Crystal structure of poly[(4-aminopyridine-κN)(N,N-dimethylformamide-κO)(μ3-pyridine-3,5-dicarboxylato-κ3 N:O 3:O 5)copper(II)]

An amino-functionalized chiral metal–organic framework with (10,3)-a topology has been constructed via the assembly of the achiral triconnected building block pyridine-3,5-dicarboxylate and a triconnected CuII centre.


Chemical context
Research on metal-organic frameworks (MOFs) has attracted much attention in recent years not only for their great potential applications, such as in gas storage, separation, fluorescence and magnetism, but also for their intriguing topologies and structural diversity (Allendorf et al., 2009). Of special interest is the rational design and synthesis of chiral networks, which offer great potential in non-linear optics, asymmetric catalysis, and chiral separation (Evans & Lin, 2002;Zhang & Xiong, 2012). Therefore, a logical target for synthesis would be a default structure that possesses chirality. The (10,3)-a network meets these requirements since it is mutually chiral and regarded as the default three-dimensional structure for the assembly of triconnected building blocks (Eubank et al., 2005;Han et al., 2013a).
On the other hand, amino-functionalized porous metalorganic frameworks have also attracted much attention. Recent research on amino-functionalized MOFs revealed that they have high CO 2 adsorption capacity at lower pressure due to the potential interaction between amino groups and CO 2 (Couck et al., 2009). Amino-functionalized MOFs can also act as reaction active sites for the post-synthesis modification of metal-organic frameworks (Shultz et al., 2011).

Structural commentary
The asymmetric unit of the title compound, Cu(3,5-PDC)(4-APY)(DMF), contains one Cu II ion, one 3,5-PDC anion, one 4-apy molecule and one DMF molecule. As shown in Fig. 1, each Cu II ion adopts a square-pyramidal (CuN 2 O 3 ) coordin-ation geometry. In the equatorial plane, the Cu II ion is coordinated by two oxygen atoms and one nitrogen atom, respectively, of three crystallographically independent 3,5-PDC ligands, and one nitrogen atom of a terminal 4-APY ligand. The oxygen atom of a terminal DMF molecule is bonded to the Cu II ion in the axial position to complete the square-pyramidal coordination geometry. The bond lengths and bond angles around the Cu II ion are in good agreement with similar structures (Eubank et al., 2005;Lu et al., 2006). The axial Cu-O DMF bond length [2.396 (4) Å ] is longer than the equatorial Cu-O carboxylate and Cu-N 4-APY bonds due to the Jahn-Teller effect of the Cu 2+ atom.

Figure 2
Crystal packing of the title compound viewed along the a axis, showing hydrogen bonds as dashed lines.
dinated 4-APY and DMF ligands are oriented to the interior of the channels and thus prevent self-interpenetration. The (10,3)-a topology leads to an enantiopure network of the title compound (Eubank et al., 2005;Han et al., 2013a), despite being formed solely from achiral molecular units.

Supramolecular features
By introducing 4-aminopyridine as co-ligand, the aminofunctionalized chiral metal-organic framework was successfully designed and synthesized. Additionally, the -NH 2 group of the 4-APY ligand can act as the donor N-H groups to form hydrogen bonds (Han et al., 2011). In the three-dimensional structure of the title compound, weak N-HÁ Á ÁO hydrogen bonds are observed (Table 1) in which the acceptors are provided by the non-coordinating oxygen atoms of the carboxylate groups of the 3,5-PDC ligands.

Synthesis and crystallization
The title compound was prepared by a solvothermal method. A mixture of pyridine-3,5-dicarboxylic acid (0.0339 g, 0.2 mmol), 4-aminopyridine (0.0098 g, 0.10 mmol) and Cu(NO 3 ) 2 Á3H 2 O (0.0484 g, 0.20 mmol) in 6 ml DMF solution was stirred at room temperature for 30 minutes, and subsequently sealed in a 25 ml Teflon-lined stainless steel reactor. The reactor was heated at 363 K for 3 d. A crop of blue, blockshaped single crystals of the title compound was obtained after cooling the solution to room temperature. The yield was approximately 70% based on Cu salt.

Poly[(4-aminopyridine-κN)(N,N-dimethylformamide-κO)(µ 3 -pyridine-3,5-dicarboxylato-κ 3 N:O 3 :O 5 )copper(II)]
Crystal data Absolute structure: Flack (1983), 1619 Friedel pairs Absolute structure parameter: 0.00 (2) Special details Geometry. All e.s.d.'s (except the e.s.d. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell e.s.d.'s are taken into account individually in the estimation of e.s.d.'s in distances, angles and torsion angles; correlations between e.s.d.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell e.s.d.'s is used for estimating e.s.d.'s involving l.s. planes. 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 R-factors(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.