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BY 4.0 license Open Access Published by De Gruyter Open Access September 25, 2019

Complex formation in a liquid-liquid extraction-chromogenic system for vanadium(IV)

  • Kiril B. Gavazov EMAIL logo , Vassil B. Delchev , Nikolina P. Milcheva and Galya K. Toncheva
From the journal Open Chemistry


The azo dye 4-(2-thiazolylazo)orcinol (TAO) and the cationic ion-pair reagent 2,3,5-triphenyl-2H-tetrazolium chloride (TTC) were examined as constituents of a water-chloroform extraction-chromogenic system for vanadium(IV). The effects of TAO concentration, TTC concentration, pH and extraction time were examined. Under the optimum conditions the extracted complex has a composition of 1:2:1 (V:TAO:TTC). The absorption maximum, molar absorptivity and constant of extraction were determined to be λmax=544 nm, ε544=1.75×104 dm3 mol–1 cm–1 and Log Kex=4.1. The ground state equilibrium geometries of the possible monoanionic VIV-TAO 1:2 species were optimized by the HF method using 3-21G* basis functions. Their theoretical time dependent electronic spectra were simulated and compared with the experimental spectrum. The best fit was obtained for the structure in which one of the TAO ligands is tridentate, but the other is monodentate (bound to VIV through the oxygen which is in the ortho-position to the azo group) and forms a hydrogen bond N–H...O=V through its protonated heterocyclic nitrogen. Based on this unusual structure, which can explain some peculiarities of the complex formation between VIV and commonly used azo dyes, the ground state equilibrium geometry of the whole ternary 1:2:1 complex was computed at the HF and BLYP levels.

1 Introduction

Vanadium occupies position 23 in the periodic table. It is the fifth most abundant transition element in the Earth’s crust and has applications in many industries. Apart from natural processes, such as volcanic activity, wild forest fires, continental dust and marine aerosols formation, vanadium compounds can enter the biosphere as a result of burning crude petroleum and coal, mining, processing of ores, steel refining, production of dyes, glasses, ceramics and alloys, and its use as a catalyst in large-scale processes [1, 2]. Various hazards are associated with elevated concentrations of vanadium in the environment [3]. On the other hand, health disorders can arise from vanadium deficiency [2, 4]. The window between beneficial and toxic action of this element is not well defined and depends on many factors, including the oxidation state. For example VV can be almost as toxic as lead, cadmium, and mercury [5], while vanadyl(IV) sulfate is a common supplement (Vana Trace) used to enhance weight training in athletes [6, 7]. Uncertainties in the correlation between toxicity–oxidation state and toxicity–ligand system [8, 9, 10], along with vanadium’s stereochemical flexibility [11, 12] define the necessity for thorough investigations on its coordination chemistry. In addition, it is important to develop reliable methods for determining VIV and VV in their joint presence [13, 14, 15].

In a previous paper [16] we characterized the coordination compound formed in a liquid-liquid extraction-chromogenic system containing VV, 4-(2-thiazolylazo)orcinol (TAO) and 2,3,5-triphenyl-2H-tetrazolium chloride (TTC). We discussed the special features of the extracted 2:2:2 complex and highlighted the differences between it and other complexes with related azo dyes (AD) [17, 18, 19, 20]. Here we describe a similar water-chloroform extraction system containing VIV instead of VV.

The structural formulae of the reagents TAO and TTC are shown in Figure 1.

Figure 1 Structural formulae of the reagents TAO (a) and TTC (b).
Figure 1

Structural formulae of the reagents TAO (a) and TTC (b).

It should be noted that ternary complexes of VIV with ADs have been less studied. Moreover, the conclusions drawn are not always convincing enough due to the possibility of oxidation of VIV by the oxygen in air [21, 22]; lower stability of the complexes [23, 24]; inconsistencies associated with the opinion that ADs, such as 4-(2-thiazolylazo)resorcinol (TAR) and 4-(2-pyridylazo) resorcinol (PAR), act as tridentate ligands [25, 26, 27, 28]; the experimentally determined molar VIV:AD ratios (e.g. 1:2) [21, 24, 29]; the stability of the V=O bond, which is likely to remain unaffected in the process of complex formation [30] and the common coordination numbers of VIV (5 and 6) [31].

