Crystal structure of bis(N,N′-dimethylthiourea-κS)bis(thiocyanato-κN)cobalt(II)

In the crystal structure of the title compound, Co(NCS)2(N,N′-dimethylthiourea)2, discrete tetrahedral complexes are found in which the thiocyanate anions are N-terminally coordinated, while the coligands are S-terminally coordinated and are connected via weak hydrogen bonds into a three-dimensional structure.


Chemical context
Investigations on the synthesis, crystal structures and magnetic properties of coordination polymers based on transition-metal thiocyanates have become of increasing interest in recent years, which is due to the high structural diversity of the thiocyanate anion and its ability to mediate reasonable magnetic exchange. Although many different transition-metal thiocyanate coordination compounds are known, our main interests focus on first-row transition-metal thiocyanate compounds with the general composition M(NCS) 2 (L) 2 in which paramagnetic metal cations M are connected by pairs of -1,3 bridging thiocyanate anions into chains, while the remaining coordination sites are occupied by neutral coligands L, forming an octahedral coordination polyhedron. In most of these compounds, linear chains are formed in which all ligands are trans (Prananto et al., 2017;Werner et al., 2015a;Mautner et al., 2018;Rams et al., 2020;Jin et al., 2007), but other isomers are also possible. This includes compounds in which either the N-bonding thiocyanates, the S-bonding thiocyanates or the coligands are trans, while the other ligands are cis. These motifs are found, for example, in [M(NCS) 2 (4-benzylpyridine) 2 ] n (M = Mn, Ni, Cd) (Suckert et al., 2015;Jochim et al., 2018;Neumann et al., 2018a), [M(NCS) 2 (4-nitropyridine N-oxide) 2 ] n (M = Mn, Co, Cd) (Shi et al., 2006(Shi et al., , 2007Mautner et al., 2016) and [M(NCS) 2 (4-benzoylpyridine) 2 ] n (M = Co, Ni) Jochim et al., 2018), in which different ciscis-trans configurations can be found around the metal centers. The last possible isomer, in which all ligands are cis, is not found for the composition M(NCS) 2 (L) 2 , although it was observed in more ligand-deficient compounds of composition Co(NCS) 2 (L), with L representing either 4-methylpyridine N-oxide or 4-methoxypyridine N-oxide (Zhang et al., 2006a,b).
In those cases in which either only the N-bonding or the S-bonding thiocyanate are cis, corrugated chains are formed instead of linear ones, which is also the case if all ligands are cis. Furthermore, in some compounds a mixture of these configurations can be found, as in [M(NCS) 2 (4-chloropyridine) 2 ] n (M = Co, Ni, Cd) Jochim et al., 2018;Goher et al., 2003) or the high-temperature modification of [Ni(NCS) 2 (4-aminopyridine) 2 ] n (Neumann et al., 2018b), in which an alternating arrangement of all-trans and cis-cistrans-coordinated metal cations is present.
For some compounds of composition M(NCS) 2 (L) 2 a completely different structure is observed, in which the metal cations are connected into layers of different topologies by the thiocyanate anions. Only one topology is known for cobalt compounds of this composition, in which every two cobalt cations are connected by a pair of -1,3-bridging thiocyanate anions, forming dimers, which are connected to four adjacent dimers by single -1,3-bridging thiocyanate anions (Wö hlert & Nä ther, 2013;Suckert et al., 2016Suckert et al., , 2017Werner et al., 2015b). Nevertheless, with other metal cations different layer topologies are known in which, for example, trimers are formed instead of dimers, which are connected by single -1,3thiocyanate bridges (Kozísková et al., 1990;Kabešová et al., 1990) or in which each metal cation is directly connected to four neighboring metal cations by single -1,3-thiocyanate bridges (McElearney et al., 1979;Werner et al., 2015c;Ðaković et al., 2010). Although it is not clear by which parameters the formation of either chains or layers is promoted, a third isomer of composition Co(NCS) 2 (L) 2 exists in which the thiocyanate anions are N-terminally coordinated, forming discrete tetrahedral complexes (Prananto et al., 2017;Neumann et al., 2018c;Hannachi et al., 2019). In most of these compounds, electron-rich coligands are found, which might indicate that this type of compound is formed when a strong donor is used as coligand.
Most of the aforementioned compounds contain N-donor coligands, but in the course of our systematic work we became interested in the influence of S-donor coligands based on thiourea derivatives on the magnetic properties of Co(NCS) 2 chain compounds, in which the metal cations are linked by pairs of -1,3-bridging thiocyanate anions. With nickel, two compounds with the composition Ni(NCS) 2 (ethylenethiourea) 2 (Nardelli et al., 1966) and Ni(NCS) 2 (thioacetamide) 2 (Capacchi et al., 1968) are known. We prepared the corresponding cobalt compound with ethylenethiourea, which is isotypic to the nickel analogue (Jochim et al., 2020a). Only one additional polymeric compound with the composition [Co(NCS) 2 (thiourea) 2 ] n is known, in which the cobalt cations are connected by the sulfur atoms of the coligands, while the thiocyanate anions are N-terminally coordinated (Rajarajan et al., 2012;Muthu et al., 2015). In further work we prepared Co(NCS) 2 (tetramethylthiourea) 2 , which surprisingly consists of discrete tetrahedral complexes (Jochim et al., 2020b).
To further investigate this structural behavior, we used N,N 0 -dimethylthiourea as coligand, which is between thiourea and tetramethylthiourea considering the substitution with methyl groups, and during these investigations the title compound was obtained. Its IR spectrum shows that the C-N stretching vibration is found at 2064 cm À1 , which is indicative of N-terminally bonded thiocyanate anions, pointing to the formation of a tetrahedral complex, as proven by single crystal X-ray diffraction (see Fig. S1 in the supporting information). Although the experimental XRPD pattern is very similar to that calculated for the title compound, some additional reflections are found, indicating some contamination (Fig. S2). This is surprising, because all samples were prepared in different ways and using a different ratio of Co(NCS) 2 and dimethylthiourea leads to an identical XRPD pattern. In the thermogravimetry curve ( Fig. S3), two mass losses of 37.2% and 26.3% are observed, which in total is more than expected for the emission of both coligand molecules (54.3%), indicating the decomposition of the thiocyanate anions. This was proven by PXRD of the residue isolated after the first mass loss, which is mostly amorphous but which contains a small amount of crystalline CoS (Fig. S4). At low temperatures, an additional endothermic event can be found in the differential thermoanalysis curve, which is not accompanied by a mass change of the sample. This event corresponds to melting of the sample and is not reversible upon cooling, which was shown by differential scanning calorimetry (Fig. S5) and thermomicroscopy (Fig. S6).

