Solid-state and solution properties of the lanthanide complexes of a new nonadentate tripodal ligand derived from 1,4,7-triazacyclononane

The synthesis of the potentially nonadentate ligand 1,4,7-tris[(6-carboxypyridin-2-yl)methyl]-1,4,7-triazacyclononane (H3tpatcn), a new derivative of 1,4,7-triazacyclononane N-functionalised with three pyridinecarboxylate arms, is described. The complexes of four lanthanide ions (Nd, Eu, Gd, Lu) of this ligand have been prepared and structurally characterised. These complexes, which have high water solubility, show highly rigid C3 symmetric solution structures. All the complexes present mononuclear nine-coordinated solid-state structures. The coordination polyhedron is a slightly distorted tricapped trigonal prism. The NMRD (Nuclear Magnetic Relaxation Dispersion) profiles measured for the [Gd(tpatcn)] complex indicate that the second-sphere contribution arising from the presence of water molecules tightly hydrogen-bonded to the carboxylate moieties on the surface of the complex are not large enough to explain the very high relaxivity of the previously reported [Gd(tpaa)(H2O)2] complex (H3tpaa = α,α′,α″-nitrilotri(6-methyl-2-pyridinecarboxylic acid)). In fact the low-field relaxivity of [Gd(tpatcn)] more likely points to a favorable electronic relaxation rate