Inner-sphere water and hydrogen bonds in lanthanide DOTAM complexes. A neutron diffraction study

A tight network of hydrogen bonds is observed by neutron diffraction for Ln(DOTAM)(H2O)(triflate)3·3H2O (Ln=Eu, Nd) at different temperatures by means of VIVALDI and D19 techniques. [Display omitted] •Ln(DOTAM)(H2O)(triflate)3·3H2O (Ln=Eu, Nd) complexes studied by neutron diffraction.•The network of hydrogen bonds observed.•High accuracy for Eu-H and Nd-H distances.•Relevance for r calculations.•Asymmetry in the OH bonds for the coordinated water. Two isostructural complexes Ln(DOTAM)(H2O)(triflate)3·3H2O (Ln=Eu, Nd) have been characterized by neutron diffraction analysis at different temperatures by means of two techniques, white-beam Laue at 65K and monochromatic at 20K, 65K and 220K. The high accuracy in the hydrogen-atom localization revealed the H-bonds network formed by the metal-coordinated water, the water molecules, the anions, and the macrocycle, as well as the non-symmetric orientation of the coordinated water with respect to the Ln-Ow (w=water) axis. The accurate determination of Nd-H and Eu-H distances allows derivation of an averaged Gd-H value (2.974Å at 220K) that is in the range of the smaller values commonly used in relaxivity calculations for Gd-based contrast agents.