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 Å, b = 22.504 Å, c = 23.975 Å, and β = 97.05°). The Eu3+ cation is bound to four macrocyclic nitrogen atoms (mean Eu-N = 2.627 Å) and four amide oxygen atoms (mean EU-Oamide = 2.335 Å) 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 Å) 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 lifetimes (τm 298) of 789 ± 50 μs (in water as solvent; a line shape analysis of the Eu3+-bound water resonance gave a τm 298 = 382 ± 5 μs). Slow water exchange was also evidenced by the water proton relaxivity of Gd(4) (R1 = 2.2 mM-1 s-1, a value characteristic of pure outer-sphere relaxation at 25 °C). With increasing temperature, the inner-sphere contribution gradually increased due to accelerated chemical exchange between bound water and bulk water protons. A fitting of the relaxation data (T1) to standard SBM theory gave a water proton lifetime (τm 298) of 159 μs, somewhat shorter than the value determined by high-resolution 1H and 17O NMR of Eu(4). Exchange of the bound water protons in Gd(4) with bulk water protons was catalyzed by addition of exogenous phosphate at 25 °C (R1 increased to 10.0 mM-1 s-1 in the presence of 1500-fold excess HPO4 2-).
ASJC Scopus subject areas
- Inorganic Chemistry