Crystal structure, Hirshfeld surface analysis and DFT studies of tetrakis(μ-3-nitrobenzoato-κ2 O 1:O 1′)bis[(N,N-dimethylformamide-κO)copper(II)] dimethylformamide disolvate

The title compound, [Cu2(C7H4NO4)4(C3H7NO)2]·(C3H7NO)2, is a binuclear copper(II) complex located on an inversion center midway between the two copper(II) cations. The asymmetric unit consists of one CuII cation, two 3-nitrobenzoato ligands, and two dimethylformamide (DMF) molecules, one of which coordinates to the CuII cation and one is a solvate molecule. The carboxylate groups of the ligands bridge two CuII cations, completing a distorted octahedral O5Cu coordination environment.

The title compound, [Cu 2 (C 7 H 4 NO 4 ) 4 (C 3 H 7 NO) 2 ]Á(C 3 H 7 NO) 2 , is a binuclear copper(II) complex located on an inversion center midway between the two copper(II) cations. The asymmetric unit consists of one Cu II cation, two 3-nitrobenzoato ligands, and two dimethylformamide (DMF) molecules, one of which coordinates to the Cu II cation and one is a solvate molecule. The carboxylate groups of the ligands bridge two Cu II cations with a Cu-Cu distance of 2.6554 (6) Å , completing a distorted octahedral O 5 Cu coordination environment. The dihedral angles between the carboxylate and the aromatic ring planes of the two independent ligands are different from one another, viz. 5.2 (3) and 23.9 (3) . The three-dimensional structure is consolidated by weak C-HÁ Á ÁO interactions and stabilized bystacking interactions between the aromatic rings. The complex and the free ligand were further characterized by Fourier-transform infrared spectroscopy (FT-IR), and the energies of the frontier molecular orbitals of the complex were determined by DFT calculations at the B3LYP/def2-TZVP level of theory.

Chemical context
Copper complexes have been explored extensively due to the fact that copper is a bio-essential element responsible for numerous bioactivities in living organisms (Tapiero & Tew, 2003). Moreover, it is well known that Cu II complexation plays an important role in the pharmacological profile of antimicrobial activities (Haiduc & Silvestru, 1989;Linder & Goode, 1991). The first syn-syn bridged binuclear structure of a large number of copper(II) carboxylates with general formula [Cu(RCOO) 2 (L)] 2 (L = co-ligand), was reported for simple copper(II) acetate monohydrate ( Van Niekerk & Schoening, 1953). This classical structure consists of a binuclear [Cu 2 O 8 ] unit in which each copper(II) atom is surrounded by four oxygen atoms of carboxylate groups in an almost square-planar coordination. An additional ligand, here the O atom of a water molecule, is attached in an apical position at longer Cu-O distances. The Cu-Cu contact completes a distorted octahedral coordination sphere around each copper(II) atom. This motif is also observed in polymeric copper(II) carboxylates, where the apical ligand has two coordination centers and links dimeric units (Rao et al., 1983;Zhu et al., 2003). In the situation where the apical ligand is absent, a zigzag polymeric structure is formed with direct ISSN 2056-9890 bonding between [Cu 2 O 8 ] units via the metal and one of the basal oxygen atoms of the neighbouring unit (Drożdżewski et al., 2004).
Copper(II) carboxylates, including copper(II) benzoates have been studied extensively from different points of view, because the carboxylato ligands exhibit different binding modes that are related to their properties, e.g. the basicity of the anion or the position of substituents on the aromatic ring. A bulky benzene ring substituent in an ortho position to the carboxylate group is believed to prefer the dimeric copper(II) benzoate structure (Harrison et al., 1972;Ueyama et al., 1996). In general, copper(II) carboxylates exhibit a dimeric paddlewheel cage structure. More than 500 crystal structures containing the Cu 2 (OOCR) 4 core have been determined on the basis of X-ray data and can be found in the Cambridge Structural Database (Groom et al., 2016), of which more than 250 are of the type [Cu 2 (OOCR) 4 (L) 2 ], where L is an apical ligand with an oxygen, a nitrogen, a chlorine or a phosphorus donor atom. We report here on the molecular and crystal structure of a similar binuclear copper(II) complex, [Cu 2 (C 7 H 4 NO 4 ) 4 (C 3 H 7 NO) 2 ]Á(C 3 H 7 NO) 2 , further characterized by infrared spectroscopy and DFT calculations.

