Synthesis and crystal structure of hydrated μ-oxalato-bis{bis[3-methyl-5-(pyridin-2-yl)-1H-1,2,4-triazole]iron(II)} bis(toluenesulfonate) 2.75-hydrate

A bis-bidentate oxalate bridging anion connects two FeII ions further surrounded by bidentate pyridyl-triazole ligands.


Chemical context
The study of coordination compounds based on substituted 1,2,4-triazoles and 3d and 4d transition metals allows the design of supramolecular structures that can find applications in various fields such as molecular magnetism, catalysis, electrochemistry or cluster engineering (Zhang et al., 2017;Zakharchenko et al., 2019;Chen et al., 2015;Petrenko et al., 2020Petrenko et al., , 2021. The presence of the pyridine ring in such triazole systems leads to the formation of interesting isolated metalorganic frameworks that demonstrate promising magnetic properties, making them suitable for application as moleculebased magnets Han et al., 2017;Li et al., 2015;Huang et al., 2015). Moreover, a combination of 3d 4 -3d 7 metals with N-donor bridging ligands may form coordination compounds with switchable spin states (Aromí et al., 2011;Kucheriv et al., 2021). This phenomenon is called spin crossover. Changes in the external temperature, pressure, magnetic field, light radiation or the presence of a guest alters the magnetic, electrical, mechanical and optical properties significantly in these compounds (Gü tlich & Goodwin, 2004). Therefore, the synthesis and crystallographic characterization of these complexes are of current interest.
On the other hand, the ability of the oxalate anion to generate homobinuclear complexes is well known (Craig et al., 2010;Selmi et al., 2021;Karimpour et al., 2013;Paine et al., 2007). The coordination chemistry of oxalato-bridged binuclear Fe II complexes with pyridyl-triazole chelating ligands is less studied. A few examples with a similar type of ligand indicate that complexes of this kind possess interesting magnetic and oxidizing properties (de Ruiter et al., 2008;Oliveira et al., 2018). In order to continue research in this field and in the course of our studies dedicated to the investigation of triazoles and, in particular, 3-methyl-5-(pyrid-2-yl)-2H-1,2,4-triazole (metrzpy) (Zakharchenko et al., 2017;Zakharchenko, Khomenko, Doroschuk, Raspertova, Fesych et al., 2021;Zakharchenko, Khomenko, Doroshchuk, Raspertova, Shova et al., 2021), we report herein the synthesis and crystal structure of a new binuclear iron(II) complex with this ligand.

Structural commentary
The structure of the title compound is built up from dinuclear [Fe 2 (metrzpy) 4 (C 2 O 4 )] 2+ complex cations, p-toluenesulfonate anions and co-crystallized water molecules in a 1:2:2.75 ratio. It crystallizes in the triclinic space group P1 with two complex molecules per unit cell. Each iron(II) ion has an N 4 O 2 coordination environment in a distorted octahedral geometry provided by two chelating metrzpy ligands in cis positions and a bidentate bridging oxalate anion (Fig. 1, Table 1). The reduced values of the angles subtended at the iron atom by the metrzpy and oxalate ligands are the main factors behind this distortion. The Fe-N and Fe-O bond lengths vary in the ranges 2.150 (3)-2.209 (3) Å and 2.123 (2)-2.171 (2) Å , respectively. The Fe1Á Á ÁFe2 separation across the oxalate bridge of 5.576 (6) Å is in good agreement with previously reported values for other oxalate-bridged iron(II) complexes. The sets of coordinating atoms (O1/O2/N2/N6 for Fe1 and O3/ O4/N10/N14 for Fe2) defining the mean equatorial planes are co-planar within 0.22 and 0.20 Å , while the displacement of the metal atom from these planes is 0.015 (1) and 0.037 (1) Å , respectively. The dihedral angle formed by each plane and the mean plane of the oxalate atoms is of 9.74 (6) for Fe1 and 10.04 (7) for Fe2.

Supramolecular features
All the species present in the structure are interconnected via a system of O-HÁ Á ÁO and N-HÁ Á ÁO hydrogen bonds (Table 2), which determines the formation of a two-dimensional architecture, as shown in Fig. 2. Further analysis has shown that the main crystal-structure motif consists of the parallel packing of 2D layers consolidated by thestacking interactions observed between triazole and pyridine rings of

Figure 2
Two-dimensional supramolecular network viewed along the a axis.

Synthesis and crystallization
The triazole ligand was prepared according to a synthesis described in the literature (Zakharchenko et al., 2017). Single crystals of [Fe 2 (C 2 O 4 )(metrzpy) 4 ](CH 3 C 6 H 4 SO 3 ) 2 Á2.75H 2 O were obtained by the liquid-to-liquid diffusion technique using a layering tube. The bottom was filled with Fe(CH 3 C 6 H 4-SO 3 ) 2 Á6H 2 O (50.6 mg, 0.1 mmol) in 2 ml of water. The middle was filled with a solution of 2 ml methanol/water (1:1) containing ascorbic acid (35.2 mg, 0.2 mmol). Then the top was filled with a solution of metrzpy ligand (32.0 mg, 0.2 mmol) in 2 ml of methanol. Afterwards, the tube was sealed with parafilm and light brown square-plate single crystals were formed within 3 days in relative high yield (ca 50%).

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