In the interpretation of the obtained experimental results, we used information for the complex formation between VIV and ADs in the presence [32, 33, 34, 35] or absence [23, 35,36,37,38] of auxiliary reagents. We suggested several structures of the chromogenic anionic part of the extracted species. To evaluate them, we compared the theoretically simulated spectra and the experimentally obtained spectrum. This approach is known to give good results [39] and can be especially useful for complexes that are easily altered in attempts to be isolated in solid state.

For stabilization of VIV reducing reagents, such as ascorbic acid (АА), are often added [14, 33]. However, АА can enter the coordination entity and form ternary complexes in similar systems [33]. On the other hand, the reducing reagent can convert the tetrazolium salt to formazan [40]. In order to avoid the possibility of such side processes, no additional stabilizing reagents of VIV were used in the present work.

2 Methods

2.1 Reagents and Apparatus

Stock VIV aqueous solution (5×10–2 mol dm–3) was prepared from VOSO4⋅5H2O (Fluka, purum) and standardized by potassium permanganate titration. Working 2×10–4 mol dm–3 solutions with pH 2-3 were prepared every day by an appropriate dissolution of standard solution in the presence of H2SO4. TAO (95%, Sigma-Aldrich Chemie GmbH) was dissolved in dilute KOH; the obtained neutral aqueous solution was at concentration of 3×10–3 mol dm–3. TTC (p.a. Loba Feinchemie GMBH) was dissolved in water (cTTC = 4.7×10–3 mol dm−3) and stored in a dark place. The acidity of the aqueous medium was set by the addition of buffer solution, prepared by mixing 2.0 mol dm–3 aqueous solutions of CH3COOH and NH4OH. The pH was checked using a Hanna HI-83141 (Romania) and a WTW InoLab 7110 (Germany) instruments. Distilled water and additionally distilled commercial chloroform (p. a.) were used throughout the work. Absorbance measurements were performed using a Camspec M508 or a Ultrospec3300 pro UV-Vis spectrophotometers (UK), equipped with 1 cm path-length glass cells. All experiments were performed in an air conditioned laboratory at 22±1°C.

2.2 Procedure

Solutions of VIV, TAO, buffer (2 cm3; pH 3.5 – 6.4) and TTC were placed into a separatory funnel. The resulting mixture was diluted with water to a total volume of 10 cm3; the corresponding concentrations of VIV, TAO and TTC in the aqueous phase were 2×10–5 mol dm–3, (0.60 – 14.3)×10–4 mol dm–3 and (0.24 – 14.1)×10–4 mol dm–3 respectively. Then 10 cm3 of chloroform was added and the funnel was shaken for a fixed time period. After a short wait for phase separation (ca. 10 – 15 seconds), a portion of the organic extract was transferred (through a filter paper impregnated with chloroform) into the spectrophotometer cell. The absorbance was measured against a simultaneously prepared blank solution.

2.3 Theoretical calculations

The ground-state equilibrium geometries of the anionic complexes were optimized at the Hartree-Fock (HF) level of theory using 3-21G* basis functions. The charge and the spin multiplicity were set as –1 and doublet, respectively. Subsequently, at the same level, the vertical excitation energies of the anions were calculated. The structure of the 2,3,5-triphenyl-2H-tetrazolium cation (TT+) was also optimized. Then the whole ternary complex was constructed and optimized at the same level (HF) and by means of BLYP functional based on DFT theory. All calculations were performed with the GAUSSIAN 03 program package (Gaussian, Inc., Wallingford CT, 2004). The output files were visualized by means of the ChemCraft program (v. 1.8, b. 523a,

Ethical approval: The conducted research is not related to either human or animal use.

