Tetrakis(μ-4-methylbenzoato-κ2 O:O′)bis[(N,N-diethylnicotinamide-κN 1)zinc(II)]

In the centrosymmetric binuclear title complex, [Zn2(C8H7O2)4(C10H14N2O)2], the Zn atoms [Zn⋯Zn′ = 2.9494 (3) Å] are bridged by four 4-methylbenzoate (PMB) anions. The four nearest O atoms around each ZnII ion form a distorted square-planar arrangement, the octahedral coordination being completed by the pyridine N atom of the N,N-diethylnicotinamide (DENA) ligand. Each ZnII ion is displaced by 0.3530 (1) Å from the plane of the four O atoms. The dihedral angles between carboxylate groups and their adjacent benzene rings are 5.88 (10) and 11.89 (9)°, while the benzene rings are oriented at a dihedral angle of 75.19 (4)°. The pyridine ring is oriented at dihedral angles of 38.28 (4) and 49.17 (4)° with respect to the benzene rings. In the crystal structure, weak intermolecular C—H⋯O hydrogen bonds link the molecules into a three-dimensional network. π–π contacts between parallel benzene rings [centroid–centroid distance = 3.8388 (8) Å] and between parallel pyridine rings [centroid–centroid distance = 3.4855 (7) Å] may further stabilize the crystal structure.


Comment
As a part of our ongoing investigation on transition metal complexes of nicotinamide (NA), one form of niacin (Krishnamachari, 1974), and/or the nicotinic acid derivative N,N-diethylnicotinamide (DENA), an important respiratory stimulant (Bigoli et al., 1972), the title compound was synthesized and its crystal structure is reported herein.
The title dimeric complex, [Zn 2 (PMB) 4 (DENA) 2 ], has a centre of symmetry and two Zn II ions are surrounded by four PMB groups and two DENA ligands (Fig. 1). The DENA ligands are coordinated to Zn II ions through pyridine N atoms only.

Experimental
The title compound was prepared by the reaction of ZnSO 4 .H 2 O (0.90 g, 5 mmol) in H 2 O (40 ml) and DENA (1.78 g, 10 mmol) in H 2 O (10 ml) with sodium 4-methylbenzoate (1.58 g, 10 mmol) in H 2 O (250 ml). The mixture was filtered and set aside to crystallize at ambient temperature for one week, giving colourless single crystals.

Refinement
H atoms were positioned geometrically with C-H = 0.93, 0.97 and 0.96 Å, for aromatic, methylene and methyl H atoms, respectively, and constrained to ride on their parent atoms, with U iso (H) = xU eq (C), where x = 1.5 for methyl H and x = 1.2 for all other H atoms. Fig. 1. The molecular structure of the title molecule with the atom-numbering scheme. Displacement ellipsoids are drawn at the 30% probability level. Primed atoms are generated by the symmetry operator:(') 1 -x, -y, -z.

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 > σ(F 2 ) is used only for calculating Rfactors(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.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )
x y z U iso */U eq