Lanthanide(III) complex formation with diethylenetriamine in anhydrous N,N-dimethylformamide

The formation of diethylenetriamine (dien) complexes of the lanthanide(III) ions has been studied in N,N-dimethylformamide (dmf) containing 0.1 mol dm(-3) tetraethylammoniumtrifluoromethanesulfonate as constant ionic medium at 298 K. The stability constants have been obtained by potentiometry using the method of competitive reactions, silver(I) being the auxiliary central ion. The thermodynamic investigations have therefore been extended also to this metal ion. The data concerning lanthanide(III) complexation are well explained in terms of formation of two successive mononuclear complexes which are enthalpy stabilized. entropy changes being unfavourable. Dien coordinates with all N atoms both in ML and ML2 lanthanide(III) complexes; with silver(I), tridentation is achieved only when the first mononuclear complex is formed. The trends in enthalpy and entropy values of the lanthanide(III) complexes are discussed taking into account the presence of multiple solvation equilibria which involve the bare ions and the complexes in solution. Comparison with similar data previously obtained in dimethylsulfoxide (dmso) both for the lanthanide(III)- and silver(I)-dien systems, shows the remarkable influence of the solvent on the thermodynamics of complex formation. (C) 2000 Elsevier Science B.V. All rights reserved.