Complexation of trivalent lanthanide cations by different chelation sites of malic and tartric acid (composition, stability and probable structure)

2.1 Chemicals

Malic acid, tartric acid, Chrome Azurol S (H₄Ch), Gd(III) nitrate and other chemicals were commercial products (Aldrich, Prolabo,…) of the purest available and analytical grade, used as received.

2.2 Indirect photometric detection

A standard Helios γ UV–visible spectrometer controlled by Vision 32 software was used for spectrometric measurements, using quartz cells of optical path length l = 1 cm. The absorption measurements have been performed at room temperature and at wavelength λ_max = 545 nm. Stock solutions of Gd(III) nitrate and Chrome Azurol S (H₄Ch), were prepared with concentrations of respectively 10⁻² M and 10⁻³ M. In a typical experiment, a solution (v = 50 mL) of the colored sacrificial complex (Gd(III)-H₄Ch) was prepared ([Gd³⁺]/[H₄Ch] = 1.5), using it as a buffer of MESH (0.1 M) [2-(N-morpholino) sulfonic ethane acid]. The initial solution also contained a calculated amount of 1 M NaOH in order to obtain the desired pH value (pKa_(MESH) = 6.2, experimental pH range (5.50–7.50). pH values are measured with a microprocessor pH meter HANNA 210 equipped with a combined electrode glass and calibrated with commercial buffers (pH 4.00 and 7.00). Then aliquots (v = 0.100–0.200 ml) of an aqueous solution of the malic and tartric acid (C_L = 2 g/L) were added, using a Gilson micropipet of 0.200 ml. After each addition, the resulting solution was left at least 5 min in order to reach equilibrium (and thus a constant absorption value). Addition was repeated until a maximum volume of 2.00 ml of the organic acids solution was added. The change in the total volume was neglected. For fixed pH environments, the apparent formation constants log  $K_{\mathit{xyz}}^{\prime }$ was calculated from the equilibrium of the reaction of formation of these complexes. The absorption values for the undissociated (A_F) and the totally dissociated (A_I), sacrificial complex (Gd(III)-H₄Ch), are used as determined in experiments using pure Chrome Azurol S (H₄Ch), and an excess of Gadolinium(III) (performed at pH intervals of 0.20). Assuming various integers for the Gadolinium and organic acids stoichiometry, a formation constant is calculated for each added amount of ligands and corresponding absorption value. The results are rejected when a systematic variation of log  $K_{\mathit{xyz}}^{\prime }$ occurs with increasing added-up amount of ligands or when individual values of log  $K_{\mathit{xyz}}^{\prime }$ differed from the mean value by more than 2%.

2.3 IR spectroscopy

Samples were prepared by weighing the appropriate amount of malic and tartric acid (ligands) and (Gd(NO₃)₃·6H₂O) adding H₂O, mixing and finally adjusting the pH with concentrated HCl or NaOH. Concentrations of analyzed samples are 10⁻³ M. Analyses were performed using an infrared spectrometer, Fourier transform (FT-IR) Perkin Elmer BX, equipped with a DTGS detector, a splitter and a cesium iodide window. In this configuration, the interval of analysis is that the middle infrared, 6000–250 cm⁻¹ and analysis are conducted on small samples, whose size is less than 1 mm³. Liquid samples are placed between two plates of very pure salt (KBr), these plates are transparent to infrared light and the spectra relative to free ligands and complexes have been plotted for frequencies from 4400 to 400 cm⁻¹.

2.4 FT-RAMAN spectroscopy

The complexes are precepitated quickly at room temperature, at a concentration of 1 × 10⁻² M and fixed pH values (higher than the acidity constant of acids studied). The precipitates were filtered, and dried in a drying oven. The complexes were insoluble in water, methanol and ethanol and well soluble in DMSO. The Raman spectra of the ligands (malic and tartric acid) and their new Gd(III) complexes were performed using a Raman spectrometer Fourier transform (FT-Raman) VERTEX 70 with a range of measurement (4000–50) cm⁻¹, laser source NdYag (1.064 μm), a nominal power of 500 MW, detecting Ge with high sensitivity and a resolution of 4 cm⁻¹ (64 scan). The spectra relative to free ligands (carboxylate ions) and complexed (Gd(III)-carboxylate ions), have been plotted for frequencies from 3600 to 200 cm⁻¹.

