Synthesis, crystal structure and thermal properties of bis(1,3-dicyclohexylthiourea-κS)bis(isothiocyanato-κN)cobalt(II)

The crystal structure of the title compound consists of discrete tetrahedral complexes, which are linked by intermolecular N—H⋯S and C—H⋯S hydrogen bonding into chains.


Chemical context
Coordination polymers based on Co(NCS) 2 have been investigated for several years because they can show interesting magnetic properties due to the large magnetic anisotropy of Co II . This is the reason why we and others are especially interested in this class of compounds. In most cases, the Co II cations are octahedrally coordinated and linked by pairs of thiocyanate anions into chains, even if a few compounds with single thiocyanate bridges have been reported (Palion-Gazda et al., 2015). If the Co cations are alltrans or cis-cis-trans coordinated with the thiocyanate anions in the trans-position, the chains are linear and frequently show antiferromagnetic or ferromagnetic behavior or a slow relaxation of the magnetization indicative of single-chain magnetism (Wang et al., 2005;Shurda et al., 2013;Wö hlert et al., 2014;Jin et al., 2007;Prananto et al., 2017;Mautner et al., 2018;Rams et al., 2020;Jochim et al., 2020a). In the case where the Co centers are cis-cis-trans coordinated with the thiocyanate anions in the cis-position, the chains are corrugated and the magnetic exchange is suppressed (Shi et al., 2007;Bö hme et al., 2020). In some cases Co(NCS) 2 layers are observed, in which the Co cations are linked by single and double thiocyanate bridges or by single anionic ligands exclusively (Suckert et al., 2016;Werner et al., 2015a). These compounds usually show ferromagnetic behavior with low critical temperatures, which can be tuned by mixed crystal formation with Ni II cations (Wellm et al., 2018(Wellm et al., , 2020Neumann et al., 2018a).
In the case where monocoordinating co-ligands are used and the chains are linear, these compounds have the general composition Co(NCS) 2 (L) 2 (L = co-ligand) but for this composition a second structure exists, in which the Co cations are tetrahedrally coordinated and in this case, no cooperative magnetic exchange interactions can be observed. The reason why, dependent on the nature of the co-ligand, chains or complexes are formed is not clear. First of all, one can assume that the cobalt cations would prefer a tetrahedral coordination with bulky co-ligands because of steric crowding. On the other hand, we observed that strong N-donor co-ligands such as, for example, 4-(dimethylamino)pyridine would lead to the formation of tetrahedral complexes (Neumann et al., 2018b), whereas weaker donors such as 4-(4-chlorobenzyl)pyridine (Werner et al., 2015b) or 4-(3-phenylpropyl)pyridine (Werner et al., 2014;Ceglarska et al., 2021) lead to the formation of chains. In the case of intermediate donor ligands like 4-methoxypyridine, both isomers can be obtained, chains and discrete complexes (Mautner et al., 2018;Rams et al., 2020).
In the course of our systematic work, we became interested in S-donor co-ligands and with thiourea we obtained a compound with the desired chain structure showing antiferromagnetic ordering but no slow relaxation of the magnetization (Jochim et al., 2020a). In further work, we obtained two compounds with 1,3-dimethylthiourea (and 1,1,3,3-tetramethylthiourea) but in this case, tetrahedral discrete complexes were obtained (Jochim et al., 2020b,c). To investigate the influence of the co-ligand in more detail we used 1,3dicyclohexylthiourea as the co-ligand and we obtained crystals of the title compound, which were characterized by single crystal X-ray diffraction, which proves the formation of a discrete complex even with this ligand. Investigations using X-ray powder diffraction show that the title compound was obtained as a pure phase (Fig. 1). The CN stretching vibration is observed at 2038 cm À1 , which is typical for thiocyanates that are only terminal bonded to metal cations in a tetrahedral coordination (Fig. S1). Measurements using simultaneously differential thermoanalysis (DTA) and thermogravimetry reveal the decomposition of the title compound starting at about 227 C, which is accompanied with an endothermic event in the DTA curve (Fig. S2). The experimental mass loss of 37.7% is in a reasonable agreement with that calculated for the removal of one 1,3-dicyclohexylthiourea ligand of 36.6%. The mass loss in the second step is higher than expected for the removal of the second 1,3-dicyclohexylthiourea ligand, but in this temperature region the thiocyanate anions also decompose. Additional measurements using differential scanning calorimetry show a small endothermic event before the compound decomposes (Fig. S3). To check if this event corresponds to some transition, the residue formed after the endothermic signal (see point 'x 0 in Fig. S3) was isolated and investigated by XRPD measurements, which shows that the powder pattern is identical to that of the pristine material but of lower crystallinity (Fig. S4).