3 Results and discussion

3.1 Absorption spectra

VV forms an anionic 1:1-chelate with TAO in a water-ethanol medium [16]. In the presence of tetrazolium salt, a neutral chloroform-extractable ion-associate is formed. During its extraction dimerization occurs, the final product being a 2:2:2 complex [16, 41]. When the tetrazolium salt is TTC, the optimum pH interval for complex formation is 4.8–5.2. The absorption maximum in chloroform lies at 545 nm and its position is slightly affected by changes in pH and reagent concentrations [16].

Absorption spectra of VIV–TAO–TTC extracts in the same solvent are shown in Figure 2, curves 1–3. In contrast to the system with VV (curve 4), spectral differences are observed with changing experimental conditions. Except for competitive TAO-TTC ion-pair formation [42, 43], several other reasons can cause spectral differences: isomerism, hydrolysis and changes in the composition and/or oxidation state [44, 45]. In order to clarify the running processes, we collected additional information for the effects of pH and reagent concentrations.

Figure 2 Absorption spectra of VIV (1 - 3) and VV (4) extracts against blank; cV = 2×10-5 mol dm-3, extraction time t = 2 min.(1) cTAO = 5.4×10-4 mol dm-3, cTTC = 1.4×10-3 mol dm-3, pH 4.9;(2) cTAO = 1.1×10-3 mol dm-3, cTTC = 1.4×10-3 mol dm-3, pH 5.2;(3) cTAO = 4.5×10-4 mol dm-3, cTTC = 9.4×10-4 mol dm-3, pH 5.0;(4) cTAO = 4.5×10-4 mol dm-3, cTTC = 9.4×10-4 mol dm-3, pH 5.2.
Figure 2

Absorption spectra of VIV (1 - 3) and VV (4) extracts against blank; cV = 2×10-5 mol dm-3, extraction time t = 2 min.

(1) cTAO = 5.4×10-4 mol dm-3, cTTC = 1.4×10-3 mol dm-3, pH 4.9;

(2) cTAO = 1.1×10-3 mol dm-3, cTTC = 1.4×10-3 mol dm-3, pH 5.2;

(3) cTAO = 4.5×10-4 mol dm-3, cTTC = 9.4×10-4 mol dm-3, pH 5.0;

(4) cTAO = 4.5×10-4 mol dm-3, cTTC = 9.4×10-4 mol dm-3, pH 5.2.

3.2 The effect of pH and reagents concentrations

The effect of pH is shown in Figure 3 for two different TAO concentrations. At cTAO = 3.4×10–4 mol dm–3 and cTTC = 9.4×10–4 mol dm–3 (the optimum concentrations for VV extraction [16]) the absorbance of the VIV–TAO–TTC complex is maximal for pH-values between 5.0 and 5.9 (series 2). This interval is different and wider than those for the VV complex mentioned above. At higher TAO concentrations, the optimum pH-interval is expanded even more (series 1). These observations are consistent with the literature [35] (devoted to VIV/VV-PAR complexes) and indicate that different complexes are formed with VV and VIV. In addition, it can be concluded from Figure 3 that the complexes of VIV are at least two.

Figure 3 Absorbance of the complex (1, 2) and blank (1’) in chloroform vs pH of aqueous phase; cV = 2×10-5 mol dm-3, extraction time t = 2 min, λ = 544 nm.(1, 1’) cTAO = 5.4×10-4 mol dm-3, cTTC = 7.1×10-4 mol dm-3;(2) cTAO = 3.4×10-4 mol dm-3, cTTC = 9.4×10-4 mol dm-3.
Figure 3

Absorbance of the complex (1, 2) and blank (1’) in chloroform vs pH of aqueous phase; cV = 2×10-5 mol dm-3, extraction time t = 2 min, λ = 544 nm.

(1, 1’) cTAO = 5.4×10-4 mol dm-3, cTTC = 7.1×10-4 mol dm-3;

(2) cTAO = 3.4×10-4 mol dm-3, cTTC = 9.4×10-4 mol dm-3.

