Poly[[tetraaqua(μ6-2,2′-diiodobiphenyl-4,4′,5,5′-tetracarboxylato)dizinc(II)] dihydrate]

In the title compound, {[Zn2(C16H4I2O8)(H2O)4]·2H2O}n, two crystallographically independent ZnII atoms are each located on a twofold rotation axis. Both ZnII atoms are in distorted octahedral coordination geometries: one is coordinated by six O atoms from four carboxylate groups, while the other is coordinated by two carboxylate groups and four water molecules. The tetracarboxylate ligand molecules connect the ZnII atoms, completing a three-dimensional metal–organic framework. O—H⋯O hydrogen bonds link the metal–organic framework with the uncoordinated water molecules.

In the title compound, {[Zn 2 (C 16 H 4 I 2 O 8 )(H 2 O) 4 ]Á2H 2 O} n , two crystallographically independent Zn II atoms are each located on a twofold rotation axis. Both Zn II atoms are in distorted octahedral coordination geometries: one is coordinated by six O atoms from four carboxylate groups, while the other is coordinated by two carboxylate groups and four water molecules. The tetracarboxylate ligand molecules connect the Zn II atoms, completing a three-dimensional metal-organic framework. O-HÁ Á ÁO hydrogen bonds link the metalorganic framework with the uncoordinated water molecules.

Comment
Interest in the self-assembled construction of coordination polymers is rapidly increasing not only owing to their potential applications in gas storage, ion-exchange, catalysis, electrical conductivity, nonlinear optics and magnetism, but also because of their fascinating diversified architectures and topologies (Cordes et al., 2006;Garay et al., 2007). Multicarboxylate ligands, such as 1,4-benzenedicarboxylate (Williams et al., 2005), 1,3,5-benzenetricarboxylate (Noro et al., 2007) and biphenyl-3,3',4,4'-tetracarboxylate (Weng et al., 2007), have been extensively employed in the construction of novel metalorganic complexes with multidimensional networks and interesting properties. In view of the excellent coordination capability of multicarboxylate anions and the solubility of diaryliodonium salts, we employed 2,2'-diiodobiphenyl-4,4',5,5'-tetracarboxylic acid (H 4 L), as an organic building unit to generate three dimensional metal-organic framework. In this paper, we describe the synthesis of a novel zinc(II) complexes, namely, [Zn 2 (L)(H 2 O) 4. 2H 2 O] n , by reaction of Zn(NO 3 ) 2 .6H 2 O and H 4 L via hydrothermal method, which was characterized by IR, elemental analysis and X-ray single-crystal analysis.
To the best of our knowledge, transition metal coordination polymers based on diiodobiphenyl tetracarboxylate has never been reported before. This work may provide useful information for the further design of metal-organic frameworks with interesting architectures using diiodobiphenyl tetracarboxylate as versatile multidentate ligands.
The title complex crystallizes in the monoclinic system, space group C2, with two crystallographically independent Zn II atoms each located on a twofold axis. As shown in Fig. 1, both Zn II atoms are in distorted octahedral configurations and are connected by carboxylate groups. The Zn1 center is coordinated by four carboxylate groups, that is, 4(4')-COOfrom two different L 4ligands in a monodentate fashion (O1 and O1 ii ) and 5(5')-COOfrom other two L 4ligands in a bischelating fashion (O3 i , O4 i , O3 iii and O4 iii ). In contrast, the Zn2 center is coordinated by four O atoms of water (O5, O6, O5 ii and O6 ii ) and two carboxylate groups: 4(4')-COOfrom two different L 4ligands in a monodentate fashion (O2 and O2 ii ). The Zn-O bond lengths fall in the range of 1.953 (6)-2.368 (7) Å, similar to those in other zinc-tetracarboxylate coordination polymers (Wang et al., 2007). Hence, the L 4ligand acts as a octadentate ligand, linking six different Zn II atoms to form a three-dimensional metal-organic framework (Fig. 2); the 5,5'-carboxyl groups adopt a bidentate bridging mode, while the 4,4'-carboxyl groups exhibit a bis(monodentate) bridging mode. Within the L 4ligand, the two phenyl rings are almost perpendicular to each other with the dihedral angle of 88.6 (1)°, and the dihedral angles between 4,5(4',5')-carboxylate groups and the plane of correspondingly linked phenyl rings are respectively 72.0 (1) and 27.3 (1)°.
There are various O-H···O hydrogen bonds associated with the coordinated water molecules, uncoordinated water molecules and carboxylate O atoms in the title complex, linking the metal-organic framework with the uncoordinated water molecules.
supplementary materials sup-2 Experimental All chemicals were of analytical grade and used without further purification. According to the reported procedure (Beringer et al., 1953;Qiu et al., 2007), H 4 L was prepared by iodine substitution of the N-methyl protected 3,3',4,4'-biphenyltetracarboxylicdianhydride, hydrolysis by 3M KOH and acidification by 6.5 M HCl to pH 1.0. The hydrothermal reaction was performed in a 25 ml Teflon-lined stainless steel autoclave under autogenous pressure. An solution of H 4 L (58 mg, 0.1 mmol), Zn(NO 3 ) 2 .6H 2 O (59 mg, 0.2 mmol), 2 ml EtOH and 8 ml H 2 O was adjusted to pH 7 with 1 M NaOH solution and then heated at 140 °C for 3 days. After the sample was cooled slowly to room temperature, colourless crystals were obtained   Fig. 1. A view of the title complex, depicting the Zn II coordination environment. Displacement ellipsoids are drawn at the 30% probability level. The solvent water molecules and H atoms have been omitted for clarity [symmetry codes: (i) -x + 1/2, y -1/2, -z + 1; (ii) -x, y, -z + 1; (iii) x -1/2, y -1/2, z].  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.