Poly[[{μ3-dihydrogen [(pyridin-4-ylmethylimino)bis(methylene)]diphosphonato-κ5 O:O′,N,O′′:N′}copper(II)] dihydrate]

In the title polymer, {[Cu(C8H12N2O6P2)]·2H2O}n, the geometry of the five-coordinate CuII ion can best be described as slightly distorted square-pyramidal formed by one N and two O atoms of an N(CH2PO3H)2 group and one N atom from a pyridine ring. The elongated apex of the pyramid is occupied by one O atom from a third diphosphonate ligand. The interconnection of Cu2+ ions by the diphosphonate ligands results in the formation of a double-chain array along the b axis, in which the two sub-chains are interlocked by pairs of PO3 groups. The outside of each sub-chain is decorated by other PO3 groups. These double chains are further assembled into a three-dimensional supramolecular architecture via a large number of O—H⋯O hydrogen bonds between the phosphonate groups and lattice water molecules.

In the title polymer, {[Cu(C 8 H 12 N 2 O 6 P 2 )]Á2H 2 O} n , the geometry of the five-coordinate Cu II ion can best be described as slightly distorted square-pyramidal formed by one N and two O atoms of an N(CH 2 PO 3 H) 2 group and one N atom from a pyridine ring. The elongated apex of the pyramid is occupied by one O atom from a third diphosphonate ligand. The interconnection of Cu 2+ ions by the diphosphonate ligands results in the formation of a double-chain array along the b axis, in which the two sub-chains are interlocked by pairs of PO 3 groups. The outside of each sub-chain is decorated by other PO 3 groups. These double chains are further assembled into a three-dimensional supramolecular architecture via a large number of O-HÁ Á ÁO hydrogen bonds between the phosphonate groups and lattice water molecules.

Comment
During the past few decades, the syntheses of metal phosphonates with various structures has attracted much attention, owing to their potential applications in areas such as catalysis, ion exchange, intercalation chemistry, and material chemistry (Maeda, 2004;Mao, 2007;Shimizu et al., 2009). The strategy of attaching functional groups such as amine, hydroxyl, carboxylate, sulfonate, and sulfone groups to the phosphonic acid has proven to be an effective method for the isolation of a variety of metal phosphonates with new structures (Drumel et al., 1995;Mao et al., 2002;Liang & Shimizu, 2007;Du et al. 2006Du et al. , 2010b. Recently, we are interested in the combination of multiple functional groups to phosphonic acid as a more complex ligand. Herein, we report a copper(II) phosphonate based on an amino-bis(methyl-phosphonic acid) ligand, which contains pyridyl group as an additional functional group. As far as we are aware, only one layered cobalt(II) phosphonate has been reported based on the same ligand (Song & Mao, 2005).
The title compound (I) features a one-dimensional double-chain structure. The formula of it contains one Cu 2+ ion, one H 2 L 2anion and two lattice water molecules. Cu(1) ion is five-coordinate and its coordination geometry can be described as a slightly distorted square-pyramid ( Fig. 1): the square plane is formed by one N and two O atoms of a N(CH 2 PO 3 H) 2 group as well as one N atom of a pyridyl group from two H 2 L 2ligand, and the prolonged apex of the pyramid is occupied by one O atom from a third H 2 L 2ligand. The H 2 L 2ligand in compound (I) acts as a pentadentate chelating and bridging ligand.
It chelates one Cu 2+ ion by its N(CH 2 PO 3 H) 2 group in a tridentate fashion (2O and 1 N), and also bridges with other two Cu 2+ ions via its pyridyl group and a third O atom (Scheme 1). The two phosphonate groups of the H 2 L 2ligand both are 1H-protonated as the requirement for charge balance and also as indicated by two much longer P-O bonds. It is worthy of note that the strongly basic N atom in the H 2 L 2ligand is not protonated but bonded to a Cu 2+ ion, which is rarely observed for phosphonic acid ligands containing a tertiary amine group (Yang et al., 2008;Du et al., 2009Du et al., , 2010a.
The interconnection of the Cu 2+ ions by the HL 2anions results in the formation of a one-dimensional double-chain array along the b-axis, in which the two sub-chains are inter-locked by pairs of P(1)O 3 groups and the outside of each sub-chain is decorated by P(2)O 3 groups. It is worthy of note that such two sub-chains are related by inversion centers, and the shortest Cd···Cd distance between them is 5.170 (4) Å while that in each sub-chain is 9.000 (1) Å. These double-chains are further assembled into a three-dimensional supramolecular architecture via a large number of hydrogen bonds between the phosphonate groups and lattice water molecules ( Fig. 3

Poly[[{µ 3 -dihydrogen [(pyridin-4-ylmethylimino)bis(methylene)]diphosphonatoκ 5 O:O',N,O'':N'}copper(II)] dihydrate]
Crystal data [Cu(C 8  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.