Poly[bis(N,N-dimethylformamide-κO)(μ4-naphthalene-1,5-disulfonato)magnesium(II)]

The structure of the title compound, [Mg(C10H6O6S2)(C3H7NO)2]n, consists of MgO6 octahedra ( symmetry) connected to naphthalene-1,5-disulfonate ligands ( symmetry) in the equatoral plane, forming a two-dimensional network propagating parallel to (010). The coordination sphere of the Mg atom is completed by the O atoms of two N,N-dimethylformamide (DMF) molecules in the axial positions. The title compound represents the first time the naphthalene-1,5-disulfonate anion is bound directly to a Mg2+ atom. Disorder over two positions was found in the DMF molecule in a 0.518 (8):0.482 (8) ratio.


Comment
Interest in metal organic coordination polymers (CPs) and frameworks (MOFs) arises due to the potential applications including size/shape catalysis and gas storage/separation (Cheetham et al., 2006;Kitagawa et al., 2004;Rosseinsky, 2004;Rowsell & Yaghi, 2004). This is owing to the fact that these materials have similar properties to zeolites with the added possiblity of structural design. Three factors need to be considered during the sythesis of CPs and MOFs; the coordination sphere of the metal center, the size, shape and functionality of the organic linker and the synthetic conditions. Previous studies have looked at the interaction between a variety of metal atoms and 1,5-naphthalenedisulfonic acid.
A search of the literature results in examples of direct bonding between this ligand and every alkali and alkali earth metal except Mg 2+ . Due to perodic trends we suspected that Mg 2+ and naphthalene-1,5-disulfonate should have a direct bonding interaction, which indicates a change in synthetic conditions compared to the previous studies was needed. The previous efforts used water as the solvent resulting in structures where the Mg 2+ atoms are bound solely to water molecules and interacted with 1,5-naphthalenedisulfonic acid through hydrogen bonds (Cody & Hazel, 1977;Morris et al., 2003;Shakeri & Haussühl, 1992). In our study we used N,N-dimethylformamide (DMF) as the solvent with the thought that DMF is a large solvent molecule and is unlikely to complete for the coordination sphere of the Mg 2+ atom.
The title compound consists of MgO 6 octahedra linked by 1,5-naphthalenedisulfonate ligands to form a two-dimensional network parallel to (010). The Mg 2+ atom is bound to four O atoms from four separate naphthalene-1,5-disulfonate ligands in the equitorial plane and two O atoms from two symmetry equivalent N,N-dimethylformamide molecules in the axial positions ( Figure 1). The average equitorial Mg-O bond length is 2.031 Å whereas the axial bond length is slightly longer at 2.156 Å ( Table 1). The individual MgO 6 octahedra are connected by the naphthalene-1,5-disulfonate linkers in a bridging bidentate fashion to form chains along the [100] direction, which are further connected by the linker molecules to form an overall 2-dimensional structure ( Figure 2).

Experimental
All of the starting materials are available commercially and were used without any further purification.  allowed to refine independent of the C atoms with U iso (H)=1.5U eq (C) for the methyl groups and U iso (H)=1.2U eq (C) for all others. The highest peak (~0.6 e -A -3 ) can be found within 0.89 Å of C2 and the deepest hole (~-0.5 e -A -3 ) within 0.73 Å of S1. The near equal values of the highest peak and deepest hole indicate that there is not any remaining unmodelled electron density. The crystal remained stable throughout the low temperature data collection.

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.
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 Rfactors(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.