Solvatochromism of Copper(II) Complexes Derived from Trifluoroacetylacetone and Dinitrogen Ligands

Wolfgang Linert ^{1, †, *}, Azza. A. Abou-Hussein ^2, †, Nelly H. Mahmoud ^2, †

1. Institute of Applied Synthetic Chemistry, Vienna University of Technology, Getreidemarkt, 9/163-AC, 1060 Vienna, Austria

2. Department of Chemistry, Girl’s College for Arts, Science and Education, Ain Shams University, Cairo, Egypt

† These authors contributed equally to this work.

* Correspondence: Wolfgang Linert

Academic Editor: Maxim L. Kuznetsov

Special Issue: Coordination Chemistry and Metal Complexes

Received: September 18, 2019 | Accepted: November 07, 2019 | Published: November 19, 2019

Advances in Chemical Research 2019, Volume 1, Issue 4, doi:10.21926/acr.1904001

Recommended citation: Linert W, Abou-Hussein AA, Mahmoud NH. Solvatochromism of Copper(II) Complexes Derived from Trifluoroacetylacetone and Dinitrogen Ligands. Advances in Chemical Research 2019; 1(4): 001; doi:10.21926/acr.1904001.

© 2019 by the authors. This is an open access article distributed under the conditions of the Creative Commons by Attribution License, which permits unrestricted use, distribution, and reproduction in any medium or format, provided the original work is correctly cited.

1. Introduction

Solvatochromism is a type of chromotropism that has several applications. For example, when the donor number (DN) of the solvent is the dominant responsible factor, bathochromic shifts are observed [1,2,3,4,5]. Furthermore, copper is considered to be one of the most important transition metal elements in human bodies, where its ioncomplexes play a fundamental role in bio-catalytic processing [6,7,8,9,10,11]. Comprehensive studies have focused on the mixed ligand Cu (II) [12,13,14,15,16,17,18,19]. It was reported that solvatochromism of transition metal complexes were ideal probes for understanding how transition metals acted as a catalyst or were solubilized, and how the interaction of solute and solvent could influence reactivity, reaction rate, and stability constants [20,21]. Accordingly, solution equilibria involved in the formation of ternary complexes of copper have been studied in order to investigate the protonation constants of the ligand and stability constants of the complexes [22,23,24].

The present study is an extension of our research on the formation of copper complexes in a ternary system [25,26,27]. Mononuclear copper complexes of 1,1,1-Trifluoro–2,4-pentanedione (HTFAA) and dinitrogen ligands (Bipy, Phen, Dphen) were synthesized to obtain six new copper complexes, which were evaluated by different spectroscopic methods, elemental and thermal analyses, as well as measurements of conductivity. The solvatochromic properties were studied to investigate the effect of solvents on structural change of the complex. The synthesized complexes were used as catalysts for the decomposition of hydrogen peroxide (H₂O₂), as H₂O_2­ is an important chemical substance with several industrial applications. The stability constants of the complexes were determined. The antimicrobial activity of mixed-ligand complexes of copper was studied against two pathogenic bacteria (Staphylococcus aureus and Pseudomonas fluorescens). Inaddition, to study the antifungal activity of the complexes, their influence on a type of fungus (Fusarium oxysporum) was investigated.

Figure 1 Scheme.

2. Materials and Methods

2.1 Materials

All the chemicals were of analytical reagent grade and were obtained from Merck or Sigma Aldrich and used without purification. Copper perchlorate hexahydrate (Cu (ClO₄)₂.6H₂O), copper tetrafluoroborate hexahydrate (Cu (BF₄)₂.6H₂O), 1,1,1-Trifluoro-2,4-pentanedione (HTFAA), 2,2´-bipyridine (Bipy), 1,10´-phenanthroline (Phen), 4,7-diphenyl-1,10´-phenanthroline (Dphen), and anhydrous sodium carbonate (Na₂CO₃) were used for this study. Organic solvents used for spectral studies were nitrobenzene (NB), acetonitrile (AN), methanol (MeOH), ethanol (EtOH), acetone (Ac), methylene chloride (DCM), N,N-dimethylformamide (DMF), and dimethyl sulfoxide (DMSO). Hydrogen peroxide was purchased from Adwic.

