Interaction of Rare-Earth Metals and Some Perfluorinated β-Diketones

In this work, we demonstrate the fundamental relationships between stability constants and periodic, acid–base, and structural parameters for complexes of some 1,3-diketones. The four analogues of hexafluoroacetylacetone—2-thenoyltrifluoroacetone, 2-furoyltrifluoroacetone, benzoyltrifluoroacetone, and 2-naphthyltrifluoroacetone—have been studied as chelating ligands for 16 rare-earth metals (Sc, Y, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu) in aqueous solutions. Systems have been investigated spectrophotometrically using a multiwave nonlinear least-squares regression algorithm for data processing. Conditional stability constants were obtained for a wide pH region (2.0–5.4) at constant ionic strength (I = 0.5 M, NaCl). To receive the apparent ("true") equilibrium parameters, acid–base and keto–enol characteristics of the studied ligands have been described and revised for specific conditions. Dissociation constants were obtained in citrate–phosphate buffer media and protonation parameters were received in concentrated hydrochloric acid by the Cox–Yates method. The apparent formation constants for monocomplex species were obtained as thermodynamic invariants (depend only on the temperature) for each ligand and lie from 4.2 to 12.7 logarithmic units. Although the studied ligands have similar values of pKa, the stabilities of their complexes vary considerably. Systematic analysis of 64 apparent stability constants demonstrates that the force of interaction between the metals and nonsymmetric β-diketones increases as 2-furoyltrifluoroacetone < 2-thenoyltrifluoroacetone < benzoyltrifluoroacetone < 2-naphthyltrifluoroacetone. The studied ligands display varying degrees of the correlation between the periodic parameters and formation constants. Naphthyltrifluoroacetone and its complexes with heavy lanthanides exhibit a clear trend in properties with increasing ionic potential. In general, the received set of data can be described from purely electrostatic grounds within the framework of the periodic law. Spectral, keto–enol, acid–base, and complexing properties were reproduced using density functional theory modeling and explain some of the regularities discovered.