2.1. Synthesis
The synthesis of the ligand is presented in
Scheme 1. We have previously reported the preparation of 2,5-bis(chloromethyl)-1,3,4-thiadiazole (
1) [
19]. Thioacetic acid S-pyridine-2-ylmethyl ester (
2) was synthesized as described in literature [
21]. Finally, the ligand (
L = 2,5-bis[(2-pyridylmethyl)thio]methyl-1,3,4-thiadiazole) was obtained according to [
21] by treating
2 with sodium ethanolate and thereafter reacting with
1 in a nucleophilic substitution.
The iron(II) and cobalt(II) complexes, [MII(L)2](ClO4)2, have been synthesized in a stochiometric reaction of the ligand (L) with the corresponding perchlorate salt (Fe(ClO4)4 × xH2O and Co(ClO4)2 × 6H2O) in methanol. The compounds were obtained as single crystals (C1 and C2) suitable for X-ray diffraction experiments via slow evaporation of the complex solutions. The iron(II) complex reaction was performed under nitrogen atmosphere and by using absolute methanol. The dried complexes are stable to air, no oxidation was observed.
2.2. Variable Temperature Magnetic Susceptibility Measurements
Variable temperature magnetic susceptibility measurements were carried on dried samples in the temperature range of 300–400 K for
C1 and of 10–400 K for
C2 in an applied magnetic field of 1000 Oe (0.1 T) and with a scan rate of 1.5 K/min. The temperature-dependent magnetic susceptibility date of the samples
C1 and
C2 are shown in
Figure 1.
C1 shows a
χΜT value of 0.15 cm
3Kmol
−1 at 300 K accounting for a diamagnetic iron(II) ion in the [LS] state. Also, the structural data obtained by X-ray crystallography at 173 K (described below) confirms an LS state of the iron(II) indicating that no spin crossover occurs until 300 K. Raising the temperature to 400 K, the
χΜT value slightly increases to 0.36 cm
3Kmol
−1. Although this rise is no evidence of a spin crossover, it is at least a strong indication. The diamagnetic nature of
C1 at room temperature is further confirmed by the
1H-NMR spectra of the complex, shown along with that of the ligand in
Figure S3 in the supporting information.
At low temperature, compound C2 shows a χΜT value of 0.47 cm3Kmol−1, which accounts for a cobalt(II) ion in the [LS] state in accordance with the single X-ray structure analysis at 120 K. With increasing temperature, the χΜT product remains almost constant until 100 K, then raises up to 2.20 cm3Kmol−1 at 250 K. This is explained by a gradual spin transition of the complex from [LS] to [HS] with a transition temperature T1/2 of 175 K. No magnetic hysteresis is observed. In fact, when using a cooling/heating rate of 1.5 K/min, the χΜT vs. T curves for the heating or cooling mode cannot be distinguished. The χΜT values for [LS] and [HS] cobalt(II) are slightly higher than the calculated ones ([LS] = 0.38 cm3Kmol−1 and [HS] = 1.88 cm3Kmol−1) using the spin-only formalism. This is expected because the spin-only formalism does not take into account orbit angular momentum contribution.
It is known from literature that cobalt(II) complexes with N-donor ligands, which form with iron(II) only [LS] complexes, might show SCO and is well studied for terpyridine complexes [
2,
4,
22]. However, to the best of our knowledge, the cobalt(II) complex reported here is the only one showing this phenomena with Co(II) in a N
4S
2 coordination.
2.3. Crystal Structures
The complex [Fe
II(
L)
2](ClO
4)
2 (
C1) crystallizes in the monoclinic space group P2
1/c at 173 K. The crystal structure of complex [Co
II(
L)
2](ClO
4)
2 (
C2) was measured at two different temperatures (120 K and 250 K) to confirm the spin crossover phenomenon. For both temperatures, the monoclinic space group is P2
1/c. In all three structures,
C1 (@173 K) and
C2 (@120 K) and
C2 (@250 K) the complex cation consists of one metal ion and two ligand molecules, showing pseudo centrosymmetry as sketched in
Figure 2. Each ligand contributes with one of the tridentate N
2S binding pockets to the N
4S
2 octahedral coordination sphere. The second potentially donating binding pocket is not coordinating.
