Crystal structure of bis(tetramethylthiourea-κS)bis(thiocyanato-κN)cobalt(II)

The title compound consists of discrete complexes with the composition Co(NCS)2(tetramethylthiourea)2, in which the CoII cations are tetrahedrally coordinated by two N-bonded thiocyanate anions and two tetramethylthiourea ligands. These complexes are linked by intermolecular C—H⋯S hydrogen bonds into layers that are parallel to the ab plane.

In the course of systematic investigations on the synthesis of Co(NCS) 2 coordination compounds with different thiourea ligands, the title compound, [Co(NCS) 2 (C 5 H 12 N 2 S) 2 ], was obtained. In this compound the Co II cations are coordinated by two crystallographically independent N-bonded thiocyanate anions and two tetramethylthiourea ligands into discrete complexes that are located in general positions and show a strongly distorted tetrahedral geometry. Intermolecular C-HÁ Á ÁS hydrogen bonds of different strength can be observed between the discrete complexes, which are connected by pairs of hydrogen bonds into zigzag-like chains that elongate in the b-axis direction. These chains are additionally linked by strong C-HÁ Á ÁS hydrogen bonds along the a-axis direction, resulting in the formation of layers that are parallel to the ab plane. There is also one weak intramolecular C-HÁ Á ÁS hydrogen bond between two neighbouring thiourea ligands within the complexes. Comparison of the experimental PXRD pattern with that calculated from the single-crystal data prove that a pure phase has been obtained. Thermoanalytical investigations reveal that this compound melts at 364 K and decomposes upon further heating.

Chemical context
The thiocyanate anion is a very versatile ligand, which can coordinate in many different ways to metal cations, leading to compounds with a variety of coordination networks (Buckingham, 1994;Haasnoot et al., 1984;Barnett et al., 2002;Bhowmik et al., 2010;Abedi et al., 2016). This ligand is also able to mediate reasonable magnetic exchange (Palion-Gazda et al., 2015), which is one reason why we have been interested in transition-metal thiocyanate coordination compounds for many years. In this context, we are especially interested in compounds of the general composition [M(NCS) 2 (L) 2 ] n , in which paramagnetic first-row transition-metal cations M such as Mn II , Fe II , Co II or Ni II are octahedrally coordinated by two N-and two S-bonding thiocyanate anions and two coligands L that usually consist of pyridine derivatives. Depending on the nature of the coligand, the metal cations are connected into chains by pairs of -1,3 coordinating anionic ligands (Mautner et al., 2018;Prananto et al., 2017;Shurdha et al., 2013;Jin et al., 2007;Bö hme et al., 2020), or they are linked into layers with different layer topologies (Werner et al., 2015a;Neumann et al., 2018a;Suckert et al., 2016). The chain compounds show either ferromagnetism (Neumann et al., 2019), antiferromagnetism  or they represent antiferromagnetic phases of single-chain magnets (Mautner et al., 2018;Rams et al., 2017Rams et al., , 2020Werner et al., 2015b), whereas the layer compounds are in most cases ferromagnets (Suckert et al., 2016). For this composition a third structure type is known, in which the metal cations are tetrahedrally coordinated, forming discrete complexes with only N-terminally bonded thiocyanate anions (Neumann et al., 2018b). For some coligands, at least two of the three isomers can be obtained. With 4-acetylpyridine as coligand, for example, the chain as well as the layer isomer can be prepared, with the latter representing the thermodynamically stable form at room temperature (Werner et al., 2015a). If 4-methoxypyridine is used as coligand, the chain isomer as well as the tetrahedral discrete complex can be obtained, and in this case the chain compound is thermodynamically stable at room temperature (Mautner et al., 2018;Rams et al., 2020). Finally, different polymorphic modifications can also be obtained for discrete complexes (Neumann et al., 2018b).
However, in all previous work we exclusively used N-donor coligands for the synthesis of Co(NCS) 2 thiocyanate coordination polymers, and to investigate the influence of the coligand on the structure and the magnetic behaviour, we became interested in donor ligands that can coordinate via a sulfur atom, including thiourea derivatives. With thiourea, the crystal structure of one Co(NCS) 2 coordination compound has already been already reported, and in this case the Co cations are linked by pairs of thiourea sulfur atoms, whereas the thiocyanate anions are only terminally N-bonded (Rajarajan et al., 2012). Independent of this, we used ethylenethiourea as coligand and obtained a compound with the composition [Co(NCS) 2 (ethylenethiourea) 2 ] n . Single-crystal structure determination proves that, in this case, the Co II cations are connected by pairs of thiocyanate ligands into chains, which corresponds exactly to the desired structure . In contrast to the analogous compounds with N-donor coligands, this compound shows antiferromagnetic ordering but no relaxations of single chains. To investigate this in more detail, we used tetramethylthiourea as coligand and obtained crystals of the title compound Co(NCS) 2 (tetramethylthiourea) 2 . Surprisingly, this compound consists of discrete complexes, in which the Co II cations are tetrahedrally coordinated, which is also reflected in its IR spectra, where the C-N stretching vibration of the thiocyanate anion is observed at 2048 cm À1 (see Fig. S1 in the supporting information). The powder diffraction pattern reveals that a pure product has been obtained (Fig. S2). The thermogravimetric (TG) curve shows that all tetramethylthiourea ligands are emitted in one step (theoretical mass loss: 60.2%), which is accompanied by an exothermic peak in the differential thermoanalysis (DTA) curve (Fig. S3). However, two endothermic peaks are observed at low temperatures where the sample mass does not change. To investigate this phenomenon in more detail, measurements using differential scanning calorimetry (DSC) were performed, which prove that the first endothermic signal is reversible with some hysteresis, pointing to some structural transition ( Fig. S4), which could explain why no changes in the PXRD pattern are observed after cooling. The second endothermic peak is irreversible, but thermomicroscopic measurements prove that this event corresponds to melting, which means that upon cooling no crystallization is observed (Figs. S5 and S6).

