Alkaline Earth Metal Organotriphosphonates:  Inorganic−Organic Polymeric Hybrids from Dication−Dianion Association

Abstract

Mg 2+ ( 1 ), Ca 2+ ( 2 ), Sr 2+ ( 3 ), or Ba 2+ ( 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(CH 2 PO 3 H) 3 (H 2 O) 3 ]} n units. Mg 2+ 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)(H 2 O)} n, the two crystallographically independent Ba 2+ centers are 9- and 10-coordinated by phosphonate and H 2 O oxygen atoms. All bridging oxygen atoms belong to phosphonate moieties that act as chelates for one Ba 2+ and form 4-membered rings. The crystal structure of {Ba(H 4 AMP)(H 2 O)} n is characterized by the complete absence of hydrogen bonds.