Comment

In recent decades, pyridinecarboxamides, as multidentate heterocyclic ligands, have been widely used as spacers to construct intriguing metal-containing assemblies in coordination chemistry (Yue et al., 2005; Adarsh & Dastidar, 2011; Uemura et al., 2002). Among them, the U-shaped symmetric bispyridine-bisamides, which combine rigidity with flexibility, play an important role in engineering metal-containing macrocycles, nanocages and porous frameworks (Burchell et al., 2004; Wu et al., 2008; Wang et al., 2012). In particular, the labile conformations of U-shaped symmetric bispyridine-bisamides, especially the nonplanar conformers, can be recognized and used to direct self-assembly with metal cations. For example, using N²,N⁶-bis(pyridin-3-yl)pyridine-2,6-dicarboxamide and N²,N⁶-bis[(pyridin-3-yl)methyl]pyridine-2,6-dicarboxamide (L) ligands to assemble with Ni^II and Ag^I, a chiral tetragonal molecular cage (Li et al., 2012) and homochiral coordination polymers (Wu et al., 2010) have been obtained, respectively. In addition, the regular hydrogen bonding occurring from the amide groups of these ligands can further organize the resulting metal-containing arrays into higher-dimensional networks (Burchell et al., 2004), or it can anchor smaller guests in the cavities of the resulting frameworks (Guo et al., 2012). In this paper, we report the reaction of the multifunctional ligand L with zinc(II) cations, and report the title zinc(II) coordination polymer {[Zn(L)₂(H₂O)₂](NO₃)₂}_n, (I).

In (I), each Zn^II centre is coordinated by four N atoms from the outer pyridine rings of four L ligands in a plane with the two water O atoms perpendicular to that. The Zn^II atom sits on an inversion centre and has a slightly distorted octahedral coordination geometry (Fig. 1). The Zn-N and Zn-O bond lengths cover the range 2.1017 (19)-2.321 (2) Å (Table 1).

In (I), the U-shaped L ligands act as bidentate bridging ligands to link the Zn nodes into a one-dimensional centrosymmetric double-chain structure along the c axis (Fig. 2a), in contrast with the two-dimensional corrugated sheet of uniform (4,4)-connected topology found in the Co^II and Ni^II polymers of L (Yao et al., 2012; Guo et al., 2012). Owing to coordination, the outer pyridine arms of the L ligands in (I) are twisted out of the plane of the central pyridine ring [the dihedral angles between the planes of the outer rings and the central ring of L are 90.2 (3) and 87.8 (1)°]. As a result, the L ligands lower the C_2v symmetry to pseudo-C₂ symmetry, and adopt helical conformations very similar to those in the Co^II and Ni^II polymers (Yao et al., 2012; Guo et al., 2012).

The polymeric chain of (I) is based on 32-membered metallated macrocycles built from a pair of enantiomers (R and S conformers) sharing two communal Zn^II cations. For each macrocycle there are two nitrate anions, which are involved in multiple N-HO hydrogen-bond interactions (Fig. 2a and Table 2). The separation of adjacent Zn nodes bridged by L is 11.233 (2) Å. Interestingly, the double-chain structure consists of two chiral chains individually constructed of S and R conformers bridging the metal centres, as shown in Fig. 2.

In (I), the double-chain structures are extended into a two-dimensional supramolecular framework in the bc plane through interchain O-HO hydrogen-bond interactions arising from the coordinated water and the carboxamide O atoms of L (Fig. 3 and Table 2). The layers are linked into a three-dimensional supramolecular framework through weak interlayer C-HO hydrogen-bond interactions (Fig. 4).

Thermogravimetric analysis (TGA) of (I) was carried out in air from 303 to 957 K at a heating rate of 10 K min^-1 using a crystalline sample. As shown in Fig. 5, (I) is stable up to 420 K. On heating, the compound suffers a continuous weight loss in a range 420-868 K, indicating the complete decomposition of the framework. The remaining 8.29% may be ZnO, in agreement with the theoretical value of 8.84%.

The solid-state photoluminescence behaviours of L and (I) were investigated at room temperature, and the results are presented in Fig. 6. The free ligand L displays strong luminescence, with a single broad band centred at 397 nm, corresponding to excitation at 352 nm. When excited at 390 nm, (I) displays a very intense emission with a peak maximum at 449 nm. In comparison with the fluorescence of the free ligand L, the emission of (I) is blue-shifted by 52 nm and its intensity is increased. The emission behaviour of complex (I) probably originates from a metal-perturbed intraligand transition, as reported for Zn^II or other d¹⁰ metal complexes with N-donor ligands, based on the position and shape of the emission band (Li et al., 2009).

Experimental

Ligand L was prepared according to the method of Yang et al. (2012). For the preparation of (I), a solution of L (69.4 mg, 0.2 mmol) in methanol (12 ml) was added dropwise to a solution of Zn(NO₃)₂·6H₂O (29.7 mg, 0.1 mmol) in methanol (12 ml). After stirring for 30 min, the resulting mixture was filtered. The filtrate was allowed to evaporate at room temperature for one week, and colourless crystals of (I) were obtained in 72% yield. IR (KBr pellet, , cm^-1): 3256 (s), 3058 (w), 166 (v), 1549 (s), 1436 (m), 1380 (v), 1257 (m), 745 (m), 708 (m), 683 (m).

The H atoms of the coordinated water molecules and those on N atoms were located from difference Fourier maps and subsequently constrained to ride on their parent atoms with their U_iso values allowed to refine freely. All other H atoms were generated geometrically and allowed to ride on their parent atoms, with C-H = 0.95 (aromatic) or 0.99 Å (methylene) and U_iso(H) = 1.2U_eq(parent).

Data collection: SMART (Siemens, 1996); cell refinement: SAINT (Siemens, 1994); data reduction: SAINT; program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: SHELXTL (Sheldrick, 2008); software used to prepare material for publication: SHELXL97.

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Supplementary data for this paper are available from the IUCr electronic archives (Reference: SF3189 ). Services for accessing these data are described at the back of the journal.

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