catena-Poly[[triaquamagnesium]-μ2-malonato]

In the title compound, [Mg(C3H2O4)(H2O)3]n, the metal atom is in an octahedral environment. The octahedra are connected by malonate anions, forming chains along the c-axis direction. O—H⋯O hydrogen bonds link these chains into a three-dimensional network.

In the title compound, [Mg(C 3 H 2 O 4 )(H 2 O) 3 ] n , the metal atom is in an octahedral environment. The octahedra are connected by malonate anions, forming chains along the c-axis direction. O-HÁ Á ÁO hydrogen bonds link these chains into a threedimensional network.

Comment
The malonates of the divalent metals Zn, Ni, and Co are isostructural and crystallize as dihydrates in the monoclinic space group C2/m (Walter-Levy et al., 1973;Ray & Hathaway, 1982;Delgado et al., 2004;Zheng & Xie, 2004). The metals are located on sites with 2/m symmetry, octahedrally surrounded by six O atoms. The water molecules and the malonate ligand have mirror symmetry. Overall, this leads to a two-dimensional coordination network in which the layers are interlinked by O-H···O hydrogen bonds.
In this context we set out to synthesize the corresponding Mg(II) complex. Interestingly, the title compound is not isostructural to the above mentioned Zn, Ni, and Co complexes but crystallizes as a trihydrate in the orthorhombic space group Pna2 1 . All atoms are located on general positions without symmetry. The magnesium centers are surrounded by six O atoms in a slightly distorted octahedral geometry. Three O atoms are from the deprotonated malonate ligand, and three O atoms are from the coordinated water molecules (Figure 1). The Mg-O distance to O7 is the largest. O7 is donor of two and acceptor of one hydrogen bond. Water O atoms O5 and O6 do not accept hydrogen bonds. According to the definition by Ptasiewicz-Bak et al. (1999), water molecule O5 is trigonally coordinated, and water molecules O6 and O7 in tetrahedral direction. The angles between the planes of the water molecules and the O-Mg bonds are 7(4), 29 (3), and 42 (3)° for O5, O6, and O7, respectively.
While the malonate dianion has no crystallographic symmetry, it still has an approximate mirror symmetry with an r.m.s. deviation of 0.20 Å (Pilati & Forni, 1998). By coordination to the Mg, a six-membered chelate ring is formed ( Figure 2). According to the algorithm by Evans & Boeyens (1989), the conformation of the ring can be described as linear combination of 75% boat, 23% twist-boat, and 2% chair conformation.
In the title compound, the malonate dianion acts as a bridging ligand, which connects the Mg octahedra into a onedimensional chain in the direction of the c axis ( Figure 3). This distance between the Mg centers is consequently the length of the c axis [6.0920 (4) Å]. The one-dimensional coordination chains are linked by O-H···O hydrogen bonds into a three-dimensional network ( Table 2). All three water molecules act as hydrogen bond donors. The non-coordinated O2 accepts two hydrogen bonds. Each of the coordinated O atoms O1, O3, and O4 accept one hydrogen bond, respectively, and one hydrogen bond is accepted by the water molecule at O7.

Experimental
Crystals were prepared according to the method by Delgado et al. (2004). 2.15 g (10.0 mmol) of magnesium acetate tetrahydrate (Fluka) were dissolved in 20 ml water. This solution was slowly added to a solution of 1.16 g (11.1 mmol) of malonic acid (Fluka) in 20 ml water. The resulting mixture was concentrated by evaporation at 333 K and normal pressure. After standing at room temperature over night, the crystals were obtained.

Refinement
The crystal consisted of two fragments and was consequently integrated with two orientation matrices. The two matrices are related by a rotation angle of 8.6 ° about an axis approximately collinear with the c axis. Only the non-overlapping reflections were used for the structure refinement.