Poly[bis[μ2-(dimethylazaniumyl)methylenediphosphonato]magnesium]

The title compound, [Mg(C3H10NO6P2)2]n, synthesized by a hydrothermal method, adopts a one-dimensional polymeric chain structure and is isotypic with the previously reported Cd complex based on the ligand N,N-dimethylaminomethane-1,1-diphosphonic acid (H4 L). The asymmetric unit contains one half Mg2+ ion and one H3 L − anion. The unique Mg2+ ion lies on an inversion center and is octahedrally coordinated by O atoms from six phosphonate groups of four different H3 L − anions. Each H3 L − anion, with one protonated N atom and two phosphonate OH groups, serves as a tridentate ligand. Two of its six phosphonate O atoms chelate to a Mg2+ cation in a bidentate fashion, while a third O atom bridges to a neighbouring Mg2+ ion. The interconnection of Mg2+ ions by the H3 L −anions leads to the formation of a polymer chain along the a axis in which the adjacent Mg2+ ions are doubly bridged by two equivalent H3 L − anions. These discrete chains are further assembled into a three-dimensional supramolecular network via O—H⋯O and N—H⋯O hydrogen bonds involving the non-coordinated phosphonate O atoms and the protonated N atoms.

The title compound, [Mg(C 3 H 10 NO 6 P 2 ) 2 ] n , synthesized by a hydrothermal method, adopts a one-dimensional polymeric chain structure and is isotypic with the previously reported Cd complex based on the ligand N,N-dimethylaminomethane-1,1diphosphonic acid (H 4 L). The asymmetric unit contains one half Mg 2+ ion and one H 3 L À anion. The unique Mg 2+ ion lies on an inversion center and is octahedrally coordinated by O atoms from six phosphonate groups of four different H 3 L À anions. Each H 3 L À anion, with one protonated N atom and two phosphonate OH groups, serves as a tridentate ligand. Two of its six phosphonate O atoms chelate to a Mg 2+ cation in a bidentate fashion, while a third O atom bridges to a neighbouring Mg 2+ ion. The interconnection of Mg 2+ ions by the H 3 L À anions leads to the formation of a polymer chain along the a axis in which the adjacent Mg 2+ ions are doubly bridged by two equivalent H 3 L À anions. These discrete chains are further assembled into a three-dimensional supramolecular network via O-HÁ Á ÁO and N-HÁ Á ÁO hydrogen bonds involving the non-coordinated phosphonate O atoms and the protonated N atoms.
Data collection: SMART (Bruker, 2008); cell refinement: SAINT (Bruker, 2008); data reduction: SAINT; program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: SHELXTL (Sheldrick, 2008) and DIAMOND (Brandenburg, 1999); software used to prepare material for publication: SHELXTL. Comment Among many of the phosphonate ligands studied so far, methylenediphosphonic acid and its derivatives are quite unique because they feature a close connection of two phosphonate moieties via one carbon atom, which facilitate their combined coordination ability to act as a [CP 2 O 6 ] unit rather than two [CPO 3 ] units. As a result, they show diversified coordination capabilities with metal ions and thus lead to the formation of new structural types. Recently, by using such ligand types, i.e. N,N-dimethylaminomethane-1,1-diphosphonate, we have isolated a series of diphosphonate complexes of metals such as Al III , Fe III , Cd II , Pb II and Ba II , which exhibit variable structures such as zero-dimensional, one-dimensional, double-1dimensional, double-2-dimensional, and three-dimensional structures (Du et al., 2009(Du et al., , 2010a. As an expansion of our previous work, we have also obtained a one-dimensional magnesium(II) diphosphonate, namely [Mg(C 6 H 20 N 2 O 12 P 4 )] n , which is isostructural with the previously reported cadmium(II) complex based on the same ligand and shows a one-dimensional chain structure. The asymmetric unit contains a half Mg 2+ cation and one H 3 Lanion. The unique Mg 2+ cation lies on an inversion center and is octahedrally coordinated by the O atoms of six phosphonate groups from four H 3 Lanions. The Mg-O [2.0448 (11) -2.1879 (11) Å] bond lengths are comparable to those reported for other Mg II phosphonate complexes (Lutz & Muller, 1995;Distler et al., 1999;Stock & Bein, 2004). The unique H 3 Lanion, with one protonated N atom and two phosphonate OH groups, serves as a tridentate ligand. By using three of its six phosphonate O atoms, it chelates in a bidentate fashion with one Mg 2+ cation and also bridges to a second Mg 2+ ion. The interconnection of Mg 2+ cations by the H 3 Lanions leads to the formation of a one-dimensional chain along the a-axis, in which the adjacent Mg 2+ ions are doubly bridged by two equivalent H 3 Lanions. These discrete one-dimensional chains are further assembled into a three-dimensional supramolecular network via O-H···O and N-H···O hydrogen bonds involving the non-coordinated phosphonate O atoms and the protonated N atoms.

Experimental
For the preparation of (I), a mixture of Mg(NO 3 ) 2 (0.20 mmol) and H 4 L (0.50 mmol) and ethanol (3 ml) in 10 ml distilled water, was sealed into a Parr Teflon-lined autoclave (23 ml) and heated at 393 K for 3 d. Colorless block-shaped crystals were collected in ca 55% yield based on Mg. Analysis calculated for C 6 H 20 N 2 O 12 Mg 1 P 4 : C 15.65, H 4.38, N 6.08%; found: C 15.59, H 4.48, N 6.03%.

Refinement
The N-bound and the tertiary C-bound H atoms were positioned geometrically and refined using a riding model:  Fig. 1. A view of the selected unit of (I), showing the atom-numbering scheme. Displacement ellipsoids are drawn at the 30% probability level. Hydrogen atoms have been omitted for clarity. [Symmetry codes: (i) x -1, y, z; (ii) -x + 1, -y + 1, -z + 1; (iii) -x, -y + 1, -z + 1; (iv) x + 1, y, z.] Fig. 2. A view of the chain structure of (I) along the a-axis. The CPO 3 tetrahedra are shaded in purple. Mg, N and C atoms are drawn as cyan, blue and grey circles, respectively. Hydrogen atoms have been omitted for clarity. Fig. 3. A view of the three-dimensional supramolecular structure of (I) down the a-axis. The MgO 6 octahedra and CPO 3 tetrahedra are shaded in cyan and purple, respectively. N, C and H atoms are drawn as blue, grey and green circles, respectively. Hydrogen bonds are represented by dashed lines.

Poly[bis[µ 2 -(dimethylazaniumyl)methylenediphosphonato]magnesium]
Crystal data [Mg(C 3   Refinement. Refinement of F 2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F 2 , conventional R-factors R are based on F, with F set to zero for negative F 2 . The threshold expression of F 2 > σ(F 2 ) is used only for calculating Rfactors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F 2 are statistically about twice as large as those based on F, and R-factors based on ALL data will be even larger.