New Aspects of Complex Formation in the Gadolinium(III)-Citric Acid System in Aqueous Solution

• Published in: Comments on modern chemistry. Part A, Comments on inorganic chemistry Comments on inorganic chemistry, 2022-03, Vol.42 (2), p.109-144
• Main Authors: Ivanova, Valentina Yu, Shurygin, Igor D., Chevela, Vladimir V., Ajsuvakova, Olga P., Semenov, Vyacheslav E., Bezryadin, Sergey G.
• Format: Article
• Language: English
• Subjects: Aqueous solutions; Citric acid; Complex formation; Complexation; complexes stability; Coordination compounds; Gadolinium; gadolinium(III); Inorganic chemistry; Magnetic induction; Magnetic relaxation; NMR; Nuclear magnetic resonance; polynuclear complexation; Protonation; Relaxation method (mathematics); Titration
• ISSN: 0260-3594; 1548-9574
• DOI: 10.1080/02603594.2021.1976759

The complexation of gadolinium(III) with citric acid in aqueous solutions was studied by pH-metric titration, proton magnetic relaxation, and mathematic simulation in the pH range 2.0-10 at [Gd 3+ ]: [H 4 Cit] = 1:1, 1:2, 1:3 ([Gd 3+ ] = 1.3, 2.6, 5.0, 10.0, and 20.0 mmol L −1 ). In the process of simulation, the equilibrium composition, a model obtained from previously known works and including mono- and bis-citrate complexes of gadolinium(III) was taken as a basis. In this work, it is shown that a satisfactory description of the experimental data set of two independent physico-chemical methods is achieved only with the additional inclusion of citrate complexes of gadolinium(III) with higher degree of protonation and some new polynuclear complexes. The complex [GdH 4 Cit] 3+ with the molecular form of the citrate ligand, and the polynuclear complexes [Gd 2 (HCit) 2 ]°, [Gd 2 Cit 2 ] 2- , [Gd 6 (OH) 2 Cit 6 ] 8- , [Gd 6 (OH) 3 Cit 6 ] 9- were first detected at the molar ratio of 1:1. At two- and three-fold excess of citric acid, mononuclear bis- and tris-citrate complexes with different degrees of protonation were found, and binuclear tetrakis- and hexakis-citrate complexes [Gd 2 (HCit)Cit 3 ] 9- and [Gd 2 (HCit) 4 Cit 2 ] 14- at the pH > 7.5. On the example of this manuscript, the importance of applying the NMR relaxation method for identifying polynuclear complexation in systems containing paramagnetic ions is shown.