Lanthanide(III) Complexes with Ligands Derived from a Cyclen Framework Containing Pyridinecarboxylate Pendants. The Effect of Steric Hindrance on the Hydration Number

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• 作者：Aurora Rodr铆guez-Rodr铆guez ; David Esteban-G贸mez ; Andr茅s de Blas ; Teresa Rodr铆guez-Blas ; Marianna Fekete ; Mauro Botta ; Rapha毛l Tripier ; Carlos Platas-Iglesias
• 刊名：Inorganic Chemistry
• 出版年：2012
• 出版时间：February 20, 2012
• 年：2012
• 卷：51
• 期：4
• 页码：2509-2521
• 全文大小：541K
• 年卷期：v.51,no.4(February 20, 2012)
• ISSN：1520-510X

文摘

Two new macrocyclic ligands, 6,6鈥?((1,4,7,10-tetraazacyclododecane-1,7-diyl)bis(methylene))dipicolinic acid (H₂DODPA) and 6,6鈥?((4,10-dimethyl-1,4,7,10-tetraazacyclododecane-1,7-diyl)bis(methylene))dipicolinic acid (H₂Me-DODPA), designed for complexation of lanthanide ions in aqueous solution, have been synthesized and studied. The X-ray crystal structure of [Yb(DODPA)](PF₆)路H₂O shows that the metal ion is directly bound to the eight donor atoms of the ligand, which results in a square-antiprismatic coordination around the metal ion. The hydration numbers (q) obtained from luminescence lifetime measurements in aqueous solution of the Eu^III and Tb^III complexes indicate that the DODPA complexes contain one inner-sphere water molecule, while those of the methylated analogue H₂Me-DODPA are q = 0. The structure of the complexes in solution has been investigated by ¹H and ¹³C NMR spectroscopy, as well as by theoretical calculations performed at the density functional theory (DFT; mPWB95) level. The minimum energy conformation calculated for the Yb^III complex [螞(位位位位)] is in good agreement with the experimental structure in solution, as demonstrated by the analysis of the Yb^III-induced paramagnetic ¹H shifts. The nuclear magnetic relaxation dispersion (NMRD) profiles recorded for [Gd(Me-DODPA)]⁺ are typical of a complex with q = 0, where the observed relaxivity can be accounted for by the outer-sphere mechanism. However, [Gd(DODPA)]⁺ shows NMRD profiles consistent with the presence of both inner- and outer-sphere contributions to relaxivity. A simultaneous fitting of the NMRD profiles and variable temperature ¹⁷O NMR chemical shifts and transversal relaxation rates provided the parameters governing the relaxivity in [Gd(DODPA)]⁺. The results show that this system is endowed with a relatively fast water exchange rate k_ex²⁹⁸ = 58 脳 10⁶ s^鈥?.