Triaza-macrocyclic complexes of aluminium, gallium and indium halides: fast 18F and 19F incorporation via halide exchange under mild conditions in aqueous solution

Abstract

Rapid and complete fluorination of the complexes [MCl3(L)] (L = Me3-tacn, BzMe2-tacn, M = Al, Ga, In) occurs at room temperature via reaction of a MeCN solution of the complex with 3 mol. equivs. of KF in water. The Ga and In complexes are also readily fluorinated using R4NF (R = Me or nBu) in MeCN solution, whereas no reaction occurs with the Al species under these conditions. The distorted octahedral fac-trifluoride coordination at M is confirmed in solution by multinuclear (19F, 27Al, 71Ga and 115In) NMR spectroscopic studies, leading to sharp resonances with 19F-71Ga and 19F-115In couplings evident. The [MF3(L)] are extremely stable in aqueous solution and at low pH; they crystallise as tetrahydrates, [MF3(Me3-tacn)]·4H2O, with extended H-bonding networks formed through both F···H-O and O···H-O contacts. [InF3(BzMe2-tacn)]·1.2H2O also shows intermolecular F···H-O hydrogen bonding contacts. The prospects for developing this coordination chemistry further to take advantage of the high metal-fluoride bond energies to enable rapid, late-stage fluorination of large macromolecules under mild conditions for PET imaging applications in nuclear medicine are discussed. This work also demonstrates that F-18 radiolabelling to form [F-18] [GaF3(BzMe2-tacn)] is effected readily at room temperature in aqueous MeCN over 30-60 mins on addition of 2.99 mol equivs. of [19F]-KFaq and 0.4 mL [18F]-KFaq (100 – 400 MBq) to [GaCl3(BzMe2-tacn)] with ca. 30% incorporation.

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Identifiers

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

ORCID for William Levason: orcid.org/0000-0003-3540-0971

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