Crystal structure of tetraaquabis(1,3-dimethyl-2,6-dioxo-3,7-dihydro-1H-purin-9-ido)magnesium

The title complex, [Mg(C7H7N4O2)2(H2O)4], lies across an inversion centre and the MgII atom is coordinated in a slightly distorted octahedral environment by four aqua ligands in the equatorial sites and imidazole ring N atoms in the axial sites. In the crystal, O—H⋯O and O—H⋯N hydrogen bonds link complex molecules, forming a three-dimensional network incorporating (8) and (18) graph-set motifs.


Chemical context
Co-crystallization represents a crystal engineering approach for modifying properties of active pharmaceutical ingredients (APIs) (Sun, 2013). Metal coordination is an alternative strategy without changing chemical structures of APIs (Ma & Moulton, 2007). Theophylline is a methylxanthine drug in the treatment of asthma and chronic obstructive pulmonary disease (Barnes, 2003). In this study, we reacted theophylline with the Mg II ion in a basic solution to give rise to a tetraaqua mononuclear Mg II complex, (I).

Structural commentary
The molecular structure of (I) is shown in Fig. 1. The complex lies across an inversion centre and the Mg II atom is coordinated in a slightly distorted octahedral environment (Table 1) by four aqua ligands in the equatorial sites and two 1,3-dimethyl-2,6-dioxo-3,7-dihydro-1H-purin-9-ide ligands, through imidazole ring N atoms [N1 and N1(Àx + 1, Ày, Àz + 1)], in the axial sites. The symmetry-unique purine ring system is essentially planar, with a maximum deviation of 0.030 (2) Å for N3 and the bonded methyl C atoms C4 and C5 deviate from this mean plane by À0.118 (3) and 0.136 (2) Å , respectively.

Figure 3
Part of the crystal structure, showing the overall three-dimensional hydrogen-bonded structure (dashed lines).

Figure 2
Part of the crystal structure, showing hydrogen bonds in two dimensions (dashed lines).  Begum & Manohar, 1994). The title compound is the first crystal structure reported to date of a complex of theophylline with an alkaline-earth metal.

Synthesis and crystallization
Theophylline (180 mg, 1 mmol) was dissolved in water (20 ml). An aqueous solution (15 ml) of NaOH (40 mg, 1 mmol) was added slowly. MgCl 2 Á6H 2 O (102 mg, 0.5 mmol) in water (15 ml) was then added. The resulting solution was kept in air and, after several days, colourless block-shaped crystals were obtained.

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. Refinement. Refinement of F 2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F 2 , conventional R-factors R are based on F, with F set to zero for negative F 2 . The threshold expression of F 2 > σ(F 2 ) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F 2 are statistically about twice as large as those based on F, and R-factors based on ALL data will be even larger.