Ab initio structure determination of [(dimethylamino)methylene]bis[phosphonic acid] dihydrate from X-ray powder diffraction data: Comparison with thecorresponding monohydrate and unhydrated form

The structure of [(dimethylamino)methylene]bis[phosphonic acid] dihydrate ( C3H11NO6P2. 2H(2)O; 1) was solved ab initio from synchrotron powder X-ray d iffraction data. The structure determination was based on direct methods co mbined with difference Fourier techniques, and the refinement was carried o ut 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 monoclini c, space group P2(1)/c, Z = 4, with a = 10.6644(1), b = 9.1599(1), c = 10.5 213(1) Angstrom, and beta = 98.353(1)degrees The structure analysis indicat es two nonequivalent P-atoms in the molecule of 1 which are also observed i n the corresponding monohydrate 2 and unhydrated form 3. All three compound s exhibit extended H-bonding networks which result in remarkably different P-31-NMR spectra. The [(dimethylamino)methylene]bis[phosphonic acids] 1-3 c rystallize in the betaine-type structure which, therefore, contains two non equivalent P-atoms. The -P(=O)(OH)(2) and -P(=O)(OH)O- groups of 1-3 are in volved in a number of strong H-bonds which can be characterized by the diff erent P-31-NMR chemical shifts of the two P-atoms of 1-3.