1,2-Hydroxypyridonates as Contrast Agents for Magnetic Resonance Imaging: TREN-1,2-HOPO

1,2-Hydroxypyridinones (1,2-HOPO) form very stable lanthanide complexes that may be useful as contrast agents
for magnetic resonance imaging (MRI). X-ray diffraction of single crystals established that the solid-state structures
of the Eu(III) and the previously reported [Inorg. Chem. 2004, 43, 5452] Gd(III) complex are identical. The recently
discovered sensitizing properties of 1,2-HOPO chelates for Eu(III) luminescence [J. Am. Chem. Soc. 2006, 128,
10 067] allow for direct measurement of the number of water molecules coordinated to the metal center. Fluorescence
measurements of the Eu(III) complex corroborate that, in solution, two water molecules coordinate the lanthanide
(q = 2) as proposed from the analysis of NMRD profiles. In addition, fluorescence measurements have verified the
anion binding interactions of lanthanide TREN-1,2-HOPO complexes in solution, studied by relaxivity, revealing
only very weak oxalate binding (KA = 82.7 ± 6.5 M-1). Solution thermodynamic studies of the metal complex and
free ligand have been carried out using potentiometry, spectrophotometry, and fluorescence spectroscopy. The
metal ion selectivity of TREN-1,2-HOPO supports the feasibility of using 1,2-HOPO ligands for selective lanthanide
binding [pGd = 19.3 (2), pZn = 15.2 (2), pCa = 8.8 (3)].