2.2 Physical Measurements

Microanalyses of carbon, hydrogen, and nitrogen were carried out on a Perkin-Elmer 2400 Series II Analyzer, which gave results having good agreement with the calculated values. Electronic spectra of the metal complexes in DMF were analyzed on a UV-VIS Perkin-Elmer Model Lambda 900. NIR-, IR-, and mid-range FTIR spectra of the compounds were recorded as KBr-pellets within the range of 400-4000 cm^–1, using a Perkin–Elmer 16PC FTIR spectrometer. Sodium hydroxide (0.02 mol dm^–3) and nitric acid (0.02 mol dm^–3) solutions were standardized using primary standard sodium carbonate. Analyses of the metals in the complexes were carried out according to a standard method [28]. Double-distilled water was used throughout all the experiments. Mass spectra were recorded on a Shimadzu-GC-Ms-QP mass spectrometer (model 1000 EX) using a direct inlet system at 220 °C and 70 eV, at the Micro Analytical Center, Cairo University, Egypt. The magnetic susceptibilities of the complexes were measured by Gouy method at room temperature using a magnetic susceptibility balance (Johnson Matthey. Alfa Products, Model No. (MKI)).

Effective magnetic moments were calculated from the expression m_eff. = 2.828 (c_M.T)^1/2 B.M., where c_M is the molar susceptibility, corrected using Pascal’s constants for the diamagnetism of all atoms in the compounds [29]. The uncorrected melting points were measured using a Stuart melting point instrument. Molar conductance of 1.0x10⁻³ M in DMF solution was measured on the WTW.D8120 Weilheim L.F.42 conductivity meter (Germany). Thermogravimetric and its differential analysis (TGA/DTG) were carried out in dynamic nitrogen atmosphere with a heating rate of 10˚C/min, using a Shimadzu TGA–50H thermal analyzer. Protonation constants of the ligands and formation constants of their complexes at 298 K were determined using a pHmeter.

Potentiometric and pH-metric titrations were performed using Orion Star T910 pH titrator. For potentiometric analyses, a series of aqueous solutions prepared to a total volume of 50 cm³ was titrated against standardized NaOH at constant ionic strength (0.10 mol dm^–3) and temperature of 25°C (a thermostatic water bath was used to maintain the temperature). Nitrogen was passed through the reaction mixtures to exclude the adverse effect of CO₂. The values of constants for protonation and the formation of binary and ternary complexes were determined using the Irving-Rossotti equations. The SCOGS computer program was based on linear least square [30,31].

In vitro investigations of antimicrobial activity were performed at the micro-center laboratory, Cairo University. The standard agar-disc diffusion method [32] was used to evaluate the activity of the synthesized complexes against sensitive organisms, Staphylococcus aureus as a representative of Gram-positive bacteria, and Pseudomonas fluorescens as Gram-negative bacteria, in addition to a type of fungus (Fusarium oxysporum). The antibiotic chloramphenicol was used as a standard reference for Gram-negative bacteria, while cephalothin was used for Gram-positive bacteria. Cycloheximide was used as the standard antifungal reference. The tested compounds were dissolved in DMF (which does not show any inhibition activity) at final concentrations of 2mg cm^–3 and 1 mg cm^–3.

2.3 Synthesis of the Transition Metal Complexes

Complex 1 was prepared by adding a mixture of HTFAA (0.6066 cm³, 5.0 mmol) in 20 cm³ absolute ethanol and anhydrous Na₂CO₃ (1.060 g, 10.0 mmol) to an ethanolic solution of Cu (ClO₄)₂.6H₂O (1.853 g, 5.0 mmol). The mixture was stirred for about 30 min, resulting in a green solution, which was then filtered. Then, a solution of Bipy (0.781 g, 5.0 mmol) in 10 cm³ absolute ethanol was added to the filtrate drop-by-drop, with continuous stirring for an additional 30 min. The resulting solution was then filtered, left to stand overnight, and then recrystallized from DCM solvent. A dark violet precipitate was formed. The yield was 1.936 g (82%). IR (KBr): ῡ = 2976 and 2856 (CH) aromatic, 1665 (C=O), 1432 (C–O), 1300 and 985 (C–CF₃), 1135 and 762 (ClO₄), 545 (Cu–O), 477 (Cu–N) cm^–1; UV–Vis (DMF, c = 10^–3 mol dm^–3): λ_max = 570 nm.

