Studies on kinetics of complexation of lanthanide α-hydroxycarboxylates with 1,10-phenanthroline

Kinetics of the coordination reaction of lanthanide (La^III, Eu^III) α-hydroxycarboxylates LnL₃(H₂O)₂] with 1,10-phenanthroline (phen) in methanol-water (v/v, 3:2) were studied at 25°C by calorimetric titration. A one-step reaction process in accordance with the rate law has been suggested. The reaction is found to be first order for both lanthanide α-hydroxycarboxylates and phen. We have evaluated rate constants of the reactions. It is found that a linear free energy relationship exists between the stability constants of the lanthanide-α-hydroxycarboxylate-phen ternary complex and the rate constants. It is also found that a linear free energy relationship exists between the rate constants of La-hydroxycarboxylate with phen and the acid strength of α-hydroxy-acid as primary ligand, but the linear free energy relationship does not exist in the Eu-α-hydroxycarboxylate-phen ternary complex. The influence of other factors upon the reaction rate constants was also discussed.