Structural commentary
The asymmetric unit of the title compound contains two crystallographically independent complexes built up of two thiocyanate anions and two N,N 0 -dimethylthiourea molecules in general positions as well as one cobalt(II) cation, which is situated on a twofold rotational axis in both crystallographically independent complexes. Both metal cations are coordinated by two N-bonding thiocyanate anions and two S-bonding N,N 0 -dimethylthiourea molecules, forming strongly distorted tetrahedra ( Fig. 1), which becomes obvious from the tetrahedral angle variance hteti 2 = 73.2 (Co1) or 73.3 (Co2) and the mean tetrahedral quadratic elongation h tet i = 1.030 (Co1, Co2) (Robinson et al., 1971). However, all bond lengths and angles are comparable to those reported for similar compounds in the literature (Neumann et al. 2018c). Both N,N 0 -dimethylthiourea molecules are nearly planar with C Me -N-C-N torsion angles of 177.7 (3) and À0.5 (4) for the complex containing Co1 and 178.9 (3) and À1.2 (4) for that containing Co2. Both N,N 0 -dimethylthiourea planes within each complex are slightly tilted relative to each other, leading to an angle of 9.51 (10) between the normal vectors of the N,N 0 -dimethylthiourea planes in the complex containing Co1, while the corresponding angle for the complex containing Co2 is 13.52 (8) . Similar values are observed for the corresponding angles between the N,N 0 -dimethylthiourea planes of different crystallographically independent complexes.

