A structural and thermodynamic study of the complexes of U(vi) with azinecarboxylates.

Complexation of U(vi) with pyridazine-3-carboxylate (PDZ) and pyrazine-2-carboxylate (PAZ) was studied by spectrophotometry, potentiometry and microcalorimetry in 1.0 mol dm-3 NaClO4. Three complexes, [UO2L]+, UO2L2(aq) and [UO2L3]-, were identified and their stability constants (log β) and the corresponding formation enthalpies were determined. The thermodynamic parameters indicate that the formation of the three complexes is endothermic and driven exclusively by entropy. 1H and 13C-NMR data provide insight into the coordination modes of the complexes which corroborate with the thermodynamic data. Ligands chelate to U(vi) via κ2(N,O) coordination mode in complexes [UO2L]+ and UO2L2(aq). The crystal structures of four U(vi) complexes, [(UO2)(PAZ)2(H2O)]·H2O(i), [(UO2)(PDZ)2(H2O)](ii), [(UO2)(PDZ)3Na2ClO4]·2H2O(iii), and [(UO2)2(PDZ)4(H2O)2]·2H2O(iv), were determined by single-crystal X-ray diffraction and compared with the U(vi) complex with picolinate (PA) (CH6N3)[UO2(PA)3] in the literature. The structure data suggest that the carboxylates coordinate with uranium in O[double bond, length as m-dash]C-O-U mode. The strengths of the U-O-C-C-N chelate cycles in the U(vi)/L complexes decrease with the trend of PA > PDZ > PAZ, which is in great agreement with the trend of thermodynamic parameters in aqueous solutions. It is interesting that in compound II two PDZ molecules coordinate with U(vi) in cis-planar positions via κ2(N,O) mode, but in other metal complexes of the three ligands having the same κ2(N,O) coordination mode the two ligand molecules are all in trans-arrangement. In the dimeric complex IV, one ligand coordinate with U(vi) in κ2(N,O) mode, while the other does it in μ2-L-κ2(O:O') mode respectively.