Synthesis and crystal structure of a new copper(II) complex based on 5-ethyl-3-(pyridin-2-yl)-1,2,4-triazole

A new copper(II) coordination compound [Cu2(L Et)2(OAc)2(dmf)2], where HL Et = 3-(2-pyridyl)-5-ethyl-1,2,4-triazole, was synthesized and structurally characterized by single-crystal X-ray diffraction.


Chemical context
The design and construction of coordination complexes based on dinuclear copper(II) compounds have been the subject of intensive study over the past decades (Li et al., 2018;Cui et al., 2019;Doroschuk, 2016). N-containing ligands with polypyridyl (Lee et al., 2017), triazolyl (Kucheriv et al., 2016) and pyridyl moieties (Bartual-Murgui et al., 2020) have been widely used for this purpose. Much interest has been focused on functional materials with the presence of a triazole ring, which demonstrate interesting properties such as catalytic ability (Petrenko et al., 2021), anticancer activity (Muhammad & Guo, 2014) and magnetism (Kuzevanova et al., 2021). Although a variety of triazolate frameworks with intriguing topologies (Govor et al., 2010;Senchyk et al., 2012;Lysenko et al., 2016) have been synthesized to date, making rational control in the construction of coordination compounds is a great challenge in crystal engineering. Derivatives of 3-(2-pyridyl)-1,2,4-triazole are among the most widely used ligands that form stable Cu II coordination compounds. There are about 127 examples in the Cambridge Structural Database that exhibit this type of ligand, 37 of which complexes include the binuclear unit [Cu 2 (trz-py) 2 ] with a Cu-[N-N] 2 -Cu bridge. Among the reported binuclear compounds, there are few reports of 3-(2pyridyl)-1,2,4-triazole compounds obtained with copper(II) acetate (Petrenko et al., 2021;Li et al., 2010). In all cases, the equatorial coordination consists of metallocentres linked by two deprotonated triazole ligands, where additional ligands (acetate anions or solvent) axially coordinate the copper atom.

Structural commentary
The results of the X-ray diffraction study are depicted in Fig. 1. The crystal is built from discrete dinuclear units [Cu 2 (L Et ) 2 (OAc) 2 (dmf) 2 ], where the CuÁ Á ÁCu1 0 separation is of 4.0159 (8) Å . There are no co-crystallized solvent molecules in the crystals. The complex molecule has its own crystallographically imposed symmetry, being assembled around the inversion centers located at the mid-point of the Cu1Á Á ÁCu1 0 distances. Each copper(II) atom exhibits an N 3 O 2 coordination environment in a slightly distorted trigonal-bipyramidal geometry provided by three nitrogen atoms of the organic ligands and two oxygen atoms from the dmf molecule and the monodentate acetate anion.
The inner (Cu1/N2/N3-Cu1 0 /N2 0 /N3 0 ) core has an almost planar conformation in [Cu 2 (L Et ) 2 (OAc) 2 (dmf) 2 ], although for the previously described complex [Cu 2 (L Me ) 2 (OAc) 2 -(H 2 O) 2 ] [HL Me = 5-methyl-3-(2-pyridyl)-1,2,4-triazole], a twist-boat conformation was observed (Petrenko et al., 2021) for the non-planar six-membered Cu 2 N 4 metal ring. The structures were compared (Fig. 2) using OLEX2 software (Dolomanov et al., 2009). It was found that in [Cu 2 (L Me ) 2 (OAc) 2 (H 2 O) 2 ], the water molecules are axially coordinated by the central atom from one side of the Cu 2 N 4 plane, and the acetates from the other. Thus, the non-coordinated oxygen of the acetate anion is involved in an intermolecular hydrogen bond with the coordinated water molecule of an adjacent complex, giving rise to an essentially different crystal motif than was observed for [Cu 2 (L Et ) 2 -(OAc) 2 (dmf) 2 ]. In the newly reported compound [Cu 2 (L Et ) 2 (OAc) 2 (dmf) 2 ], the copper atoms coordinate the dmf molecules and acetate anions in the axial positions in such a manner that they reflect in the symmetry center, which is typical for such a kind of binuclear species. Notably, both [Cu 2 (L Me ) 2 (OAc) 2 (H 2 O) 2 ] and [Cu 2 (L Et ) 2 (OAc) 2 (dmf) 2 ] were synthesized using the same conditions. These features can be probably induced by different substituents in the 5-position of the 3-(2-pyridyl)-1,2,4-triazole ring in these two compounds, indicating that even negligible changes of the non-coordinating part of the ligand could significantly influence the structure of the complex. The non-typical molecular structure of [Cu 2 (L Me ) 2 (OAc) 2 (H 2 O) 2 ] is supported by the formation of intermolecular hydrogen bonds. In the case of [Cu 2 (L Et ) 2 -(OAc) 2 (dmf) 2 ], branching of the non-coordinated part leads to the formation of a less-hindered structure of higher symmetry, similar to those of the previously described 37 compounds, indicating a small difference in the energies of these two topologies, which is probably the result of the formation of additional intermolecular contacts.

Supramolecular features
Further analysis of the structure showed that the crystal structure motif is characterized as a parallel packing of discrete supramolecular chains running along the b-axis direction (Fig. 3). Within a chain, the complex molecules interact through weak C-HÁ Á ÁO hydrogen bonds, where the pyridine H atom acts as acceptor, and the acetate O atom as donor (Table 1, Fig. 4).

Figure 4
Partial view of the crystal packing showing hydrogen-bond contacts between adjacent molecules.

Figure 3
One-dimensional coordination network viewed along the b-axis.

Funding information
This work was supported by grants 22BF037-06 obtained from the Ministry of Education and Science of Ukraine. 1.5 (Dolomanov et al., 2009); software used to prepare material for publication: Olex2 1.5 (Dolomanov et al., 2009).