Structural commentary
The asymmetric unit of the title compound contains two crystallographically independent tetramethylthiourea molecules, two thiocyanate anions and one Co II cation in general positions (Fig. 1). The Co II cations are coordinated by two Nbonded thiocyanate anions and two tetramethylthiourea molecules into discrete complexes, with bonds lengths and angles similar to those reported in the literature (Table 1) View of the asymmetric unit of the title compound with the atom labelling and displacement ellipsoids drawn at the 50% probability level. Table 1 Selected geometric parameters (Å , ). coordination polyhedra around the Co II cations can be described as strongly distorted tetrahedra (Table 1), which is also obvious from the tetrahedral angle variance hteti 2 = 81.0 and the mean tetrahedral quadratic elongation h tet i = 1.036 (Robinson et al., 1971). The C-N bond lengths between the thioketone C and the amino groups are significantly shorter than those between the amino groups and the methyl C atoms, which points to some degree of double-bond character of the former. This is expected as thioketones are subject to thioketone-enthiole tautomerism similar to the tautomerism found for regular ketones, which is also supported by the fact that the CNMe 2 groups are planar with angles close to 120 (Devillanova, 2007). The NMe 2 groups of the same coligand are twisted against each other with angles of 45.74 (9) and 46.32 (8) for the two crystallographically independent tetramethylthiourea coligands.

Supramolecular features
As can be seen in Table 2, two sets of hydrogen bonds can be found for which the D-HÁ Á ÁA angles are either relatively near to 180 (174.0 and 166.2 ) or far from 180 (142.8 and 140.8 ), which is indicative for strong or relatively weak hydrogen bonds, respectively. Although nearly all of these hydrogen bonds are intermolecular bonds between the thiocyanate sulfur and a C-H hydrogen atom from an adjacent complex, in one case relatively weak intramolecular hydrogen C-HÁ Á ÁS bonding between two different tetramethylthiourea molecules of the same discrete complex is found. Each complex is connected to two different neighbouring complexes by pairs of C-HÁ Á ÁS NCS hydrogen bonds between the tetramethylthiourea coligands and the thiocyanate anions. This leads to the formation of zigzag-like chains along the b-axis direction (Fig. 2), which are further connected by additional single C-HÁ Á ÁS NCS hydrogen bonds into layers that are parallel to the ab plane (Fig. 3). These layers are stacked along the c-axis direction with no pronounced intermolecular interactions between them (Fig. 4).  Table 2 Hydrogen-bond geometry (Å , ). (ii) Àx; y À 1 2 ; Àz þ 1 2 ; (iii) Àx þ 1; y þ 1 2 ; Àz þ 1 2 .

Figure 2
Crystal structure of the title compound with a view of the chains that run along the b-axis direction with intermolecular C-HÁ Á ÁS hydrogen bonds shown as dashed lines.

Figure 3
Crystal structure of the title compound with a view along the c axis of the layers. Intermolecular C-HÁ Á ÁS hydrogen bonds are shown as dashed lines.

Figure 4
Crystal structure of the title compound with a view in the direction of the layers along the b axis. Intermolecular C-HÁ Á ÁS hydrogen bonds are shown as dashed lines.
[Co(NCS) 2 (thiourea) 2 ] n has been reported (Rajarajan et al., 2012). However, several structures built up of discrete tetrahedral complexes with cobalt thiocyanate and a variety of Ncontaining ligands are reported in the CCDC. These include, for example, bis(3-methylpyridine)diisothiocyanatocobalt(II) (Bö ckmann et al. 2011) and bis(quinoline)diisothiocyanatocobalt(II) (Mirčeva & Golič, 1990). It is noted that, in several cases, pyridine or imidazole derivatives are used that have large substituents adjacent to the coordinating N atoms, which might enforce the formation of a tetrahedral complex for steric reasons.

Synthesis and crystallization
General Co(NCS) 2 and tetramethylthiourea were purchased from Sigma Aldrich and were used without further purification.
Synthesis A suspension of Co(NCS) 2 (0.50 mmol, 87.5 mg) and tetramethylthiourea (1.00 mmol, 132.23 mg) in 0.75 mL water was stored at 281 K. After a few days, deep-blue-coloured crystals were obtained, which were filtered off, and ground into powder or used for single-crystal structure determination. It is noted that no crystalline product could be obtained from an analogous reaction at room temperature. Elemental analysis calculated for C 12 H 24 N 6 CoS 4 (439.56 g mol À1 ) C 32.79, H 5.50, N 19.12%, S 29.18, found: C 32.37, H 5.38, N 18.75, S 28.64.

Experimental details
Elemental analysis was performed using an EURO EA elemental analyser fabricated by EURO VECTOR Instruments.
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.
DTA-TG measurements were performed in a dynamic nitrogen atmosphere (100 sccm) in Al 2 O 3 crucibles using a STA-PT 1600 thermobalance from Linseis. The instrument was calibrated using standard reference materials.
The DSC measurements were performed with a DSC 1 Star System with STARe Excellence Software from Mettler-Toledo AG. The instrument was calibrated using standard reference materials.
Thermomicroscopic measurements were performed using a hot-stage FP82 from Mettler and a BX60 microscope from Olympus, using the software analysis package from Mettler.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 3. All non-hydrogen atoms were refined anisotropically. The C-H atoms were positioned with idealized geometry (allowed to rotate but not to tip) and refined isotropically with U iso (H) = 1.5U eq (C).

Bis(tetramethylthiourea-κS)bis(thiocyanato-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.