Similarly, Complex 2 was prepared using a solution of Phen (0.901 g, 5.0 mmol) in 10 cm³ absolute ethanol. A dark violet precipitate was obtained. The yield was 1.829 g (67.1%). IR (KBr): ῡ = 2917 and 2854 (CH) aromatic, 1682 (C=O), 1375 (C–O), 1325 and 957(C–CF₃), 1025 and 823 (ClO₄), 524(Cu–O), 459 (Cu–N) cm^–1; UV–Vis (DMF, c = 10^–3 mol dm^–3): λ_max = 594 nm.

The dark violet precipitate of complex 3 was prepared in the same way as complex 1, using a solution of Dphen (1.662 g, 5.0 mmol) in 10 cm³ absolute ethanol. The yield was 2.360 g (72.8%). IR (KBr): ῡ = 2949 and 2863 (CH) aromatic, 1634 (C=O), 1456 (C–O), 1300 and 967 (C–CF₃), 1034 and 745 (ClO₄), 578 (Cu–O), 426 (Cu–N) cm^–1; UV–Vis (DMF, c = 10^–3 mol dm^–3): λ_max = 620 nm.

Complex 4 was prepared in the same way as complex 1, using an ethanolic solution of Cu (BF₄)₂.6H₂O (1.726 g, 5.0 mmol) as a precursor. A violet precipitate was formed. The yield was 1.731 g (75.3%). The melting point was 295˚C. IR (KBr): ῡ = 2989 and 2863 (CH) aromatic, 1665 (C=O), 1366 (C–O), 1300 and 985 (C–CF₃), 1135 and 782 (BF₄), 546 (Cu–O), 466 (Cu–N) cm^–1; UV–Vis (DMF, c = 10^–3 mol dm^–3): λ_max = 562 nm; MS (70 eV): m/z = 372 (M⁺).

The dark violet precipitate of complex 5 was prepared by adding a mixture of HTFAA (0.6066 cm³, 5.0 mmol) in 20 cm³ absolute ethanol and anhydrous Na₂CO₃ (1.060 g, 10.0 mmol) to an ethanolic solution of Cu (BF₄)₂.6H₂O (1.726 g, 5.0 mmol). The mixture was stirred for about 30 min, resulting in a green-colored solution, which was then filtered. Then, a solution of Phen (0.901 g, 5.0 mmol) in 10 cm³ absolute ethanol was added to the filtrate drop-by-drop, with continuous stirring for an additional 30 min. The resulting solution was filtered, left to stand overnight, and then recrystallized using DCM solvent. The yield was 1.575 g (59.2%). Melting point was 260 °C. IR (KBr): ῡ = 2981 and 2823 (CH) aromatic, 1677 (C=O), 1335 (C–O), 1195 and 918 (C–CF₃), 1100 and 765 (BF₄), 574 (Cu–O), 463 (Cu–N) cm^–1; UV–Vis (DMF, c = 10^–3 mol dm^–3): λ_max = 518 nm; MS (70 eV): m/z = 396 (M⁺).

Complex 6 was prepared in the same manner as complex 5, using a solution of Dphen (1.662 g, 5.0 mmol) in 10 cm³ absolute ethanol. A violet precipitate was obtained. The yield was 2.283 g (71.8%). The melting point was 283 °C. IR (KBr): ῡ = 2956 and 2845 (CH) aromatic, 1670 (C=O), 1422 (C–O), 1323 and 967 (C–CF₃), 1154 and 663 (BF₄), 522 (Cu–O), 461 (Cu–N) cm^–1; UV–Vis (DMF, c = 10^–3 mol dm^–3): λ_max = 621 nm; MS (70 eV): m/z = 549 (M⁺).

