Crystal structure of poly[[(acetato-κO){μ3-N-[(pyridin-4-yl)methyl]pyrazine-2-carboxamidato-κ4 N:N 1,N 2:N 4]copper(II)] dihydrate]: a metal–organic framework (MOF)1

The title compound, a hydrated copper acetate complex of the ligand N-[(pyridin-4-yl)methyl]pyrazine-2-carboxamide, has a metal-organic framework (MOF) structure with a 10 (3) network topology. The water molecules are located in the cavities of the framework and linked to it by O—H⋯O hydrogen bonds.

The title compound, [Cu(C 11 H 9 N 4 O)(CH 3 CO 2 )]Á2H 2 O (CuL), is a hydrated copper acetate complex of the ligand N-[(pyridin-4-yl)methyl]pyrazine-2carboxamide (HL). Complex CuL has a metal-organic framework (MOF) structure with a 10 (3) network topology. The ligand coordinates in a bidentate and a bis-monodentate manner, bridging three equivalent Cu II atoms via the pyridine N atom and the second pyrazine N atom. The Cu II atom has a fivefold coordination sphere, CuN 4 O, being coordinated to three N atoms of the ligand and the acetate O atom in the equatorial plane and to the second pyrazine atom in the apical position. This gives rise to a fairly regular square-pyramidal geometry. In the crystal, the water molecules are linked to each other and to the three-dimensional framework via O-HÁ Á ÁO hydrogen bonds. There are also a number of C-HÁ Á ÁO hydrogen bonds present within the framework.

Chemical context
The ligand N-[(pyridin-4-yl)methyl]pyrazine-2-carboxamide (HL) is one of a series of ligands which were synthesized in order to study their coordination behaviour towards first-row transition metals (Cati, 2002;Cati et al., 2004;Cati & Stoeckli-Evans, 2014). HL is expected to coordinate in a bidentate and possibly a monodentate manner, with eventual bridging of metal atoms to construct two-or three-dimensional networks. A excellent review on the subject of coordination polymers and network structures has been published by Batten et al. (2009).

Figure 3
A view of the 10 (3) network topology of the title metal-organic framework (MOF) structure, illustrating the 3-connected three-dimensional nets with ten-membered rings.

Supramolecular features
The three-dimensional network of the title MOF structure has a 10 (3) network topology (Fig. 3). It is one of the most commonly encountered 3-connected three-dimensional nets with ten-membered rings (Wells, 1984). It is a cubic (10,3)-a net, also known as the srs (SrSi 2 ) net, which is chiral [note that the Flack x parameter = À0.01 (3)]. Such structures contain fourfold helices along the three axes all of the same hand (Batten et al., 2009). In the crystal of CuL, the water molecules are located in the cavities of the MOF structure. They are hydrogen bonded to one another and to the ligand and acetate carbonyl O atoms (Table 1 and Fig. 4). There are also a number of C-HÁ Á ÁO hydrogen bonds present within the framework (Table 1).

Database survey
A search of the Cambridge Structural Database (Version 5.35, last update November 2013; Allen, 2002) indicated that no complexes of the ligand HL have been described previously. The analogous ligand N-[(pyridin-2-yl)methyl]pyrazine-2carboxamide has been described as well as a number of metal complexes. These include the mononuclear copper acetate complex (Mohamadou et al., 2012). Here this ligand coordinates in a tridentate manner but in a number of other complexes it coordinates in a bis-monodentate manner via the pyridine N atom and a pyrazine N atom; for example, in two polymeric mercury chloride complexes (Khavasi et al., 2010), and a polymeric silver tetrafluoroborate complex (Hellyer et al., 2009).

Synthesis and crystallization
The synthesis of the ligand N-[(pyridin-4-yl)methyl]pyrazine-2-carboxamide (HL) has been described elsewhere (Cati, 2002;Cati & Stoeckli-Evans, 2014). Complex CuL was prepared by adding Cu(acetate) 2 ÁH 2 O (64 mg, 0.318 mmol) to a hot solution (323 K) of HL (68 mg, 0.318 mmol) in dry methanol (25 ml). In 2 min a precipitate appeared and heating was stopped and the mixture stirred as the temperature decreased to room temperature. After 30 min the precipitate was filtered off and washed with dry methanol. It was then dissolved in a mixture of water (12 ml) and methanol (15 ml

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2. The water H atoms were located in difference Fourier maps were refined with distance restraints: O-H = 0.84 (2) and HÁ Á ÁH = 1.35 (2) Å with U iso (H) = 1.5U eq (O). The C-bound H atoms were included in calculated positions and treated as riding atoms: C-H = 0.95 Å with U iso (H) = 1.2U eq (C).

Figure 4
A view along the c axis of the crystal packing of complex CuL, with the hydrogen bonds involving the water molecules shown as dashed lines (see Table 1 for details; H atoms not involved in these hydrogen bonding have been omitted for clarity). program(s) used to refine structure: SHELXL2013 (Sheldrick, 2008); molecular graphics: Mercury (Macrae et al., 2008); software used to prepare material for publication: SHELXL2013 (Sheldrick, 2008), PLATON (Spek, 2009) and publCIF (Westrip, 2010).

Special details
Geometry. All e.s.d.'s (except the e.s.d. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell e.s.d.'s are taken into account individually in the estimation of e.s.d.'s in distances, angles and torsion angles; correlations between e.s.d.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell e.s.d.'s is used for estimating e.s.d.'s involving l.s. planes.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )
x y z U iso */U eq  (13)