Crystal structure of tetraaquabis(1,3-dimethyl-2,6-dioxo-7H-purin-7-ido-κN 7)cobalt(II)

The complex title molecule lies across an inversion centre and exhibits a slightly distorted octahedral coordination environment for the CoII atom.

The title complex, [Co(C 7 H 7 N 4 O 2 ) 2 (H 2 O) 4 ], comprises mononuclear molecules consisting of a Co II ion, two deprotonated theophylline ligands (systematic name: 1,3-dimethyl-7H-purine-2,6-dione) and four coordinating water molecules. The Co II atom lies on an inversion centre and has a slightly distorted octahedral coordination environment, with two N atoms of two trans-oriented theophylline ligands and the O atoms of four water molecules. An intramolecular hydrogen bond stabilizes this conformation. A three-dimensional supramolecular network structure is formed by intermolecular O-HÁ Á ÁO and O-HÁ Á ÁN hydrogen bonds.

Chemical context
Theophylline (systematic name: 1,3-dimethyl-7H-purine-2,6dione) belongs to the family of xanthines, which are purine derivatives. It is related to dietary xanthines caffeine and theobromine and is an important pharmacologic compound (Shukla & Mishra, 1994). Usually, synthetic drugs of theophylline are used for the treatment of disorder in the physiological functions of the pulmonary system (Childs, 2004) because theophylline is a bronchodilator that is given against asthma and bronchospasm in adults (Chen et al., 2007).
The complexing ability of theophylline has been studied towards modelling metal interactions with the guanine base of nucleic acids (Orbell et al., 1988). Theophylline can be deprotonated in basic or neutral media. In the majority of cases, the resulting anionic ligand is monodentate and coordinates through the N7 atom of theophylline (Marzilli et al., 1973;Begum & Manohar, 1994;Bombicz et al., 1997;Buncel et al., 1985), while only in a few cases has a different coordination behaviour been reported, e.g. through the N9 atom of the imidazole ring (Aoki & Yamazaki, 1980). In addition, deprotonated theophylline may act as a bidentate ligand, where the ISSN 2056-9890 initial metal bonding to N7 is supplemented by coordination to the O6 atom, forming an N7/O6 chelate (Cozak et al., 1986).
In this study, we reacted theophylline with the Co II ion to yield the title complex, [Co(C 7 H 7 N 4 O 2 ) 2 (H 2 O) 4 ].

Structural commentary
The molecular structure of the title complex is shown in Fig. 1. The complex lies across an inversion centre, with the Co II atom being coordinated in a slightly distorted octahedral environment by four aqua ligands in the equatorial sites and the imidazole ring N atoms of two 1,3-dimethyl-2,6-dioxo-7Hpurin-7-ide ligands [N1 and N1 i ; see Table 1 for symmetry code], in the axial sites. The Co-O bond lengths are shorter than the Co-N bond length (Table 1). The purine ring system is essentially planar, with a maximum deviation of 0.029 Å for N5; methyl atoms C10 and C12 deviate from this mean plane by À0.117 and 0.12 Å , respectively. The molecular conformation is stabilized by an intramolecular O-HÁ Á ÁO hydrogen bond between a water molecule (O15) and a carbonyl O atom (O13) ( Table 2), leading to an S(7) graph-set motif (Bernstein et al., 1995).

Supramolecular features
In the crystal, the mononuclear units are connected into a layered arrangement parallel to (010). The coordinating water molecules are involved in various hydrogen-bonding interactions (Table 2), including R 2 4 (8) graph-set motifs that are formed through (O14Á Á ÁO11 ii = 2.817 (2) Å and O14Á Á ÁO11 iii = 2.773 (2) Å ; see Table 2 for symmetry codes) between a coordinating water molecule and the carbonyl groups of symmetry-related theophylline ligands (Fig. 2). In addition, water molecule O15 is hydrogen bonded to the nonmethylated N atom of the imidazole group (O15Á Á ÁN3 iv = 2.799 (2) Å ; see Table 2 for symmetry code), leading to an overall threedimensional network.

Synthesis and crystallization
Theophylline (360 mg, 2 mmol) was dissolved in water (20 ml). An aqueous solution (10 ml) of NaOH (80 mg, 2 mmol) was added slowly. CoCl 2 Á6H 2 O (237 mg, 1 mmol) in water (10 ml) was then added. Pink single crystals of the title compound suitable for X-ray analysis were obtained after several months by slow evaporation of the solvent at room temperature.

Refinement
Details of data collection and structure refinement are summarized in Table 3. The calculated strategy was based on monoclinic chiral symmetry, with a completeness of 100%, an average multiplicity of 11.4 and no missing reflections. However, some reflections were still missing after data collection, thus reducing the completeness to less than 100%. All H atoms were located in a difference map, but those attached to C atoms were repositioned geometrically. The H atoms were refined with soft restraints on bond lengths and angles to regularize their geometry (  Symmetry code: (i) Àx; Ày þ 1; Àz þ 1.

Figure 2
The crystal structure, showing the overall three-dimensional hydrogenbonded network (hydrogen bonds as dashed lines).

Figure 1
The molecular structure of the title complex. Primed atoms are related to the nonprimed atoms by the inversion centre of the title compound (symmetry code: Àx, Ày + 1, Àz + 1). Displacement ellipsoids are drawn at the 50% probability level.