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

The title compound, [Ni(NCS)2(C5H12N2S)2], consists of discrete complexes with a distorted all-trans square-planar coordination geometry that are linked by intermolecular C—H⋯S hydrogen bonds into corrugated layers parallel to the ac plane.

In the course of our investigations regarding transition-metal thiocyanates with thiourea derivatives, the title compound, [Ni(NCS) 2 (C 5 H 12 N 2 S) 2 ], was obtained. The asymmetric unit consists of one thiocyanate anion and one tetramethylthiourea molecule on general positions, as well as one Ni II cation that is located on a twofold rotational axis. In this compound, discrete complexes are formed in which the Ni II cations are surrounded by two trans-N-bonding thiocyanate anions as well as two trans-S-bonding tetramethylthiourea molecules within a distorted square-planar coordination geometry. The discrete complexes are linked by pairs of weak C-HÁ Á ÁS hydrogen bonds between the thiocyanate S and one of the tetramethylthiourea methyl hydrogen atoms into chains along the crystallographic a-and c-axis directions, which are combined into layers parallel to the ac plane. X-ray powder diffraction proves that a pure crystalline phase was obtained and measurements using thermogravimetry and differential thermoanalysis reveal that the compound decomposes at about 408 K, where all tetramethylthiourea molecules are lost.

Chemical context
Many thiocyanate coordination compounds are reported in the literature, which mostly consist of discrete complexes containing non-bridging N-terminally coordinated thiocyanate anions, while compounds in which the metal cations are bridged by these anionic ligands are comparatively rare. Despite this fact, a variety of coordination modes can be found for bridging thiocyanate anions, which leads to metal-thiocyanate networks with different dimensionalities and topologies (Wö hlert et al., 2014;Lin, 2008;Li et al., 2014;Suckert et al., 2016). If these compounds contain paramagnetic metal cations, they are of special interest, because thiocyanate anions can mediate magnetic exchange and thus cooperative magnetic phenomena can be expected (Palion-Gazda et al., 2015;Mekuimemba et al., 2018;Mousavi et al., 2020;Mautner et al., 2018). Our interest focuses mainly on transition-metal thiocyanates with the general composition [M(NCS) 2 (coligand) 2 ] n with M = Mn II , Fe II , Co II or Ni II that consist of linear chains, in which the metal cations are connected by pairs of N-and S-bonding thiocyanate anions into centrosymmetric M 2 (NCS) 2 units, while the remaining sites of the coordination octahedron are occupied by neutral coligands forming a coordination environment in which all ligands are trans (Wö hlert et al., 2014;Werner et al., 2014Werner et al., , 2015Prananto et al., 2017). In this context, it is noted that the Co II compounds are of special interest, because either ferromagnetic behavior or a slow relaxation of the magnetization is observed (Werner et al., 2015;Neumann et al., 2019;Rams et al., 2017. Besides these chain compounds with an alltrans coordination environment, several other isomers with different cis-cis-trans arrangements of the ligands can be found in which either the coligand, the N-bonding or the S-bonding thiocyanate are trans, while the other ligands are cis (Maji et al., 2001;Shi et al., 2007;Rams et al., 2017). For most of these compounds, corrugated chains are observed in which the magnetic exchange is low or negligible Jochim et al., 2018). In the case of [M(NCS) 2 (4-chloropyridine) 2 ] n (M = Co, Ni), two isomeric compounds are observed that contain either linear or corrugated chains, which allowed investigations on the influence of the chain geometry on the magnetic behavior, because both compounds contain the same coligand and thus all differences in the magnetic behavior can be attributed to the structural changes Jochim et al., 2018).
However, to investigate the magnetic properties of transition-metal thiocyanate compounds in more detail, the influence of the coligands on the structural and magnetic behavior must be investigated systematically. Most of these compounds contain N-donor coligands, whereas compounds with, for example, O-or S-donor coligands are rare (Groom et al., 2016;Amzel et al., 1969;Shurdha, et al., 2013). This is the reason why we became interested in transition-metal thiocyanate compounds with thiourea derivatives, where a few compounds have been reported for which either octahedral (Amzel et al., 1969) or tetrahedral complexes (Jochim et al., 2020a,b) are observed. Furthermore, some polymeric compounds have been reported in which the metal cations are connected by either the coligands (Nardelli et al., 1966a) or the thiocyanate anions into chains (Nardelli et al., 1966b;Jochim et al., 2020c). In the course of our systematic investigations we became interested in tetramethylthiourea as coligand, which upon reaction with nickel thiocyanate leads to the formation of the title compound [Ni(NCS) 2 (C 5 H 12 N 2 S) 2 ] that consists of discrete complexes, in which the thiocyanate anions are N-terminally coordinated. Phase pure powders of the title compound could easily be obtained, which is confirmed by X-ray powder diffraction ( Fig. S1 in the supporting information). The C-N stretching band of the thiocyanate anion can be found at 2080 cm À1 , which proves the presence of termin-ally bonded thiocyanate anions, in accordance with the results from single crystal X-ray diffraction (Fig. S2). Investigation of the thermal behavior of the title compound shows that it decomposes at about 408 K in one discrete step of 59.0%, which is in agreement with the mass loss calculated for the loss of all coligand molecules of 60.2% (Fig. S3).