The fact that both pH and cTAO play an important role in the complex formation mechanism can also be seen in Figure 4a. The saturation series 1 (for pH 5.2) is atypical; a well-defined fall of the absorbance is visible for cTAOca. (6 – 8)×10–4 mol dm–3. However, the series 2 (for pH 4.9) is simpler. It is characterized by a maximum at cTAO = 5.4×10–4 mol dm–3. This concentration is about 2.5 times smaller than the concentration needed to obtain the same absorbance at pH 5.2.

Figure 4 Absorbance of extracted species vs concentration of TAO (a) and TTC (b); cV = 2×10-5 mol dm-3, extraction time t = 2 min, λ = 544 nm.a)cTTC = 9.4×10-4 mol dm-3, pH 5.2 (1) or 4.9 (2);b)cTAO = 1.2×10-3 mol dm-3 (1) or 5.4×10-4 mol dm-3 (2), pH 5.2 (1) or 4.9 (2).
Figure 4

Absorbance of extracted species vs concentration of TAO (a) and TTC (b); cV = 2×10-5 mol dm-3, extraction time t = 2 min, λ = 544 nm.

a)cTTC = 9.4×10-4 mol dm-3, pH 5.2 (1) or 4.9 (2);

b)cTAO = 1.2×10-3 mol dm-3 (1) or 5.4×10-4 mol dm-3 (2), pH 5.2 (1) or 4.9 (2).

Figure 4b shows the effect of cTTC on the absorbance. The series for both pH values are almost identical. They allow for a reliable determination of the molar TTC : VIV ratio by different methods.

3.3 Complex stoichiometry

The mobile equilibrium method [46] (Figure 5, lines 2 and 2’) and the straight-line method of Asmus [47] showed that nTTC : nV = 1:1 irrespective of pH. The same two methods were used to determine the TAO-to-VIV molar ratio at pH 4.9. They demonstrate that nTAO : nV = 2:1 (Figure 5, line 1). However, the determination of nTAO : nV for pH 5.2 was impeded by the above-mentioned absorbance fall. The results for the low TAO concentrations, (1.4 – 4.3)×10-4 mol dm-3, indicate that nTAO : nV = 1:1 (Figure 4, line 1’).

Figure 5 Determination of the R-to-VIV molar ratio by the mobile equilibrium method. The experimental conditions are given in Fig. 3.(1) R = TAO, pH 4.9;(1’) R = TAO, pH 5.2;(2) R = TTC, pH 4.9;(2’) R = TTC, pH 5.2.Straight line equations: (1) y = 1.94x + 7.35; (1’) y = 1.03x + 3.51; (2) y = 1.07x + 4.32; and (2’) y = 1.07x + 4.22.
Figure 5

Determination of the R-to-VIV molar ratio by the mobile equilibrium method. The experimental conditions are given in Fig. 3.

(1) R = TAO, pH 4.9;

(1’) R = TAO, pH 5.2;

(2) R = TTC, pH 4.9;

(2’) R = TTC, pH 5.2.

Straight line equations: (1) y = 1.94x + 7.35; (1’) y = 1.03x + 3.51; (2) y = 1.07x + 4.32; and (2’) y = 1.07x + 4.22.

The application of the Bent-French limited logarithm method [48] (Figure 6, straight line 1) for the same concentration interval led to a slope of 0.65. This slope is significantly smaller than the integer value of 1. On the other hand, it is known that the method gives best results (integer values) when (i) the complex is unstable, (ii) the metal concentration is low and (iii) the reagent excess is small [44, 45]. If these conditions are not met, the obtained slope is smaller. Consequently, the molar nTAO : nV ratio is 1:1, a result consistent with the mobile equilibrium method (Figure 5, line 1’). For the concentration interval (4.3 – 5.7)×10-4 mol dm-3 (or –Log cTAO between 3.37 and 3.24) the slope is different (1.6; straight line 2). This value shows that most probably nTAO : nV = 2:1. To the best of our knowledge, there are no reports for coordination compounds of VIV/V and ADs with nAD : nV > 2 : 1 and such ratios are characteristic for heavier elements e. g. EuIII, HoIII, HfIV and ThIV [27].

