Pure and Applied Chemistry

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 M^II (Mn, Fe, Co, Ni, Cu, Zn, Cd, and VO²⁺) and M^III (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.