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Pure Appl. Chem., 2008, Vol. 80, No. 6, pp. 1303-1316

Chemical speciation of polynuclear complexes containing [Ln2M3L6] units

Julia Torres1, Carlos Kremer1 and Sixto Domínguez2

1 Estrella Campos Department, Faculty of Chemistry, University of the Republic, CC 1157, Montevideo, Uruguay
2 Department of Inorganic Chemistry, University of La Laguna, Tenerife, Canary Islands, Spain

Abstract: Polynuclear complexes are widely regarded as potential materials in separation, catalysis, gas storage, and molecular recognition. Those complexes including lanthanide (Ln) ions in the heterobimetallic assembly enhance the structural flexibility and increase the thermodynamic stability. Up to now, the studies on these heteropolynuclear complexes have been mostly focused on lanthanide-copper compounds in the solid state. We have expanded the studies to solution chemistry, to know if such polynuclear species can also exist in aqueous solution. In this work, potentiometric titrations (25.0 °C, and I = 0.50 M Me4NCl) were carried out for systems containing a Ln ion (La, Ce, Sm), a bridging ligand (oda or ida), and a second metal ion. We found that polynuclear units can be formed with MII (Mn, Fe, Co, Ni, Cu, Zn, Cd, and VO2+) and MIII (Fe, Al) ions, and can be predominant in aqueous solution. Species like [Ln2M3L6] or [Ln2M3L6(OH)n]n- (n = 3, 6) can exist in solution, representing the same 2:3:6 stoichiometry found in the solid state. The presence of an amino group in the ida ligand makes the mononuclear complexes with 3d metal ions more stable, preventing to some extent the formation of polynuclear complexes.