Figure 6 Application of the Bent-French limited logarithm method for determination of the TAO-to-VIV molar ratio at pH 5.2. cV = 2×10-5 mol dm-3, cTTC = 9.4×10-4 mol dm-3, extraction time t = 2 min, λ = 544 nm.
Figure 6

Application of the Bent-French limited logarithm method for determination of the TAO-to-VIV molar ratio at pH 5.2. cV = 2×10-5 mol dm-3, cTTC = 9.4×10-4 mol dm-3, extraction time t = 2 min, λ = 544 nm.

From the experimental results, it is not possible to determine the complex composition in the region of the absorbance fall, i.e. cTAOca. (6 – 8)×10-4 mol dm-3, nor to draw explicit conclusions about its origin. Assuming that there is no change in the composition with further increase in cTAO (nTAO : nV = 2:1), two explanations seem possible: (i) change in the chromophore system (isomerism, hydrolysis, protonation, etc.) or (ii) decrease in the extraction rate due to formation of non-extractable charged complexes. The second explanation is not very likely because the 2,3,5-triphenyl-2H-tetrazolium cation (TT+) is known to form neutral chloroform-extractable complexes with high-charge anions of the type [VO2(PAR)2]3– [20] and [VO2(TAR)2]3– [19].

3.4 State of the reactants and possible formulae of the extracted species

In order to determine the most likely formulas of the extracted complexes, it is necessary to know the state of the reactants in the reaction mixture.

TAO is a triprotic acid (H3TAO+) giving up its first proton (that at the heterocyclic nitrogen) in an acidic medium. Under the experimental pH it probably exists mainly as H2TAO with two protonated oxygen atoms. According to Menek et al. [49], the pK2 and pK3 values of TAO for aqueous medium are 5.7 and 11.8, respectively (pK2 refers to the p-OH and pK3 refers to the o-OH). However, in the presence of a cationic surfactant these pKa values get lower due to ion-pair formation [49].

TTC dissociates to TT+ in aqueous solutions and shown in our previous work [50], this salt also decreases the pKa values of reagents involved in the complexation processes.

[VO(H2O)5]2+ is the most common VIV form in acidic aqueous solution. With increasing pH, hydrolyzed species, such as [VO(OH)(H2O)4]+ and [(VOOH)2(H2O)n]2+, can be stable [30]. At pH 5.0, water-insoluble VO(OH)2 starts to form, which turns into soluble [(VO)2(OH)5] and [VO(OH)3] species with further increase in alkalinity [51].

The experimentally determined TTC-to-VIV molar ratio (1:1), means that the extracted ternary complexes are ion-associates between TT+ and monoanions. The following anionic species seem possible taking into consideration the experimental conditions: [VIV(OH) (TAO)2], [VIVO(HTAO)(TAO)] or [VIVO(OH)(HTAO)2] (when nTAO : nV = 2 : 1), and [VIVO(OH)TAO]- (when nTAO : nV = 1 : 1). Since the molar absorbability under the optimal extraction-spectrophotometric conditions is relatively high (see Table 1), the existence of [VIVO(OH)(HTAO)2], in which both the TAO ligand are protonated, is not very likely. The same applies to the complex [VIVO(OH)TAO], which dominates only at low TAO concentrations and pH values higher than ca. 5.

Table 1

Extraction-spectrophotometric optimization of the VIV – TAO – TTC – water – chloroform system.

ParameterOptimization rangeOptimal value/rangeFigure
Wavelength, nmVisible range545Fig. 1
pH of the aqueous phase3.6 – 6.44.9Fig. 2
Concentration of TAO, mol dm–3(0.60 – 14.3)×10–45.4 ×10–4Fig. 3a
Concentration of TTC, mol dm–3(0.24 – 14.1)×10–49.4×10–4Fig. 3b
Extraction time, sec.5 – 240120-

3.5 Possible structures and HF optimization

When discussing the possible structures and their optical properties, it should be taken into account that: (i) the V=O bond is strong and difficult to break [30]; (ii) square pyramidal complexation is a favored coordination mode, with the VO bond projecting vertical to the plane of the remaining coordinating atoms [30]; (iii) VIV-TAR complex in water-ethanol medium most probably contains a coordinated -OH group instead of bare oxygen [23]; (iv) ligands, similar to TAO are considered tridentate [25, 26, 27, 28] and (v) the spectral characteristics in the visible range of the ion-association complexes of TT+ are governed primarily by the anionic moiety [40].

