Thermodynamic parameters of the complexation of trivalent lanthanides by monocyclic and bicyclic cryptands and the tripodal ligand tris(3,6-dioxaheptyl)amine in propylene carbonate

Abstract

Complexation of lanthanide cations by the monocyclic trioxa (2.1) and tetraoxa (2.2) cryptands, the bicyclic tetraoxa (2.1.1), pentaoxa (2.2.1), and hexaoxa (2.2.2) cryptands, and the related tripodal ligand tris(3,6-dioxaheptyl)amine (tdoha) in propylene carbonate has been investigated at various temperatures by a competitive potentiometric method with Ag⁺ as auxiliary cation. Values of the stability constants of the 1 : 1 cryptates and the 1 : 1 and 1 : 2 tdoha complexes are given for eight cations from La³⁺ to Yb³⁺, as well as the complexation enthalpies and entropies for La³⁺, Er³⁺, Pr³⁺, and Eu³⁺ complexes. The ligand tdoha is a weaker complexing agent than all cryptands but a stronger one than related crown ethers. For all nitrogen-containing ligands, the stabilities of the complexes increase with the atomic number of the lanthanides, whereas they decrease for the polyethers. All complexes are enthalpy stabilized, the entropic contributions being favourable to the complexation in some cases, and unfavourable in some others. The macrobicyclic effect is important if 2.2.1 is compared to 2.1, and entirely of enthalpic origin, but much weaker if 2.2.2 is compared to 2.2, due to unfavourable enthalpic contributions counterbalancing favourable entropic factors.