3.1 Indirect photometric detection study

The complexation reaction of x gadolinium ions (Gd³⁺) with y malate ions and y tartrate ions, and z protons (H⁺), is given by expression (I):

3.2 Formation of the colored sacrificial complex Gd-Chrome Azurol (HCh³⁻)

The majority of organometallic complexes studied are colorless. Thus the study of the formation of the sacrificial complex Gd(III)-HCh³⁻ is very important, because this steps decisive parameters (composition and stability) of these new complexes gadolinite. In this sense and in the recent work published (Riri et al., 2011) we saw that the composition and stability of the sacrificial complex were determined colorful, they found a complex type (3,2,3) and stability constant log K₃₂₃ = 16.27 by the following reaction:

3.3 The composition and stability of the Gd(III)-acids system

If the studied ligands (⁻OOC–CH₂–CHOH–COO⁻ (malate) and ^–OOC–CHOH–CHOH–COO⁻ (tartrate)) are added to a colored solution of the sacrificial complex (3,2,3), this Chrome Azurol S (HCh³⁻) complex will dissociate. To calculate the complex concentration of the HCh³⁻ ions, the absorption values of the totally complexed (A_F), and the completely dissociated (A_I) HCh³⁻ ions have to be determined. Then the following equations can be used:

Knowing [(3,2,3)] and [Gd³⁺], the concentration of the gadolinium complex under study [(x,y,z)] (balanced reaction I) can be determined using the gadolinium balance equation:

In a similar way, the concentration of the free ligand is obtained by Eq. (8):

3.4 Determination of the composition and the stability constant of these new gadolinium complexes

With an aim of determining the composition and the stability constant of the gadolinium–malate and gadolinium–tartrate complexes, we monitored the evolution of the absorbance during the disappearance of the sacrificial complex by the addition of increasing quantities of dicarboxylate ions. For a given volume (50 ml) of a solution containing sacrificial complex (3,2,3) (10⁻⁴ M), we added increasing quantities of solution (⁻OOC-CH₂-CHOH-COO⁻ (malate) and ^–OOC–CHOH–CHOH–COO⁻ (tartrate)), with known concentration. The spectrophotometric study carried out with fixed wavelength (λ_max = 545 nm), showed a reduction in the absorbance of the solution progressively with the addition of the dicarboxylate ions (Fig. 1). The dissociation of the sacrificial complex, relating to the reduction in the absorbance by the addition of malate and tartrate ions solution, is done in favor of the formation of the colorless complex between Gd³⁺ and dicarboxylate ion species.

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Figure 1

Formation of colored sacrificial complex (Gd₃HCh₂) and decomposition of this complex by addition of malic and tartric acid.

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The curves C2 and C3 in Fig. 1, clearly show that the absorbance decreases and stabilizes. This stability indicates that all Gd³⁺ ions, initially present in the solution have reacted to added malate and tartrate ions. Knowing the concentration of gadolinium ions and the quantity of (⁻OOC–CH₂–CHOH–COO⁻ and ^–OOC–CHOH–CHOH–COO⁻) ions from the added volume of the malic and tartric acid solution, necessary to reach this stage of absorption, we could determine the molar ratio q (gadolinium/carboxylate), involved in the complexation reaction. The preceding experiment was carried out for different values of pH between 5.50 and 7.50, the way in which the absorption decreases, depends on the pH of the medium and on the formation constant of the detected complex (Gadolinium–acid), as well as on the absorption values (A_I) and (A_F) of the free and totally complexed Chrome Azurol S (HCh³⁻). Analyzing the experimental data with the computer program written from the balanced reaction (I), the results confirm the reproducibility of the molar ratio “q” and to determine the composition and apparent stability constant $K_{\mathit{xyz}}^{\prime }$ (Table 1). The ratio q = [Gd³⁺]/[Ligand] = x/y is determined experimentally from a computer program written in Microsoft Excel and expressed from the reaction of complex (reaction I). This program is not a theoretical modeling. But this is a translation of the reaction steps of the balance (I) for ease of calculation. Fig. 2 Represents an example of calculating the ratio q at pH 5.60 of tartric acid complexed with gadolinium, where we inserted the two experimental values: the volumes added (V_a) of ligand and the absorbance (A) correspond for each added volume V_a, to seek the values of x and y to the value of conditional constant (log K′) which remains constant. The calculations are repeated for different experimental pH between 5.50 and 7.50. We obtained a ratio q = x/y for the two acids equal 3/2. We conducted several studies of the complexation of acids with the gadolinium ion, and we obtained values of q depending on the nature of the acid studied (q = 2/1, 2/2, 3/2, 5/2...).