Structural commentary
[Cu 2 (C 7 H 4 NO 4 ) 4 (C 3 H 7 NO) 2 ]Á(C 3 H 7 NO) 2 crystallizes as a dimethylformamide disolvate (Fig. 1). The neutral [Cu 2 (C 7 H 4-NO 4 ) 4 (C 3 H 7 NO) 2 ] complex is centrosymmetric, with the inversion center located midway between the two Cu II cations. The asymmetric unit comprises one Cu II cation, two 3-nitrobenzoato ligands and two dimethylformamide molecules, one ligating and one as a solvent. The complex displays a paddle-wheel-shaped binuclear structure. Each Cu II cation is coordinated by four carboxylate oxygen atoms, forming the base of a slightly distorted square pyramid supplemented by a fifth oxygen atom of the dimethylformamide molecule at the apical position (Fig. 1). The overall distorted octahedral coordination environment is completed by the neighbouring Cu II cation with a Cu-Cu distance of 2.6554 (6) Å . This distance is close to that reported for similar binuclear complexes (Wang et al., 2018).
In the binuclear complex, the carboxylate groups of the 3-nitrobenzoato ligands link the two Cu II cations with short Cu-O distances [from 1.9620 (17) to 1.9751 (16) Å ; Table 1] whereas the distance to the O atom of the dimethylformamide ligand is elongated [2.1453 (17) Å ] . The carboxylate groups of the 3-nitrobenzoato ligands adopt a bidentate syn-syn bridging mode (Su et al., 2015;Wang et al., 2018), with dihedral angles between the carboxylate planes and the aromatic rings of 5.2 (3) and 23.9 (3) , respectively.

DFT calculations
Theoretical calculations were carried out by the hybrid density functional theory (DFT) at the B3LYP level of theory (Becke, 1988;Lee et al., 1988) using Aldrich's def2-TZVP basis set, which has been successfully tested in one of our previous Three-dimensional Hirshfeld surface of the title compound mapped over d norm .
studies (Ibragimov et al., 2021). Input files for the DFT calculations using the ORCA 4.2.0 program package (Neese, 2012) were generated by Avogadro (Hanwell et al., 2012) using the CIF of the title compound. Results of these calculations were analyzed with the aid of Avogadro and Multiwfn (Lu & Chen, 2012). Homonuclear Cu II complexes form a closed system in which [Ar]d 9 electrons of two neighbouring Cu II cations are paired with each other. Such a system is usually characterized by a singlet ground state. However, triplet and quintet electronic states are also possible, depending on the nature of the ligand molecules. The bond lengths and angles of the complex were therefore fully optimized in the singlet, triplet and quintet electronic spin states with the result that the singlet electronic state was found to be the energetically optimal structure. Calculated and experimentally determined bond lengths and angles are compared in Tables S1 and S2 in the supporting information, and mean absolute errors (MAE), largest errors (LE) and the correlation coefficients R 2 were determined. The very low values of MAE and LE, and also the high R 2 coefficient of 0.997 reveal the suitability of the applied method for calculation of the electronic structure parameters of the complex.
Calculations of electron densities on atoms in the highest occupied (HOMO) and lowest unoccupied (LUMO) molecular orbitals (MO), as well as the energies of the frontier MOs (FMO) were carried out. The charge distributions on atoms and on the FMOs, as well as the energy of FMOs are one of the main parameters of the electronic structure of chemical compounds (Karelson et al., 1996;Rauk, 2001;Miar et al., 2021). The energy of the HOMO is related to the electron-donating ability of a molecule and the energy of the LUMO is related to the electron-accepting ability of a molecule. The parameter for chemical hardness () is calculated based on the HOMO-LUMO energy gap ( = ÁE/2). The shape of the FMOs and the HOMO-LUMO energy gap of the complex are displayed in Fig. 5. The contribution of both Cu II d orbitals in the HOMO and LUMO are 58.14% and 52.72%, respectively. The contribution of the p orbitals of the eight oxygen atoms of the 3-nitrobenzoate ligands in the HOMO and LUMO are 37.15% and 38.06%, respectively. A higher contribution of Cu II d electrons (56.74%) was observed in the case of second occupied MO (E HOMO-1 = À7.0 eV), and the next unoccupied MO (E LUMO+1 = À2.74 eV) consists of the antibonding p orbitals of the 3-nitrobenzoate fragment.
The complex has a very low HOMO-LUMO energy gap, which can be seen from Fig. 5 and also from the total density of state diagram (TDOS, Fig. S1) of the complex. The low energy gap is caused by a significant decreasing of the energy level of the LUMO of the complex. In other words, the electron accepting ability of the complex is very high and thus the molecule becomes more susceptible to nucleophile attack which makes this complex interesting in chemistry and physics due to its electrical properties and light absorption at a low energy level.
Atomic charge analysis (Fig. S2) shows that the largest negative charges and the largest positive charges are located on oxygen atoms and carbon atoms of the carboxylic group, respectively.

