Synthesis, spectroscopy and supramolecular structures of two magnesium 4-nitrobenzoate complexes

The aqueous reaction of [Mg(H2O)6]Cl2 with the in situ generated sodium salt of 4-nitro-benzoic acid (4-nbaH) and N-methylimidazole (N-MeIm) results in the formation of the dinuclear complex [Mg(H2O)(N-MeIm)2(4-nba)2]2 1 (4-nba = 4-nitro benzoate), while the direct reaction of [Mg(H2O)6]Cl2 with sodium 4-nba leads to the formation of the mononuclear complex, [Mg(H2O)6](4-nba)2·2H2O 2. In the centrosymmetric dimer 1, each Mg atom is coordinated to an aquo ligand, two monodentate N-MeIm ligands and a monodentate 4-nba ligand. The second 4-nba ligand functions as a bridging bidentate ligand, linking the metal centers and completes the hexacoordination around each Mg(II). The dimeric molecules of 1 are linked into a one-dimensional chain along b with the aid of intra-as well as intermo-lecular H-bonding interactions between the coordinated water and the free oxygen atom of the monodentate 4-nba ligand. In the mononuclear complex [Mg(H2O)6](4-nba)2·2H2O 2, the Mg(II) is located on an inversion center and its structure consists of an octahedral [Mg(H2O)6]2+ dication, a free uncoordinated 4-nba anion and a lattice water molecule. One of the H atoms attached of the lattice water is disordered over two positions. The hexaaquomagnesium(II) dication, the 4-nba anion and the lattice water molecule are linked by intra- and intermolecular H-bonding interactions resulting in the formation of alternating layers of [Mg(H2O)6]2+ dications and 4-nba anions in the crystallographic bc plane. The lattice water molecules are situated between the cations, while the 4-nba anions are arranged antiparallel to each other along b.