Alkaline earth metal organotriphosphonates: Inorganic-organic polymeric hybrids from dication-dianion association

Mg2+ (1), Ca2+ (2), Sr2+ (3), or Ba2+ (4) salts react with amino-tris(methylenephosphonic acid) (H(6)AMP) in a 1: 1 ratio at low pH by replacing two of the six phosphonic acid protons, while the amino N maintains its proton, thus forming a zwitterion (H(4)AMP(2-)). All four structures are polymeric and substantially different from one another. In all structures, each phosphonate group is singly deprotonated, whereas the N atom is protonated. 1 consists of {Mg[HN(CH2PO3H)(3)(H2O)(3))}(n) units. Mg2+ ions form zigzag chains bridged by two of the three phosphonate groups. The third phosphonate group is noncoordinated and involved in hydrogen bonding. 3 is a three-dimensional (3D) polymer with the composition [Sr(H(4)AMP)](n). The Sr-atoms are seven-coordinate, with five monohapto and one chelating AMP ligands. In the 3D polymer 4, {Ba(H(4)AMP)(H2O)}(n), the two crystallographically independent Ball centers are 9- and 10-coordinated by phosphonate and H2O oxygen items. All bridging oxygen atoms belong to phosphonate moieties that act as chelates for one Ball and form 4-membered rings. The crystal structure of {Ba(H(4)AMP)(H2O)}(n) is characterized by the complete absence of hydrogen bonds.