Structural commentary
The asymmetric unit of the title compound consists of one Co II cation that is located on a twofold rotation axis, one thio-

Figure 2
Crystal structure of the title compound with labeling and displacement cyanate anion and one 1,3-dicyclohexylthiourea ligand that occupies general positions. The Co II cations are fourfold coordinated by two terminal N-bonded thiocyanate anions and two sulfur atoms of 1,3-dicyclohexylthiourea ligands each (Fig. 2). The Co-N and Co-S distances are comparable to that observed in other Co(NCS) 2 compounds with thiourea derivatives (Table 1, Jochim et al., 2020a,b). The bond angles deviate from the ideal values, revealing that the tetrahedra are slightly distorted (see supporting information). Both hexane rings of the 1,3-dimethylthiourea ligand are in a chair conformation (Figs. 2 and 3). There are two symmetryequivalent intramolecular N-HÁ Á ÁN hydrogen bonds between the amino H atom of the 1,3-dicyclohexylthiourea ligand and the N atoms of the thiocyanate anions (Table 2 and Fig. 3). The N-HÁ Á ÁN angle is close to linearity, indicating that this is a relatively strong interaction (Table 2).

Supramolecular features
In the crystal structure of the title compound the discrete complexes are linked into chains by two intermolecular N-HÁ Á ÁS hydrogen bonds related by the twofold rotation axis between the N-H H atoms and the thiocyanate S atom of a neighboring complex (Fig. 4, Table 2). The discrete complexes are additionally linked by two symmetry-equivalent C-HÁ Á ÁS hydrogen bonds, which might correspond to a weak interaction (Fig. 4, Table 2). These chains elongate along the b-axis direction and each chain is surrounded by six neighboring chains in a pseudo-hexagonal manner (Fig. 5).

Figure 3
View of the discrete complex with intramolecular N-HÁ Á ÁN hydrogen bonding shown as dashed lines.
There are also several crystal structures with Co(NCS) 2 reported, in which the Co II cations are tetrahedrally coordinated by two terminal N-bonded thiocyanate anions and two N-donor co-ligands, for example two polymorphic modifications of bis(4-dimethylaminopyridine)bis (

Synthesis
Co(NCS) 2 was purchased from Merck. 1,3-Dicyclohexylthiourea was purchased from Alfa Aesar. All chemicals were used without further purification. Blue-colored single crystals suitable for single-crystal X-ray analysis were obtained after storage of 0.25 mmol Co(NCS) 2 (43.8 mg) and 0.50 mmol 1,3dicyclohexylthiourea (120.2 mg) in 2.0 ml ethanol at 333 K over night.

Experimental details
The data collection for single crystal structure analysis was performed using an XtaLAB Synergy, Dualflex, HyPix diffractometer from Rigaku with Cu-K radiation.
The IR spectrum was measured using an ATI Mattson Genesis Series FTIR Spectrometer, control software: WINFIRST, from ATI Mattson.
The PXRD measurement was performed with Cu K 1 radiation ( = 1.540598 Å ) using a Stoe Transmission Powder Diffraction System (STADI P) equipped with a MYTHEN 1K detector and a Johansson-type Ge(111) monochromator.
Thermogravimetry and differential thermoanalysis (TG-DTA) measurements were performed in a dynamic nitrogen atmosphere in Al 2 O 3 crucibles using a STA-PT 1000 ther-mobalance from Linseis. The instrument was calibrated using standard reference materials.
The DSC experiments were performed using a DSC 1 star system with STARe Excellence software from Mettler-Toledo AG under dynamic nitrogen flow in Al pans.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 3. All non-hydrogen atoms were refined anisotropically. The C-bound H atoms were positioned with idealized geometry and were refined isotropically with U iso (H) = 1.2 U eq (C) using a riding model.

Bis(1,3-dicyclohexylthiourea-κS)bis(isothiocyanato-κN)cobalt(II)
Crystal data Special details Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes.