Structural characterization of two salts derived from tetrafluorophthalic acid and isopropylamine.

An anomalous acid salt (2) in the 2:1 ratio of tetrafluorophthalic acid and isopropylamine was prepared from bis(isopropylammonium) tetrafluorophthalate (1). Both salts were characterized by ¹³C NMR spectroscopy in solution and in solid state, as well as X-ray crystallography. Crystallographic data revealed that molecules of 2 contain a tetrafluorophthalic acid component and a tetrafluorohydrogenphthalate unit. The geometrical arrangement in the crystal structure of 2 can be attributed to a network of intermolecular N–H···O, O–H···O, and C–H···O hydrogen bonds. Fluorine atoms were not involved in such interactions. The ¹³C NMR chemical shifts of solutions 1 and 2, as well as tetrafluorophthalic acid are discussed. In case of the salt 2, only four different signals were observed for the aromatic and CO carbons and thus, all carboxyl/carboxylate groups appear to be equivalent in solution. In the solid-state NMR spectrum of 2, a splitting into three resonances arising from the ipso-carbons was observed, in agreement with the anomalous acid salt structure.Markus Pietsch, Hans-Georg Häcker, Gregor Schnakenburg, Wilfried Hoffbauer, Martin Nieger and Michael Gütscho