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

Abstract

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 OC–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.