Structural investigations of unsubstituted polymethylenediphosphonic acids. 1. The crystal and molecular structure of methylenediphosphonic and ethane-1,2-diphosphonic acids

A previously discussed correlation between an increase in hydrogen-bond strength with a decrease in P-O(H) distance in methylene-substituted diphosphonic acids is not upheld for hydrogen bonding in unsubstituted polymethylenediphosphonic acids. Strong asymmetric hydrogen bonds link -(OH)P(=O)OH moieties into infinite chains spiralling about 2/sub 1/ axes and cross linking the molecules to form three-dimensional molecular chains in both methylenediphosphonic acid (PCP), CH/sub 2/(PO/sub 3/H/sub 2/)/sub 2/, and ethane-1,2-diphosphonic acid (PC/sub 2/P), (CH/sub 2/)/sub 2/(PO/sub 3/H/sub 2/)/sub 2/. PCP and PC/sub 2/P show short P-C bond distances (1.793(2) and 1.786(2) A, respectively as compared with substituted diphosphonic acids (approximately 1.83 A), PCP crystallizes in the monoclinic space group, P/sub 1//c, with unit cell dimensions a = 7.836(4), b = 5.497(3), c = 13.766(7) A, ..beta.. = 103.60(2)/sup 0/, and Z = 4. PC/sub 2/P crystallizes in the monoclinic space group P2/sub 1//c with unit cell dimensions, a = 5.856(2), b = 5.272(2), c = 11.665(4) A, ..beta.. = 100.69(2)/sup 0/, and Z = 2. Three-dimensional x-ray data were collected for both compounds using a GE XRD-490 automated diffractometer. Full-matrix least-squares refinement of PCP gave a final R/sub F/ = 0.036 for a total of 1017 unique reflections, while a similar refinement ofmore » PC/sub 2/P gave a final R/sub F/ = 0.046 for 1019 unique reflections. Implications for cation coordination and liquid-liquid extraction chelation sites are discussed for ionized PCP and PC/sub 2/P.« less