Gadolinium(III)-Based Dual H/ F Magnetic Resonance Imaging Probes.

We present two novel octadentate cyclen-based ligands bearing one (L ) or two (L ) phenylacetamide pendants with two CF groups either at positions 3 and 5 (L ) or 4 (L ). The corresponding Gd complexes possess one coordinated water molecule, as confirmed by luminescence lifetime measurements on the Eu and Tb analogues. A detailed H and O relaxometric characterization has revealed the parameters that govern the relaxivities of these complexes. The water-exchange rate of the mono-amide derivative GdL (k =1.52×10  s ) is faster than that determined for the bis-amide complex GdL (k =0.73×10  s ). H and F NMR studies have indicated that the complexes are present in solution almost exclusively as the square-antiprismatic (SAP) isomers. F NMR relaxation studies indicated Gd⋅⋅⋅F distances of 7.4±0.1 and 9.1±0.1 Å for GdL and GdL , respectively. Phantom MRI studies revealed the favorable properties of GdL as a dual H/ F magnetic resonance imaging (MRI) probe, whereas the shorter Gd⋅⋅⋅F distance of GdL reduces the signal-to-noise ratio due to the very short transverse relaxation time of the F NMR signal.