Composition, Stability and Probable Structure of a Colourless Organometallic Complex (Gd(III)-Malic Acid)

Abstract

<p align="justify"> The formation of colourless gadolinium complexes (<i>x</i>,<i>y</i>,<i>z</i>), between x gadolinium ions, y ligands and z protons, of some organic acids, has been studied in aqueous solution. In this work we present the results of investigations on the interaction of the gadolinium ion (<i>Gd³⁺</i>) with malic acid (C₄H₆O₅, <i>a</i>-hydroxyl dicarboxylic acid), in dilute aqueous solution for pH values between 5.5 and 7.5. Colourless gadolinium complexes of malate ions have no absorption band UV-visible, the indirect photometric detection (IPD) technique was used and studies have identified a major tri-nuclear complex of malate ion (<a name="OLE_LINK2"></a>^﹣OOC-CH2-CHOH-COO^﹣). The formation of this new colourless complex is derived from three Gd(III) ions that react with two malate ions and two hydronium ions (H₃O⁺), giving for this colourless complex, a (3,2,2) composition and apparent stability constant depends on the acidity of the medium, with <i>logK'322</i> = 18.88 ± 0.05 at pH = 6.30. To complement previous results and to propose a probable structure for this new complex detected in solution, studies of IR spectroscopy have been conducted to identify the chelation sites for both ligands. The results were analysed and show that this organometallic gadolinium complex, contains two different sites, respectively, two lateral tetradentate mono-nuclear sites and a single central bidentate mono-nuclear site. From these results, the reaction of formation, the stability constant and the probable structure of this new colourless organometallic gadolinium complex are proposed. </p

Novel gadolinium complexes in aqueous solution : characterization, identification and probable structures

“…In this sense and in the my works published [7][8][9][10] we have demonstrated, experimentally, that the composition and stability of the colored sacrificial complex were determined, we found a complex type (3, 2, 3) and stability constant 16.27 323 logK  in pH range 5.50 -7.50, and the probable reaction for this complex faormation is :…”

“…The absence of doubling new peaks, which appeared, gives us the idea that complexes structures are symmetrical. So the general formulas of the three characterized complexes are : [8][9][10], [28], [34]. The complexes identified, in this work, are the type (2,2,2) for system Gd-mandelate and (3,2,4) for Gd-Serine complex.…”

“…Table 3 based on bibliographic studies very powerful [8], [9], [25][26][27]. Among the Raman spectra obtained are the free mandelic acid and its complex formed in solution, which is represented in Figure 5.…”

“…Thus, the importance of IPD technique was also revealed by its adaptation to other techniques of separation and determination, such as liquid chromatography [3], [4], capillary electrophoresis [5] and continuous flow analysis (FIA) [6]. Some studies show that this technique is very effective in identifying some colorless gadolinium [7], [8], [9] and tungstate complexes of sugars and organic acids [10]. The detection or monitoring of certain diseases, sometimes requires injection gadolinium complexes because of the interesting electro nic and magnetic properties of this ion [11], [12].…”

“…In this structure, there is formation of two identical sites tridentate mononuclear, each site requires one proton H + . The OH group in α position participate in chelations sites without protonation [9], [28], [34], [35]. This structure is the most probable, because, if we assume that the mandelate does not rotate to form this symmetrical structure, we obtained a new structure with two sites : the first one is bidentate mononuclear, that not requires H + protons (Gd between OH in α position of two ligands) and the second site is mononuclear tetradentate with the intervention of one H + proton (2 nd Gd between the four oxygens of two functions COOH), that is which means the formation of this complex requires only one H + proton.…”