catena-Poly[[[tetraaquacadmium(II)]-μ-3,3′-[p-phenylenebis(oxymethylene)]bis(1-pyridinioacetate)] dinitrate hemihydrate]

In the title polymeric coordination complex, {[Cd(C22H20N2O6)(H2O)4](NO3)2·0.5H2O}n, obtained from the self-assembly of the flexible double betaine 3,3′-[p-phenylenebis(oxymethylene)]bis(1-pyridinioacetate) with cadmium nitrate, both the octahedrally coordinated CdII cation and the substituted betaine ligand lie on inversion centres. The chains constructed through the trans-related acetate groups of the ligand are inter-connected via O—H⋯O hydrogen bonds involving coordinated aqua ligands, the nitrate anions and the partial-occupancy (0.25) water molecule of solvation, forming a three-dimensional structure.

In the title polymeric coordination complex, {[Cd(C 22 H 20 -N 2 O 6 )(H 2 O) 4 ](NO 3 ) 2 Á0.5H 2 O} n , obtained from the selfassembly of the flexible double betaine 3,3 0 -[p-phenylenebis(oxymethylene)]bis(1-pyridinioacetate) with cadmium nitrate, both the octahedrally coordinated Cd II cation and the substituted betaine ligand lie on inversion centres. The chains constructed through the trans-related acetate groups of the ligand are inter-connected via O-HÁ Á ÁO hydrogen bonds involving coordinated aqua ligands, the nitrate anions and the partial-occupancy (0.25) water molecule of solvation, forming a three-dimensional structure.

Related literature
For betaine-metal coordination compexes, see: ; . For the structure of the copper(II) complex with the ligand employed here, see: Pan & Lian (2010).
Financial support from Zhengzhou University is greatly appreciated.
Supplementary data and figures for this paper are available from the IUCr electronic archives (Reference: ZS2059).

Comment
The coordinative bond approach has been widely used in the construction of supramolecular coordination compounds. As a parallel development, it is possible to use highly directional hydrogen bonds as a means of controlling self-assembly. Double betaines, comprising two betaine moieties and having two terminal anionic carboxylate substituent groups are of great utility for linkage to a broad array of metal ions, generating a variey of supramolecular entities, ranging from discrete complex units, through networks linked by hydrogen bonding, to metallo-supramolecular systems   (Table 1), to form a three-dimensional structure (Fig.   2).

Refinement
The H atoms of the water molecule were located in a difference map but were constrained in the refinement. Other H atoms were positioned geometrically and refined using a riding model with C-H = 0.93, 0.97 Å and O-H = 0.85 Å, with U iso (H) = 1.2U eq (C).