Potentiometric Study of Lanthanide Salicylaldimine Schiff Base Complexes

Formation of complexes between the lanthanide ions and N,N'-bis(salicylidene)-4-methyl-1,3-phenylenediamine ligand was studied in solution by pH potentiometry. The potentiometric titration was performed at 25.00 A degrees C in 0.1 mol center dot dm(-3) NaClO4 ionic strength and in DMSO:water (30:70 v:v) solvent mixture. N,N'-bis(salicylidene)-4-methyl-1,3-phenylenediamine ligand (H2L) occurs in three forms: fully or partially deprotonated and unionized. Computer analysis of potentiometric data indicated that in solution the lanthanide (Ln) complexes exist as LnL(2), Ln(HL)(2) and Ln(H2L)(2) species. This observation appears to be in contrast to the solid-state behavior of these complexes prepared in a self-assembly process and structurally defined. Stability constants for La3+, Eu3+, Gd3+, Tb3+, Ho3+ and Lu3+ (Ln(3+)) complexes were determined. The order of stabilities of LnL(2) species in terms of metal ions is La3+ > Eu3+ a parts per thousand Gd3+ = Tb3+ < Ho3+ < Lu3+ with a prominent "gadolinium break".