Synthesis, crystal structure and Hirshfeld surface analysis of tetraaquabis(isonicotinamide-κN 1)cobalt(II) fumarate

In the complex cation, the CoII atom, located on an inverse centre, is coordinated by two isonicotinamide and four water molecules in a distorted O4N2 octahedral geometry. The fumarate anion is located on another inverse centre and is linked to neighbouring complex cations via O—H⋯O and O—H⋯N hydrogen bonds and weak C—H⋯O hydrogen bonds. In the crystal, the complex cations are further linked by O—H⋯O, N—H⋯O an weak C—H⋯O hydrogen bonds, forming a three-dimensional supramolecular architectecture.


Chemical context
Metal carboxylates have attracted intense attention because of their interesting framework topologies (Rao et al., 2004). Among metal carboxylates, fumarate dianions (fum) have good conformational freedom and they possess some desirable features such as being versatile ligands because of the four electron-donor oxygen atoms they carry, and their ability to link inorganic moieties (Zheng et al., 2003). Moreover, metal fumarates exhibit interesting structural varieties.
Dicarboxylic acids such as fumaric acid and amides have been particularly useful in creating many supramolecular structures involving isonicotinamide and a variety of carboxylic acid molecules (Vishweshwar et al., 2003;Aakerö y et al., 2002). Dicarboxylic acid ligands are utilized in the synthesis of a wide variety of metal carboxylates. For this reason they have been investigated extensively, both experimentally and computationally. We describe herein the synthesis, structural features and Hirshfeld surface analysis of the title salt.

Structural commentary
The molecular structure of the title compound is illustrated in Fig. 1. The Co II cation and midpoint of the C C bond of the fumarate anion are each located on an inversion centre. In the complex cation, the Co II atom is coordinated to two isonicotinamide ligands and four water molecules in a distorted N 2 O 4 octahedral geometry. The fumarate anion interacts with neighboring complex cations via O-HÁ Á ÁO and N-HÁ Á ÁO hydrogen bonds and weak C-HÁ Á ÁO hydrogen bonds (Table 1).

Hirshfeld surface analysis
Crystal Explorer 17.5 (Turner et al., 2017) was used to analyse the interactions in the crystal and fingerprint plots mapped over d norm (Figs. 3 and 4) were generated. The contact distances to the closest atom inside (d i ) and outside (d e ) of the Hirshfeld surface are used to analyse the intermolecular interactions via the mapping of d norm . The molecular Hirshfeld surfaces were obtained using a standard (high) surface resolution with the three-dimensional d norm surfaces mapped over a fixed colour scale of À1.227 (red) to 1.279 (blue). Many studies on Hirshfeld surfaces can be found in the literature (see, for example, Ş en et al., 2018;Yaman et al., 2018).

Figure 2
Packing of the title compound in the unit cell. Dashed lines indicate hydrogen bonds.

Figure 3
The Hirshfeld surface of the title compound mapped with d norm . The red spots indicate the regions of the donor-acceptor interactions.

Figure 1
The molecular structure of the title compound, showing the atom labelling. Displacement ellipsoids are drawn at the 50% probability level.

Synthesis and crystallization
An aqueous solution of fumaric acid (26 mmol, 3 g) in water was added to a solution of NaOH (52 mmol, 2.07 g) while stirring. A solution of CoCl 2 Á6H 2 O (25 mmol, 6.19 g) in water was added. The reaction mixture was stirred for an hour at room temperature. The pink mixture was filtered and left to dry. The pink crystals (0.88 mmol, 0.20 g) were dissolved in water and added to an aqueous solution of isonicotinamide (1.75 mmol, 0.21 g). The resulting suspension was filtered and allowed to crystallize for five weeks at room temperature yielding orange block-shaped crystals suitable for X-ray diffraction analysis.

Refinement
Crystal data, data collection and structure refinement details are summarized in        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.