Synthesis and properties of metal trifluoride complexes with amide-functionalised tacn macrocycles and radiofluorination of [GaF 3 (L1)] by 18 F/ 19 F isotopic exchange.

Three amide-functionalised tacn macrocyclic derivatives (tacn = 1,4,7-triazacyclononane) are reported, two tris-amide derivatives, L1 containing three -CH 2 C(O)NHPh pendant arms, L2 containing three -CH 2 CH 2 C(O)NH i Pr pendant arms, and one mono -amide, L3, containing i Pr groups on two of the tacn amine functions and a single -CH 2 C(O)NHPh function on the third. The reactions of these new ligands towards [MF 3 (dmso)(OH 2 ) 2 ] (M = Al, Ga) and towards FeF 3 ·3H 2 O in alcoholic solution afford the complexes [MF 3 (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. [GaF 3 (L1)], [GaF 3 (L3)] and [FeF 3 (L3)] are also characterised by single crystal X-ray analysis. The corresponding reactions involving [InF 3 (dmso)(OH 2 ) 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, [InF 3 (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 MF 3 moieties is evident in the crystal structures. In the isostructural [MF 3 (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 MF 3 fragments being strong H-bond acceptors. Radiofluorination of [GaF 3 (L1)] by 18 F/ 19 F isotopic exchange in MeOH at 3 μmol mL -1 precursor concentration and using aqueous [ 18 F]F - in target water (75% : 25%) with gentle heating (80 °C, 10 min) gave ca . 20% radiochemical yield of [Ga 18 FF 2 (L1)]. In contrast, no 18 F incorporation occurs with [GaF 3 (L3)] under any of the conditions explored.