Modellsysteme für die Gallium-Extraktion, II / Model Systems for Gallium Extraction, II

The introduction of a methyl substituent in the 2-position of 8-oxy-quinoline strongly alters its ligand properties in the complex formation with gallium(III). From aqueous solutions (ammonium acetate buffer, acetic acid, gallium nitrate) a pale-green complex (2-MeOx)₂GaOAc is precipitated instead of the tris(oxinate) chelate. According to a single crystal X-ray diffraction study, the gallium atom is pentacoordinate in this complex, with two chelating 2-MeOx and one monocoordinate acetate ligands. Similar acetato complexes were generated with equally hindered 2,4-dimethyl- and 2,5,7-trimethylquinoline, as well as with 2-ethyl- and 2-n-propylquinoline ligands. Retaining the 2-MeOx ligand, acetate OAc could be exchanged for trifluoroacetate, propionate, cyanoacetate, and (in binuclear complexes) equivalents of succinic, maleic, fumaric, and acetylenedicarboxylic acid. From this series, crystal and molecular structures have also been determined for (2-MeOx)₂GaOC(O)CH₂CN and [(2-MeOx)₂GaOC(O)CHJ₂]2 (with crystal nitrobenzene). In the cyanoacetate, the nitrile function is not involved in intra/intermolecular metal coordination. In the binuclear succinate the molecule has a crystallographical center of inversion. As in the cyanoacetate, the environment of the two equivalent gallium centers of the succinate is very similar to that of the acetate complex.