3. Results and Discussion

Unfortunately, we were unable to obtain crystals that were suitable for the determination of the X-ray structure, and formation of the desired complexes was confirmed by measuring spectroscopic data, elemental analysis, and thermal analysis. It is worth mentioning that it was not possible to determine the melting points, mass spectra, and thermal analyses of the perchlorate complexes, due to their detonation effect. The ternary metal complexes were prepared in the molar ratio of 1:1:1 from HTFAA, dinitrogen bases (Bipy, Phen, and Dphen), and copper ions associated with BF₄^- or ClO₄^- as counter anions, in absolute ethanol (Figure 1). HTFAA represented a monobasic bidentate dioxygen ligand, while all the dinitrogen bases were neutral bidentate ligands. All the copper complexes carried a positive charge due to the presence of α-hydrogens of the methylene group. Table 1 presents the results of the characterization of the synthesized Cu (TFAA)(di-imine)X complexes.

Table 1 Data on the characterization of the synthesized Cu(TFAA)(diimine)X complexes.

3.1 IR Spectra

Infrared spectra of the complexes were recorded to obtain information on the modes of binding of the ligand with the corresponding metals. The IR spectra of all the synthesized complexes were generally similar, and only minor differences related to the counterions were observed (supplementary material S1). Perchlorate complexes 1, 2, and 3 showed two sharp stretching vibrational bands in the range of 1025–1135 cm^–1 and 745–762 cm^–1, corresponding to the antisymmetric stretching and bending vibrations of uncoordinated perchlorate. This behavior indicated that the ClO₄^–ion was ionic and had an expected T_d symmetry [33]. Tetrafluoroborate complexes 4, 5, and 6 showed similar behavior as the counterion, where the strong absorption bands at 1100–1154 cm^–1 were assigned to antisymmetric ν(BF₄⁻) stretching mode [34]. The bands at 1634–1682 cm^–1 and 1574–1615 cm^–1 can be assigned to the stretching vibration of νC=O and νC=C, respectively. The observed downshift compared to the free ligands at 1725 cm^–1 and 1645 cm^–1 (assigned to νC=O and νC=C, respectively) was due to the chelation of the dioxygen atoms of HTFAA, suggesting coordination via the carbonyl group upon formation of the six-membered chelate rings [35]. Peaks between 1195 and 1325 cm^–1, and between 918 cm^–1 and 985 cm^–1, could be assigned to various vibrational modes of the C–CF₃ bond overlapping with the peaks around 800 cm^–1, which represented the out-of-plane bending modes of the C–H bond between the two carbonyl groups [36]. The bands ascribed to the stretching vibration of ν(CH₃) were observed in the range 3023–3142 cm^–1. One of the most characteristic features of the IR spectra of these complexes is the frequency of the dinitrogen base ligands. Changes were observed in the spectrum of Bipy, Phen, and Dphen on coordination with the metal ion. Characteristic bands of the azomethine group (νC=N) at the free dinitrogen at 1628, 1617, and 1605 cm^–1 for Bipy, Phen, and Dphen were shifted to lower frequencies on chelation [37]. New copper complexes showed bands at 522–578 and 426–477 cm^–1 due to ν(Cu-O) and ν(Cu-N), respectively [38]. Due to the larger change in the dipole moment for Cu-O compared to Cu-N, ν(Cu-O) always appears at a higher frequency than ν(Cu-N). The absence of a characteristic broad band of stretching frequencies of the ν(OH) of water molecules in the range of 3100–3700 cm^–1 was also confirmed by elemental analysis and TGA data. The infrared spectroscopy data revealed that the binding of ligands to the metal ions was achieved by the nitrogen atoms of the dinitrogen ligands, as well as the oxygen atoms of HTFAA.

3.2 Electronic Spectra, Magnetic Moments, Molar Conductivity Measurements, and Solvatochromism

The electronic spectra of the ligand and its transition metal complexes in a DMF solution (10^–3 M), with their assignments, magnetic moments, and molar conductivity measurements are presented in Table 1. In the electronic spectra of the ligands, four absorption bands at 272–275, 339–341, 327–331, and 421–430 nm were characteristic. The former two bands corresponded to ¹L_a→ ¹A and ¹L_b→ ¹A transitions of the aromatic ring of the ligands Phen, Bipy, and Dphen [39]. The third band corresponded to the π→ π* transition of the carbonyl group of the dioxygen ligand, as well as the phenyl rings of 4,7-diphenyl-1,10-phenanthroline. The last band corresponded to the n→ π* transitions of the oxygen and nitrogen atoms that overlapped with the transfer of intermolecular charge from the aromatic ring.

The synthesized copper mixed ligand complexes were soluble in a large number of organic solvents. Absorption spectra of all the complexes were characterized by a broad band in the visible region, which was attributed to the promotion of an electron in the low energy orbitals to the vacant d_x²_–y² orbital of the copper (II) ion (d⁹) (Figure 2). The positions of the λ_max of complexes, along with the ν_max, are summarized in Table 2. In case of lower DN of the solvents, neither the solvent nor the counter ion was coordinated with the metal ion, due to which a square planar geometry occurred [40]. As the Lewis basicity of the solvent increased, its coordination ability became stronger and the structure of the complexes changed from square planar to square pyramidal, which finally changed to octahedral geometry. All these features have been discussed in terms of ligand field theory. The bathochromic nature of the complexes was not only affected by solvent basicity, but also by steric hindrance and the nature of the counter anion. Dphen showed more positive solvatochromism than Phen and Bipy in the order of l_max (Dphen > Phen > Bipy). This was because higher steric hindrance made the complex more solvatochromic since ligands with large steric hindrance are likely to bind more weakly, shifting the d-d band to red [41,42,43]. It has also been reported that ClO₄^–, BF₄^–are classified as non-coordinating anions and should not interfere with the coordination of the solvent medium. However, the presence of counter ions also possibly exerted a slight effect on the absorbance of the wavelength of the complexes. With an increase in the size of the counter ion, the wavelength at the copper complexes decreased and the emission energy of BF₄ complex was lower than that of ClO₄ complex [44]. Table 2 summarizes the absorption maxima of the complexes in different solvents.

Table 2 Absorption maxima of the complexes in various solvents in the presence of various anions.

Figure 2 UV-Vis absorption spectra of copper mixed ligand complexes in different solvents, where (a) 1,2-Dichloromethan (DCM), (b) Nitrobenzene (NB), (C) Acetonitrile (AN), (d) Methanol (MeOH), (e) Dimethylformamide (DMF), (f) Dimethylsulfoxide(DMSO).

Regression analysis of band maxima of complexes 1–6 against the DN of solvents is shown in Figure 3. The regression analysis indicated a good correlation and also confirmed the solvatochromic behavior (ν_max/10³ = v^o +a (DN), where ν_max represented the measured d-d absorption frequency; v^o was the extrapolated frequency, and ‘a’, the slope, represented the sensitivity of the complex toward the solvent). It was observed that the ν_max values decreased almost linearly with an increase in the solvent donor ability. These results confirmed the significant contribution of DN in the solvatochromism of the complexes, owing to the coordination of polar solvent molecules and the axial site of the copper center with different strength, consequently leading to a change in the geometry of the complex from square planar to octahedral. The negative slope indicated that the strength of the Cu-O bond decreased with an increase in the donor strength of the coordinated ligand. It was observed that complex 5 had the lowest slope value. This could be attributed to the fact that the dinitrogen ligands are coordinated to the copper center, causing a large steric hindrance to the axial coordination of the entering solvent, resulting in less solvatochromism.

Figure 3 Dependence of the v_max/1000 for complexes (a) [Cu (TFAA)(Bipy)]ClO₄ 1, [Cu (TFAA)(Phen)]ClO₄ 2 and [Cu (TFAA)(Dphen)]ClO₄ 3, (b) [Cu (TFAA)(Bipy)]BF₄ 4, [Cu (TFAA)(Phen)]BF₄ 5 and [Cu (TFAA)(Dphen)]BF₄ 6 on the solvent donor number (DN) values.

