First Cobalt(II) Spin Crossover Compound with N4S2-Donorset

Herein we report the synthesis and characterization of a novel bis-tridentate 1,3,4-thiadiazole ligand (L = 2,5-bis[(2-pyridylmethyl)thio]methyl-1,3,4-thiadiazole). Two new mononuclear complexes of the type [MII(L)2](ClO4)2 (with M = FeII (C1) and CoII (C2)) have been synthesized, containing the new ligand (L). In both complexes the metal centers are coordinated by an N4S2-donorset and each of the two ligands is donating to the metal ion with just one of the tridentate pockets. The iron(II) complex (C1) is in the low spin [LS] state below room temperature and shows an increase in the magnetic moment only above 300 K. In contrast, the cobalt(II) complex (C2) shows a gradual spin crossover (SCO) with T1/2 = 175 K. To our knowledge, this is the first cobalt(II) SCO complex with an N4S2-coordination.


Introduction
Switching metal complexes between two different electronic states, high spin [HS] and low spin [LS], by external stimuli such as temperature, light irradiation or pressure is known as spin crossover (SCO). Due to the molecular bistability and the associated change in the optical and magnetic properties upon switching, these compounds can possibly be used in memory storages, displays and sensors [1][2][3][4][5][6][7][8]. However, abruptness of the property changes and the occurrence of a thermal hysteresis is necessary for future applications. Both depend on the cooperative interactions between the metal centers in the solid state. 'Intermolecularly', the cooperativity can be enhanced by hydrogen bonding or π-π-stacking interactions between the complexes. 'Intramolecularly', in polynuclear complexes, the spin-bearing metal centers can be bridged via organic ligands, leading to close proximity and a stronger communication of these metal centers [5,[9][10][11][12][13]. Although SCO coordination polymers often show large thermal hysteresis [14,15], research on discrete polynuclear, and in particular in dinuclear SCO systems recently increased because the latter have better reproducibility and easier characterization. The dimeric structural motif as the simplest and smallest model for investigating intramolecular cooperative interactions also offers potential access to three states ([HS-HS], [HS-LS] and [LS-LS]) [4,16,17]. However, the design of ligands that simultaneously act as a bridge and induce a suitable ligand field is a difficult task.
Our group recently reported on the synthesis and characterization of symmetrical dinuclear iron(II) compounds with bridging ligands based on the 1,3,4-oxadiazole as well as on the 1,3,4-thiadiazole backbone [18][19][20]. For the thiadiazole-based ligand 2,5-bis[(2-pyridylmethyl)amino]methyl-1,3,4-thiadiazole with pyridyl donor sidearms, the complexes are in the [LS-LS] state at low temperatures and show a gradual but incomplete spin crossover only above room temperature [19]. Inspired by the fruitful work of S. Brooker et al. [21], we herein report the modification of the previous reported ligand [19] by replacing the amino for thioether linkages. The longer C-S bonds compared to the C-N bonds longer C-S bonds compared to the C-N bonds give greater flexibility of the ligand, and thus, should enable the population of the [HS-LS] and/or the [HS-HS] state at elevated temperatures. However, rather than obtaining the dimeric [Fe2(µ-L)2] 4+ complex cation, we exclusively isolated two new mononuclear complexes [M II (L)2](ClO)4 (with M = Fe II (C1) and Co II (C2) and L = 2,5-bis[(2pyridylmethyl)thio]methyl-1,3,4-thiadiazole). While the iron(II) complex (C1) remains in the [LS] state as well, the cobalt(II) complex shows a gradual SCO.

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 χT value of 0.15 cm 3 Kmol −1 at 300 K accounting for a diamagnetic iron(II) ion in the [LS] state. Also, the structural data obtained by Xray 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 χT value slightly increases to 0.36 cm 3 Kmol −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 1 H-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 χT value of 0.47 cm 3 Kmol −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 χT product remains almost constant until 100 K, then raises up to 2.20 cm 3 Kmol −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 χT vs. T curves for the heating or cooling mode cannot be

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 χ M T value of 0.15 cm 3 Kmol −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 χ M T value slightly increases to 0.36 cm 3 Kmol −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 1 H-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 χ M T value of 0.47 cm 3 Kmol −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 χ M T product remains almost constant until 100 K, then raises up to 2.20 cm 3 Kmol −1 at 250 K. This is explained by a gradual spin transition of the complex from [LS] to [HS] with a transition temperature T 1/2 of 175 K. No magnetic hysteresis is observed. In fact, when using a cooling/heating rate of 1. 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 N4S2 coordination.

Crystal Structures
The complex [Fe II (L)2](ClO4)2 (C1) crystallizes in the monoclinic space group P21/c at 173 K. The crystal structure of complex [Co II (L)2](ClO4)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 P21/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 N2S binding pockets to the N4S2 octahedral coordination sphere. The second potentially donating binding pocket is not coordinating.  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 4 S 2 coordination.

