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Thermodynamic and Structural Trends in Hexavalent Actinyl Cations: Complexation of Dipicolinic Acid with NpO 2 2+ and PuO 2 2+ in Comparison with UO 2 2+
Author(s) -
Xu Chao,
Tian Guoxin,
Teat Simon J.,
Liu Guokui,
Rao Linfeng
Publication year - 2013
Publication title -
chemistry – a european journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.687
H-Index - 242
eISSN - 1521-3765
pISSN - 0947-6539
DOI - 10.1002/chem.201302119
Subject(s) - chemistry , dipicolinic acid , isothermal microcalorimetry , inorganic chemistry , actinide , metal , stability constants of complexes , spectrochemical series , neptunium , ligand (biochemistry) , chemical stability , crystallography , ligand field theory , nuclear chemistry , enthalpy , ion , aqueous solution , biochemistry , botany , physics , receptor , spore , organic chemistry , quantum mechanics , biology
Abstract The complexation of NpO 2 2+ and PuO 2 2+ with dipicolinic acid (DPA) has been investigated in 0.1 M NaClO 4 by spectrophotometry, microcalorimetry, and single crystal diffractometry. Formation of 1:1 and 1:2 (metal/ligand molar ratio) complexes of DPA with NpO 2 2+ and PuO 2 2+ were identified and the thermodynamic parameters were determined and compared with those of UO 2 2+ . All three hexavalent actinyl cations form strong 1:1 DPA complexes with slightly decreasing but comparable stability constants from UO 2 2+ to PuO 2 2+ , whereas the stability constants of the 1:2 complexes (log β 2 ) decrease substantially along the series (16.3 for UO 2 L 2 2− , 15.17 for NpO 2 L 2 2− , and 14.17 for PuO 2 L 2 2− at 25 °C). The enthalpies of complexation for the 1:2 complexes become less exothermic from UO 2 L 2 2− (−28.9 kJ mol −1 ), through NpO 2 L 2 2− (−27.2 kJ mol −1 ), and to PuO 2 L 2 2− (−22.7 kJ mol −1 ). The trends in the thermodynamic parameters are discussed in terms of the effective charge of the cations and the steric constraints in the structures of the complexes. In addition, the features of the absorption spectra, including the wavelength and intensity of the absorption bands, are related to the perturbation of the ligand field and the symmetry of the actinyl complexes.