Poly[diaqua[μ6-4,4′-(1,4-phenylene)bis(2,6-dimethylpyridine-3,5-dicarboxylato)]dilead(II)]

The asymmetric unit of the title Pb-based coordination polymer, [Pb2(C24H16N2O8)(H2O)2]n, consists of one PbII cation, half of a 4,4′-(1,4-phenylene)bis(2,6-dimethylpyridine-3,5-dicarboxylate (L 4−) ligand and one coordinating water molecule. The centers of the benzene ring of the ligand and the four-membered Pb/O/Pb/O ring are located on centers of inversion. The PbII ion is coordinated in form of a distorted polyhedron by seven O atoms from four separate L 4− ligands and by one water O atom. The PbO7 polyhedra share O atoms, forming infinite zigzag [PbO4(H2O)]n chains along [100] that are bridged by L 4− ligands, forming a two-dimensional coordination network parallel to (001). O—H⋯O hydrogen bonds involving the water molecule are observed.

Data collection: APEX2 (Bruker, 2010); cell refinement: SAINT-Plus (Bruker, 2008); data reduction: SAINT-Plus; 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: PLATON (Spek, 2009 In recent years, the chemistry of novel metal-organic hybrid coordination polymers has been the subject of intensive research, due to their interesting molecular structures and their potential as a new class of solid-state materials applied in catalysis, molecular recognition, gas storage, drug delivery, and so on (Liu et al., 2002;O′Keeffe et al., 2008). Generally speaking, the diversity of potential applications in the framework structures of such materials greatly depends on the selection of the metal centers and organic spacers. Recently, carboxylate groups are frequently exploited in the design, syntheses, and crystallization of coordination frameworks, because they exhibit diverse coordination modes, which can enhance the robustness of the architectures. Furthermore, the flexibility of carboxylate groups is always efficient to form fascinating structures. In this paper, we choose a new flexible and multidentate carboxylate ligand, 4,4′-(1,4-phenylene)bis(2,6-dimethylpyridine-3,5-dicarboxylic acid) (H 4 L).
Up to date, research on coordination polymers has focused on transition metal ions as coordination centers, while less concentration has been given to heavy p-block metal ion, e.g. lead(II). In contrast to transiton metal ions, lead(II), with its large radius, flexible coordination environment, and variable stereochemical activity, provides unique opportunities for the formation of unusual structures with interesting properties (Harrowfield et al.., 2004;Yang et al.., 2007). In addition, the intrinsic features of lead(II), the presence of a 6 s 2 outer electron configuration, inspire chemists extensive interest in coordination chemistry, photophysics, and photochemistry. Herein, we report a new photoluminescent complex [Pb(L) (H 2 O)] n (1) from the flexible 4,4′-(1,4-phenylene)bis(2,6-dimethylpyridine-3,5-dicarboxylic acid) (H 4 L) and lead salt.
As shown in Fig.1, each H 4 L ligand connects six crystallographically equivalent Pb atoms. The carboxylato group with O1 and O2 coordinates three lead atoms producing two Pb 2 O 2 rings that share one common lead atom. The other carboxylate moiety with donor atoms O3 and O4 coordinates one lead atom in a chelating mode. Notably, the resulting PbO 7 polyhedra share the O1 #4 , O1 #5 , O2 #2 , O2 #3 atoms to form infinite zigzag chains composed of [PbO 4 (H 2 O)] n in which adjacent Pb atoms are coplanar and Pb···Pb distances are 4.077 Å and 4.161 Å respectively (Fig. 2). Another interesting structural feature of complex 1 is that the zigzag [PbO 4 (H 2 O)] n chains are bridged by H 4 L ligands to form a twodimensional (2-D) coordination network (Fig. 3). The photoluminescence spectrum of compound 1 was measured in the solid state at room temperature, as shown in Fig.   4. At room temperature the photoluminescent emission maximum of free H 4 L was observed at 426 nm (upon λ Ex, max = 208 nm). For compound 1, excitation at 380 nm leads to strong photoluminescence with an emission maximum at λ = 465 nm.
The emission peak of complex 1 is red-shifted by about 40 nm compared to that of the pure H 4 L ligand, which can be assigned to the ligand-metal charge transfer (LMCT) (Hu et al., 2010;Zhang et al., 2011;Zhang et al., 2012).

Experimental
A mixture of Pb(NO 3 ) 2 × 6 H 2 O (66 mg), H 4 L (40 mg) and DMF (6 ml) was sealed in a 25 ml Teflon-lined stainless steel reactor. The mixture was heated to 373 K for 3 days and then cooled to room temperature. The crystal samples were washed with methanol to yield 18 mg of compound 1.

Refinement
Methyl H atoms were constrained to an ideal geometry (C-H = 0.96 Å), with U iso (H) =1.5U eq (C), but were allowed to rotate freely. Other H atoms attached to C atoms were refined using a riding model [C-H = 0.93 Å (CH) and U iso (H) = 1.2U eq (parent atom)].

Poly[diaqua[µ 6 -4,4′-(1,4-phenylene)bis(2,6-dimethylpyridine-3,5-dicarboxylato)]dilead(II)]
Crystal data [Pb 2 (C 24   where P = (F o 2 + 2F c 2 )/3 (Δ/σ) max < 0.001 Δρ max = 1.15 e Å −3 Δρ min = −1.16 e Å −3 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.