Keeping the above in mind, we optimized the ground state equilibrium geometry of the monoanionic chelates which match to the 2 : 1 (TAO : VIV) composition requirement. We started the optimization process with two different mutual arrangements of the ТАО ligands – coplanar and perpendicular. In the course of optimization, we found that some of the structures with perpendicular ligands (TAO2– or HTAO) are unstable; the oxygen atom of the VO group releases, which in turn changes the charge requirement.

The ground state equilibrium geometries of the remaining structures are shown in Figure 7 where one of the TAO ligands is tridentate and the second is monodentate. In all three structures, VIV occupies the center of a square pyramid with oxygen O18 at the apex and nitrogen and oxygen atoms (N12, N9, O7 and O32) in the distorted basal plane. N12, N9 and O7 belong to the tridentate ligand. The monodentate TAO is bound to VIV through O 32, the oxygen atom in ortho position to the azo group.

Figure 7 Ground state equilibrium geometries of the monoanionic chelates.
Figure 7

Ground state equilibrium geometries of the monoanionic chelates.

In structure I, the oxygen in the para position to the azo group of the monodentate TAO is protonated (O33–H44). In this structure, the two TAO ligands are almost coplanar and the coordination polyhedron is the least distorted.

In structure II, the oxygen of the VO group is protonated (O18–H21) and the hydrogen atom forms a weak H-bond (H21...N37 = 2.227 Å) with the heterocyclic nitrogen of the monodentate ligand.

Structure III appears to be the most stable. А stronger hydrogen bond N37–H44...O18–V17 (1.688 Å) is visible in it. This bond is part of a 10-membered ring (consisting of V17, O18, H 44, N37, C36, N35, N34, C29, C30 and O32), which additionally stabilizes the coordination entity. It is important to note that the existence of a hydrogen atom (H44) attached to the heterocyclic nitrogen is unusual since it should be the first hydrogen that separates [27, 49, 52] from the isolated ligand (H3TAO+) by increasing the pH of the solution. As mentioned above, under the optimum acidity (pH 4.9), the reagent is predominantly in its neutral form (H2TAO) with hydrogen atoms at the two oxygens.

3.6 Comparison between theoretical and experimental spectra

The next step in elucidating the structure of the extracted complex was the comparison between the normalized experimental spectrum (Figure 8, full red line) and theoretical spectra of structures I, II and III. The theoretical HF/3-21G* spectra were modeled by calculated TD electron transition lines and a Lorentzian broadening of the bands. A scale factor of 1.1 was applied. This relatively small factor provides good correlation between the experimental spectrum and that of the structure III (dotted green line). The spectra of structures I and II are rather different. The corresponding maxima are blue shifted by almost 170-180 nm. Most probably they are not formed in noticeable amounts at the optimum extraction conditions. However, we consider that structure II, which has some similarities to structure III (10-membered ring involving a H-bond), is responsible for the abnormal course of series 1 in Figure 4a due to complications induced by the presence of vanadium as VO(OH)2 at pH > 5.

Figure 8 Comparison of the normalized experimental spectrum (full line) and theoretical HF/3-21G* spectra of Structures I, II and III (dotted lines) scaled by a factor of 1.1.
Figure 8

Comparison of the normalized experimental spectrum (full line) and theoretical HF/3-21G* spectra of Structures I, II and III (dotted lines) scaled by a factor of 1.1.

