Equilibrium and NMR studies on GdIII, YIII, CuIIand ZnIIcomplexes of various DTPA–N,N″-bis(amide) ligands. Kinetic stabilities of the gadolinium(iii) complexes

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Abstract
Three DTPA-derivative ligands, the non-substituted DTPA–bis(amide) (L0), the mono-substituted DTPA–bis(n-butylamide) (L1) and the di-substituted DTPA–bis[bis(n-butylamide)] (L2) were synthesized. The stability constants of their Gd3+ complexes (GdL) have been determined by pH-potentiometry with the use of EDTA or DTPA as competing ligands. The endogenous Cu2+ and Zn2+ ions form ML, MHL and M2L species. For the complexes CuL0 and CuL1 the dissociation of the amide hydrogens (CuLH−1) has also been detected. The stability constants of complexes formed with Gd3+, Cu2+ and Zn2+ increase with an increase in the number of butyl substituents in the order ML0 < ML1 < ML2. NMR studies of the diamagnetic YL0 show the presence of four diastereomers formed by changing the chirality of the terminal nitrogens of their enantiomers. At 323 K, the enantiomerization process, involving the racemization of central nitrogen, falls into the fast exchange range. By the assignment and interpretation of 1H and 13C NMR spectra, the fractions of the diastereomers were found to be equal at pH = 5.8 for YL0. The kinetic stabilities of GdL0, GdL1 and GdL2 have been characterized by the rates of the exchange reactions occurring between the complexes and Eu3+, Cu2+ or Zn2+. The rates of reaction with Eu3+ are independent of the [Eu3+] and increase with increasing [H+], indicating the rate determining role of the proton assisted dissociation of complexes. The rates of reaction with Cu2+ and Zn2+ increase with rising metal ion concentration, which shows that the exchange can take place with direct attack of Cu2+ or Zn2+ on the complex, via the formation of a dinuclear intermediate. The rates of the proton, Cu2+ and Zn2+ assisted dissociation of Gd3+ complexes decrease with increasing number of the n-butyl substituents, which is presumably the result of steric hindrance hampering the formation or dissociation of the intermediates. The kinetic stabilities of GdL0 and GdL1 at pH = 7.4, [Cu2+] = 1 × 10−6 M and [Zn2+] = 1 × 10−5 M are similar to that of Gd(DTPA)2−, while the complex GdL2 possesses a much higher kinetic stability.

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