Poly[(μ5-2,6-dimethylpyridine-3,5-dicarboxylato)zinc]

In the polymeric title complex, [Zn(C9H7NO4)]n, the ZnII cation is located on a twofold rotation axis and is coordinated by five 2,6-dimethylpyridine-3,5-dicarboxylate (mpdc) anions in a distorted ZnNO4 trigonal–bipyramidal geometry. The mpdc anion is also located on the twofold rotation axis and bridges five ZnII cations, forming the three-dimensional polymeric complex. Weak C—H⋯π interactions are present in the crystal structure.

In the polymeric title complex, [Zn(C 9 H 7 NO 4 )] n , the Zn II cation is located on a twofold rotation axis and is coordinated by five 2,6-dimethylpyridine-3,5-dicarboxylate (mpdc) anions in a distorted ZnNO 4 trigonal-bipyramidal geometry. The mpdc anion is also located on the twofold rotation axis and bridges five Zn II cations, forming the three-dimensional polymeric complex. Weak C-HÁ Á Á interactions are present in the crystal structure.
Cg is the centroid of the pyridine ring.
Data collection: CrystalClear (Rigaku, 2005); cell refinement: CrystalClear; data reduction: CrystalClear; program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: DIAMOND (Brandenburg, 2008) and ORTEP-3 (Farrugia, 1997); software used to prepare material for publication: PLATON (Spek, 2009).  Recently, research on metal-organic frameworks (MOFs) has become of increasing interest (Long & Yaghi, 2009). However, it is still a great challenge to assemble a predicted structure because there are numerous influences that can play decisive roles on the structure and crystal packing. Fortunately, these uncertainties can be reduced by the use of well selected spacers that have the ability to aggregate metal ions into different secondary building units (Zhao et al., 2003). Herein we reports an interesting five-connected zeolite-like coordination polymer based on highly-substituted pyridinedicarboxylates.

Structure Reports Online
The title compound is a three-dimensional framework built from Zn cations that are linked by mpdc anions. From this arrangement cavities are formed. Zn1 is coordinated by four oxygen atoms from four different CO 2 groups of mpdc ligands and one pyridyl nitrogen atom from another mpdc ligand. The mpdc ligand bridges five different Zn atoms and favors the construction of the structure with zeolite-like topology.The topology of the title compound is identical with the reported [Cd(mpdc)] n (Huang et al., 2007), but the coordination sphere of cation, the binding mode of the carboxylate group and the synthesis condition are different.
The combination of the dramatic twists between two carboxylate groups in mpdc ligands results in the formation of the intersecting double-stranded helical chain comprised of [Zn(CO 2 ) 2 ] n (Zn atoms as nodes).

Experimental
All chemicals were of reagent grade and used as purchased without further purification. A mixture of Zn(NO 3 ) 2 .6H 2 O (450 mg, 1.5 mmol), H 2 mpdc (97.5 mg, 0.5 mmol), (Et) 3 N 0.07 mL and H 2 O 10 mL was sealed in a 25 ml stainless steel reactor with Teflon liner and directly heated to 180 °C for 3 days, and then cooled to room temperature. The crystal samples were washed with methanol to give the title compound in about 35% yield (based on H 2 mpdc ligand).

Refinement
Constraint instruction 'delu 0.001 Zn1 O1' was used in the refinement. All H atoms were placed in geometrically idealized positions (C-H = 0.93 Å) and treated as riding on their parent atoms, with U iso (H) = 1.5U eq (C) for methyl H atoms and 1.2U eq (C) for aromatic H atom.