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Figure 2

Example of program image used to calculate the ratio q the stability and log K′moy at pH 5.60.

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The data-processing treatment of the preceding experimental results, shows that these tri-nuclear detected complexes are formed between the gadolinium ions and carboxylate ions (malate and tartrate), resulting from the interaction of three Gd³⁺ ions equivalents with two equivalents of carboxylate species, so a molar ratio q = x(Gd³⁺)/y(L²⁻) = 3/2 in all pH range between 5.50 and 7.50.

Therefore, for the complexation reaction of Gd³⁺ ions with malate and tartrate ions at this pH range, these results and the Eqs. ()()()(1)–(3) allow us to write the following expressions.

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Figure 3

Evolution of log  $K_{32z}^{\prime }$ for gadolinium complexes.

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The Eq. (12) allowed us to calculate the stability constants of complexing reaction of acids studied with gadolinium ion, the experimental results obtained by the IPD for gadolinium–malate (log K₃₂₂ = 31.52 ± 0.05) and gadolinium–tartrate (log K₃₂₃ = 36.70 ± 0.05).

Consequently, the spectrophotometric results concerning the interaction of the Gd³⁺ ion with tartrate and malate show that the complexation reaction uses three Gd³⁺ ions, two carboxylates and fixation of two and three H⁺ protons. According to the literature (Hummel et al., 2002), the Gd³⁺ ion can be presented in various hydrolyzed forms in aqueous solution and these new tri-nuclear gadolinium complexes detected in solution at an experimental pH range are probably formed from the hydrolyzed form $\text{Gd}{(\text{OH})}_2^{+}$ . In order to confirm our results, to have more information on the nature of these new gadolinium complexes and likely to propose a probable structure for these tri-nuclear species, we carried out IR and Raman spectroscopy investigations.

4.1 Infrared spectral study

IR spectroscopic studies are used to identify different groups of malic acid and tartric acid (ligands) involved in chelation sites to form gadolinium complexes which are detected in solution. Table 2 presents the bibliographic data (Colthup et al., 1990; Jhon et al., 1979), theory vibration value of various groups: –OH, –C⚌O, –COO– and –C–C⚌O and the experimental vibration value of analyzed spectra of two free carboxylic acids and their complexes studied in this work.

The experimental spectra obtained declared in Table 2 for the analyzed samples, clearly indicate that the vibration bands of the groups:–OH,–C⚌O and COO–, have seriously reduced passing the free to complexed ligands. So, for each of the two ligands involved in the formation of these detected tri-nuclear complexes, the four oxygen atoms of the ionized carboxylic groups, participate in chelation sites, and the OH group in α,β-positions of the ionized group (–COO–) of tartric and malic acid (Riri et al., 2011). Moreover, the vibration Infra red of Gd(III)-O which is formed in the detectable complexes is located in 150–780 cm⁻¹ region with weak spectra (Kostova et al., 2007; Roeges, 1994).

