title: Lanthanide complexes based on a 1,7-diaza-12-crown-4 platform containing picolinate pendants: a new structural entry for the design of magnetic resonance imaging contrast agents.
abstract: We have synthesized a new macrocyclic ligand, N,N'-Bis[(6-carboxy-2-pyridyl)methyl]-1,7-diaza-12-crown-4 (H 2bp12c4), designed for complexation of lanthanide ions in aqueous solution. The X-ray crystal structure of the Gd (III) complex shows that the metal ion is directly bound to the eight donor atoms of the bp12c4 ligand, the ninth coordination site being occupied by an oxygen atom of a carboxylate group of a neighboring [Gd(bp12c4)] (+) unit, while the structure of the Lu (III) analogue shows the metal ion being only eight-coordinate. The hydration numbers obtained from luminescence lifetime measurements in aqueous solution of the Eu (III) and Tb (III) complexes suggest an equilibrium in aqueous solution between a dihydrated ( q = 2), ten-coordinate and a monohydrated ( q = 1), nine-coordinate species. This has been confirmed by a variable temperature UV-vis spectrophotometric study on the Eu (III) complex. The structure of the complexes in solution has been investigated by (1)H and (13)C NMR spectroscopy, as well as by theoretical calculations performed at the DFT (B3LYP) level. The results indicate that the change in hydration number occurring around the middle of the lanthanide series is accompanied by a change in the conformation adopted by the complexes in solution [Delta(lambdalambdalambdalambda) for q = 2 and Lambda(deltalambdadeltalambda) for q = 1]. The structure calculated for the Yb (III) complex (Lambda(deltalambdadeltalambda)) is in good agreement with the experimental structure in solution, as demonstrated by the analysis of the Yb (III)-induced paramagnetic (1)H shifts.
authors.affiliation: Departamento de Quimica Fundamental, Universidade da Coruna, Campus da Zapateira, Alejandro de la Sota 1, 15008 A Coruna, Spain.
authors.forename: Marta
authors.initials: M
authors.lastname: Mato-Iglesias
authors.forename: Adrián
authors.initials: A
authors.lastname: Roca-Sabio
authors.forename: Zoltán
authors.initials: Z
authors.lastname: Pálinkás
authors.forename: David
authors.initials: D
authors.lastname: Esteban-Gómez
authors.forename: Carlos
authors.initials: C
authors.lastname: Platas-Iglesias
authors.forename: Eva
authors.initials: E
authors.lastname: Tóth
authors.forename: Andrés
authors.initials: A
authors.lastname: de Blas
authors.forename: Teresa
authors.initials: T
authors.lastname: Rodríguez-Blas
mesh_terms: D003287:Contrast Media / Q000138:chemical synthesis* / Q000737:chemistry*; D017666:Deuterium Oxide / Q000737:chemistry; D015195:Drug Design; D005063:Europium / Q000737:chemistry; D028581:Lanthanoid Series Elements / Q000737:chemistry*; D008024:Ligands; D008163:Luminescent Measurements; D047028:Macrocyclic Compounds / Q000737:chemistry*; D008279:Magnetic Resonance Imaging; D009682:Magnetic Resonance Spectroscopy; D008968:Molecular Conformation; D009942:Organometallic Compounds / Q000138:chemical synthesis* / Q000737:chemistry; D010848:Picolinic Acids / Q000737:chemistry*; D011789:Quantum Theory; D014867:Water / Q000737:chemistry; D015018:Ytterbium / Q000737:chemistry; D043844:Crown Ethers
chemical_list: D003287:Contrast Media; D017666:Deuterium Oxide; D005063:Europium; D028581:Lanthanoid Series Elements; D008024:Ligands; D047028:Macrocyclic Compounds; D009942:Organometallic Compounds; D010848:Picolinic Acids; D014867:Water; D015018:Ytterbium; C533505:N,N'-Bis((6-carboxy-2-pyridyl)methyl)-1,7-diaza-12-crown-4; D043844:Crown Ethers; C030614:picolinic acid