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 2 S binding pockets to the N 4 S 2 octahedral coordination sphere. The second potentially donating binding pocket is not coordinating. 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 N4S2 coordination.

Crystal Structures
The complex [Fe II (L)2](ClO4)2 (C1) crystallizes in the monoclinic space group P21/c at 173 K. The crystal structure of complex [Co II (L)2](ClO4)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 P21/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 N2S binding pockets to the N4S2 octahedral coordination sphere. The second potentially donating binding pocket is not coordinating.   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.  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. 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).
Molecules 2020, 25, x FOR PEER REVIEW 5 of 10 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 dorbitals from t2g 5 eg* 2 in the [HS] state to t2g 6 eg* 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 N4S2 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].  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 4 S 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].
Molecules 2020, 25, x FOR PEER REVIEW 5 of 10 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 dorbitals from t2g 5 eg* 2 in the [HS] state to t2g 6 eg* 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 N4S2 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].

General Methods and Materials
All chemicals were purchased from Alfa Aesar, Deutero, Fisher Chemicals, TCI, Sigma-Aldrich and Acros Organics and used without further purification. Absolute solvents were dried according to known procedures and used freshly distilled [40]. The NMR spectra were recorded at room temperature with a Bruker Avance DSX 400 and analyzed with the program MestReNova [41]. Magnetic susceptibility measurements were performed on a Quantum Design SQUID magnetometer MPMSXL in a temperature range between 10-400 K with an applied field of 1 kOe. ESI and FD mass spectra as well as elemental analysis (C,H and N) were measured at the microanalytical laboratories of the Johannes Gutenberg University Mainz. X-ray diffraction data were collected at 173 K with STOE STADIVARI and at 120 K with a STOE IPDS 2T at the Johannes Gutenberg University Mainz. The structures were solved with ShelXT [42] and refined with ShelXL [43] implemented in the program Olex2 [44]. Caution! The prepared perchlorate complexes are potentially explosive. Even though no explosions occurred, only small amounts should be prepared and handled with care.

Complex Synthesis
To a yellow solution of the ligand (L) (0.1 mmol) in methanol (3 mL), an almost colorless solution of the corresponding metal(II) salt [0.1 mmol, Fe(ClO 4 ) 2 × xH 2 O or Co(ClO 4 ) 2 × 6H 2 O] in methanol (3 mL) was added. Slow evaporation at room temperature of the obtained orange solutions resulted in the formation of crystals suitable for X-ray diffraction after several hours. The iron(II) complex was prepared under nitrogen atmosphere and by using dried solvents.

Conclusions
In conclusion, using the novel bis-tridentate 1,3,4-thiadiazole ligand (L = 2,5-bis[(2-pyridylmethyl)thio]methyl-1,3,4-thiadiazole), we were able to synthesize and characterize two new complexes [M II (L) 2 ](ClO 4 ) 2 (with M = Fe II (C1) and Co II (C2)). Due to the fact that mononuclear complexes were obtained, rather than the expected dinuclear ones, we assume this is due to the fact that the sulphur donor atoms of the thioether linkages are large compared to the nitrogen donor atoms of the amino linkages reported by Herold [19]. Thus, the cis-coordination of two sulphur donor atoms is unfavorable. The magnetic data of the mononuclear compound together with the single crystal X-ray structure analysis reveal a [LS] state for the iron(II) complex (C1) until 400 K. The cobalt(II) compound (C2) shows a gradual SCO between 100 K and 250 K from [LS] to [HS] state with a transition temperature T 1/2 of 175 K. To our knowledge, this is the first cobalt(II) complex with a N 4 S 2 coordination environment, showing SCO behavior, that has been reported.  Figure S5: IR spectrum of dried [Fe II (L) 2 ](ClO 4 ) 2 (C1). Figure S6: IR spectrum of dried [Co II (L) 2 ](ClO 4 ) 2 (C2). Figure S7: Molecular structure of [Fe II (L) 2 ](ClO 4 ) 2 (C1) with thermal ellipsoids at 173 K. b) Asymmetric unit without hydrogens, solvent molecules and counter ions. Color code: Fe dark red, N blue, S yellow, C grey, H light grey, Cl green and O red. Figure S8: Molecular structure of [Co II (L) 2 ](ClO 4 ) 2 (C2) with thermal ellipsoids at 120 K. Color code: Co dark blue, N blue, S yellow, C grey, H white, Cl green and O red. Figure S9: Molecular structure of [Co II (L) 2 ](ClO 4 ) 2 (C2) with thermal ellipsoids at 250 K. Color code: Co dark blue, N blue, S yellow, C grey, H white, Cl green and O red. Table S1: Crystallographic parameters for the discussed crystal structures of C1 and C2. CCDC 1980403 for C1 (@173 K), 1980401 for C2 (@120 K), 1980402 for C2 (@250 K) contain the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.