Crystal structure of catena-poly[[copper(II)-μ2-salicylato-[diaquacopper(II)]-μ2-salicylato] dihydrate]

Two independent copper(II) cations with coordination numbers of 4 and 6 are bridged by dianionic salicylate anions into chains extending parallel [001]. O—H⋯O hydrogen-bonding interactions involving both the coordinating and the lattice water molecules result in the formation of a three-dimensional network.

The title compound, {[Cu 2 (C 7 H 4 O 3 ) 2 (H 2 O) 2 ]Á2H 2 O} n , contains two copper(II) cations in special positions (one on a twofold rotation axis and one on an inversion centre) and the the salicylate ligand in its dianionic form. By four-and six-coordinate metal coordination, chains are formed parallel to [001], which are extended by O-HÁ Á ÁO hydrogen bonding into sheets extending parallel to (100). These sheets are weakly connected by O-HÁ Á ÁO hydrogen bonding via the non-coordinating lattice water molecules into a three-dimensional network.

Chemical context
Salicylic acid (2-hydroxybenzoic acid, H 2 Sal) has two acidic hydrogen atoms and the corresponding pK a values are 2.853 (9) and 12.897 (7) (Farajtabar & Gharib, 2010;García et al., 1982). Titration studies with Cu 2+ indeed indicate the formation of complexes with the monoanionic ligand HSal À as well as with the dianionic ligand Sal 2À (Dahlund & Olin, 1988;Furia & Porto, 2002). From the literature, crystal structures of copper salicylate are only known with the monoanionic HSal À ligand. They occur as a tetrahydrate (Hanic & Michalov, 1960;Rissanen et al., 1987) and as a dihydrate (Jagner et al., 1976), the latter being described as an order-disorder structure. In an attempt to crystallize copper(II) salicylate we obtained a mixture of crystals (see Synthesis and Crystallization), among which was the title compound (

Structural commentary
The asymmetric unit of (I) is shown in Fig. 1. The two copper(II) cations are located on special positions with twofold rotation symmetry (Cu1, Wyckoff position c) and inversion symmetry (Cu2, Wyckoff position b). Cu2 is fourcoordinated in a square-planar configuration with donor atoms O2 of the carboxylate and O3 of the deprotonated hydroxy group. The two pairs of Cu-O distances are 1.905 (2) Å and are the shortest in the present structure (Table 1). As a consequence of the inversion symmetry, the fourfold coordination environment is exactly planar with an angle sum of 360.0 (2) . Cu1 has an environment of six oxygen atoms (Fig. 2). The Cu1-O1 distance to a carboxylate oxygen atom, and the Cu1-O4 distance to the coordinating water molecule are in the expected range. The Cu1-O2 distance of 2.332 (2) Å is rather long, which indicates only a weak interaction. The twofold rotation axis bisects the O1-Cu1-O1 i and the O4-Cu1-O4 i angles [symmetry code: (i) 1 À x, y, 1 2 À z]. This allows the five atoms Cu1, O1, O4, O1 i and O4 i to deviate significantly from planarity. The sum of the cis angles is 382.8 (3) and the dihedral angle between the O1-Cu1-O1 i /O4-Cu1-O4 i planes is 49.86 (14) . If one decides to consider Cu1 as four-coordinated, the coordination environment is consequently best described as halfway between square-planar and tetrahedral with approximate D 2d symmetry [ 4 parameter = 0.52; 6 = 94.60 (11) ; Yang et al., 2007]. The O2-Cu1-O2 i angle is nearly perpendicular to the twofold axis and thus at 176.45 (12) nearly linear. A description as a six-coordinated metal cation can nevertheless only be called very distorted due to the non-planarity of the equatorial atoms.
The Cu1-O2 bond fails the Hirshfeld rigid-bond test (Hirshfeld, 1976) with Á m.s.d.a. of 0.0200 (13) Å 2 as calculated with the PLATON software (Spek, 2009). A similar effect has been observed in bidentate Zn-O(carboxylate) complexes and was attributed to the strain in the fourmembered chelate ring (Lutz & Spek, 2009). In the present case, it can also be ascribed to the weakness of the interaction, which allows a rather uncorrelated movement of Cu1 and O2. In fact, O2 is bridging between Cu1 and Cu2 and the O2-Cu2 bond is much stronger than O2-Cu1. Á m.s.d.a. for O2-Cu2 is only 0.0007 (13) Å 2 and inconspicuous.
The salicylate dianion is located on a general position. It is essentially planar with a maximum deviation of 0.054 (3) Å from the least-squares plane. This small deviation involves the carboxylate group with torsion angles of À3.4 (5) for C7-C2-C1-O1 and À2.5 (6) for C3-C2-C1-O2. The C-OH bond length of 1.359 (2) Å in Cu(HSal) 2 Á4H 2 O (Rissanen et al., 1987)  The asymmetric unit in the crystal structure of (I) in a view along [010]. Displacement ellipsoids are drawn at the 50% probability level. H atoms are drawn as spheres with an arbitrary radius. Additional bonds to O atoms outside the asymmetric unit indicate the completed coordination environments of the two copper(II) cations. Table 1 Selected geometric parameters (Å , ).

