Poly[[(μ-benzene-1,4-dicarboxylato)bis[μ-4-(1H-1,3,7,8-tetraazacyclopenta[l]phenanthren-2-yl)benzoato]dizinc] tetrahydrate]

In the title complex, [Zn2(C8H4O4)(C20H11N4O2)2]·4H2O, the ZnII atom is six-coordinated by two carboxylate O atoms from one bidentate benzene-1,4-dicarboxylate (1,4-BDC) ligand, two carboxylate O atoms from two different monodentate 4-(1H-1,3,7,8-tetraazacyclopenta[l]phenanthren-2-yl)benzoate (HNCP) ligands and two HNCP N atoms. The ZnII atoms are bridged by the centrosymmetric 1,4-BDC ligands, forming an extended single-chain structure. Neighbouring single chains are connected by the HNCP ligands from two opposite directions, resulting in a sheet. In addition, there are N—H⋯O hydrogen-bonding interactions between adjacent layers. As a result, the polymeric sheets are further extended into a three-dimensional supramolecular structure.


Comment
Coordination polymers with a variety of supramolecular structures have been studied extensively because of their novel topologies and potential applications as functional materials (Eddaoudi et al., 2001). 1,10-Phenanthroline (phen), as a common organic ligand, has been widely used in the construction of metal-organic coordination polymers (Chen & Liu, 2002). The phen derivative 4-(1H-1, 3, 7, 8-Tetraaza-cyclopenta [l]phenanthren-2-yl)-benzoic acid (HNCP) with both phenanthroline ring and carboxylate groups, is a good building block for construction of coordination polymers. Moreover, the long-conjugated system and the carboxylate groups are inclined to form π-π stacking interactions and hydrogen bonding interactions, which are important factors in the formation of supramolecular architectures. However, to date, only a handful of supramolecular architectures based on HNCP molecules have been described (Yongqin et al., 2007;Hsu et al., 2005). We selected benzene-1,4-dicarboxylic acid (1,4-BDC) as a linker and HNCP as a secondary ligand, generating a new coordination polymer, [Zn 2 (1,4-BDC) (NCP) 2 (H 2 O) 4] (I), which is reported here.
In compound (I), the Zn atom is coordinated by two N atoms from one HNCP ligand, two O atoms from one 1,4-BDC ligand, and two O atoms from two different HNCP ligands in a distorted octahedral coordination (Fig. 1). The single unique 1,4-BDC species is generated from the atoms of the asymmetric unit by inversion. Two Zn II centers are bridged by the carboxylate groups of HNCP ligands to furnish a binuclear unit with a Zn II ···Zn II distance of 3.2224 (9) Å. Neighbouring Even though the H atoms pertaining to water molecules could not be confidently found, and accordingly were not included in the model, the short O1···O2W i 2.891 (9)Å; O1W···O2W ii 2.886 (14)Å; O1W···N4: 2.899 (10)Å distances, (i): x, -1+y, -1+z; (ii): 1-x,-y,1-z might indicate the formation of hydrogen bonds between these atoms.
Besides, there are hydrogen bonding interactions between adjacent layers. The imidazole nitrogen atoms of HNCP ligands act as hydrogen bond donors, while the carboxylate oxygen atoms of 1,4-BDC ligands from the neighboring layer act as hydrogen bond acceptors (N3-H···O3) ( Table 1). As a result, these two-dimensional polymeric sheets are further extended into three-dimensional supramolecular structures through these hydrogen-bonding interactions (Fig. 3).

Experimental
HNCP was prepared according to the literature method (Yongqin et al., 2007). Other reagents were commercially available and used without further purification. Zn(CH 3 COO)2 (0.2 mmol), 1,4-BDC (0.1 mmol), and HNCP (0.1 mmol) were mixed in 10 ml deionized water. And its pH value was controlled in the range of 7-8 with 1 mol/L NaOH solution. Then, the supplementary materials sup-2 resulting precursor was placed in 25 ml Teflon-lined stainless steel reactor, and heated at 433 K for 3 d. Cooling slowly to room temperature, the yellow block crystals of the title complex suitable for X-ray diffraction analysis were obtained.

Refinement
All C-and N-attached H atoms were positioned geometrically (N-H = 0.86Å and C-H = 0.93 Å) and refined as riding, with U iso (H)= 1.2U eq (C). Hydrogen atoms corresponding to water molecules could not be confidently located and were not included in the model.

Figures
Fig . 1. The asymmetric unit of (I), together with further atoms to complete the Zn1 coordination and the 1,4-BDC ligand. Displacement ellipsoids are drawn at the 30% probability level.

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.
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 > 2sigma(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.