Crystal structure and Hirshfeld surface analysis of diaquabis(isonicotinamide-κN)bis(2,4,6-trimethylbenzoato-κO 1)nickel(II) dihydrate

In the title NiII complex, the divalent Ni ion occupies a crystallographically imposed centre of symmetry and is coordinated by two O atoms from the carboxylate groups of two 2,4,6-trimethylbenzoate ligands, two N atoms from the pyridyl groups of two isonicotinamide ligands and two water molecules in a slightly distorted octahedral geometry. Hirshfeld surface analysis indicates that the most important contributions for the crystal packing are from H⋯H (59.8%), O⋯H/H⋯O (20.2%) and C⋯H/H⋯C (13.7%) interactions.


Chemical context
Nicotinamide (NA) is a derivative of nicotinic acid, also called niacin. A deficiency in this vitamin leads to loss of copper from the body, giving rise to a condition known as pellagra disease. Victims of pellagra show unusually high serum and urinary copper levels (Krishnamachari, 1974). The crystal structure of NA was first determined in 1954 (Wright & King, 1954). The NA ring is the reactive part of nicotinamide adenine dinucleotide (NAD) and its phosphate (NADP), which are the major electron carriers in many biological oxidation-reduction reactions (You et al., 1978). Another nicotinic acid derivative, N,N-diethylnicotinamide (DENA), is an important respiratory stimulant (Bigoli et al., 1972). Transition-metal complexes with ligands of biochemical interest, such as imidazole and some N-protected amino acids, often show interesting physical and/or chemical properties, which lead to applications in biological systems (Antolini et al., 1982). There have been many reports of the crystal structures of metal complexes with benzoic acid derivatives, which are of interest because of the number of different coordination modes exhibited by the carboxylic acid groups. These include Co and Cd complexes with 4-aminobenzoic acid (Chen & Chen, 2002;Amiraslanov et al., 1979;Hauptmann et al., 2000), Co complexes with benzoic acid (Catterick et al., 1974), 4-nitro-benzoic acid  and phthalic acid (Adiwidjaja et al., 1978) and Cu complexes with 4-hydrochlorobenzoic acid . Mn complexes closely related to the title compound have also been reported, e.g. diaquabis(4-nitrobenzoato)bis(1H-1,2,4-triazol-3-amine)manganese(II) (Zhang et al., 2013) and diaquabis(1Himidazole)bis(4-nitrobenzoato)manganese(II) (Xu & Xu, 2004).
The crystal structures of anhydrous zinc(II) carboxylates are diverse and include one-dimensional (Guseinov et al., 1984;Clegg et al., 1986a), two-dimensional (Clegg et al., 1986b(Clegg et al., , 1987 and three-dimensional (Capilla & Aranda, 1979) polymeric motifs of different types, while discrete monomeric complexes with octahedral or tetrahedral coordination geometry are found if water or other donor molecules are coordinated to Zn ( van Niekerk et al., 1953;Usubaliev et al., 1992). Pertinent to the present work, the structure-functioncoordination relationships of the arylcarboxylate ion in Zn II complexes of benzoic acid derivatives have been studied and shown to depend on the nature and position of the substituted groups on the benzene ring, the nature of the additional ligand, molecule or solvent, and the pH and temperature of synthesis Nadzhafov et al., 1981;Antsyshkina et al., 1980;Adiwidjaja et al., 1978;Catterick et al., 1974).
The structures of a number of mononuclear complexes of divalent transition-metal ions with both nicotinamide (NA) and benzoic acid derivatives as ligands have been previously reported and include [Ni(C 7 Tercan et al., 2009]. In this work, to enable comparison with the above Ni II compounds and develop structure-function-coordination relationships, we describe the synthesis of diaquabis(isonicotinamide-N)bis(2,4,6-trimethylbenzoato-O 1 )nickel(II) dihydrate, [Ni(C 10 H 11 O 2 ) 2 (C 6 H 6 N 2 O) 2 (H 2 O) 2 ]Á2H 2 O, and report its molecular and crystal structures, along with a Hirshfeld surface analysis.

Hirshfeld surface analysis
A Hirshfeld surface analysis (Hirshfeld, 1977;Spackman & Jayatilaka, 2009) of the title complex was carried out to investigate the locations of the atoms with potential to form hydrogen bonds and the quantitative ratios of these interactions. Conventional mapping of d norm (Fig. 3), together with graphical representation of the Hirshfeld surface (Fig. 4) suggest the locations of the donors and acceptors of intermolecular contacts, which are represented in Fig. 3 as brightred spots near respective atoms. According to the analysis results, the most important interaction is HÁ Á ÁH contributing 59.8% to the overall crystal packing. The next most important interactions are OÁ Á ÁH/HÁ Á ÁO and CÁ Á ÁH/HÁ Á ÁC contributing 20.2% and 13.7%, respectively. The weakest intermolecular contacts contributing to the cohesion of the structure are CÁ Á ÁC, NÁ Á ÁH/HÁ Á ÁN, CÁ Á ÁO/OÁ Á ÁC and CÁ Á ÁN/NÁ Á ÁC, found to contribute only 3.0, 2.3, 0.6 and 0.4%, respectively. The overall two-dimensional fingerprint plot, Fig. 4a View of the hydrogen bonding and packing of the title complex along the a axis. Non-bonding H atoms have been omitted for clarity. (McKinnon et al., 2007) are illustrated in Fig. 4 b-h, respectively, together with their relative contributions to the Hirshfeld surface, where the significant OÁ Á ÁH/HÁ Á ÁO interactions are indicated by the pair of wings in the two-dimensional fingerprint plot with a prominent long spike at d e + d i $1.0 Å (Fig. 4c). The presence of these interactions may also be shown by the Hirshfeld surface mapped as a function of curvedness (Fig. 5).

Refinement
The experimental details including the crystal data, data collection and refinement are summarized in Table 2. H atoms of NH 2 groups and water molecules were located in difference Fourier maps and refined freely. The C-bound H atoms were positioned geometrically with C-H = 0.93 and 0.96 Å for aromatic and methyl H atoms, respectively, and constrained to ride on their parent atoms, with U iso (H) = k Â U eq (C), where k = 1.5 for methyl H atoms and k = 1.2 for aromatic H atoms. The maximum and minimum residual density peaks were found at 0.83 and 0.78 Å from atoms O1 and O4, respectively.  View of the three-dimensional Hirshfeld surface of the title complex plotted over d norm in the range À0.7129 to 1.3644 au.

Diaquabis(isonicotinamide-κN)bis(2,4,6-trimethylbenzoato-κO 1 )nickel(II) dihydrate
Crystal data 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 > 2sigma(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.