Studies of Size-Based Selectivity in Aqueous Ternary Complexes of Americium(III) or Lanthanide(III) Cations

Abstract

Spectrophotometric and calorimetric titrations were used to determine the equilibrium constants (log10 K 111) and enthalpies of formation (ΔH 111) for aqueous ternary complexes of the form M(La)(Lb) (M = Nd3+, Sm3+, Tb3+, Ho3+, Er3+, or Am3+; La = DTPA5−, DO3A3−, or CDTA4−; Lb = oxalate (Ox), malonate (Mal), or iminodiacetate (IDA)). Inner-sphere ternary complexes were readily formed with the septadentate DO3A (1,4,7,10-tetraazacyclododecane-1,4,7-triacetic acid) and hexadentate CDTA (trans-1,2-diaminocyclohexanetetraacetic acid) ligands, whose binary complexes have residual metal-coordinated water molecules that are readily displaced by the smaller secondary ligands. The stability constants for the formation of lanthanide–CDTA complexes with Ox, Mal, and IDA generally increase with decreasing ionic radius when steric hindrance is minimal, with the trend in the M(CDTA)− formation constants overshadowing any size-based reversal in the stepwise ternary complexation constants. Similar ternary complexes with DO3A showed little increase in thermodynamic stability compared to analogous CDTA complexes and no preference for larger Ln cations. The octadentate DTPA (diethylenetriaminepentaacetic acid) ligand proved too large to form ternary complexes to a measurable extent with any of the secondary ligands investigated, despite the presence of one residual inner sphere water molecule.