Poly[diaqua-μ3-4-nitrophthalato-copper(II)]

In the title complex, [Cu(C8H3NO6)(H2O)2]n, the two carboxylate groups of the 4-nitrophthalate dianion ligands have monodentate and 1,3-bridging bonding modes, respectively. The Cu atom shows an approximate square-pyramidal coordination as it is bonded to O atoms from the carboxylate groups of three 4-nitrophthalate ligands and two O atoms of the non-equivalent coordinated water molecules. Other Cu atoms in the coordination polymer are connected into a two-dimensional layer in the ab plane. The layers are aggregated to a three-dimensional structure through interlayer hydrogen bonding involving an O atom of a nitro group. The whole three-dimensional structure is further maintained and stabilized by intralayer hydrogen bonds between the O atoms of the carboxylate groups and the coordinated water molecules.

In the title complex, [Cu(C 8 H 3 NO 6 )(H 2 O) 2 ] n , the two carboxylate groups of the 4-nitrophthalate dianion ligands have monodentate and 1,3-bridging bonding modes, respectively. The Cu atom shows an approximate square-pyramidal coordination as it is bonded to O atoms from the carboxylate groups of three 4-nitrophthalate ligands and two O atoms of the non-equivalent coordinated water molecules. Other Cu atoms in the coordination polymer are connected into a twodimensional layer in the ab plane. The layers are aggregated to a three-dimensional structure through interlayer hydrogen bonding involving an O atom of a nitro group. The whole three-dimensional structure is further maintained and stabilized by intralayer hydrogen bonds between the O atoms of the carboxylate groups and the coordinated water molecules.
The author thanks Tianjin Polytechnic University for financial support.

Comment
Aromatic dicarboxylate ligands such as phthalate (phth) and substituted phthalatehave been used in the construction of polymeric metal complexes because they can act as a bis-monodentate, bis-bidentate and combined modes of coordination to form short bridges via one carboxylato end or long bridges via the benzene ring and lead to a great variety of structures (Biagini Cingi et al., 1978;Guo and Guo, 2007;Wang et al., 2009;Ma et al., 2004;Baca et al., 2003Baca et al., , 2004Yang et al., 2003;Fu et al., 2006). We have used the 4-nitrophthalate dianion as a ligand, and have obtained the title novel five-coordinate 4-nitrophthalate-copper complex, (I), which forms a three-dimensional supramolecular network through O-H···O hydrogen bonding.
The asymmetric unit in the structure of (I) comprises one Cu atom, one complete 4-nitrophthate dianion and two nonequivalent water molecules, and is shown in Fig. 1 in a symmetry-expanded view, which displays the full coordination of the Cu atom. Selected geometric parameters are given in Table 1.
The Cu atom exhibits an approximate square pyramidal environment (the τ value being 0.171, Addison et al., 1984), with atoms O1, O4 i (see Fig. 1 for symmetry codes) of two non-equivalent 4-nitrophthalate dianions and O7 and O8 atoms of coordinated water molecules in a planar arrangement, with the mean Cu-O(eq) bond distance being 1.961 (3)  In the present structure, monodentate, bidentate 1,3-bridging bonding and 1,6-bridging bonding modes via the benzene ring are present (Fig. 2). The O1 atom binds in a monodentate fashion, while the O3 and O4 atoms display both monodentate and bidentate 1,3-bridge bonding to link two Cu atoms. The O1 and O3 (or O4) atoms adopt a 1,6-bridging bonding mode via the benzene ring to connect with two other Cu atoms.
The Cu atoms are further interconnected by three O atoms from three 4-nitrophthalate dianions into a two-dimensional layer in the ab plane. The mean planes of the carboxylate groups of O1/C1/O2 and the benzene ring make a dihedral angle of 72.4 (5)°, and the value of a dihedral angle for the carboxylate groups of O3/C8/O4 is 14.5 (5)°; the two C-O bond distances (O1-C1 and O2-C1) of the monodentate carboxylate group are 1.278 (5) and 1.241 (5) Å, respectively, and the two C-O bond distances (O3-C4 and O4-C4) of the 1,3-bridging bonding carboxylate group are 1.253 (5) and 1.266 (6) Å, respectively. These indicate that the mesomeric effect for the 1,3-bridging bonding carboxylate group is somewhat greater than that of the monodentate carboxylate group.
The two water molecules within the coordination sphere of the Cu atom, and the nitro group (O5/N1/O6) in the present structure engage in distinct hydrogen bonding interactions (see Table 2). Within each layer, the non-coordinated O2 atom is involved in forming strong O8-H8B···O2 ii (Brown, 1976) and weak O7-H7A···O2 v hydrogen bonds. These play an important role in the propagation of the two-dimensional layer structure, due to the formation of different hydrogen bonded supplementary materials sup-2 ring graph set motifs (Bernstein et al., 1995), such as an S(8), and two 10-membered R 2 2 (10) motifs (Fig.3). The neighbouring layers are linked together via weak O7-H7B···O6 vi hydrogen bonding interactions. These also result in the aryl rings of the 4-nitrophthalato ligands stacking weakly in an offset fashion along the c direction with centroid to centroid distances in the range 4.55 (4)Å -4.97 (2)%A. Thus, the three-dimensional connectivity of the structure is achieved.

Experimental
Copper(II) oxide (0.32 g 4 mmol) was added to a stirred solution of 4-nitrophthalic acid (0.53 g, 2.5 mmol) in boiling water (20.0 ml) over a period of 40 min. After filtration, slow evaporation over a period of a week at room temperature provided green needle-like crystals of (I).

Refinement
All water H atoms were found in difference Fourier maps. However, during refinement, they were fixed at O-H distances of 0.85 Å, with U iso (H)=1.2 U eq (O). The H atoms of C-H groups were treated as riding, with C-H = 0.93 Å and U iso (H) = 1.2 U eq (C). Fig. 1. A view of the structure of (I), showing the atom-numbering scheme and coordination polyhedra for the Cu atoms; displacement ellipsoids were drawn at the 30% probability level [Symmetry codes: (i) x -1/2, y, -z + 1/2; (ii) -x + 1, y -1/2, -z + 1/2].

Poly[diaqua-µ 3 -4-nitrophthalato-copper(II)]
Crystal data [Cu(C 8   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 Rfactors(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.