The main focus of magnetochemical research is situated in the study of molecular functional materials exhibiting bistability in which physical properties can be triggered by a change of temperature, pressure, light or magnetic field, such as spin crossover compounds (SCO) or single-molecule magnets (SMM), where the latter are nanomagnets characterized by the slow relaxation of the magnetization. The widespread basis for the study of these phenomena originates in its potential technological applications in many areas of everyday life. In this case, SMM compounds are studied as materials for storage devices, quantum computing, sensors, and spintronics [
1,
2]. Additionally, SCO compounds may find potential in many applications such as pressure or optical switches [
3], gas sensors [
4], pressure, or temperature sensors [
5]. Therefore, the magnetic properties of 3d elements are of much interest to the scientific community. Among all of the 3d metals, Co(II) possesses great predispositions for the formation of compounds exhibiting SMM behavior due to relatively large spin-orbit coupling and the possibility to vary the coordination numbers from 2 to 8 [
6]. Likewise, Fe(II) ranks among elements whose compounds are commonly known for SCO behavior [
7]. The tuning of physico-chemical properties of both SMM and SCO phenomena can be achieved by means of several factors. In SMMs, complexes honing of magnetization reversal barrier quantified by
Ueff parameter and magnetic blocking temperature (
TB) is achieved by the strengthening of magnetic anisotropy of the easy-axis type. The modification of
Ueff is accomplished by increasing magnitude of the zero-field splitting parameter
D, due to a simple relationship among them,
U = |
D|(S
2 − 1/4) for half-integer spin and
U = |
D|(S
2) for integer spin, and it holds that values of ZFS (zero-field splitting) parameters are dictated by the coordination number, the ligand field, and the symmetry of the coordination polyhedron. In general, higher |
D|-values should yield higher
Ueff and correspondingly higher
TB [
8,
9,
10,
11]. However, large zero-field splitting parameter
E causes an increase of the tunneling rate of the magnetization. In SCO compounds, a proper ligand field and cooperativity between neighboring molecules can allow a transition between low spin (LS) and high spin (HS) state and vice versa, which is triggered by the change of temperature or pressure, eventually by the light. Since many of these parameters in both SMM and SCO relate to the coordination environment, search for suitable organic ligands is crucial for improving properties of these molecular compound classes. Miscellaneous organic ligands have been employed to study or enhance the magnetic properties of Fe(II) and Co(II) coordination compounds. Such ligands frequently include five or six-membered heterocycles containing at least one nitrogen atom. Examples of these heterocycles cover substituted diazoles, triazoles, tetrazoles, pyridines, pyrimidines, pyrazines, triazines, and others [
12,
13,
14]. In our search for suitable ligands, we were inspired by numerous publications with interesting magnetochemical results encompassing substituted triazoles, and more specifically 4-amino-3,5-di-2-pyridyl-4
H-1,2,4-triazole (abpt). Our co-workers participated in publishing Co(II) field-induced single-ion magnets incorporating the abpt ligand. The first publication in 2014 introduced [Co(abpt)
2(tcm)
2] (tcm = tricyanomethanide), which was identified as the field-induced single-ion magnet with large ZFS parameters and positive
D = 48 cm
−1. The energy value for spin reversal barrier
Ueff = 86.2 K was the highest at the time among Co(II) complexes with transversal magnetic anisotropy [
15]. The research continued in series of two publications involving other pseudohalide analogs—[Co(abpt)
2(solv)
2]X
2 (solv = H
2O and X = tcap, solv = H
2O and X = nodcm, solv = CH