The magnetic data of the investigated copper complexes were in the range of 1.90–1.97 B.M., suggesting a square planar structure for all the complexes [45]. The values of molar conductance observed for solutions of the mixed ligand complexes in DCM were in the range of 20–30 ohm^–1 cm² mol^–1. On the other hand, in DMF, the values of molar conductance for the same complexes were observed to be in the range of 90–99 ohm^–1 cm² mol^–1. In DCM, the values for the complexes were in the ratio of 1:1 electrolyte. In DMF, they were higher, due to the fact that the DMF solvent replaced the anions, although the ratio was still 1:1 electrolyte, as the maximum value of molar conductance for 1:1 electrolyte in DMF was about 100 Ohm^–1 cm² mol^-1 [46]. The mass spectra of tetrafluoroborate-mixed copper complexes are shown in supplementary material S2. The molecular ion peaks were in accordance with the formula-derived weights of the complexes (Table 1). The fragmentation patterns of the complexes are described in supplementary material S3. The mass spectra of the perchlorates were not performed due to the possibility of detonation of the perchlorate compound.

3.3 Thermal Analysis

The thermal decomposition of the complexes was studied using TGA analysis to determine whether the solvent molecules were within the inner or outer coordination of the central metal ion[47]. The analysis also provided information on the degradation of the complexes, as presented schematically in supplementary material S4.

Thermal curves obtained for all the complexes were highly similar in behavior, with all the complexes being stable at temperatures up to 180 °C, and subsequently showing a three-stage pattern of decomposition (supplementary material S5).

The first stage involved the removal of BF₃ and CF₄ gas molecules in the temperature range of 190–350^oC [48,49]. The second stage involved the loss of carbon monoxide and propyne gases in the temperature range of 395–530^oC. The last stage involved the removal of the organic ligand at temperatures greater than 550 °C.

Several methods have been employed for the evaluation of kinetic parameters from TGA data. In the present study, the kinetic parameters (E, A, ∆S, ∆H, and ∆G) for the decomposition stages of all the complexes were calculated using the Coats-Redfern equation [50]. The plots of log[(–log (1–α)/T²)] versus [1000/T] for all decomposition stages were linear, where the activation energy was calculated from the slope (supplementary material S6). All the results are presented in Table 3.

The negative values of ∆S indicated a more ordered activated state. The values of activation energy for the third stage of decomposition were observed to be higher than those for the first and the second stages, indicating that the rates of decomposition during the earlier stages were lower than that for the third stage. Based on the values of energy of activation, the thermal stabilities of the complexes were in the order: Bipy ˃ Phen ˃ Dphen.

Table 3 Temperature of decomposition and the kinetic parameters of complexes 4, 5 and 6.

The increase in values of ΔG through the stages of decomposition of the complex is because of the increasing values of TΔS from one stage to another, indicating that the rate of removal of the subsequent ligand will be lower than that of the precedent ligand [51]. Thismay be attributed to the structural rigidity of the complex that remains after the expulsion of one or more ligands, as compared with the precedent complex. Thus, more energy (TΔS)is required for its rearrangement before undergoing any change.

3.4 Catalytic Decomposition of Hydrogen Peroxide

The decomposition of H₂O₂, catalyzed by copper complexes, was kinetically monitored by removing aliquots of the reaction mixture at predetermined intervals of time and titrating the undecomposed H₂O₂ with standard KMnO₄ solutions (0.05 M) that were standardized with COONa₂ (primary standard). The reaction was carried out in aqueous phosphate buffer pH7.0 at room temperature in a temperature-controlled cell. Potassium permanganate was used as an oxidizing agent to determine the amount of H₂O₂ in the solution. The initial concentration of H₂O₂ at a pHof 7.0 was 0.2 M. The studied range of the catalyst was 0.3×10^–4–1.3×10^–4 M copper complexes at constant H₂O₂ concentration, pH, and temperature [52]. Acidification by H₂SO₄ effectively halted the alkaline decomposition at the desired time and titrations with potassium permanganate in the acidic media could be carried out even one hour later without significant discrepancy.