FT-IR analysis
The FT-IR (ATR) spectrum of dimethylformamide (see   Analysis of the IR spectra of the starting compounds and the product shows significant changes in the wavenumbers of absorption bands of characteristic groups in the IR spectrum of the product. Upon complexation, some absorption bands disappear, and some of them undergo a high-frequency or lowfrequency shift. For example, the C O band of 3-nitrobenzoic (1689 cm À1 ) and dimethylformamide (1662 cm À1 ) shifts to the lowfrequency region and is observed at 1600 cm À1 . Likewise, the absorption band of the nitro group (1525, 1350 cm À1 ) of 3-nitrobenzoic acid is shifted to the low-frequency region (1514, 1348 cm À1 ) after complexation. At the same time, the absorption band of the -CH 3 groups of dimethylformamide at 1384 cm À1 undergoes a high-frequency shift and is observed at 1396 cm À1 . On the other hand, the absorption band of the C-O group at 1288 cm À1 of 3-nitrobenzoic acid does not occur in the IR spectrum of the complex.  (Kabbani et al., 2004), KELXEF (Stachová et al., 2006), NIDSEY (Hö kelek et al., 1998) and PABNEP (Xu et al., 2020)], and with a p-nitro group also gave four hits [AQNBCU (Usubaliev et al., 1980), BOVPIN (Jassal et al., 2015), QIXQIX01 (Li & Zhou, 2010) and VIHNAD (Song et al., 2013)]. In the dinitrobenzoate complex NIDSEY, [Cu 3 {(NO 2 ) 2 C 6 H 3 COO} 6 (CH 3 -OH) 2 ], comprising three copper(II) atoms, two of them are fivecoordinate, being surrounded by square-pyramids of carboxylate O atoms and forming a paddle-wheel-shaped binuclear structure, whereas the third copper(II) ion has a square-planar environment.

Synthesis and crystallization
The crystals were grown from low-cost standard materials. 3-Nitrobenzoic acid (20.0 mg, 0.12 mmol) and CuSO 4 Á5H 2 O (20 mg, 0.056 mmol) were mixed and stirred at room temperature for 1 h. Then, in a gradual way, dimethylformamide (DMF; 0.78 mmol) was added dropwise to the stirred mixture throughout 60 min at 303 K, immediately after which the solution was cooled down and kept for several hours. Darkish blue single crystals suitable for X-ray analysis were grown by slow evaporation at ambient temperature for one week and collected by filtration. They were finally washed with pure DMF. Yield: 70%.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 3. Hydrogen atoms bonded to carbon atoms were placed in calculated positions and refined to ride on their parent atoms with C-H = 0.93 Å and U iso (H) = 1.2U eq (C) for aromatic hydrogen atoms, and with C-H = 0.96 Å and U iso (H) = 1.5U eq (C) for methyl hydrogen atoms.

Tetrakis(µ-3-nitrobenzoato-κ 2 O 1 :O 1′ )bis[(N,N-dimethylformamide-κO)copper(II)] dimethylformamide disolvate
Crystal data where P = (F o 2 + 2F c 2 )/3 (Δ/σ) max = 0.001 Δρ max = 0.38 e Å −3 Δρ min = −0.65 e Å −3 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.

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