Crystal structure and Hirshfeld surface analysis of aquabis(nicotinamide-κN 1)bis(2,4,6-trimethylbenzoato-κ2 O,O′)cadmium(II)

The CdII cation, located on a twofold rotation axis, is coordinated by two 2,4,6-trimethylbenzoate anions, two nicotinamide ligands and a water molecule in a distorted pentagonal–bipyramidal geometry.

The asymmetric unit of the title complex, [Cd(C 10 H 11 O 2 ) 2 (C 6 H 6 N 2 O) 2 (H 2 O)], contains one half of the complex molecule, with the Cd II cation and the coordinated water O atom residing on a twofold rotation axis. The Cd II cation is coordinated in a bidentate manner to the carboxylate O atoms of the two symmetry-related 2,4,6-trimethylbenzoate (TMB) anions and to the water O atom at distances of 2.297 (2), 2.527 (2) and 2.306 (3) Å to form a distorted pentagonal arrangement, while the distorted pentagonal-bipyramidal coordination sphere is completed by the two pyridine N atoms of the two symmetryrelated monodentate nicotinamide (NA) ligands at distances of 2.371 (3)  (16) and R 6 6 (18) ring motifs, forming a three-dimensional architecture. The Hirshfeld surface analysis of the crystal structure indicates that the most important contributions for the crystal packing are HÁ Á ÁH (56.9%), HÁ Á ÁC/CÁ Á ÁH (21.3%) and HÁ Á ÁO/OÁ Á ÁH (19.0%) interactions.

Chemical context
Nicotinamide (NA) is one form of niacin. A deficiency of this vitamin leads to loss of copper from the body, 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 by 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). The 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 show interesting physical and/or chemical properties, through which they may find applications in biological systems (Antolini et al., 1982). Crystal structures of metal complexes with benzoic acid derivatives have been reported extensively because of the varieties of the coordination modes. For example, Co and Cd complexes with 4-aminobenzoic acid (Chen & Chen, 2002), Co complexes with benzoic acid (Catterick et al., 1974), 4-nitrobenzoic acid  and phthalic acid (Adiwidjaja et al., 1978), and Cu complexes with 4-hydrochloric acid  have been described. ISSN 2056-9890 The structure-function-coordination relationships of the arylcarboxylate ion in Cd II complexes of benzoic acid derivatives change depending 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 . When pyridine and its derivatives are used instead of water molecules, the structure is completely different (Catterick et al., 1974).
The structures of some mononuclear complexes obtained from the reactions of transition metal(II) ions with nicotinamide (NA) and some benzoic acid derivatives as ligands have been determined previously, e.g. [Zn(C 7 , 2009c]. The structure determination of the title compound, (I), a cadmium complex with two 2,4,6-trimethylbenzoate (TMB) and two nicotinamide (NA) ligands and one coordinated water molecule, was undertaken in order to compare the results obtained with those reported previously. In this context, we synthesized the Cd II -containing title compound and report herein its crystal and molecular structures along with the Hirshfeld surface analysis.

Structural commentary
The asymmetric unit of the crystal structure of the mononuclear title complex contains half of a Cd II cation (site symmetry 2), one 2,4,6-trimethylbenzoate (TMB) anion and one nicotinamide (NA) molecule together with half of a water molecule (point group symmetry 2), the TMB and NA ligands coordinating in bidentate and monodentate manners, respectively (Fig. 1) . This confirms the presence of the Cd II cation with a small deviation from the basal plane. The distorted pentagonal-bipyramidal coordination sphere is completed by the two pyridine N atoms (N1 and N1 i ) of the two symmetry-related monodentate nicotinamide (NA) ligands at distances of 2.371 (3) Å in the axial positions (Fig. 1).
The near equalities of the C1-O1 [1.249 (4) Å ] and C1-O2 [1.253 (3) Å ] bonds in the carboxylate groups indicate delocalized bonding arrangements, rather than localized single and double bonds. The O2-C1-O1 bond angle [121.7 (3) ] seems to be slightly decreased than that present in a free acid The molecular structure of the title complex with the atom-numbering scheme. Unlabelled atoms are related to labelled atoms by the symmetry operation (1 À x, y, 1

