Structural and thermodynamic aspects of hydration of Gd(iii) systems

摘要

X-ray crystal structures of Gd(III) and Lu(III) aqua ions as well as their complexes with polyaminopolycarboxylates (EDTA, CDTA, EGTA, DTPA, DOTA) were determined: [Gd(H2O)(9)](CF3SO3)(3), [Gd(H2O)(8)]Cl-3 center dot C10H20O5, [Lu(H2O)(8)]Cl-3 center dot C12H24O6 center dot 4H2O, [C(NH2)(3)][Gd(EDTA)(H2O)(3)], [C(NH2)(3)](2)[Lu(EDTA)(H2O)(2)]ClO4 center dot 6H(2)O, [C(NH2)(3)][Lu(CDTA)(H2O)(2)]center dot 6H(2)O, [C(NH2)(3)][Gd(EGTA)(H2O)]center dot 2H(2)O, [C(NH2)(2)(N2H4)][Gd(HDTPA)(H2O)]center dot 2H(2)O, Na[Gd(DOTA)(H2O)]center dot 4H(2)O, and K-2[Lu(DOTA)]Cl center dot 4.6H(2)O. The weighted sums of UV absorption spectra of appropriate crystals were used to reproduce the spectra of the Gd(III) aqueous solutions in the temperature range 276-363 K. It was shown that in aqueous solution the Gd(III)-EGTA, Gd(III)DTPA and Gd(III)-DOTA complexes exist as almost pure monohydrate [GdL(H2O)] n-species, while in the case of the Gd(III) aqua ion, Gd(III)-EDTA and Gd(III)-CDTA systems the equilibria between variously hydrated species were found. The derived molar fractions of these species were used to determine the Delta G, Delta H and Delta S of hydration. It was shown that these thermodynamic functions may be derived not only from the spectra of the hypersensitive transitions, but from other f-f transitions as well. Next the Delta G, Delta H and Delta S values of hydration for the other Ln(III)-EDTA systems (where Ln = Pr, Nd, Sm, Eu) were determined. It was found that the Delta G(298) values of the dehydration reaction for Ln(III)-EDTA complexes (where Ln = Pr, Nd, Sm, Eu, Gd, Ho, Er) were almost linearly dependent on the number of 4f electrons in the whole series of lanthanides. Moreover, it was shown that the point, where the ratio of [LnL(H2O)(n)] : [LnL(H2O)(n-1)] is equal to 1, shifts along the lanthanide series depending on the ligand denticity - the higher the ligand denticity, the farther the point of the equimolar ratio in the lanthanide series. The presented r