Supramolecular features
The discrete complexes are connected via relatively weak N-HÁ Á ÁS NCS hydrogen bonds (Table 1), forming chains along the [101] direction in which the two crystallographically independent discrete complexes alternate. For both complexes, either only the thiocyanate anions (Co1) or the N,N 0 -dimethylthiourea molecules (Co2) are involved in the formation of this structure. Two neighboring chains are connected into double chains by pairs of additional N-HÁ Á ÁS NCS hydrogen bonds involving only the complexes containing Co1 (Fig. 2). In a similar way, chains built up of an alternating sequence of complexes containing either Co1 or Co2 are formed, which run along the a-axis direction, but in this case, in the complexes containing Co1, only the N,N 0 -dimethylthiourea molecules participate in the N-HÁ Á ÁS NCS hydrogen bonds, while in the complex containing Co2, only the thiocyanate anions are involved. Within these chains, additional C-HÁ Á ÁS hydrogen bonds between the N,N 0 -dimethylthiourea molecules of adjacent complexes can be found (Fig. 3). The two types of chain are connected via C-HÁ Á ÁS hydrogen bonds between the N,N 0 -dimethylthiourea molecules, forming layers parallel to the ac plane (Fig. 4), which are further connected into a three-dimensional network by additional N-HÁ Á ÁS NCS hydrogen bonds. View of the two crystallographically independent molecules in the asymmetric unit of the title compound with atom labeling and displacement ellipsoids drawn at the 50% probability level. Symmetry codes: (i) Àx + 1, y, Àz + 3 2 ; (ii) Àx, y, Àz + 1 2 . Table 1 Hydrogen-bond geometry (Å , ). Symmetry codes: (i) Àx þ 1 2 ; y þ 1 2 ; Àz þ 3 2 ; (ii) x; Ày þ 1; z þ 1 2 ; (iii) Àx; Ày þ 1; Àz þ 1; (iv) x À 1 2 ; y À 1 2 ; z; (v) Àx þ 1 2 ; Ày þ 3 2 ; Àz þ 1.

Figure 2
Crystal structure of the title compound with a view of the chains that extend along [101]. Intermolecular N-HÁ Á ÁS hydrogen bonding is shown as dashed lines.

Figure 3
urea is reported, which has the composition Cu(NCS)(N,N 0dimethylthiourea)[propane-1,3-diylbis(diphenylphosphine)] (Wattanakanjana et al., 2015). In this compound, the Cu I cations are tetrahedrally coordinated by one thiocyanate anion, one N,N 0 -dimethylthiourea molecule and one propane-1,3-diylbis(diphenylphosphine) molecule, with both phosphines coordinating to the metal center. In total, only 49 compounds containing a transition-metal cation and N,N 0 -dimethylthiourea are known, none of which contains cobalt. In fact, 28 of these compounds involve more chalcophilic second or third row transition-metal cations, while nearly half of the remaining compounds contain copper. Considering compounds that contain cobalt thiocyanate, over one thousand compounds are found, most of which show either an octahedral or a tetrahedral coordination geometry with the octahedral coordination geometry being the most common. Nevertheless, approximately 200 compounds with tetrahedral coordination geometry are known, making it the second most common coordination geometry for cobalt thiocyanate compounds.

Synthesis and crystallization
General N,N 0 -dimethylthiourea was purchased from Sigma Aldrich, while Co(NCS) 2 was purchased from Alfa Aesar. All chemicals were used without further purification.
Synthesis Single crystals were obtained by reacting Co(NCS) 2 (0.15 mmol, 26.3 mg) with N,N 0 -dimethylthiourea (0.3 mmol, 31.3 mg) in 0.4 mL of water. After approximately one week, deep-blue crystals were obtained, which were suitable for single crystal analysis. The same procedure was used to obtain powder samples using a higher amount of Co(NCS) 2 (1.00 mmol, 175.1 mg), N,N 0 -dimethylthiourea (2.0 mmol, 208.4 mg) and water (0.75 mL). The resulting crystals were ground into powder. In some cases, no crystallization of the mixture occurred, which was prevented by storing the reaction mixture at 281 K. Elemental analysis calculated for C 8 H 16 N 6 CoS 4 (383.45 g/mol) C 25.06%, H 4.21%, N 21.92%, S 33.45%, found: C 24.79%, H 4.34%, N 21.75%, S 32.16%.
Experimental details Elemental analysis was performed using a EURO EA elemental analyzer 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 an FP82 hot stage from Mettler and a BX60 microscope from Olympus, using the analysis software package from Mettler. Crystal structure of the title compound with a view perpendicular to the layers parallel to the crystallographic ac plane. Intermolecular N-HÁ Á ÁS and C-HÁ Á ÁS hydrogen bonds are shown as dashed lines.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2. All non-hydrogen atoms were refined anisotropically. The C-H H atoms were positioned with idealized geometry and refined isotropically with U iso (H) = 1.5U eq (C), allowing them to rotate, but not to tip. The N-H H atoms were located in the difference map, their bond lengths were set to ideal values and finally they were refined using a riding model [U iso (H) = 1.2U eq (N)]. As the crystal studied was twinned by non-merohedry, a twin refinement using data in HKLF 5 format was performed. The corresponding files were generated by PLATON (Spek 2020   SHELXL2018/3 (Sheldrick, 2015); molecular graphics: XP (Sheldrick, 2008) and DIAMOND (Brandenburg & Putz, 1999); software used to prepare material for publication: publCIF (Westrip, 2010).