Crystal structure of di-μ-hydroxido-bis[aqua(1,10-phenanthroline-κ2 N,N′)copper(II)] naphthalene-2,6-dicarboxylate hexahydrate

The cations and anions of the title compound are organized through π–π stacking between the aromatic rings of the 1,10-phenanthroline and the naphthalene-2,6-carboxylate into a two-dimensional structure. The extensive O—H⋯H hydrogen bonds further connect the cations, anions and lattice water molecules into a three-dimensional network.


Chemical context
The designed arrangement of molecules through intermolecular interactions is one of the main purposes of crystal engineering. Among these interactions are hydrogen bonds andstacking (Hunter & Sanders, 1990).stacking interactions are ubiquitous in biological systems, and organic molecules (Riley & Hobza, 2013;Klä rner & Schrader, 2013), and are present in many metal complexes (Janiak, 2000). Nevertheless, relatively few systems have been designed to be organized mainly byinteractions (Putta et al., 2014;Sebaoun et al., 2014;Valdé s-Martínez et al., 2005). In most cases, they are secondary interactions helping to stabilize the network, not the main tool in the organization of the molecules in the crystal. We have proved that it is possible to obtain designed non-centrosymmetric crystals throughstacking interactions (Serrano-Becerra et al., 2009).
As part of a systematic study of the possible organization of copper coordination compounds controlled bystacking interactions, we decided to use aromatic amines, as blocking ligands, and naphthalene-2,6-dicarboxylate as a possible bridging ligand between the [Cu(ammine)] moieties, as long as all of them may forminteractions. The reactions were ISSN 2056-9890 done in water -the tendency of carboxylates to form hydrogen bonds with water is well known, as is their tendency to coordinate to Cu II complexes -so these structures will give us an opportunity to evaluate the importance of waterÁ Á Á hydrogen bonding versusinteractions as the main interaction controlling the organization of the molecules in the crystal.
During these studies, the title compound was unexpectedly obtained. Its molecular and crystal structure are described herein.

Structural commentary
The asymmetric unit of the title compound contains half of a [(phen)(H 2 O)Cu(OH) 2 Cu(H 2 O)(phen)] (phen is 1,10-phenanthroline) dimer, half of an naphthalene-2,6-dicarboxylate anion and three lattice water molecules. The Cu II cation is pentacoordinated with a square-pyramidal geometry, the phen coordinates as a bidentate ligand through the N atoms, the hydroxide groups bridge the two Cu II cations and a water molecule is coordinated in the apical position (Fig. 1). The carboxylate group of the naphthalene-2,6-dicarboxylate anion is twisted at 12.4 (3) with respect to the naphthalene ring system.

Supramolecular features
An extensive network of hydrogen bonds is formed (Table 1) in the crystal. Atom O4 of the coordinating water molecule acts as a hydrogen-bond donor to O6 of a water molecule and carboxylate atom O1. The bridging hydroxide group hydrogen bonds to atom O5 of a water molecule and acts as a hydrogenbond acceptor with water oxygen atom O7. The carboxylate atom O1 forms three hydrogen bonds while carboxylate atom O2 forms two hydrogen bonds. Water oxygen atoms O6 and O7 form hydrogen bonds with each other as well as with the carboxylate O atoms. The hydrogen-bond network extends into a three-dimensional structure, see Fig. 2. The presence of a free naphthalene-2,6-dicarboxylate with four hydrogenbond acceptors requires the presence of water molecules, but the tendency of the aromatic rings in the ligands to form interactions may also observed and this is an important factor in the organization of the molecules in the crystal (Fig. 2). Two phenanthroline units from two adjacent cations lie parallel, on top of each other, the distance between the centroids of the ligand rings N7-C8-C10-C17-C18 and C15-C19-C20-N16-C14-C13 being 3.4990 (16) Å .

Synthesis and crystallization
Naphthalene-2,6-dicarboxylic acid (0.021 g, 0.10 mmol) was suspended in 10 ml of water; while stirring and heating, a The structure of the title compound showing the atom-labelling scheme. Displacement ellipsoids are drawn at the 50% probability level. H atoms are shown as circles of arbitrary radius.

D-HÁ
concentrated solution of KOH was added until a transparent solution was obtained. A second solution was prepared by mixing 1,10-phenanthroline (0.018 g, 0.10 mmol) in MeOH (5 ml) and Cu(NO 3 ) 2 Á3H 2 0 (0.018 g, 0.21 mmol) dissolved in water (5 ml). Both solutions were mixed and stirred under reflux for a period of 3 h. The clear-blue solution was slowly evaporated at room temperature. Blue crystals of the title compound were obtained after several days. The yield was not determined due to the poor stability of the compound out of solution.