Two N,N′-bis(pyridin-4-yl)pyridine-2,6-dicarboxamide coordination compounds

The title compounds both contain a central metal atom in a distorted octahedral geometry coordinated equatorially by four oxygen atoms from water molecules. In the MnII complex, the axial positions are occupied by a nitrogen atom from the ligand and an oxygen atom from the sulfate anion, whereas in the CdII complex they contain two nitrogen atoms from two different ligands and the sulfate anion only serves as the charge-balancing ion. π–π stacking between pyridine rings plays a crucial role in the molecular self-assembly of the two structures.

In complexes (I) and (II), the heterocyclic nitrogen ligand (H 2 L 1 ) acts as a monodentate ligand. The results indicate that the rational design and selection of ligands with heterocyclic nitrogen systems is an effective synthetic strategy to construct complexes via self-assembly. The asymmetry of the [N,N 0bis(pyridin-4-yl)pyridine-2,6-dicarboxamide ligand has resulted in some novel structures (Li et al., 2012). Further study is ongoing.
When MnSO 4 ÁH 2 O is replaced by CdSO 4 Á8/3H 2 O, complex (II) is obtained, which crystallizes in the monoclinic space group C2/c. As illustrated in Fig. 2, Cd II also shows an octahedral environment coordinating four oxygen atoms from four water molecules and two axial nitrogen atoms from two symmetry-related H 2 L 1 ligands (Table 2). In contrast to complex (I), the sulfate group does not coordinate to the cadmium(II) atom, but balances the compound charge as a Table 1 Selected geometric parameters (Å , ) for (I).

Figure 2
The molecular structure of the title complex (II) with displacement ellipsoids shown at the 50% probability level. Symmetry code: (i) Àx, y, Àz + 1 2 .

Figure 1
The molecular structure of the title complex (I) with displacement ellipsoids shown at the 50% probability level.  (Table 4).
In complexes (I) and (II), the heterocyclic nitrogen ligand (H 2 L 1 ) acts as a monodentate ligand. The results indicate that the rational design and selection of ligands with heterocyclic nitrogen systems is an effective synthetic strategy to construct complexes via self-assembly. The asymmetry of the [N,N 0bis(pyridin-4-yl)pyridine-2,6-dicarboxamide ligand has resulted in some novel structures. Further study is ongoing.

Figure 3
The weak interactions between molecules of complex (I).interactions are shown as red dashed lines, hydrogen-bonding interactions as blue dashed lines.

Figure 4
The molecular packing of (I) viewed along the a axis.

Figure 5
The weak interactions between molecules of complex (II) with hydrogenbonding interactions andinteractions shown as blue and red dashed lines, respectively.

Synthesis and crystallization
The heterocyclic nitrogen ligand (H 2 L 1 ) was prepared using a modified literature procedure (Qin et al., 2003;Li et al., 2012).
In the preparation of complex (I), H 2 L 1 (0.1 mmol, 0.032 g) in N,N 0 -dimethylformamide solution (4 mL) was gradually added to MnSO 4 . H 2 O (0.1 mmol, 0.017 g) in a mixed solution (3 mL, water-methanol v/v = 1/3). After standing for 5 min, the suspension was filtered and the filtrate was kept at room temperature in the dark. One week later, colourless single crystals suitable for X-ray diffraction were obtained. Complex (II) was prepared with the same procedure employed for (I) except that CdSO 4 . 8/3H 2 O (0.1 mmol, 0.026 g) was used instead of MnSO 4 . H 2 O.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 5. Water H atoms were located in a difference-Fourier map and freely refined. All other H atoms were positioned gemetrically and refined using a riding model with bond lengths of 0.93 Å (C-H, aromatic), 0.83 Å (N-H) and 0.85 Å (O-H), and with U iso (H) = 1.2-1.5U eq (C/N/O).  (Sheldrick, 2015b); software used to prepare material for publication: SHELXL2014/7 (Sheldrick, 2015b).

Tetraaqua[N,N′-bis(pyridin-4-yl)pyridine-2,6-dicarboxamide]sulfatomanganese(II) dihydrate (I)
Crystal data (18)  Special details Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes.