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Abstract:
We present two novel octadentate cyclen-based ligands containing one (L1) or two (L2) phenylacetamide pendants that feature 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 the luminescence lifetime measurements recorded on the Eu(III) and Tb(III) analogues. A detailed 1H and 17O relaxometric characterisation 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). 1H and 19F NMR studies indicate that the complexes are present in solution almost exclusively as the square antiprismatic (SAP) isomers. 19F NMR relaxation studies provided Gd···F distances of 7.4 + 0.1 and 9.1 + 0.1 Å for GdL1 and GdL2, respectively. Phantom MRI studies revealed the favourable properties of GdL2 as a dual 1H/19F MRI probe, while the shorter Gd···F distance of GdL1 reduces the signal to noise ratio due to the very short transverse relaxation time of the 19F NMR signal.
