Lanthanides(III) and Silver(I) complex formation with triamines in DMSO: The effect of ligand cyclization

• Published in: Inorganica Chimica Acta 2020-04, Vol.503, p.119392, Article 119392
• Main Authors: Luigi Zanonato, Pier, Di Bernardo, Plinio, Melchior, Andrea, Busato, Matteo, Tolazzi, Marilena
• Format: Article
• Language: English
• Subjects: Aqueous solutions; Calorimetry; Complex formation; Complexation; Constants; Cyclic triamine; Density functional theory; DFT; Diamines; Enthalpy; Ethane; Heat measurement; Lanthanides; Lanthanides(III); Ligands; Mathematical analysis; Methylation; Nonaqueous solvent; Silver; Silver(I); Solvation; Stability; Stability constants; Thermodynamics
• ISSN: 0020-1693; 1873-3255
• DOI: 10.1016/j.ica.2019.119392

[Display omitted] •Thermodynamics of formation for Ln(III) ions and TACN and DMDIEN is studied in DMSO.•1:1 species are always formed: with cyclic TACN also 1:2 species exist from Tb to Lu.•Negative enthalpies and entropies are found and results discussed in comparison with DIEN.•Thermodynamic/DFT integrated data show that reactions depend on different complex solvation. The thermodynamic functions of complexation between lanthanides(III) and 1,4,7-triazacyclononane (TACN) together with those of some of the above metal ions with the TACN-linear-analogue,N′-methyl-N′′-(2-(methylamino)ethyl)ethane-1,2-diamine (DMDIEN), are determined in dimethylsulphoxide (DMSO). The study aims to investigate the effect of ligand cyclization and methylation on the stability and nature of the complexes in non-aqueous solution. The stability constants of Ln3+/TACN complexes were obtained by means of potentiometric titrations at 298 K and I = 0.1 mol dm−3, using Ag+ as competitive ion. Ln3+-ligand complexation enthalpies were obtained by calorimetric titrations, extended also to the silver(I)-TACN system, whose thermodynamic data in DMSO are not published. Calorimetry was applied also to obtain simultaneously enthalpy values and stability constants of the Ln3+/DMDIEN systems, for which the difference in stabilities between Ag+ and Ln3+-DMDIEN complexes did not allow the competitive potentiometric method to work. The Ag+ ion forms mono- and poly-nuclear AgiLji+ complexes with TACN (i:j =  1:1, 1:2 and 2:3), whereas all lanthanide ions form with TACN both 1:1 complexes and 1:2, from Tb3+ to Lu3+. Only 1:1 species were detected for the Ln3+-DMDIEN systems. The 1:1 TACN complexes show an extra stability both with respect to those of DIEN and those of the linear analogue DMDIEN. Negative enthalpies favor the formation of all complexes, while entropies partially counteract the complexation. The results for TACN systems are compared with those here obtained for DMDIEN and for DIEN, reported previously. Density Functional Theory (DFT) calculations, carried out in presence of DMSO introduced as implicit solvent, were performed on some of the Ln3+ complexes with TACN and DIEN to provide insights into the structures and related solvation energies of the species formed in solution. Calculations support a tri-dentate coordination of both ligands. The effects of ligand methylation and cyclization on the stability constants, enthalpies and entropies of complex formation are discussed. The the