New hypodiphosphates of the alkali metals: Synthesis, crystal structure and vibrational spectra of the hypodiphosphates(IV) M2[(H2P2O6)(H4P2O6)] (M=Rb and Cs)

The new hypodiphosphates(IV) Rb2[(H2P2O6)(H4P2O6)] (1) and Cs2[(H2P2O6)(H4P2O6)] (2) were synthesized by soft chemistry reactions from aqueous solutions of hypophosphoric acid and the corresponding heavy alkali-metal carbonates. Their crystal structures were determined by single crystal X-ray diffraction. Both compounds crystallize isotypic in the triclinic space group P-1 with one formula unit in the unit cell. The structures are built up by discrete (H2P2O6)2− and (H4P2O6) units in staggered conformation for the P2O6 skeleton and the corresponding alkali-metal cations. In the (H2P2O6)2− ion the hydrogen atoms are in a “trans–trans” conformation. O·H–O hydrogen bonds between the (H2P2O6)2− and (H4P2O6) groups consolidate the structures into a three-dimensional network. The FT-Raman and 31P and 1H and MAS NMR spectra of the title compounds have been recorded and interpreted, especially with respect to their assignment to the (H2P2O6)2− and (H4P2O6) groups. Thermogravimetric data of 2 have been interpreted in terms of a thermal decomposition model.