Crystal structure and Hirshfeld surface analysis of a copper(II) complex containing 2-nitrobenzoate and tetramethylethylenediamine ligands

In the title complex, [Cu(2-nitrobenzoate)2(tmeda)], the central metal atom has distorted square-planar geometry with one oxygen atom each from two 2-nitrobenzoate ligands and two TMEDA ligand nitrogen atoms.

The reaction of copper(II) sulfatepentahydrate with 2-nitrobenzoic acid and N,N,N 0 ,N 0 -tetramethylethylenediamine (TMEDA) in basic solution produces the complex bis(2-nitrobenzoato-O)(N,N,N 0 ,N 0 -tetramethylethylenediamine-2 N,N 0 )copper(II), [Cu(C 7 H 4 NO 4 ) 2 (C 6 H 16 N 2 )] or [Cu(2-nitrobenzoate) 2 -(tmeda)]. Each carboxylate group of the 2-nitrobenzoate ligand is coordinated by Cu II atom in a monodentate fashion and two TMEDA ligand nitrogen atoms are coordinate by the metal center, giving rise to a distorted square-planar coordination environment. In the crystal, metal complexes are linked by centrosymmetric C-HÁ Á ÁO hydrogen bonds, forming ribbons via a R 2 2 (10) ring motif. These ribbons are linked by further C-HÁ Á ÁO hydrogen bonds, leading to two-dimensional hydrogen-bonded arrays parallel to the bc plane. Weakstacking interactions provide additional stabilization of the crystal structure. Hirshfeld surface analysis, d norm and two-dimensional fingerprint plots were examined to verify the contributions of the different intermolecular contacts within the supramolecular structure. The major interactions of the complex are OÁ Á ÁH/HÁ Á ÁO (44.9%), HÁ Á ÁH (34%) and CÁ Á ÁH (14.5%).

Chemical context
Copper(II) carboxylate complexes continue to be of considerable interest on account of their biological properties such as antibacterial (Melník et al., 1982), antifungal (Kozlevčar et al., 1999), cytotoxic and antiviral activities (Ranford et al., 1993). Carboxylate ligands are versatile and can coordinate to metal centers in different modes such as monodentate, bidentate and bridging fashions. The bidentate coordination can be either symmetrical bidentate chelating, having the same C-O bond lengths, or asymmetrical bidentate chelating, having different C-O bond lengths. Carboxylate ligands have been used to generate units for developing supramolecular architectures. Copper is one of essential metals for human life. In the human body, various enzymes are copper-dependent such as Cytochrome c oxidase, superoxide dismutase, ferroxidases, monoamine oxidase, and dopamine -monoxygenase (Brewer, 2009;Balamurugan & Schaffner, 2006). In this work, a new copper(II) complex involving 2-nitrobenzoic acid and N,N,N 0 ,N 0 -tetramethylethylenediamine was synthesized, characterized by single crystal X-ray and studied by Hirshfeld surface analysis.

Structural commentary
Copper(II) acetate reacts with 2-nitrobenzoic acid and N,N,N 0 ,N 0 -tetramethylethylenediamine (TMEDA) to give the mono-nuclear copper(II) complex (I). The asymmetric unit of the title compound contains one half of the metal complex, the central metal being located on the special position 4e (1/2, y, 1/ 4). The Cu II atom has a distorted square-planar geometry with one oxygen atom each from two nitrobenzoic acid ligands and two TMEDA ligand nitrogen atoms (Figs. 1 and 2). The two nitro groups of the rings are oriented trans to each other, being symmetry-related to each other through a twofold axis. The structure of the complex is shown in Fig. 1. The Cu1-N1 and Cu1-O1 bond distances are 2.0269 (13) and 1.9589 (11) Å , respectively. The structural parameters of the TMEDA ligand, i.e. Cu-N bond lengths, are in agreement with a work reported by Gumienna-Kontecka et al. (2013). The C4-O1 and C4-O2 distances in the carboxyl group are 1.2772 (19) and 1.2388 (18) Å , respectively. Selected bond lengths are given in Table 1.

Database survey
A search of the Cambridge Structural Database (CSD, version 5.41, update of November 2019; Groom et al., 2016) for the title complex revealed four hits: View of the two-dimensional hydrogen-bonded network in the structure of [Cu(2-nitrobenzoate) 2 (tmeda) showing C9-H9Á Á ÁO4 hydrogen bonds [described by an R 2 2 (10) ring motif] as green dashed lines and C3-H3AÁ Á ÁO3 hydrogen bonds as blue dashed lines.  Hydrogen-bond geometry (Å , ).

Hirshfeld surface analysis
Hirshfeld surface analysis and the associated two-dimensional fingerprint plots (Spackman & Jayatilaka, 2009) are very important for explaining the intermolecular contacts in the crystal structure (Demirciog lu et al., 2019;Ilmi et al., 2020). We performed the Hirshfeld surface analysis with Crystal-Explorer17 (Turner et al., 2017). Fig. 3 shows the Hirshfeld surface mapped over d norm (-0.2250 to 1.2935 a.u.) and the molecular electrostatic potentials (-0.2173 to 0.1248). In Fig. 3a, the red spots correspond to the OÁ Á ÁH contacts. The electrostatic potential (Fig. 3b) shows donor (red) and acceptor (blue) regions. OÁ Á ÁH/HÁ Á ÁO (44.9%) contacts, seen as a pair of spikes of scattered points in the fingerprint plot, make the largest contribution to the total Hirshfeld surface in [Cu(2-nitrobenzoate) 2 (tmeda)] (Fig. 4). The second most important interaction is HÁ Á ÁH, contributing 34% to the overall crystal packing, which is shown in the 2D fingerprint of the (d i , d e ) points related to the H atoms. Two symmetrical wings on the left and right sides are shown in the graph of CÁ Á ÁH/HÁ Á ÁC interactions (14.5%). The Hirshfeld surface analysis confirms the importance of H-atom contacts in establishing the packing. The large number of OÁ Á ÁH, HÁ Á ÁH and CÁ Á ÁH interactions suggest that van der Waals interactions and hydrogen bonding play the major role in the crystal packing.

Synthesis and crystallization
An aqueous solution of sodium 2-nitrobenzoate (5 mmol, 0.9 g) was added to an aqueous solution of CuSO 4 Á5H 2 O (2.5 mmol, 0.6 g) under stirring. Tetramethylethylenediamine (2.5 mmol, 0.3 g) was added and the color changed from light blue to violet. The mixture was filtered and the filtrate was allowed to stand for slow evaporation. Single crystals suitable for X-ray were obtained after several days.

Bis(2-nitrobenzoato-κO)(N,N,N′,N′-tetramethylethylenediamine-κ 2 N,N′)copper(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.