3OH and X = pcp) (tcap = 1,1,3,3-tetracyano-2-azapropenide, nodcm = nitrodicyanomethanide, pcp = 1,1,2,3,3-pentacyanopropenide), [Co(abpt)
2(X)
2] (X = nca, NCSe, ndcm, N
3) (nca = nitrocyanamide, ndcm = nitroso-dicyanomethanide). The analysis of DC measurements revealed strong magnetic anisotropy and
D in 31–41.4 cm
−1 range with the exception of [Co(abpt)
2(N
3)
2] where
D = −24.1cm
−1. Subsequent inquiry of AC data showed field-induced slow relaxation of the magnetization and
Ueff in the scope of 71.6–108 cm
−1 [
16,
17]. Thus, we have decided to explore 1,3,4-oxadiazoles, oxygen analogs of abpt, which are relatively unexplored in a magnetochemical area. Moreover, the fact that 4
H-1,2,4-triazole analog 1,3,4-oxadiazole contains more electron-withdrawing oxygen instead of nitrogen should have a notable effect on magnetic properties. Generally, studies involving a 1,3,4-oxadiazole heterocycle focus on crystal engineering, which covers the design and preparation of building blocks, distinguished crystal structures, and understanding of intermolecular interactions in supramolecular structure. Moreover, in these studies, nitrogen of the 1,3,4-oxadiazole ring do not coordinate, and, instead, substituents (e.g., pyridine, pyrazine) in position 2 or 5 provide suitable donor atoms (
Scheme 1). The structures of [Co(3-bpo)(dca)
2]
n (dca = dicyanamide, 3-bpo = 2,5-bis(3-pyridyl)-1,3,4-oxadiazole) and [Co(L1)
2(SCN)
2(H
2O)
2](L1)
2(H
2O)
6 (L1 = 2,5-bis(2-pyrazinyl)-1,3,4-oxadiazole) may serve as a good example [
18,
19]. Studies comprising of Co(II) complexes with 1,3,4-oxadiazole derivatives in which cobalt atom is directly bound to a nitrogen atom of 1,3,4-oxadiazole were focused mainly on biological activities of these compounds, and, sporadically, DC magnetic properties were also reported [
20,
21,
22,
23,
24,
25]. Likewise, a few coordination compounds of Fe(II) with 1,3,4-oxadiazole directly bonded through its nitrogen atom appear in literature, e.g., [Fe(2-bpo)
2(H
2O)
2](ClO
4)
2 (2-bpo = 2,5-bis(2-pyridyl)-1,3,4-oxadiazole) was prepared and found to be in the high spin state in the 2-300 K range [
26]. Magnetochemically focused study exploring Light-Induced Excited Spin-State Trapping (LIESST) and SCO in 2,5-bis(2-pyridyl)-1,3,4-chalcadiazoles describes magnetic properties of [Fe(2-bpo)
2(NCS)
2]. Despite the fact that the complex contains two NCS coordinated anions creating stronger ligand field, the occurrence of SCO was not observed in the 2–300 K temperature scale [
27]. The very first SCO compounds based on 1,3,4-oxadiazole were synthesized and, subsequently, published in 2016 by C. Köhler and E. Rentschler in series of [Fe
II2(μ-L)
2]Y
4·
xCH
3CN (L = 2,5-bis{[(2-pyridylmethyl)amino]-methyl}-1,3,4-oxadiazole; Y = ClO
4−, BF
4− and CF
3SO
3−;
x = 4 for ClO
4− and
x = 2 for BF
4−, CF
3SO
3−). The dinuclear complex with the BF
4– anion remained in the high-spin (HS) state in the 10–300 K range whereas, in the case of ClO
4− and CF
3SO
3− anions, SCO was observed. The critical temperature
T1/2 occurs somewhere around 150 K for both anions and, in the case of [Fe
II2(μ-L)
2](CF
3SO
3)
4·2CH
3CN, steep hysteresis of about 26 K accompanies the transition. XRD confirmed the [HS-HS] to [LS-HS] spin transition, where only one of the Fe(II) atoms undergoes SCO [
28]. These complexes were also studied by theoretical methods (DFT and CASSCF/CASPT2) [
29]. The above-mentioned studies show the great potential of 1,3,4-oxadiazole ligands in the molecular magnetism.
Herein, the 2-(furan-2-yl)-5-pyridin-2-yl-1,3,4-oxadiazole (fpo) was utilized in the preparation of [M(fpo)2(H2O)2](ClO4)2 (M = Fe(II) for (1) Co(II) for (2)) coordination compounds, which were characterized by single-crystal X-ray analysis, FT-IR spectroscopy, and EPR spectroscopy. Both static and dynamic magnetic properties of these complexes were investigated and the analysis was supported by DFT and CASSCF/NEVPT2 theoretical methods showing the importance of the second coordination sphere on the zero-field splitting parameters.