1H and 17O NMR Detection of a Lanthanide-Bound Water Molecule at Ambient Temperatures in Pure Water as Solvent

Lanthanide complexes of a tetra-amide derivative of DOTA (structure 4 in text) with four extended carboxymethyl esters have been characterized by X-ray crystallography and multinuclear NMR spectroscopy. [Eu(4)(H2O)](triflate)3 crystallized from water in the monoclinic, P21/c space group (a = 10.366 A, b = 22.504 A, c = 23.975 A, and β = 97.05°). The Eu3+ cation is bound to four macrocyclic nitrogen atoms (mean Eu−N = 2.627 A) and four amide oxygen atoms (mean Eu−Oamide = 2.335 A) in a square antiprismatic geometry with a twist angle of 38.5° between the N4 and O4 planes. A single bound water molecule (Eu−OW = 2.414 A) occupies a typical monocapped position on the O4 surface. In pure water, resonances corresponding to a single Eu3+-bound water molecule were observed in the 1H (53 ppm) and 17O (−897 ppm) NMR spectra of [Eu(4)(H2O)](triflate)3 at 25 °C. A fit of the temperature-dependent Eu3+-bound 1H and 17O water resonance line widths in acetonitrile-d3 (containing 4% v/v 17O enriched water) gave identical...