Figure 2
The environment of the Cu1 atom with C 2 symmetry. Because the Cu1-O2 distance is rather long, the metal atom can either be described as fourcoordinated (green) or as six-coordinated (red). In both cases, the coordination geometry is severely distorted. [Symmetry code: (i) 1 À x, y, One of the water molecules (O4) coordinates to the Cu1 copper(II) ion, while the other water molecule (O5) is present as non-coordinating lattice water.

Supramolecular features
Compound (I) forms coordination chains extending parallel to [001] with a CuÁ Á ÁCu distance of 4.0478 (4) Å . The coordinating water molecule O4 acts as a donor of hydrogen bonds with the carboxylate oxygen O1 and the deprotonated hydroxy oxygen O3 as acceptors (Table 2). This extends the onedimensional coordination polymer into a two-dimensional hydrogen-bonded network parallel to (100) (Fig. 3). Between the hydrogen-bonded layers there are solvent-accessible channels along [010] at the positions x = 0.25 and z = 0.25, which is the intersection of two glide planes. By symmetry, there are four channels per unit cell with a volume of 59 Å 3 each, as calculated with the PLATON software (Spek, 2009). Each channel is occupied by two non-coordinating water molecules O5 per unit cell. The O5 water molecules are linked to each other by cooperative hydrogen bonding, forming chains along [010]. A second hydrogen bond for O5 involves the coordinating water molecule O4. The lattice water molecules thus connect the described (100) layers into a threedimensional hydrogen-bonded network. It should be noted that the hydrogen bonds O5Á Á ÁO4 are rather long (Table 2) and therefore the link between the layers appears to be weak. In a more systematic approach the packing can be subjected to a topological analysis using TOPOS (Blatov et al., 2014). In this process, molecular entities are abstracted as nodes. Cu1 is a node with a coordination number of 4 (linked to two salicylate ligands and two water molecules). Cu2 has a coordination number of 2 (two salicylate ions). The salicylate ion has four neighbours (two copper ions and two hydrogen bonds). Water molecule O4 is connected to four nodes (one copper ion and three hydrogen bonds). The lattice water O5 has a coordination number of 3 (three hydrogen bonds). A plot of the simplified structure is given in Fig. 4.

Synthesis and crystallization
0.55 g (4 mmol) salicylic acid were suspended in 8 ml water. With a concentrated NaOH solution the pH value was adjusted to approximately 5. A solution of 0.5 g (2 mmol) copper(II) sulfate pentahydrate in 10 ml water was added. Crystals appeared after a few days of standing. From the unitcell determinations it became clear that the mixture of crystals  Table 2 Hydrogen-bond geometry (Å , ). Symmetry codes: (ii) x; y À 1; z; (iii) x; Ày; z À 1 2 ; (iv) Àx þ 3 2 ; y þ 1 2 ; z.

Figure 3
The packing of (I) in the crystal, in a view along [010], showing the hydrogen-bonded layers (black) parallel to (100). The layers are linked via non-coordinating hydrate water molecules (red) into a threedimensional network. C-H hydrogen atoms have been omitted for clarity.

Figure 4
Simplified net as prepared with the TOPOS software (Blatov et al., 2014). Copper cations are shown in green. Nodes derived from the salicylate dianion are displayed in purple and have a coordination number of four. Nodes derived from the water molecules are drawn in grey and have coordination numbers of three and four for the lattice and coordinating water molecules, respectively.
contained at least three species: colourless salicylic acid, green Cu(HSal) 2 Á2H 2 O, and brown crystals of (I). The crystals of (I) are thin plates with <100> being the small dimension. A possible explanation for the form is a two-dimensional hydrogen-bonded network in the structure which extends parallel to (100), as discussed above.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 3. The diffraction data appeared to contain reflections of a small second crystal fragment related by a ca 2 rotation about hkl = (413) with respect to the main fragment. Two orientation matrices were used for the integration with the Eval15 software (Schreurs et al., 2010). A large isotropic mosaicity of 1.4 was assumed for the prediction of the reflection profiles. Only the non-overlapping reflections were used for structure solution and refinement.
All hydrogen atoms were located in difference Fourier maps. C-H hydrogen atoms were refined with a riding model. O-H hydrogen atoms were kept fixed at their located positions.

catena-Poly[[copper(II)-µ 2 -salicylato-[diaquacopper(II)]-µ 2 -salicylato] dihydrate]
Crystal data 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.
Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )