Extractive Spectrophotometric Determination and Theoretical Investigations of Two New Vanadium(V) Complexes

Abstract

Two new vanadium (V) complexes involving 6-hexyl-4-(2-thiazolylazo)resorcinol (HTAR) and tetrazolium cation were studied. The following commercially available tetrazolium salts were used as the cation source: tetrazolium red (2,3,5-triphenyltetrazol-2-ium;chloride, TTC) and neotetrazolium chloride (2-[4-[4-(3,5-diphenyltetrazol-2-ium-2-yl)phenyl]phenyl]-3,5-diphenyltetrazol-2-ium;dichloride, NTC). The cations (abbreviated as TT⁺ and NTC⁺) impart high hydrophobicity to the ternary complexes, allowing vanadium to be easily extracted and preconcentrated in one step. The complexes have different stoichiometry. The V(V)-HTAR-TTC complex dimerizes in the organic phase (chloroform) and can be represented by the formula [(TT⁺)[VO₂(HTAR)]]₂. The other complex is monomeric (NTC⁺)[VO₂(HTAR)]. The cation has a +1 charge because one of the two chloride ions remains undissociated: NTC⁺ = (NT²⁺Cl^-)⁺. The ground-state equilibrium geometries of the constituent cations and final complexes were optimized at the B3LYP and HF levels of theory. The dimer [(TT⁺)[VO₂(HTAR)]]₂ is more suitable for practical applications due to its better extraction characteristics and wider pH interval of formation and extraction. It was used for cheap and reliable extraction-spectrophotometric determination of V(V) traces in real samples. The absorption maximum, molar absorptivity coefficient, limit of detection, and linear working range were 549 nm, 5.2 × 10⁴ L mol^-1 cm^-1, 4.6 ng mL^-1, and 0.015-2.0 μg mL^-1, respectively.

Keywords: 6-hexyl-4-(2-thiazolylazo)resorcinol; HF and B3LYP calculations; liquid–liquid extraction; spectrophotometric determination; tetrazolium; vanadium.

Figure 1

Structural formulae of the reagents TTC (a), NTC (b), and HTAR (c).

Figure 2

Absorption spectra of the V(V)-HTAR-TTC complex (1) and V(V)-HTAR-NTC complex (2) against corresponding blanks. The spectra of the blanks (1′ and 2′) are recorded against chloroform. c_V(V) = 2 × 10⁻⁵ mol L⁻¹, c_HTAR = 8 × 10⁻⁵ mol L⁻¹ (1, 1′) or 4 × 10⁻⁵ mol L⁻¹ (2, 2′), c_TTC =2.4 × 10⁻⁴ mol L⁻¹, c_NTC =1.4 × 10⁻⁴ mol L⁻¹, and pH 5.0 (ammonium acetate buffer).

Figure 3

Absorbance of the complexes (1, 2) and blanks (1′, 2′) in chloroform vs. pH of aqueous phase. c_V(V) = 2 × 10⁻⁵ mol L⁻¹, c_HTAR = 8 × 10⁻⁵ mol L⁻¹ (1, 1′) or 4 × 10⁻⁵ mol L⁻¹ (2, 2′), c_TTC = 2.4 × 10⁻⁴ mol L⁻¹, c_NTC = 1.4 × 10⁻⁴ mol L⁻¹, λ = 549 nm (1, 1′), or 556 nm (2, 2′).

Figure 4

The effect of extraction time (t_ex). c_V(V) = 2 × 10⁻⁵ mol L⁻¹, c_HTAR = 8 × 10⁻⁵ mol L⁻¹ (1) or 4 × 10⁻⁵ mol L⁻¹ (2), c_TTC =2.4 × 10⁻⁴ mol L⁻¹, c_NTC =1.4 × 10⁻⁴ mol L⁻¹, and pH = 4.7.

Figure 5

The effect of HTAR concentration. c_V(V) = 2 × 10⁻⁵ mol L⁻¹, c_TTC =6 × 10⁻⁴ mol L⁻¹, c_NTC =4 × 10⁻⁴ mol L⁻¹, and pH = 5.0.

