Some Ternary Phenylmethoxybis ( tetrazolium ) Complexes of Vanadium ( IV , V ) and Their Constants of Association

Several liquid-liquid extraction systems containing vanadium {vanadium(IV) or vanadium(V)}, azoderivative of resorcinol {ADR: 4-(2-pyridylazo)-resorcinol (PAR) or 4-(2-thiazolylazo)-resorcinol (TAR)} and (phenylmethoxibis)tetrazolium salts {MBT: 3,3'-(3,3'-dimetoxy-4,4'-biphenylene)-bis(2,5diphenyl-2H-tetrazolium) chloride (Blue Tetrazolium, BT) or 3,3'-(3,3'-dimetoxy-4,4'-biphenylene)bis[2,5-di(4-nitrophenyl)-2H-tetrazolium] chloride (Tetranitroblue Tetrazolium, TNBT)} were studied. The optimum operating conditions and compositions of the extracted species were found. The constants of association (β) between MBT and the anionic V–ADR chelates (Chel) were calculated. Previously reported negative linear relationship between log β and the number of nitro groups included in tetrazolium cation (Nnitro) was discussed taking into account the obtained results and literature data for V–ADR–MBT complexes of the same class, in which MBT is BT or 3,3'-(3,3'-dimetoxy-4,4'-biphenylene)bis[2-(4nitrophenyl)-5-phenyl-2H-tetrazolium] chloride (Nitroblue Tetrazolium, NBT). (doi: 10.5562/cca1885)


INTRODUCTION
Tetrazolium salts (TS) are compounds possessing a five-membered ring of one carbon and four nitrogen atoms; one of these atoms bears a positive carge.0][11] That is why the estimation of constants, which characterize the process of association (Equation 1) between the tetrazolium cation T m+ and various anionic chelates [Chel] n-is worth. - Continuous investigations on IACs of nine different commercially available tetrazolium salts revealed that at least two factors influent noticeable on the values of β: molecular mass T ( ) m M  and the presence of nitrophenyl substituent(s) in the tetrazolium ring. 10,11,18- 20Alexandrov et al. 11,18 disclosed the first factor: the higher the molecular mass, the higher the association constant.However, some further investigations showed that this statement is not valid for IACs, which include NO 2 group(s) containing T m+ . 10,19,20In fact, the values of β of such IACs are considerably lower than the expected ones according to literature. 11,18Next step of investigation on the problem was to compare the values of β for IACs with T m+ , which differs only by the number of NO 2 groups (N nitro ). 21For this purpose, the association constants of IACs between some phenylmethoxybis(tetrazolium) cations (MBT 2+ ), in which N nitro = 0, 2, and 4 (Table 1), and the vanadium(V) -4-(2-pyridyl-azo)-resorcinol anionic chelate 10,21,22 [VO 2 (PAR) 2 ] 3-were determined.The obtained results suggested that a linear relationship exists between log β and N nitro : the higher N nitro , the lower log β Croat.Chem.Acta 85 (2012) 53.

Procedure for Establishing the Optimum Extraction-Spectrophotometric Conditions
Aliquots of vanadium solution (V(IV) or V(V), 0.5-2.0cm 3 ), buffer solution (3 cm 3 ), ADR solution (up to 1.5 cm 3 ) and MBT solution (up to 1.5 cm 3 ) were introduced into 125-cm 3 separatory funnels.The resulting solutions were diluted with distilled water to a total volume of 10 cm 3 .Then 10 cm 3 of chloroform was added and the funnels were shaken for 0.5-2 min.After phase separation, the organic layer was homogenized.A portion of the extract was transferred through a filter paper into a cell and the absorbance was read against a simultaneously prepared blank.

Procedure for the Determination of ADR/V and MBT/V Molar Ratios
The ADR/V and MBT/V molar ratios were determined by the procedure mentioned above according to the equilibrium shift method 26 and the method of Asmus. 26he experiments were performed at the optimum pH and optimum MBT concentration (2 × 10 -4 mol dm -3 ; at determining the ADR/V molar ratios) or optimum ADR concentration (2 × 10 -4 mol dm -3 ; at determining the MBT/V molar ratios).The concentration of V(IV) or V(V) was 10 times smaller (2 × 10 -5 mol dm -3 ).

