New barium 4-carboxyphenylphosphonates: Synthesis, characterization and interconversions

Three new barium 4-carboxyphenylphosphonates with formulae Ba(HOOCC6H4PO3H)2, BaH(OOCC6H4PO3), and Ba3(OOCC6H4PO3)2·2H2O were prepared and characterized by elemental analysis, thermogravimetric analysis, X-ray powder diffraction, energy-dispersive X-ray microanalysis, and infrared spectroscopy. It was found that depending on the acidity of the reaction medium, Ba(HOOCC6H4PO3H)2 can be converted to Ba3(OOCC6H4PO3)2·2H2O and vice versa. As an intermediate in these reactions, BaH(OOCC6H4PO3) is formed.The infrared spectra of these compounds were compared with analogous strontium compounds prepared and described previously and the position of the acidic hydrogen atom in BaH(OOCC6H4PO3) is deduced from the spectra.The structure of Ba(HOOCC6H4PO3H)2 was solved from X-ray powder diffraction data by an ab initio method using a FOX program with subsequent Rietveld refinement in the FullProf program. The compound is monoclinic, space group C2/c (No. 15), a = 49.382(1), b = 5.5196(1), c = 8.4977(2) Å, β = 127.52(1)°, and Z = 4. This compound has a layered structure built up from CPO3 phosphonate tetrahedra and BaO8 distorted tetragonal antiprisms and thus forming layers in the bc plane with organic groups pointing to the interlayer space.

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