Poly[[(μ4-benzene-1,3,5-tricarboxylato-κ4 O 1:O 1′:O 2:O 3)bis(2,2-bipyridine-κ2 N,N′)(μ2-hydroxido)dicopper(II)] trihydrate]

In the title two-dimensional coordination polymer, {[Cu2(C9H3O6)(OH)(C10H8N2)2]·3H2O}n, each of the two independent CuII atoms is coordinated by a bridging OH group, two O atoms from two benzene-1,3,5-tricarboxylate (L) ligands and two N atoms from a 2,2- bipyridine (bipy) ligand in a distorted square-pyramidal geometry. Each L ligand coordinates four CuII atoms, thus forming a polymeric layer parallel to the bc plane with bipy molecules protruding up and down. The lattice water molecules involved in O—H⋯· O hydrogen bonding are situated in the inner part of each layer. The crystal packing is consolidated by π–π interactions between the aromatic rings of bipy ligands from neigbouring layers [intercentroid distance = 3.762 (3) Å].


Introduction
The design and synthesis of metal-organic framework has been an area of rapid growth in recent years owing to the potential application and as zeolite-like material for molecular selection (Napolitano et al.,2008). Polycarboxylate ligands present very rich coordination chemistry, because of their ability to bridge transition metal ions generating various polynuclear complexes. Aromatic polycarboxylate are of high interest due to their versatility in constructing coordination complexes, and 1,3,5-benzene tricarboxylic acid have been proved to be efficacious towards preparation of metal-organic coordination complexes. Moreover, these carboxylate bridges provide a means for efficiently transmitting magnetic information. During the last decade, many reports appeared on the synthesis of coordination compounds where trianions of benzene-1,3,5-tricarboxylic acid combined with aromatic N-containing chelating ligands have been used to essemble a wide range of coordination polymers from chains, to networks (Wang et al.,2005). Usually the construction of molecular architecture depends on several factors such as coordination geometry of metal ions, organic ligands, counter ions, solvents and temperature. Due to the flexible nature of Cu II coordination sphere, assisted by the Jahn-Teller effect which can be realized either by distortion of an octahedral geometry to give a 4 +1+1 bonding, or else by a change in coordination number as an alternative means of lifting the degeneracy of unequally occupied d-orbitals so copper will be the best choice. Herein, we report the synthesis and crystal structure of a new two-dimensional Cu II complex-coordination polymer containing aromatic polycarboxylic ligand such as benzene-1,3,5-tricarboxylic acid and hetero aromatic ligand such as 2,2-bipyridine. Our interest in dimeric bifunctional materials is direct toward the effects of weak interactions between molecular units, since the stacking of bipy rings is a potential source of intermolecular exchange couplings.

Experimental
All starting materials were commercial products and were used as supplied from the Aldrich Company.

Refinement
C-bound H atoms were geometrically positioned and refined as riding. The O-bound H atoms were located on the Fourier difference map and isotropically refined. For the hydroxo group, the O-H bond distance has been restrained to 0.90 (2) supporting information sup-2 Acta Cryst. (2014). E70, m270-m271 Å.

Results and discussion
In the title complex (I), the dinuclear copper (II) coordination polymer (Fig. 1), features two very similar pyramidal CuN 2 O 3 chromophores both adopting a (4+1) slightly distorted square-pyramidal arrangement, which share one vertex occupied by a bridging hydroxide group. The hydroxide group occupies one of the basal positions of both the CuN 2 O 3 square pyramids, so that the intermetallic distance is 3.5251 (6)Å. The oxygen atoms of a syn-anti triatomic carboxylate bridge occupy the apical positions of the two coordination spheres. These Cu-O distances are very close to those reported for [Cu 2 (µ 5 -btb)(µ-OH)(µ-H 2 O)] n (btb= benzene-1,2,3-tricarboxylate) (Janiak et al., 2008) and shorter than that et al., 2011). The rest of the basal sites of each Cu II centre are occupied by a monodentate carboxylate oxygen of another BTC 3ligand, and completed by an N,N-chelating dipyridine ligand. The shortest interchain separation of the metal centres is 9.7017 (7)Å , and 9.7348 (7)Å between the layers.
As expected for Cu II in square-pyramidal geometry, the apical Cu-O bond distance is significantly longer than the remaining four distances in the Cu coordination polyhedron. This circumstance is characteristic of Jahn-Teller systems.
The BTC 3trianion acts as a tetradentate ligand with monodentate (C29/O5/O1) and (C27/O7/O8) for Cu1 and Cu2 respectively, and bridging (C21/O3/O4) carboxylate groups featuring C-O bonds almost perfectly resonant [C21-O3 =1.263 (4)Å and C21-O4 = 1.261 (4)Å]. As a consequence each BTC 3bridges three [dipy 2 Cu 2 (µ-OH)] units forming a two dimensional network growing perpendicularly to the a axis (Fig.2 ). This network can be described as a honeycomb structure (Fig. 2 ), formed by irregular hexagons sharing their edges and whose vertices are constituted by alternated tricarboxylate and bimetallic [dipy 2 Cu 2 (µ-OH)] units. The two-dimensional networks stack parallel to each other at an interplanar distance of 8 Å. This interplanar space is filled by the bipyridine moieties from the bimetallic units of two adjacent networks (Fig. 3 ). In particular, the bipyridine groups belonging to superposed bimetallic units, symmetry related by an inversion centre interact, interacts via face-to-face π-stacking: in each couple the two interacting pyridine rings are nearly parallel, with an interplanar distance of 3.57 (3) Å and a ring centroid-ring centroid offset of 2.45 (3) Å. Additional carbon carbon contacts (3.529 (6) Å) connects bipyridine moieties symmetry related by screw axis. The interactions involving all the bipyridine groups above and below the honeycomb structure provide an overall strong connection along the third packing dimension, since the stacking of bipyridine rings is a potential source of weak intermolecular exchange coupling. Further analysis of the packing structure reveals that this structure contains three water molecules in the lattice which are localized inside the honeycomb hexagons. There are short interchain water-carboxylate and water-water contacts that are indicative of a hydrogen bonding (Table 1). The hydrogen atom of the hydroxo bridge participate in classical O-H····O bonding with O5 of the carboxylate group of another molecule (Table 1). The multidimensional framework structures formed by these combination of aromatic ligands are often stabilized via noncovalent intermolecular forces, viz. hydrogen bonds and π-π interactions . In summary, benzenepolycarboxylic acids and N-containing chelating aromatic compounds have promoted the construction of multi-dimensional networks. Variation of the carboxylic acid elements along with the poly-N-chelating aromatic complexes is envisioned to produce materials, which could find potential application in self-assembled nanoscale molecular devices.

Figure 1
A portion of (1), showing the atom numbering scheme. Displacement ellipsoids are drawn at the 50% probability level.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )
x y z U iso */U eq