3.7 Ground state equilibrium geometry of the ternary complex

The results obtained allowed the modelling of the structure of the entire ion-association complex. For this purpose, we combined structure III with the pre-optimized structure of the tetrazolium cation [16, 53]. In addition to the HF calculations, we carried out calculations based on DFT. The ground state equilibrium geometry of the ionassociate optimized by the BLYP/3-21G* method is shown in Figure 9. Selected bond lengths and angles obtained by both methods are listed in Table 2. The results obtained by DFT are considered to be more reliable and are consistent with the expected length of the V=O bond (1.6 Å) [30]. The distortion of the basal plane (N12, N9, O7, O32) obtained by this method is smaller and the closest interaction contacts between the complex anion [VО(HNТАО)(TAO)] and TT+ are shorter: О26 ... H82 (2.109 Å) and N10 ... H84 (2.657 Å). It should be mentioned that these distances are shorter than the closest contacts reported for TT+-containing ionassociates involving anions, such as [PCl6], [PO2Cl2], [NbCl6], [NbOCl4(CH3CN) ], [SbCl6] and [CuCl4] [54, 55, 56, 57].

Figure 9 The optimized ground state geometry of the ion-associate.
Figure 9

The optimized ground state geometry of the ion-associate.

Table 2

Selected bond lengths (Å) and angles (°) calculated by HF and DFT methods using 3-21G* basis set.

О26 ... H822.1092.438
N10 ... H842.6573.336

3.8 Extraction-spectrophotometric characteristics

On the basis of the studies carried out, it can be concluded that the formation and extraction of the ternary complex under the optimal conditions can be expressed by the following equation:


The equilibrium constant characterizing this equation was calculated by two methods: mobile equilibrium method [46, 58] (Log Kex = 4.0 ± 0.4; N = 4) and Holme-Langmyhr method [59] (Log Kex = 4.1 ± 0.2; N = 7). As can be seen, the results are statistically identical.

The apparent molar absorptivity calculated by the saturation method was εmax = 1.75×104 dm3 mol–1 cm–1 (Figure 4). It is about 11% lower than that of the VV complex with the same reagents, εmax = 1.97×104 dm3 mol–1 cm–1 [16]. A similar decrease can be noted when comparing the εmax values of VIV and VV complexes with other ADs and monotetrazolium salts [29, 34].

4 Conclusions

The principal complex formed in the liquid-liquid extraction-chromogenic system VIV-TAO-TTC has a composition of 1:2:1 and can be represented by the formula (TT+)[VO(HNTAO)(TAO)]. The complex anion [VO(HTAO) (TAO)] has a relatively intense red coloration, while the tetrazolium cation TT+ is bulky and determines the ease of extraction into chloroform. With the help of theoretical calculations and simulated spectra, it was shown that one of the TAO ligands is monodentate. It is protonated at the heterocyclic nitrogen and the hydrogen forms a H-bond (1.695 Å) with the oxygen that is in the apex of the distorted square pyramid containing VIV in its center. Тhe closest interaction contacts between the complex anion [VО(HТАО)(TAO)] and TT+ are two weak H-bonds formed between hydrogen atoms of the phenyl substituents in the tetrazolium ring and (i) the oxygen in para-position relative to the azo group of the monodentate TAO (2.109 Å) and (ii) the noncoordinating nitrogen of the azo group of the tridentate ligand (2.657 Å).

The present investigations can assist in clarifying some ambiguities and apparent inconsistencies related to chemistry of VIV. They are a good basis for explaining experimental facts with the formation of a hydrogen bond of the type N-H...O, where N is an azo dye heterocyclic nitrogen.


Published with a grant under Project № BG05M2OP001-2.009-0025, “Doctoral training at MU-Plovdiv for Competence, Creativity, Originality, Realization and Academism in Science and Technology - 2 (DOCTORANT - 2)”, funded under the Operational Programme “Science and Education for Smart Growth”, co-funded by the Structural and Investment Funds of the EU.

  1. Conflict of interest: Authors declare no conflict of interest.


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Received: 2018-06-25
Accepted: 2019-03-30
Published Online: 2019-09-25

© 2019 Kiril B. Gavazov et al., published by De Gruyter

This work is licensed under the Creative Commons Attribution 4.0 Public License.

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