The rate of the catalytic reaction was monitored by evaluating the consumption of H₂O₂ by oxidation-reduction method [53]. The graphical linear dependence of ln a/(a-x) as a function of time (where ‘a’ is the initial concentration of H₂O₂ and ‘a-x’ is the remaining concentration after time ‘t’), using all the complexes at concentrations ranging from 0.3×10^–4–1.3×10^–4 M, showed a slope with the first-order reaction, and that the concentration of the complex required for the complete decomposition of H₂O₂ was 1.3×10^–4 M, at 25 °C and 0.2 M of H₂O₂. The steric effects of the dinitrogen ligands of the complexes were found to be nearly the same (Figure 4). Similar results were obtained for complexes 2–6, and are presented in supplementary material S7. In order to investigate the relation between the percentage of decomposition of H₂O₂ in the solution as a function of time using different initial concentrations of H₂O₂ (range 0.01–0.05 mmol), the complexes ‘1’ and ‘4’, at a constant concentration of 1.3×10^–4 M, were taken as representative examples. The results demonstrated that the perchlorate complexes had slightly higher activity than the tetrafluoroborate ones (Figure 5). From the results of the study, it could be concluded that [Cu (L)(TFAA)]X was the catalytically active complex, which could be considered to be a mimic of homogenous catalase in the aqueous solution [54,55].

Figure 4 First-order plots for the catalytic decomposition of H₂O₂ using different concentrations of [Cu (TFAA)(Bipy)]ClO₄ 1.

Figure 5 Relation between the percentage of H₂O₂ using different initial [H₂O₂] as a function of time for the system (a) [Cu (TFAA)(Bipy)]ClO₄ and (b) [Cu (TFAA)(Bipy)]BF₄ at T= 298 K.

3.5 Potentiometric Studies

The constants for protonation and formation of binary and ternary complexes were determined by Irving-Rossotti equation using a linear least-square fit[30,31]. Tetrafluoroborate copper complexes 4, 5, and 6 were taken as representative examples to investigate the stability constants of the copper mixed ligand complexes. The investigation on ionization constants of the ligands and stability con­stants of the binary and ternary complexes was based on the Irving and Rossotti method. The different solutions that were titrated against 0.02 mol dm^–3 standardized sodium hydroxide were:

(a) HNO₃ (9.54x10^–3 mol dm^–3)

(b) Solution (a) + Di-imine (5.0x10^–3 mol dm^–3)

(c) Solution (b) + Cu (II) (5.0x10^–3 mol dm^–3)

(d) Solution (a) + TFAA (5.0x10^–3 mol dm^–3)

(e) Solution (d) + Cu (II) (5.0x10^–3 mol dm^–3)

(f) Solution (e) + Di-imine (5.0x10^–3 mol dm^–3) (Figure 6).

The study was conducted in NaNO₃ at an ionic strength of 0.15 mol dm^–3 and at a temperature of 25 °C. The protonation constants of the free ligands and the stability constants of the complexes were calculated from the pH titration data. All the ligands showed one inflection point followed by a buffer region at higher pH values [56,57,58]. The formation of the ternary complexes was confirmed by comparing them with the corresponding binary complexes. The potentiometric and pH-metric titration curves of binary and ternary complexes overlapped at lower pH, where the formation of the binary system began. Then, at certain pH values, a divergence at the ternary complexes from the binary ones could be observed, confirming the formation at 1:1.1 ratio. Ternary complexes of copper metal ions in the presence of HTFAA and X were formed in two steps. Initially, the primary ligand with a higher formation constant (HTFAA (pk^a=6.53 ±0.01), a dioxygen ligand) interacted with the metal ion to form a binary system with a stability constant (log K [M-HTFAA]=5.29 ±0.02). The titration curve showed an inflection point indicating the step-wise coordination to form a binary complex at 1:1 ratio. This step was followed by interaction with the secondary ligand X to form the ternary [Cu (TFAA)(X)]BF₄ complex (1:1:1).

$K_{ML}^{M}$= formation constant for the complex ML.

$K_{MLX}^{ML}$= formation constant for the complex MLX formed by the reaction of complex ML with the free ligand X

The stability of the ternary complexes was higher than that of the binary complexes. This is expressed in terms of D log K (Eq. 3) [59,60] (Table 4). The observed stability of the ternary complexes with respect to their ligands decreased in the order: Bipy˃ Phen˃ Dphen, which could be attributed to the rotational flexibility of Bipy rather than the rigid character of Phen and the strike (bulky) factor of Dphen. The presence of fluorine atoms enhanced the stabilization of the complexes, as they acted as electron-withdrawing groups. The electron density of metal-ligand bonds in ternary systems appeared to be redistributed to increase the polarity. Hence, mixed ligand complexes could not be easily hydrolyzed by metal hydroxides, even at high pH values [61].

