Ab initio Structure Determination of [(Dimethylamino)methylene]bis[phosphonic Acid] Dihydrate from X‐Ray Powder Diffraction Data: Comparison with the Corresponding Monohydrate and Unhydrated Form

Abstract

The structure of [(dimethylamino)methylene]bis[phosphonic acid] dihydrate (C3H11NO6P2⋅2 H2O; 1) was solved ab initio from synchrotron powder X-ray diffraction data. The structure determination was based on direct methods combined with difference Fourier techniques, and the refinement was carried out using the Rietveld method. Using this high-quality diffraction pattern, it was possible to index a second phase which corresponds to the structure of the known [(dimethylamino)methylene]bis[phosphonic acid] monohydrate (2). [(Dimethylamino)methylene]bis[phosphonic acid] dihydrate (1) is monoclinic, space group P21/c, Z=4, with a=10.6644(1), b=9.1599(1), c=10.5213(1) A, and β=98.353(1)°. The structure analysis indicates two non-equivalent P-atoms in the molecule of 1 which are also observed in the corresponding monohydrate 2 and unhydrated form 3. All three compounds exhibit extended H-bonding networks which result in remarkably different 31P-NMR spectra. The [(dimethylamino)methylene]bis[phosphonic acids] 1 – 3 crystallize in the betaine-type structure which, therefore, contains two nonequivalent P-atoms. The −P(=O)(OH)2 and −P(=O)(OH)O− groups of 1 – 3 are involved in a number of strong H-bonds which can be characterized by the different 31P-NMR chemical shifts of the two P-atoms of 1 – 3.