4.2 Raman spectral study

A detailed analysis of vibrations in Raman spectroscopy was performed on the basis of comparison of experimental vibrational spectra obtained of malic acid, tartric acid and their gadolinium complexes. One of these FT-Raman spectra of free ligands and their complexes with Gd(III) are shown in Fig. 4. Tables 3 and 4 represent different Raman vibrations for free ligands (tartric and malic acid) and their complexes.

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Figure 4

Experimental FT-Raman spectra in the range 3600–200 cm⁻¹ of free Malic acid and Gd(III)–malate complex.

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The Raman spectra obtained show the OH vibrations (3450 ± 150 cm⁻¹) (Kostova et al., 2007; Roeges, 1994). We also see two bonds of OH group (3400 and 3210 cm⁻¹) for free tartric acid one of carboxylic groups and the other for OH in α position. Vibrations of C⚌O are located in (1680 ± 45 cm⁻¹) (Roeges, 1994; Sundaraganesan et al., 2007), the free malic acid presents two vibrations of C⚌O group (1677 and 1635 cm⁻¹) because this molecule is not symmetric, the free tartric acid presents one vibration C⚌O because this molecule is symmetric, these vibration bands of the free acids were not detected in the spectra of the complexes indicating that the deprotonated ligands form participates in the complexes. Vibrational band of the group O–C⚌O has been reduced passing from free ligand to complex. Therefore all the oxygen atoms are involved in the formation of these new gadolinium complexes. There are also vibration spectra (weak broad) of the complexes in (1630 ± 20 cm⁻¹) indicating the existence of water molecules (Horiba, xxxx), Intense spectra that are observed in the complex near 1060 cm⁻¹ indicate the existence of the nitrate ion (Kostova et al., 2007) and the vibrations located in 1048 cm⁻¹ for Gd–malate and 1046 cm⁻¹ for Gd–tartrate are vibrations of deformation of NO₂ (Sundaraganesana et al., 2005). Vibrations of the bonds formed of Gd(III)-oxygen carboxylic acids are located in 600, 547 cm⁻¹ with a weak spectra for Gd–Malic acid and in 748, 725 cm⁻¹ with a weak spectra for Gd–Tartric acid (Horiba, xxxx; Smith and Geoffrey, 2005). The links Gd(III)-oxygen of nitrate are in the vicinity of 190 ± 15 cm⁻¹ (Sundaraganesana et al., 2005). Different vibrations of C–C (aliphatic chains) appeared in a long field [(350 ± 70 cm⁻¹ and 1220 ± 100 cm⁻¹)] with intensities medium and intense intensities (Horiba, xxxx; Padmaja et al., 2006; Sundaraganesan et al., 2007). Bands in the 2950 ± 80 cm⁻¹ region were assigned to the vibration modes of CH and CH₂ (Gunasekaran and Ponnusaymy, 2005]. So, for each of the two malate ions involved in the formation of this detected tri-nuclear complexes, the four oxygen atoms of the ionized carboxylic groups, participate in chelation sites, and the OH group in α-position of the ionized group (–COO–). Therefore, these new tri-nuclear gadolinium complexes contain two types of sites, a single central bidentate mono-nuclear site with the participation of only OH groups in α,β-positions and two lateral tetradentate mono-nuclear sites, each consisting of four oxygen atoms of two ionized carboxylic functions, belonging to the two malate ions and two tartrate ion (–OOC-CH2-CHOH-COO– and –OOC-CHOH-CHOH-COO–) that are involved in the formation of these new detected gadolinium organometallic complexes. The OH group in α,β-positions of the carboxylic function is involved in the formation of new gadolinium complexes without ionization, a complex of europium ion with erythritol was synthesized in a ratio 2:2 (Yang and Xu, 2004), the authors showed that the four OH groups of erythritol were involved in the formation of the complex Eu-erythritol. Indeed, all these results can offer for these tri-nuclear complexes on the studied systems <<gadolinium–malic acid and gadolinium–tartric acid>>, and the developed structures of complexes obtained which are presented in Fig. 5.

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Figure 5

Probable structure for the new organometallic complexes “gadolinium–malate” and “gadolinium–tartrate”.

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