °C, the complete combustion of the salicylate ion and the formation of metal oxide in the air took place. The methylimidazolesalicylate of copper(II) decomposed similarly to the thermal decomposition of the imidazolesalicylate of copper(II). Thermogravimetric studies of synthesized compounds are important for understanding their thermal stability, which, along with other properties, is used by scientists searching for new materials.

2.3. Determination of Composition and Stability of Mono- and Bi-ligand Complexes in Solution

The stability constants of the CuL complexes (L²⁻—anion of oxyacid), required for the calculation of the stability constants of MLC, are taken from the literature or calculated based on our data of solubility methods and isomolar series. The composition of the monoligand CuL complexes, formed in the solution, was determined by using the method of isomolar series. According to the data of the isomolar series of the CuCl₂—NaHSal system (Figure 1), whose diffused maximum points to the presence of two complexes in the solution, the stability constant of the [CuSal] complex was calculated.

To calculate the stability constant of the [CuSal] complex according to the isomolar series data, the experimental data with a ratio of C_L:C_M, being in the 0.5–1.9 interval, were used. The [CuSal]⁰ complex dominance in this range was confirmed by the permanence of the stability constant of the complex (

Table 5. The data of optical density measurements and calculation of stability constant of the 1:1 [CuSal]⁰ complex in isomolar solutions of the CuCl₂—NaHSal (pH 3.5; V_cum = 6 mL; λ = 750 nm; D_∞ = 0.556; C⁰ (CuCl₂) = C⁰ (NaHSal) = 2 ∙ 10⁻² mole/L; logB₁ = 13.7; logB₂ = 16.53; K_h1 (Cu²⁺) = 3.1 ∙ 10^–8; f = 1.92 ∙ 10¹⁰; ω = 1.007).

No.	VM, mL	VL, mL	D	DM	∆D	CM, mole/L	CL, mole/L	αC	β1∙1011	logβ1
1	6.00	0.00	0.213	0.213	0.000	2.00 ∙ 10−2	0.00	–	–	–
2	4.00	2.00	0.175	0.142	0.033	1.30 ∙ 10−2	6.70 ∙ 10−3	0.0797	2.57	11.41
3	3.50	2.50	0.164	0.124	0.040	1.20 ∙ 10−2	8.30 ∙ 10−3	0.0930	2.68	11.43
4	3.25	2.75	0.155	0.115	0.040	1.08 ∙ 10−2	9.16 ∙ 10−3	0.0907	2.28	11.36
5	2.75	3.25	0.141	0.098	0.043	9.16 ∙ 10−3	1.18 ∙ 10−2	0.0940	1.83	11.26
6	2.50	3.50	0.130	0.089	0.041	8.30 ∙ 10−3	1.20 ∙ 10−2	0.0878	1.65	11.22
7	2.25	3.75	0.121	0.080	0.041	7.50 ∙ 10−3	1.25 ∙ 10−2	0.0861	1.55	11.19
8	2.00	4.00	0.112	0.071	0.042	6.70 ∙ 10−3	1.30 ∙ 10−2	0.0866	1.49	11.17

logβ₁ = 11.29 ± 0.10.

). According to the isomolar series data (Table 5), the stability constant of the [CuSal] complex was calculated by using the following formula:

β₁ = ([CuSal])/([Cu²⁺] × [Sal²⁻]) = (C_Cωf)/{(C_Cu − C_C) × (C_Sal − C_C)},

where C_C, ω, and f are the complex concentration in the equilibrium solution, the Cu²⁺ cation hydrolysis functions, and the protonization of the salicylic acid anion, respectively: C_C = {(D − D_Cu)C_Cu}/(D_∞ − D_Cu) when C_Cu < C_Sal; [Sal²⁻] = (C_Sal − C_C)/f, [Cu²⁺] = (C_Cu − C_C)/ω; α_C = C_C/C_M (C_L) complex yield. D_∞ was assessed in the solutions with a molar ratio of Cu²⁺:L²⁻ = 1:1 and a pH value varying within 3–6.