For all complex cations (
C1 and
C2 at both temperatures), the
cis-angles within the donor atoms of one ligand (N
TDA(
L,
L’)-M-N
Py(
L,
L’), N
TDA(
L,
L’)-M-S(
L,
L’) and N
Py(
L,
L’)-M-S(
L,
L’),
Figure 3) are ranging from 83° to 85°, while the
cis-angles between the donor atoms of the different ligands (N
TDA(
L)-M-N
Py(
L’), N
TDA(
L)-M-S(
L’) and N
Py(
L)-M-S(
L’) and vice versa) are ranging from 95° to 97°. This results in
trans-angles (N
TDA(
L)-M-N
TDA(
L’), N
Py(
L)-M-N
Py(
L’) and S(
L)-M-S(
L’)) of almost 180° and in a slightly distorted octahedral coordination for the metal centers, in which the axis N
Py(
L)-M-N
Py(
L’) is inclined around 5–6° from the ideal geometry (black lines) towards the ligands due to the strain within the ligand backbone.
The crystal structures further compromise two perchlorate anions to counterbalance the charge. All the complexes crystallize without any solvent molecules, which allows to investigate dried crystalline samples.
The average Fe-N bond length of 1.990 Å and Fe-S bond length of 2.264 Å in
C1 (@173 K) are in accordance with those reported in literature and account for an iron(II) ion in the [LS] state [
23,
24,
25,
26,
27,
28,
29].
Figure S7 shows the crystal structure/asymmetric unit of
C1 (@173 K). Detailed information on bond lengths and angles for
C1 and
C2 are summarized in
Table 1.
For
C2 (@250 K), the average Co-N bond length of 2.066 Å, as well as the average Co-S bond length of 2.479 Å indicate a [HS] cobalt(II) ion [
30,
31,
32,
33,
34,
35,
36]. Cooling to 120 K results in a spin crossover from the [HS] to the [LS] state for the cobalt(II) center as shown by magnetic data. The average Co-N distance decreases to from 2.066 Å to 2.002 Å, which is in accordance with literature [
37,
38,
39]. The shortening of the Co-N bond is explained by the decrease of electron density in the antibonding d-orbitals from t
2g5e
g*
2 in the [HS] state to t
2g6e
g*
1 in the [LS] state. Notably, the average Co-S distance remains about the same (2.479 Å @250 K and 2.472 Å @120 K) upon changing the electronic state of the cobalt(II) ion. This is explained by the Jahn-Teller distortion expected for a d
7-Co(II) ion in [LS] state, with four short Co-N bonds in equatorial plane and two long ‘axial’ Co-S bonds. Uponcooling down the transition from [HS] to [LS] also affects the counter ion. Ordering of one of the perchlorate anions in the crystal structure results in a phase change. While at 250 K only half of the complex is in the asymmetric unit, the entire complex cation is found at 120 K. This is accompanied by a doubling of the cell volume from 1990 Å
3 (@250 K) to 3874 Å
3 (@120 K) (see
Figure 4 and
Figures S8 and S9 in ESI).
When comparing our findings with the dinuclear structures obtained by
S. Brooker et al. [
21], the question arises, why the use of our new ligand (
L) results in mononuclear complexes? In the dimeric complexes of
S. Brooker et al. two iron(II) ions are coordinated by two ligand molecules, thus each iron(II) center has a N
4S
2 coordination sphere, and the two sulphur donor atoms are coordinating
cis to each other as depicted in
Figure 5a. Changing the 1,3,4-triazole to the 1,3,4-thiadiazole as the backbone in the thioether-linked ligand leads to a different strain and to closer proximity of the sulphur donor atoms in the
cis coordination, which is highly unfavorable. Hence, we exclusively obtained mononuclear complexes in which the sulphur donor atoms are coordinating
trans to each other (
Figure 5b). Similar findings were previously reported for iron(II) complexes with 1,3,4-triazole or 1,3,4-thiadiazole bridging ligands with an amino- rather than a thioether-linker group. Here, changing from the 1,3,4-triazole to the 1,3,4-thiadiazole backbone results in a larger angle between the intraligand donor atoms and the amine donor atoms of the two facing ligands, which are in closer proximity compared to the ones in the 1,3,4-triazole [
19].