Procedure for Determination of the Constants of Association
The constants of association β were determined according to the method of Komar-Tolmachev. 26The procedure mentioned above (procedure for establishing the optimum extraction-spectrophotometric conditions) was used with the following peculiarities: 1) The samples were run at the optimum pH, optimum ADR concentration and different vanadium concentrations (from 1 × 10 -5 to 4 × 10 -5 mol dm -3 ); 2) The MBT/V concentration ratio was maintained to be n/m (see Equation 1).

Vanadium(V)-TAR-MBT-Water-Chloroform Systems
It is known that V(V) forms ternary complexes with TAR and BT 2+ or NBT 2+ , 20 which could be represented with the general formula (MBT) 3 [VO 2 (TAR) 2 ] 2 .The absorption maximum of both the complexes is at λ = 550 nm, however there are some differences in the molar absorptivity and pH interval of maximum extraction (Table 2).The present investigations on the vanadium(V)-TAR-TNBT-water-chloroform system showed that λ max of the extracted ternary complex is also at 550 nm (Figure 2, curve 1).The TAR/V(V) and TNBT/V(V) molar ratios in it were determined to be 2 and 1.5, respectively.These experimental results give us ground to represent the complex with the general formula mentioned above.Taking into account the state of V(V) 27 and TAR 28 in aqueous solutions at the optimum pH we suggest the following scheme of complex formation:    H VO 2H (TAR)

Vanadium(IV)-PAR-MBT-Water-Chloroform Systems
The complex formation in the systems V(IV)-PAR-NBTwater-chloroform and V(IV)-PAR-BT-water-chloroform has been studied in Reference 23.A ternary complex with λ max = 559 nm and a presumable formula (NBT) 2 [VO-(OH) 2 (PAR) 2 ] has been found in the former system.At similar extraction conditions, the system with BT has been found more complicated.At least two different complex species have been detected into the organic phase.The dominant complex at the optimum extractionspectrophotometric conditions (Table 2) most probably is similar to those with NBT: (BT) 2 [VO(OH) 2 (PAR) 2 ]. 23 It should be mentioned that at pH ≥ 5.1 V(IV) in both the systems, V(IV)-PAR-NBT and V(IV)-PAR-BT, is partially oxidised by the atmospheric oxigen to V(V).Such an oxidation has been observed in other V(IV)-PAR containing systems; [29][30][31] it could be avoided if special precautions are taken: inert atmosphere, 30 low temperature and flushing the solutions with nitrogen. 32ur present investigations on the vanadium(IV)-PAR-TNBT-water-chloroform system showed that the optimum pH range for extraction of V(IV) is pH = 4.0 to 4.9 (Figure 3, curve 4), λ max appears at 560 nm (Figure 2, curve 4), and both the molar ratios PAR:V(IV) and TNBT:V(IV) are 2:1.Having in mind the state of V(IV) 33 and PAR 28,34 at the optimum acidity, and the literature 23 we suggest the following scheme of complex formation:

Vanadium(IV)-TAR-MBT-Water-Chloroform Systems
The complexes formed in the V(IV)-TAR aqueous system are more unstable than these in the systems V(V)-TAR 35 and V(IV)-PAR. 30They are considerably affected by hydrolysis at pH = 4.5 to 5.5, 24,35 and have no analytical application. 36,37On the other hand the association constants of T m+ -(V-TAR) n-are higher than those of the corresponding T m+ -(V-PAR) n-. 10,24Having in mind the tendency to hydrolysis and relative instability of the species V(IV)-TAR, it is not surprising that ternary complexes with different compositions have been obtained, varying T m+ , pH and reagent concentrations. 24ur investigations on the V(IV)-TAR-TNBTwater-chloroform system showed that the ternary complex formation could be represented as in the similar system with NBT: 24 Evidences for the formation of such a complex are the molar ratios TAR/V(IV) and TNBT/V(IV) equal to 2, and the identical character of the spectral curves of V(IV)-TAR-TNBT and V(IV)-TAR-NBT 24 in chloroform.
The conclusions drawn at studing the similar system with BT, however, were different.TAR/V(IV) and BT/V(IV) ratio in the ternary complex were 1:1 and the absorbance maximum appeared at different wavelength: 555 nm (Figure 2, curve 2).Taking into account the obtained results, we suggest the following scheme of complex formation at pH ≈ 5: In spite of the fact that V(IV) in TAR-T m+ systems has been found less susceptible to oxidation than in the corresponding systems with participation of PAR 24 the formation of minor V(V)-TAR anionic species cannot be excluded at pH ≈ 5.5.

