Complexation of NpO2+ with N-methyl-iminodiacetic acid: a comparison with iminodiacetic and dipicolinic acids

Abstract

Complexation of Np(V) with N-methyl-iminodiacetic acid (MIDA) in 1 M NaClO₄ solution was studied with multiple techniques including potentiometry, spectrophotometry, and microcalorimetry. The 1 : 2 complex, NpO₂(MIDA)₂³⁻ was identified for the first time in aqueous solution. The correlation between its optical absorption properties and symmetry was discussed, in comparison with Np(V) complexes with two structurally related nitrilo-dicarboxylic acids, iminodiacetic acid (IDA) and dipicolinic acid (DPA). The order of the binding strength (DPA > MIDA > IDA) is explained by the difference in the structural and electronic properties of the ligands. In general, the nitrilo-dicarboxylates form stronger complexes with Np(V) than oxy-dicarboxylates due to a much more favorable enthalpy of complexation.