Synthesis and properties of metal trifluoride complexes with amide-functionalised tacn macrocycles and radiofluorination of [GaF3(L1)] by 18F/19F isotopic exchange.

ABSTRACT

Three amide-functionalised tacn macrocyclic derivatives (tacn = 1,4,7-triazacyclononane) are reported, two tris-amide derivatives, L1 containing three -CH2C(O)NHPh pendant arms, L2 containing three -CH2CH2C(O)NHiPr pendant arms, and one mono-amide, L3, containing iPr groups on two of the tacn amine functions and a single -CH2C(O)NHPh function on the third. The reactions of these new ligands towards [MF3(dmso)(OH2)2] (M = Al, Ga) and towards FeF3·3H2O in alcoholic solution afford the complexes [MF3(L)] (L = L1-L3) in good yields as powdered solids. These complexes are characterised by IR and multinuclear NMR spectroscopy (diamagnetic species only) and mass spectrometry. [GaF3(L1)], [GaF3(L3)] and [FeF3(L3)] are also characterised by single crystal X-ray analysis. The corresponding reactions involving [InF3(dmso)(OH2)2] yield mixtures of products (along with F-), consistent with the M-F bond strengths decreasing as group 13 is descended. A few crystals of the target complex, [InF3(L2)], were also obtained from one such reaction. All of the complexes adopt fac-octahedral coordination via the amine N-donor atoms and retain the three fluoride ligands both in solution and in the solid state. Extensive intramolecular hydrogen-bonding involving the amide NH pendant groups and the MF3 moieties is evident in the crystal structures. In the isostructural [MF3(L3)] (M = Ga, Fe) complexes the head-to-tail C(O)NH⋯F H-bonded dimers observed in the solid state resemble those found frequently in organic compounds and that form the cornerstone of many supramolecular assemblies. This is consistent with the MF3 fragments being strong H-bond acceptors. Radiofluorination of [GaF3(L1)] by 18F/19F isotopic exchange in MeOH at 3 µmol mL-1 precursor concentration and using aqueous [18F]F- in target water (75% 25%) with gentle heating (80 °C, 10 min) gave ca. 20% radiochemical yield of [Ga18FF2(L1)]. In contrast, no 18F incorporation occurs with [GaF3(L3)] under any of the conditions explored.