Hirshfeld surface analysis
Visulization and exploration of intermolecular close contacts of a structure is invaluable, and this can be achieved using Hirshfeld surface (HS) analysis (Hirshfeld, 1977;Spackman & Jayatilaka, 2009). An HS analysis was carried out by using CrystalExplorer17.5 (Turner et al., 2017) to investigate the locations of atomÁ Á Áatom short contacts with the potential to form hydrogen bonds and the quantitative ratios of these interactions and the -stacking interactions in the crystal structure of the title complex.
In the HS plotted over d norm (Fig. 3), the white surface indicates contacts with distances equal to the sum of van der Waals radii, and the red and blue colours indicate distances shorter (in close contact) or longer (distinct contact) than the van der Waals radii, respectively (Venkatesan et al., 2016). The bright-red spots appearing near NA-O3, TMB-O1 and O2, and hydrogen atoms H2A, H2B, H41 and H8C indicate their role as the respective donors and acceptors in the dominant O-HÁ Á ÁO, N-HÁ Á ÁO and C-HÁ Á ÁO hydrogen bonds; they also appear as blue and red regions corresponding to positive and negative potentials on the HS mapped over electrostatic potential (Spackman et al., 2008;Jayatilaka et al., 2005) as shown in Fig. 4. The blue regions indicate the positive electrostatic potential (hydrogen-bond donors), while the red regions indicate the negative electrostatic potential (hydrogen-bond acceptors). The shape-index of the HS is a tool to visualize thestacking by the presence of adjacent red and blue triangles; if there are no adjacent red and/or blue  Symmetry codes: (iii) Àx þ 1; Ày þ 2; Àz þ 1; (vi) x; Ày þ 2; z þ 1 2 ; (vii) x; y; z þ 1; (viii) x; Ày þ 1; z À 1 2 .

Figure 3
View of the three-dimensional Hirshfeld surface of the title complex plotted over d norm in the range À0.6741 to 1.6440 a.u. triangles, then there are nointeractions. Fig. 5 clearly suggests that there are nointeractions in (I).
The overall two-dimensional fingerprint plot, Fig. 6a, and those delineated into HÁ Á ÁH, HÁ Á ÁC/CÁ Á ÁH, HÁ Á ÁO/OÁ Á ÁH, HÁ Á ÁN/NÁ Á ÁH, CÁ Á ÁC and OÁ Á ÁC/CÁ Á ÁO contacts (McKinnon et al., 2007) are illustrated in Fig. 6b-g, respectively, together with their relative contributions to the Hirshfeld surface. The most important interaction is HÁ Á ÁH, contributing 56.9% to the overall crystal packing, which is reflected in Fig. 6b as widely scattered points of high density due to the large hydrogen content of the molecule. The single spike in the centre at d e = d i = 1.2 Å in Fig. 6b is due to a short interatomic HÁ Á ÁH contact (Table 2). In the absence of C-HÁ Á Á interactions in the crystal, the pair of characteristic wings resulting in the fingerprint plot delineated into HÁ Á ÁC/CÁ Á ÁH contacts, with 21.3% contribution to the HS, Fig. 6c; the pair of thin edges at d e + d i $ 1.67 Å result from short interatomic HÁ Á ÁC/ CÁ Á ÁH contacts (Table 2). In the fingerprint plot delineated into HÁ Á ÁO/OÁ Á ÁH contacts, Fig. 6d View of the three-dimensional Hirshfeld surface of the title complex plotted over electrostatic potential energy in the range À0.1379 to 0.1988 a.u. using the STO-3G basis set at the Hartree-Fock level of theory. The N-HÁ Á ÁO, O-HÁ Á ÁO and C-HÁ Á ÁO hydrogen-bond donors and acceptors are viewed as blue and red regions around the atoms corresponding to positive and negative potentials, respectively.

Figure 5
Hirshfeld surface of the title complex plotted over shape-index.  The Hirshfeld surface analysis confirms the importance of H-atom contacts in establishing the packing. The large number of HÁ Á ÁH, HÁ Á ÁC/CÁ Á ÁH and HÁ Á ÁO/OÁ Á ÁH interactions suggest that van der Waals interactions and hydrogen bonding play the major roles in the crystal packing (Hathwar et al., 2015).

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 3. The H atoms of the NH 2 group and of the water molecule 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.  Data collection: APEX2 (Bruker, 2012); cell refinement: SAINT (Bruker, 2012); data reduction: SAINT (Bruker, 2012); program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: ORTEP-3 for Windows (Farrugia, 2012); software used to prepare material for publication: WinGX (Farrugia, 2012) and PLATON (Spek, 2009).

Aquabis(nicotinamide-κN 1 )bis(2,4,6-trimethylbenzoato-κ 2 O,O′)cadmium(II)
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.