title: A bridge to coordination isomer selection in lanthanide(III) DOTA-tetraamide complexes.
abstract: Interest in macrocyclic lanthanide complexes such as DOTA is driven largely through interest in their use as contrast agents for MRI. The lanthanide tetraamide derivatives of DOTA have shown considerable promise as PARACEST agents, taking advantage of the slow water exchange kinetics of this class of complex. We postulated that water exchange in these tetraamide complexes could be slowed even further by introducing a group to sterically encumber the space above the water coordination site, thereby hindering the departure and approach of water molecules to the complex. The ligand 8O2-bridged DOTAM was synthesized in a 34% yield from cyclen. It was found that the lanthanide complexes of this ligand did not possess a water molecule in the inner coordination sphere of the bound lanthanide. The crystal structure of the ytterbium complex revealed that distortions to the coordination sphere were induced by the steric constraints imposed on the complex by the bridging unit. The extent of the distortion was found to increase with increasing ionic radius of the lanthanide ion, eventually resulting in a complete loss of symmetry in the complex. Because this ligand system is bicyclic, the conformation of each ring in the system is constrained by that of the other; in consequence, inclusion of the bridging unit in the complexes means only a twisted square, antiprismatic coordination geometry is observed for lanthanide complexes of 8O2-bridged DOTAM.
authors.affiliation: Department of Chemistry, University of Texas at Dallas, P.O. Box 830660, Richardson, Texas 75083-0688, USA.
authors.forename: Jeff
authors.initials: J
authors.lastname: Vipond
authors.forename: Mark
authors.initials: M
authors.lastname: Woods
authors.forename: Piyu
authors.initials: P
authors.lastname: Zhao
authors.forename: Gyula
authors.initials: G
authors.lastname: Tircsó
authors.forename: Jimin
authors.initials: J
authors.lastname: Ren
authors.forename: Simon G
authors.initials: SG
authors.lastname: Bott
authors.forename: Doug
authors.initials: D
authors.lastname: Ogrin
authors.forename: Garry E
authors.initials: GE
authors.lastname: Kiefer
authors.forename: Zoltan
authors.initials: Z
authors.lastname: Kovacs
authors.forename: A Dean
authors.initials: AD
authors.lastname: Sherry
mesh_terms: D000577:Amides / Q000138:chemical synthesis / Q000737:chemistry*; D003432:Cross-Linking Reagents / Q000737:chemistry*; D018360:Crystallography, X-Ray; D006573:Heterocyclic Compounds, 1-Ring / Q000737:chemistry*; D007536:Isomerism; D028581:Lanthanoid Series Elements / Q000737:chemistry*; D009682:Magnetic Resonance Spectroscopy; D008958:Models, Molecular; D015394:Molecular Structure; D010100:Oxygen / Q000737:chemistry; D012996:Solutions; D014867:Water / Q000737:chemistry
chemical_list: D000577:Amides; D003432:Cross-Linking Reagents; D006573:Heterocyclic Compounds, 1-Ring; D028581:Lanthanoid Series Elements; D012996:Solutions; D014867:Water; C071349:1,4,7,10-tetraazacyclododecane- 1,4,7,10-tetraacetic acid; D010100:Oxygen