Gadolinium(III)‐Based Dual1H/19F Magnetic Resonance Imaging Probes

We present two novel octadentate cyclen-based ligands bearing one (L1 ) or two (L2 ) phenylacetamide pendants with two CF3 groups either at positions 3 and 5 (L1 ) or 4 (L2 ). The corresponding Gd3+ complexes possess one coordinated water molecule, as confirmed by luminescence lifetime measurements on the EuIII and TbIII analogues. A detailed 1 H and 17 O relaxometric characterization has revealed the parameters that govern the relaxivities of these complexes. The water-exchange rate of the mono-amide derivative GdL1 (kex298 =1.52×106 s-1 ) is faster than that determined for the bis-amide complex GdL2 (kex298 =0.73×106 s-1 ). 1 H and 19 F NMR studies have indicated that the complexes are present in solution almost exclusively as the square-antiprismatic (SAP) isomers. 19 F NMR relaxation studies indicated Gd⋅⋅⋅F distances of 7.4±0.1 and 9.1±0.1 A for GdL1 and GdL2 , respectively. Phantom MRI studies revealed the favorable properties of GdL2 as a dual 1 H/19 F magnetic resonance imaging (MRI) probe, whereas the shorter Gd⋅⋅⋅F distance of GdL1 reduces the signal-to-noise ratio due to the very short transverse relaxation time of the 19 F NMR signal.