Figure 6 potentiometric titration curve of complexes 4, 5 and 6.

Table 4 Stability constant of copper binary and ternary complexes in aqueous media together with the relative stability values of the mixed ligand complex, D log K.

3.6 Biological Studies

The antibacterial activities of HTFAA, the dinitrogen ligands, and their copper complexes were tested on Staphylococcus aureus as a Gram-positive bacterium and Pseudomonas fluorescens as a Gram-negative bacterium, in addition to a type of fungus (Fusarium oxysporum) (Figure 7).

Figure 7 Biological screening of the copper complexes against a Gram-positive bacterium, Gram-negative bacterium, and fungus.

The new complexes exhibited varying degrees of inhibition on the growth of the tested strains. It was observed that perchlorate complexes had higher inhibitory activity than tetrafluoroborate, and the inhibitory activity of the complexes was in the order Bipy˃ Phen˃ Dphen. This indicated that a decrease in the steric crowding increased the inhibitory activity of the complexes (Table 5). The enhanced activity of the complexes could be explained on the basis of Overtone’s concept [62] or Tweedy’s Chelation theory [63]. According to Overtone’s concept of cell permeability, the lipid membrane surrounding the cell favors the passage of only lipid-soluble materials, which controls the antimicrobial activity. Tweedy’s chelation theory proposes that chelation reduces the polarity of the metal atom mainly because of the partial sharing of its positive charge with the donor groups, and possible electron delocalization over the entire ring. This consequently increases the lipophilic character of the chelates, facilitating their permeation through the lipid layers of the bacterial membrane. All the complexes displayed approximately similar inhibitory capacity as the standard antibiotic, thus validating the biological efficiency of these complexes and indicating their potential application as a novel drug.

Table 5 Antimicrobial activity of copper complexes.

4. Conclusion

Six new copper complexes containing HTFAA as the dioxygen ligand and Bipy, Phen, and Dphen as dinitrogen ligands were prepared. The analysis suggested a square planar structure with a tautomeric form. From the interpretation of the elemental analysis and infrared, electronic spectra, and molar conductivity, it was possible to draw the tentative structures of the copper complexes. Figure 1 presents the suggested structure for the copper complexes.

By studying the solvatochromism of the complexes, it was evident that an increase in the donor strength of the solvents resulted in a positive solvatochromism. The perchlorate complexes showed higher activity toward the decomposition of hydrogen peroxide than the tetrafluoroborate ones. Potentiometric studies revealed the formation of ternary complexes. Thermal analysis and potentiometric studies demonstrated that the new complexes were generally stable and that their stability decreased in the order Bipy˃ Phen˃ Dphen. The complexes showed inhibitory activity against the bacteria Staphylococcus aureus and Pseudomonas fluorescens, as well as the fungus Fusarium oxysporum, indicating their potential application as a novel drug.

Additional Materials

The following additional materials are uploaded at the page of this paper.

1. Figure S1: IR spectra of copper mixed ligand complexes (1-6).

2. Figure S2: Mass spectra of complexes 4,5,6.

3. Figure S3: Fragmentation patterns of complexes 4,5,6.

4. Figure S4: TGA-DrTGA of copper complexes [Cu(TFAA)(Bipy)]BF4 4, [Cu(TFAA)(Phen)]BF4 5 and [Cu(TFAA)(Dphen)]BF4 6.

5. Figure S5: Schematic fragmentation TGA of complexes 4,5,6.

6. Figure S6: Coats-Redfern plots for complex 4, 5 and 6, where Y = log [-log (1-α)/T2].

7. Figure S7: First order plots for the catalytic decomposition of H2O2 using different weights of (b) [Cu(TFAA)(Phen)]ClO4 2, (c) [Cu(TFAA)(Dphen)]ClO4 3, (d) [Cu(TFAA)(Bipy)]BF4 4, (e) [Cu(TFAA)(Phen)]BF4 5 and (f) [Cu(TFAA)(Dphen)]BF4 6 at 25 Cº and 0.2 M H2O2.

Author Contributions

All authors are equally contributed.

Competing Interests

The authors have declared that no competing interests exist.