According to the data of the saturation curves of the Cu²⁺–L²⁻–HIm systems (Figure 2 and Figure 3), the stability constants β₁₁₁ and β₁₂₁ of the mixed-ligand complexes of the composition [Cu(HIm)_1,2L] were calculated. The saturation curve (Figure 2) is wavy, having a plateau, which is evidence of the stepwise nature of complexing. The presence of the plateau in Figure 2 (the ratio of C_Him:C_Cu is in the interval of ≈(0.7–1.2)) indicates that, in this area, the complex composition 1:1:1 ([CuHImTar]) dominates, which is confirmed by the constancy of the value of logβ₁₁₁ (

Table 6. Calculation results of the stability constant lgβ₁₁₁ in the [CuHImTar]⁰ complex according to data of the saturation curve of CuCl₂–H₂Tar–xHIm (λ_ef = 670 nm, pH 6.8; f_Im = 8.74; K_h1 (Cu²⁺) = 3.1 ∙ 10⁻⁸; logβ_CuTart = 3.03; I = 0.3; V_cum = 6 mL; C⁰_HIm = 0.075 mole/L; C_Cu = C_Tar = 0.0125 mole/L; ε₀ = D_CuTar/C_Cu = 0.042/0.0125 = 3.36; D_∞ = 0.480; ε_∞ = 38.4; C_C = α_∞C_Cu (C_L)).

No.	D670	εi	α∞i	VHIm, mL	CHIm, mole/L	CC, mole/L	logβ111
1	0.369	29.52	0.747	0.2	2.500 ∙ 10−3	1.868 ∙ 10−3	7.69
2	0.381	30.48	0.774	0.4	5.000 ∙ 10−3	3.870 ∙ 10−3	7.50
3	0.389	31.12	0.792	0.6	7.500 ∙ 10−3	5.942 ∙ 10−3	7.41
4	0.445	35.6	0.920	0.8	1.000 ∙ 10−2	9.200 ∙ 10−3	7.18
5	0.450	36.00	0.932	1.2	1.500 ∙ 10−2	1.164 ∙ 10−2	7.63
6	0.460	36.80	0.954	1.4	1.750 ∙ 10−2	1.193 ∙ 10−2	7.59
7	0.462	36.96	0.959	1.6	2.000 ∙ 10−2	1.199 ∙ 10−2	7.49

log β₁₁₁ = 7.49 ± 0.16.

).

The absence of points in Figure 3 up to the C_HIm/C_Cu ratio ≈1.7 is associated with the precipitation of low-soluble copper(II) salicylate, which is then dissolved by increasing the imidazole concentration. The stability constant β₁₂₁ of the complex with a composition of 1:2:1 ([Cu(HIm)₂Sal]) was calculated for this system.

In Table 6 and Table 7, the data for calculating the stability constants β₁₁₁ of the [CuHImTar] complex and β₁₂₁ of the [Cu(HIm)₂Sal] complex, respectively, are presented. The stability constants of MLC were calculated according to the procedure described in [26]. For equilibrium, for example, with the participation of the two complexes Cu(HIm)₂L and CuL (L²⁻ − Sal²⁻) absorbing with the same wavelength,

$$CuL+2HIm \stackrel{K}{\leftrightarrow } Cu{(HIm)}_{2}L$$

(charges are omitted for convenience), the equilibrium constant K connected with the stability constants β₁ of the monoligand CuL complex and β₁₂₁ of the mixed-ligand complex Cu(HIm)₂L by the ratio of β₁₂₁ = K ∙ β₁. When using photometric data (Table 7) for each point of the saturation curve, we have

β₁₂₁ = β₁(α_∞f²_HIm)/((1 − α_∞) × (C_HIm − 2α_∞C_Cu)²),

where α_∞ is the maximal yield of the Cu(Him)₂L complex; α_∞ = (ε_i − ε₀)/(ε_∞ − ε₀); ε_∞ = D_∞/C_Cu, ε_i = D_i/C_Cu; ε is the molar absorption factor of the corresponding particles: CuL (ε₀), Cu(Him)₂L (ε_∞), and CuL + Cu(Him)₂L (ε_i); and f_HIm = 1 + B₁[H⁺] (logB₁ = 7.69, according to the data in [27]). The stability constant of the salicylate complex [CuSal] (lgβ₁ = 11.27) was borrowed from this work (the solubility method). The stability constant of the tartrate complex of copper(II) [CuTar] (logβ₁ = 3.03) was taken from [9].

Table 7. Calculation results of stability constant lgβ₁₂₁ in the [Cu(HIm)₂Sal]⁰ complex according to data of the saturation curve of CuCl₂–NaHSal–xHIm (pH = 6.8; I = 0.3; C_Cu = C_Sal = 1 ∙ 10⁻² mole/L; V_cum = 6 mL; C⁰ (HIm) = 6 ∙ 10⁻² mole/L; ε₀ = 16.8; ε_∞ = 49.0; f_HIm = 8.77; β_CuSal = 1.86 ∙ 10¹¹; D_∞ = 0.490; C_C = α_∞C_Cu; K_h1 (Cu²⁺) = 3.1 ∙ 10⁻⁸).