Relationship Between log β and Number of NO 2 Groups
The constant of association characterizing the equations 2', 3', 4', and 5' were determined at 3:2, 2:1, 2:1, and 1:1 MBT/vanadium concentration ratios, respectively.Calculated values are shown in Table 2 and the relationships between log β and N nitro for the systems described above are represented in Figure 1, straight lines 2-4.
Straight lines 2 and 3 show the dependences for the associates between MBT 2+ and [VO 2 (TAR) 2 ] 3-or [VO-(OH) 2 (PAR) 2 ] 4-, respectively.The calculated correlation coefficients (CC) are close to -1, what supports the conclusion 21 that negative linear relationship exists between log β and N nitro : the higher N nitro , the lower log β.
The trend of decreasing log β with increasing N nitro could be seen in Figure 1 1, straight line 4' with a slope of b = -0.800)show that log β value for this hypothetical complex should be close to 12.2.The mentioned value is higher than these for the reported 10 real tetrazolium associates T n [Chel] m with n/m = 2.Such a value corresponds to a very short distance between the cationic and anionic parts of the ionassociate. 38Therefore, steric factors may act, preventing the formation of BT 2+ -complex with the abovementioned formula.
The complex with 1:1 BT 2+ -to-[VO(OH) 2 (TAR)] 2- ratio is the only experimentally established member of the (MBT)[VO(OH) 2 (TAR)] series.Calculations based on the straight-line equation log β = bN nitro + a (dashed straight line 4'' constructed arbitrary with the same slope of -0.800; Figure 1) show that the rest two members of this series should be very unstable with log 2.6 and log 1.0.
The mentioned values are lower than these for the normally existing associates T n [Chel] m with m = n = 1. 10 Having in mind these calculations one can conclude that the slope b of the stright line log β = bN nitro + a is an important characteristic of the system MBT 2+ -[Chel] n-.Probably b may serve as an indicator for the stability of [Chel] n-and the posibility of existence of BT 2+ , NBT 2+ and TNBT 2+ complexes with the same composition.[40][41][42]

CONCLUSIONS
Vanadium forms well extractable into chloroform ternary complexes with ADR and MBT.The extractionspectrophotometric properties and analytical applicability of these complexes depend on several factors, including stability of both: anionic chelate and ionassociate.Vanadium(V) forms stable anionic chelates with ADR.In presence of MBT 2+ these chelates converts into ternary complexes with colour properties depending on the stability of associate.Because, the order of stability decreases with the increasing of N nitro the complexes of BT 2+ are more appropriate for analytical applications that these of TNBT 2+ and NBT 2+ .
Vanadium(IV) forms less stable ADR anionic chelates than vanadium(V), which are affected by hydrolysis.In presence of MBT 2+ these chelates also convert into ternary complexes, however, the nature of MBT 2+ is a key factor determining the stabilization of one anionic species or another.The chemical behavior of TNBT 2+ is close to that of NBT 2+ , most probably due to the presence of NO 2 .At the same time, BT 2+ is different in some extent; it could stabilize different anionic species in the ion-associate.
The present investigations confirm the conclusions given in Reference 21 for the existence of negative linear relationship between log β and N nitro .At the same time, they contribute to a better understanding of some aspects concerning the chemistry of MBT and may serve as an improved tool for the prediction of some key characteristics of uninvestigated or partially investigated MBT complexes: stability, composition, color properties and analytical applicability.

Table 1 .
Phenylmethoxybis(tetrazolium) chlorides (MBT) used or discussed in the present study

Table 2 .
Optimum conditions(a)and characteristics of the extracted species in the vanadium azoderivative of resorcinol-(phenylmethoxybis)-tetrazolium systems