Coordination complexes of Group 13 Halides: towards a new class of PET imaging agents

Abstract

Reaction of the triaza macrocycles RMe2-tacn (R = Me, Bz) with the Group 13 chlorides MCl3 gives the solvent free distorted octahedral adducts [MCl3(RMe2-tacn)] (M = Al, Ga, In). Reaction of the trichloro complexes with aqueous KF at room temperature yields the hydrated complexes [MF3(RMe2-tacn)]?xH2O. The trifluoro complexes are formed in high yield, are stable under a range of conditions, and display significant H-bonding interactions between the coordinated fluoride ligands and the crystallised water molecules in the solid state, confirmed crystallographically.
The complex [GaCl3(BzMe2-tacn)] was successfully radiolabelled with aqueous K18F/19F at room temperature to give the complex [Ga18F19F2(BzMe2-tacn)] with 30% incorporation in 45 minutes. The purified compound is stable in phosphate buffered saline at pH 7.5 and represents an exciting alternative route towards the 18F radiolabelling of temperature sensitive biomolecules in PET imaging applications.
The complex [GaF3(BzMe2-tacn)] can act as a metalloligand towards alkali metal and ammonium salts via coordination of the metal fluoride fragment. Crystallographic analyses show that this results in a series of unusual supramolecular assemblies with Li+, Na+ and K+.
Reaction of Ga(NO)3?9H2O with 1-benzyl-1,4,7-triazacyclononane-4,7-dicarboxylic acid hydrochloride (L?HCl) gives the complex [GaCl(L)]?2H2O. Treatment of this complex with carrier free 18F- at 80 ?C gives the radiolabelled complex [Ga18F(L)] in high yield. The compound may be purified by a cartridge based method. The purified compound is stable up to at least pH 6, but loses fluoride at neutral pH.
Hydrothermal synthesis was used to access complexes of the poorly soluble hydrated Group 13 fluorides directly. Reaction of MF3?3H2O (M = Al, Ga, In) with the tridentate imine ligand 2,2’,2’’-terpyridine gives the hydrated adducts mer-[MF3(terpy)]?3H2O. The complexes display extensive H-bonding interactions in the solid state. Attempts to access the 18F labelled complex [Ga18F19F2(terpy)] via halide exchange from the preformed trichloro complex as an alternative complex type for potential use in PET imaging were not successful. Reaction of MF3?3H2O with the bidentate imines 1,10-phenanthroline and 2,2?-bipyridine (L-L) under hydrothermal conditions gives the neutral complexes [MF3(L-L)(OH2)].
A number of alternative tacn based ligands incorporating amide pendant arm functionality were synthesised and characterised as potential future architectures for metal complexes used in imaging applications.

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Identifiers

ORCID for Gillian Reid: orcid.org/0000-0001-5349-3468

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