Complexation of Np(v) with N,N-dimethyl-3-oxa-glutaramic acid and related ligands: thermodynamics, optical properties and structural aspects

Complexation of Np(V) with N,N-dimethyl-3-oxa-glutaramic acid (DMOGA) was studied in comparison with its diamide analog, N,N,N',N'-tetramethyl-3-oxa-glutaramide (TMOGA), and dicarboxylate analog, oxydiacetic acid (ODA). Thermodynamic parameters, including the stability constant and the enthalpy of complexation, were determined by spectrophotometry and calorimetry. Single-crystal structure of NpO(2)(H(2)O)(DMOGA).H(2)O(c) was identified by X-ray diffractometry using synchrotron radiation. Like ODA and TMOGA, DMOGA forms a tridentate Np(V) complex, with three oxygen atoms coordinating to the linear NpO(2)(+) moiety via the equatorial plane. The stability constants, enthalpy and entropy of complexation generally decrease in the order ODA > DMOGA > TMOGA, suggesting that the complexation is entropy driven and the substitution of a carboxylate group with an amide group reduces the strength of complexation with Np(V) due to the decrease in the entropy of complexation.