Synthesis, crystal structure and Hirshfeld surface analysis of a new copper(II) complex based on diethyl 2,2′-(4H-1,2,4-triazole-3,5-diyl)diacetate

The synthesis and crystal structure of a new dinuclear copper(II) complex based on ethyl 2,2′-(1H-1,2,4-triazole-3,5-diyl)-diacetate is reported. Additionally, the results of a Hirshfeld surface analysis of [Cu2(C6H5N3O4)2(H2O)4]·2H2O are described.


Chemical context
1,2,4-Triazole-based organic compounds have been widely used as ligands for the synthesis of transition-metal complexes (Haasnoot, 2000;Aromı ´et al., 2011;Farooq, 2021).Depending on the substituents on the azole core, the title ligands can coordinate not only in a monodentate manner (Cudziło et al., 2011;Zaleski et al., 2005), but also as a linker binding two metal ions (Drabent et al., 2001;Zhang et al., 2005) and thus play an important role in the design of new polynuclear coordination compounds.In particular, copper(II) coordination compounds based on 1,2,4-triazoles have attracted the interest of chemists due to their magnetic properties (Petrenko et al., 2020;Kaase et al., 2014), bioactivity (Herna ´ndez-Gil et al., 2013;Ferrer et al., 2004) and catalysis (Thorseth et al., 2013;Li et al., 2015).Dinuclear copper(II) complexes can promote single-and double-strand DNA cleavage in both aerobic and anaerobic conditions (Li et al., 2010).Being much cheaper than most metals, copper(II) coordination compounds are promising substances for exploration as catalysts.Previously we reported that a dinuclear Cu II complex based on 5-methyl-3-(2-pyridyl)-1,2,4-triazole as a ligand can selectively catalyse the oxidation of styrene towards benzaldehyde and of cyclohexane to KA oil (a mixture of cyclohexanol and cyclohexanone; Petrenko et al., 2021).Finally, Cu II complexes can exhibit urease inhibitory activities (Xu et al., 2015).Since dinuclear copper(II) complexes with triazole bridges can exhibit catalytic proper-ties, we decided to continue our research in this direction.Herein, we describe the synthesis, crystal structure, and results of Hirshfeld surface analysis of the title compound, [Cu 2 (C 6 H 5 N 3 O 4 ) 2 (H 2 O) 4 ]•2H 2 O, which potentially exhibits catalytic, inhibitory, and magnetic properties.

Supramolecular features
The crystal structure is built up from the parallel packing of discrete supramolecular chains running along the a-axis direction with a Cu� � �Cu separation of 6.5248 (11) A ˚(Fig. 2).Within the chain, the complex molecules interact through O-H� � �O hydrogen bonds, while the association with the interstitial water molecules occurs via O-H� � �O and N-H� � �O hydrogen bonds (Fig. 3, Table 1).Cu 2 (C 6 H 5 N 3 O 4 ) 2 (H 2 O) 4 ]•2H 2 O 977   Table 1 Hydrogen-bond geometry (A ˚, � ).

Figure 2
One-dimensional supramolecular chain running parallel to the a axis and viewed along the b axis.Solvent water molecules are shown in green, O-H� � �O and O-H� � �N hydrogen bonds are shown as red and blue dotted lines, respectively.

Figure 1
The molecular structure of the title compound with the atom labelling.
Displacement ellipsoids are drawn at the 50% probability level.
The triazole derivatives have two substituents at positions 3 and 5 of the triazole ring.The substituents containing donor atoms also participate in coordination with the copper atom.These ligands exhibit bridging functions and link two copper atoms at distances in the range of 3.85 to 4.09 A ˚. Two sixcoordinated copper atoms are involved in the formation of a six-membered ring.There are two water molecules in the axial positions of the central copper atom in the title compound and the compound JOZXAX.In other complexes, one axial position in the geometric environment of the copper atom is occupied by a water molecule, while the second axial position is typically occupied by an anion of an inorganic salt.The title compound crystallizes in the monoclinic P2 1 /n space group.Five complexes crystallized in the triclinic, P1 space group, while JOZXAX crystallized in the monoclinic C2/c space group.

Hirshfeld surface analysis
The Hirshfeld surface analysis was performed and the associated two-dimensional fingerprint plots were generated using

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2.The crystal studied was twinned by a twofold rotation around [100].The corresponding HKLF5 generated by the CrysAlis program was used for refinement.

Figure 6
The overall two-dimensional fingerprint plot and those delineated into specified interactions.

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.

Figure 5
Figure 5Hirshfeld surface representations with the function d norm plotted onto the surface for the different interactions.

Figure 4 (
Figure 4 (a) Two projections of the Hirshfeld surfaces mapped over d norm showing the intermolecular interactions within the molecule and (b) an illustration of selected O-H� � �O and O-H� � �N interactions depicted by green and yellow dashed lines, respectively.

Figure 3
Figure 3 Partial view of the crystal packing showing hydrogen-bond contacts between adjacent molecules.

Table 2
Experimental details.