No.	D670	εi	α∞i	VHIm, mL	CHIm, mole/L	CC, mole/L	logβ121
1	0.450	45.0	0.876	2.0	2.00 ∙ 10−2	8.76 ∙ 10−3	19.22
2	0.451	45.1	0.879	2.1	2.10 ∙ 10−2	8.79 ∙ 10−3	18.96
3	0.480	48.0	0.969	2.8	2.80 ∙ 10−2	9.70 ∙ 10−3	18.79
4	0.486	48.6	0.986	3.4	3.40 ∙ 10−2	9.86 ∙ 10−3	18.71
5	0.483	48.3	0.978	2.6	2.60 ∙ 10−2	9.79 ∙ 10−3	19.10
6	0.483	48.3	0.978	3.0	3.00 ∙ 10−2	9.79 ∙ 10−3	18.78

logβ₁₂₁ = 18.94 ± 0.23.

Table 7. Calculation results of stability constant lgβ₁₂₁ in the [Cu(HIm)₂Sal]⁰ complex according to data of the saturation curve of CuCl₂–NaHSal–xHIm (pH = 6.8; I = 0.3; C_Cu = C_Sal = 1 ∙ 10⁻² mole/L; V_cum = 6 mL; C⁰ (HIm) = 6 ∙ 10⁻² mole/L; ε₀ = 16.8; ε_∞ = 49.0; f_HIm = 8.77; β_CuSal = 1.86 ∙ 10¹¹; D_∞ = 0.490; C_C = α_∞C_Cu; K_h1 (Cu²⁺) = 3.1 ∙ 10⁻⁸).

No.	D670	εi	α∞i	VHIm, mL	CHIm, mole/L	CC, mole/L	logβ121
1	0.450	45.0	0.876	2.0	2.00 ∙ 10−2	8.76 ∙ 10−3	19.22
2	0.451	45.1	0.879	2.1	2.10 ∙ 10−2	8.79 ∙ 10−3	18.96
3	0.480	48.0	0.969	2.8	2.80 ∙ 10−2	9.70 ∙ 10−3	18.79
4	0.486	48.6	0.986	3.4	3.40 ∙ 10−2	9.86 ∙ 10−3	18.71
5	0.483	48.3	0.978	2.6	2.60 ∙ 10−2	9.79 ∙ 10−3	19.10
6	0.483	48.3	0.978	3.0	3.00 ∙ 10−2	9.79 ∙ 10−3	18.78

logβ₁₂₁ = 18.94 ± 0.23.

Electronic absorption spectra in the visible region (Figure 4 and Figure 5) were recorded for the aqueous systems Cu²⁺–H₂O, Cu²⁺–L²⁻, Cu²⁺–HIm, and Cu²⁺–L²⁻–(1–2)HIm (L²⁻ is tartrate and salicylate anions).

3. Discussion

Synthesized original salts of copper(II) CuTar ∙ 2H₂O and CuSal ∙ H₂O represent fine crystalline substances slightly soluble in water (determined by the authors for these salts under ionic strengths of 0.3 logK_S = −7.44 and logK_S = −13.61, respectively). Mixed-ligand salts, containing the oxycarboxylic acid anion and a neutral molecule of azole as ligands, were synthesized using the slightly soluble tartrate and salicylate of copper(II) and azoles, with the pH of the aqueous solution equal to 6.5–8.5. Figure 6 shows a distribution diagram of the neutral imidazole HIm molecule (the mentioned hydrogen atom belongs to the pyrrole nitrogen atom N₍₁₎, pK_a = 14.5) and the protonated H₂Im⁺ form [28]. The neutral imidazole HIm molecule (curve 1) has a wide area of dominance in the pH range of 5.5–13.0. Therefore, within these pH limits, the pyridine nitrogen N₍₃₎ atom of the neutral HIm molecule is assumed to participate in the reactions of the formation of metal complexes, since it contains an unshared electron pair on the sp²-hybrid orbital of the nitrogen atom. This is confirmed by the study of the IR spectra of MLS. The neutral imidazole molecule coordination by a copper ion is also possible during a competitive reaction:

H₂lm⁺ + Mⁿ⁺ ↔ MHlmⁿ⁺ + H⁺

The synthesis of mixed-ligand salts was performed at different CuL:HIm((metIm) ratios. Table 1 shows that, under the stated synthesis conditions, tartrate and copper(II) salicylate coordinate three imidazole molecules and two 2-methylimidazole molecules.

To determine the functional groups participating in complex formation with copper(II) ions, the infrared spectra of the ligands and mono- and bi-ligand salts were used. The IR spectra of tartaric and salicylic acids are characterized by narrow intensive absorption bands of the carbonyl group C=O at 1711.3 and 1654.8 cm⁻¹, respectively. The spectra of the mono- and bi-ligand salts do not have such bands. However, they contain intense bands of asymmetric and symmetric valence vibrations of the ionized carboxyl group COO⁻ involved in coordination with the metal cation. They were 1610.5 and 1322.5 cm⁻¹ for CuTar ∙ 2H₂O; 1588.8 and 1327.2 cm⁻¹ for Cu(HIm)₃Tar ∙ 2H₂O; 1622.5 and 1386.4 cm⁻¹ for CuSal ∙ H₂O; 1628.6 and 1384.4 cm⁻¹ for Cu(HIm)₃Sal; and 1600.6 and 1371.2 cm⁻¹ for Cu(metIm)₂Sal. This confirmed the connection in the salts of the copper ion with the ionized carboxyl groups of tartaric and salicylic acids. The absorption band, corresponding to the deformation vibrations of the salicylic acid oxygroup at 1480.9 cm⁻¹ in the spectrum of copper(II) salicylate, shifted to 1448.4 cm⁻¹, and in the biligand salts, it shifted to 1458 cm⁻¹ (imidazolesalicylate) and 1445.2 cm⁻¹ (methylimidazolesalicylate). This enabled the conclusions that the salicylic acid oxygroup participates in connection with the copper(II) cation in mono- and bi-ligand salts and that salicylic acid in synthesized salts behaves as bicarboxylic acid.

A shift of the intensive bands of the deformation vibrations and out-of-plane vibrations of the imidazole ring (657.9, 1053.9 cm⁻¹) in the IR spectrum of the biligand salts (654 and 1064 cm⁻¹ in imidazoletartrate; 650 and 1069 cm⁻¹ in imidazolesalicylate) was a confirmation of the imidazole coordination in the internal sphere of the complex. The absence of the absorption bands of the valence vibrations of the bonds C=C and C=N of imidazole (1829.0 and 1770.6 cm⁻¹) in the IR spectra of the biligand salts indicated a connection between a Cu²⁺ ion and the N₍₃₎ atom of imidazole.

Table 8. Results of determining stability constants of mono- and bi-ligand complexes of copper(II) with anions of tartaric and salicylic acids and imidazole in the solution.

Composition of Complex	Present Work		Source, Value Logβ1
	Determination Method	Logβ1, Logβ1i1
[CuTar]	Solubility	3.74 (s2 = 2.68 ∙ 10−3)	[9], 3.03; [10], 3.1; [8], 2.7
[CuSal]	Solubility; isomolar series	11.27(s2 = 1.77 · 10−3); 11.29 ± 0.10	[10], 10.6
[CuHImTar]	Saturation curve	7.49 ± 0.16	–
[Cu(HIm)2Sal]	Saturation curve	18.94 ± 0.23	–

shows that the stability constant value of the monoligand complex [CuSal] (logβ₁ = 11.27; logβ₁ = 11.29), which we determined using two methods, consistently conformed satisfactorily to the value offered by the literature. The agreement of the logβ1 values for the [CuTar] complex was worse, owing to the solubility method having a lower accuracy than other methods.

The saturation curves of the ternary systems Cu²⁺–Him–L²⁻ (Figure 2 and Figure 3) showed the formation of complexes of compositions 1:1:1 (CuHImTar) and 1:2:1 (Cu(Him)₂Sal) in the solution. The obtained stability constant of MLC of copper(II) with tartaric acid and imidazole (Table 8) was used to construct a diagram of the yield of particles from pH in the studied system at a molar ratio of components of 1:1:1 (C_Cu = C_Tar = C_HIm = 0.0125 mole/L; pH interval of 0–9). The calculation of the equilibrium composition of the solution and the construction of the diagram (Figure 7) were carried out using the HySS2009 program [29] taking into account Equilibria (1)–(10) and the corresponding equilibrium constants:

Equilibrium	Equilibrium Constant Logarithm
Tar2− + H+ ↔ HTar−	(1), lgB1T = 3.95
Tar2− + 2H+ ↔ H2Tar	(2), lgB2T = 6.76
HIm + H+ ↔ H2Im+	(3), lgB1I = 7.69
Cu2+ + Tar2− ↔ CuTar	(4), lgꞵ1T = 3.10 [10]
Cu2+ + 2Tar2− ↔ CuTar22−	(5), lgꞵ2T = 5.11 [30]
Cu2+ + HIm ↔ CuHIm2+	(6), lgꞵ1I = 4.33 [10]
Cu2+ + 2HIm ↔ Cu(HIm)22+	(7), lgꞵ2I = 7.57 [18]
Cu2+ + Tar2− + HIm ↔ CuTarHIm	(8), lgꞵ111 = 7.49
Cu2+ + H2O ↔ CuOH+ + H+	(9), lgKh1 = −7.53
H2O ↔ H+ + OH−	(10), lgKW = −13.8

Figure 7 shows that the mixed-ligand CuHImTar particles turn out to be the dominant forms in a wide pH range (curve 4). The compatibility in the inner sphere of the MLC of the copper(II) of the ligands, containing donor nitrogen and oxygen atoms, provides a high yield of MLC (more than 60%).

The results of [18,19] and the present research (Table 8) show that the attachment of one molecule of imidazole to the copper ion increases the stability constant logarithm of the complex ion by approximately 3.5–4 units. The authors in [31] estimate the compatibility of different ligands (L, A) in the inner sphere of the complex with a 1:1:1 composition by using the value of the k_S co-proportioning constant, which is related to the common stability constants of the complexes by the ratio of lgk_S = lgβ(MLA) − ½(lgβ(ML₂) + lgβ(MA₂)). When the ligands are compatible, k_S > 1, and the MLC stability constant is greater than the arithmetic mean among the stability constants of the monoligand complexes. The ligands can be shown to be compatible in the CuHImTar complex since lgk_S = 7.49 − (0.5 ∙ 7.57 + 0.5 ∙ 5.11) = 1.15 and k_S > 1 (lgβ(Cu(HIm)₂) = 7.57 [18], lgβ(CuTar₂) = 5.11 [30]), and lgβ(CuHImTar) > ½ (lgβ(Cu(HIm)₂) + lgβ(CuTar₂)), 7.49 > 6.34.

The electronic absorption spectra of the systems (Figure 4 and Figure 5) confirm the formation of MLC in the solution. The large value of optical density D in the system with the two ligands CuCl₂–H₂L–HIm (Figure 4 and Figure 5, curve 4) as compared to the systems CuCl₂–HIm and CuCl₂–L is evidence of the formation of new complexes. This was also confirmed by the shifting of the absorption maximums of the systems with complexes, compared to the hydrated copper ion, to the short-wave region. The substitution of water molecules in the coordination sphere of the Cu²⁺ ion by more firmly bound ligands (the best donors of electronic pairs) increases the difference in the energies of the split d-sublevels of the complex former. Its d-d-band of absorption shifts to the side of shorter wavelengths (the hypsochromic effect).

The formation of mixed-ligand solid salts and complexes in the solution is related to the affinity of the Cu²⁺ (d⁹) ion to both the donor nitrogen atoms and the oxygen atoms of ligands — imidazole and the anions of oxycarboxylic acids. The obtained biligand salts of oxycarboxylic acids, imidazole, and 2-methylimidazole hold promise when used as substances containing a metal microelement and bioactive ligands.

4. Conclusions

A method for the synthesis of new mixed-ligand copper(II) compounds of the composition Cu(HIm)_x(metIm)_xTar(Sal) from low-soluble copper(II) oxycarboxylates and azoles was developed. The composition of these compounds was confirmed using chemical, thermal, and thermogravimetric methods. It is planned to study the various biological properties of the obtained compounds in comparison with those of the initial components—copper(II) salicylate (antiseptic) and azoles (antimycotics).

An analysis of the electronic absorption spectra of the systems confirmed the formation of MLC in an aqueous solution, and an analysis of the IR spectra of the initial ligands and selected mixed-ligand copper(II) compounds allowed us to conclude the structure of the latter: the complexation with copper(II) ions involves the nitrogen atom N₍₃₎ of imidazole, the oxygen atoms of the carboxyl groups of tartaric acid, and the oxygen atoms of the carboxyl and hydroxyl groups of salicylic acid. The formation of individual mixed-ligand compounds and complexes in a solution with azole molecules and oxycarboxylic acid anions is due to the affinity of the Cu²⁺ ion (d⁹) for donor nitrogen and oxygen atoms.

The solubility constants of copper(II) tartrate and salicylate and the stability constants of the mono- and bi-ligand complexes ([CuTar(Sal)], [CuHImTar], and [Cu(HIm)₂Sal]) determined in this work complete the bank of thermodynamic values for copper(II) salts and complexes.