Heteronuclear NMR Spectroscopic Investigations of Gallium Complexes of Substituted Thiosemicarbazones Including X-Ray Crystal Structure, a New Halogen Exchange Strategy, and 18F Radiolabelling

Abstract

Five thiosemicarbazone ligands have been synthesized, and their coordination chemistry with gallium was investigated. The reaction of these thiosemicarbazones with gallium chloride in alcohol solutions in the presence of a base yielded the corresponding penta-coordinated Ga-Cl metal complexes. In contrast, the reaction of gallium nitrate with the ligands in the presence of alkoxides resulted in the formation of the corresponding Ga-alkoxides, rather than the anticipated Ga-nitrate complex. The crystal structures of gallium chloride and gallium methoxide complexes of diphenylthiosemicarbazone comprise a planar configuration of the tetradentate-coordinated thiosemicarbazone with Ga3+ ion, with the chloride or methoxide groups occupying the apical coordination site. The corresponding ethoxido complex was also prepared in an identical fashion, and NMR analysis confirmed structural similarity to the methoxido complex. Facile halogen exchange reactions of the gallium chloride complexes were achieved by treatment with silver nitrate, followed by addition of KF or KI to generate the gallium fluoride and iodide complexes, respectively. This method of exchange using halogenated inorganic salts aids the preparation of group 13 fluorides, which are notoriously insoluble in organic solvents, for complexation with organic ligands. All compounds have been fully characterized by NMR, and the X-ray crystal structures of two of the complexes are reported. Additionally, the positron-emitting isotope 18F was introduced in the structure of the diphenyl gallium thiosemicarbazone complex.