Synthesis, crystal structures and characterization of three novel main group metal diphosphonate coordination polymers of Ca(II), Sr(II) and Pb(II) with 1-aminodiphosphonic acid

Hydrothermal reactions of 1-aminoethylidenediphonic acid { CH3C(NH2)(PO3H2)2, H4aedp} with CaCl2, Sr(NO3)2 and Pb(AC)2·3H2O resulted in three new main group metal diphosphonate coordination polymers. Namely, Ca[CH3C(NH3)(PO3H)2]2[CH3C(NH3)(PO3H)(PO3H2)]2 (1), Sr[CH3C(NH2) (PO3H)2(H2O)]·H2O(2) and Pb[CH3C(NH3)(PO3H)(PO3)]·H2O(3). The calcium (II) ion in complex 1 is octahedrally coordinated by six phosphonate oxygen atoms from six diphosphonate ligands, four of them in a bidentate and two in a monodentate fashion. The neighboring calcium ions are linked by pairs of P–O–P bridges, resulting in a pseudo-one-dimensional chain along the c axis. Complex 2 is the first example of the strontium phosphonate which has not been synthesized and structurally characterized before. It features a layered structure, in which strontium(II) ions are eight coordinated by an aqua ligand and seven phosphonate oxygen atoms from four diphosphonate ligands, two SrO8 polyhedra are bridged into a demeric unit by a pair of oxygen atoms and two diphosphonate groups. And such dimers are interconnected by the P–O–P bridges and O bridges to form complicated layers parallel to the ab plane. Complex 3 also has a layered structure in which the lead (II) ions are six coordinated by six phosphonate oxygens from four equivalent diphosphonate ligands. Each pair of PbO6 octahedra forms a dimeric unit via O–O edge-sharing and such dimeric units are interconnected via O–O edge-sharing to form a one-dimensional chain along the b-axis. These 1D chains are further bridged by the phosphonate ligands to form a complicated layer parallel to the bc plane. This type of layered structure has not been reported in other lead phosphonates.