Figure 6

The effect of TS concentration. TS = TTC (1), TS = NTC (2). c_V(V) = 2 × 10⁻⁵ mol L⁻¹, c_HTAR = 1 × 10⁻⁴ mol L⁻¹ (1) or 4 × 10⁻⁵ mol L⁻¹ (2), and pH = 5.0.

Figure 7

Determination of the HTAR:V(V) (a) and TTC:V(V) molar ratios in the V(V)–HTAR–TTC complex via the mobile equilibrium method. c_V(V) = 2 × 10⁻⁵ mol L⁻¹c_TTC = 3 × 10⁻⁴ mol L⁻¹ (a), and c_HTAR = 8 × 10⁻⁵ mol L⁻¹ (b).

Figure 8

Determination of the HTAR:V(V) (a) and NTC:V(V) molar ratios in the V(V)–HTAR–NTC complex via the mobile equilibrium method. c_V(V) = 2 × 10⁻⁵ mol L⁻¹, c_NTC = 4 × 10⁻⁴ mol L⁻¹ (a), and c_HTAR = 4 × 10⁻⁵ mol L⁻¹ (b).

Figure 9

Determination of the TTC:V(V) molar ratio in the V(V)–HTAR–TTC complex via the dilution method. c_TTC = c_V(V), b₀ = 3 × 10⁻⁵ mol L⁻¹, c_HTAR = 8 × 10⁻⁵ mol L⁻¹, and pH = 4.7.

Figure 10

Job’s method of continuous variations and Likussar–Boltz approach for the determination of K_ex in the V(V)—HTAR—TTC system (a) and V(V)—HTAR—NTC system (b). k = c_V(V) + c_TS = 4 × 10⁻⁵ mol L⁻¹, c_HTAR = 8 × 10⁻⁵ mol L⁻¹ (a) or 4 × 10⁻⁵ mol L⁻¹ (b), and pH = 4.7.

Figure 11

Optimized ground-state structures of the ions: TT⁺ (B3LYP/6-311++G**) (a), NTC⁺ (B3LYP/6-31+G*) (b), and [VO₂(HTAR)]⁻ (B3LYP/6-311++G**) (c).

Figure 12

Optimized ground-state structures of the (NTC⁺)[VO₂(HTAR)]⁻ ion-association complex at the HF/3-21G theoretical level: (a) Structure 1, E₁ = −4706.2401 a.u., G₁ = −4705.3824 a.u., H₁ = −4705.2128 a.u.; (b) Structure 2, E₂ = −4706.2345 a.u., G₂ = −4705.3779 a.u., H₂ = −4705.2073 a.u.

Figure 13

Optimized ground-state structures of the (TT⁺)[VO₂(HTAR)] monomer at the HF/3-21G theoretical level: (a) Structure M1, E₁ = −3309.4733 a.u., G₁ = −3308.8904 a.u., H₁ = −3308.7693 a.u.; (b) Structure M2, E₂ = −3309.4656 a.u., G₂ = −3308.8823 a.u., H₂ = −3308.7619 a.u.

Figure 14

Optimized ground-state structures of the dimer [(TT⁺)[VO₂(HTAR)]]₂ at the HF/3-21G theoretical level: (a) Structure D1, E₁ = −6618.9481 a.u., G₁ = −6617.7571 a.u., H₁ = −6617.5383 a.u.; (b) Structure D2, E₂ = −6618.9658 a.u., G₂ = −6617.7737 a.u., H₂ = −6617.5555 a.u.; (c) Structure D3, E₃ = −6618.9454 a.u., G₃ = −6617.7531 a.u., H₃ = −6617.5359 a.u.

Figure 14

Optimized ground-state structures of the dimer [(TT⁺)[VO₂(HTAR)]]₂ at the HF/3-21G theoretical level: (a) Structure D1, E₁ = −6618.9481 a.u., G₁ = −6617.7571 a.u., H₁ = −6617.5383 a.u.; (b) Structure D2, E₂ = −6618.9658 a.u., G₂ = −6617.7737 a.u., H₂ = −6617.5555 a.u.; (c) Structure D3, E₃ = −6618.9454 a.u., G₃ = −6617.7531 a.u., H₃ = −6617.5359 a.u.