Structural commentary
The asymmetric unit consists of one Ni II cation that is located on a twofold rotational axis as well as one thiocyanate anion and one tetramethylthiourea molecule, which both occupy general positions. Each Ni II cation is fourfold coordinated by two trans N-binding thiocyanate anions and two trans S-binding tetramethylthiourea molecules into discrete complexes. (Fig. 1). The Ni-N bonds are much shorter than the Ni-S bonds and from the angles it is obvious that the Ni cation is in a square-planar coordination geometry (Table 1). Furthermore, a strong deviation from the ideal angles is found in the coordination environment of the Ni cation, which can probably be attributed to the relatively bulky NMe 2 groups of the tetramethylthiourea molecules. This is most pronounced in the N-Ni-N angle, which amounts to 167.47 (16) . In contrast, for the S-Ni-S angle a smaller deviation with a value of 173.26 (5) Table 1 Selected geometric parameters (Å , ).

Figure 1
View of the asymmetric unit of the title compound with atom labeling and displacement ellipsoids drawn at the 50% probability level. Symmetry transformations used to generate equivalent atoms: (i) Àx + 1, y, Àz + 3 2 .
C SÁ Á ÁS C torsion angle of 135.0 (2) . Furthermore, while the thiourea unit of each tetramethylthiourea ligand is planar, both NMe 2 groups are rotated out of this plane by angles of 28.8 (2) and 27.3 (2) .

Supramolecular features
In the crystal structure of the title compound, the discrete complexes are linked by two crystallographically different intermolecular C-HÁ Á ÁS hydrogen bonds between the thiocyanate S atom S1 and the methyl hydrogen atoms H13C and H12C of the tetramethylthiourea molecule. In both cases, each two neighbouring complexes are linked into pairs containing 18-membered rings that are located on centers of inversion ( Fig. 2 and Table 2). These pairs are further linked into chains, which for the hydrogen bonds between S1 and H13C proceed along the crystallographic a-axis direction and for those between S1 and H12b along the c-axis direction (Fig. 2). These two chains condense into layers parallel to the ac plane by centrosymmetric pairs of both crystallographically different C-HÁ Á ÁS hydrogen bonds (Fig. 3).

Database survey
In the Cambridge Crystallographic Database (CSD, Version 5.41, last update May 2020; Groom et al., 2016) no transitionmetal thiocyanate compounds with tetramethylthiourea are reported, but one such compound with cobalt was published recently (Jochim et al., 2020b). In this compound, discrete tetrahedral complexes are found in which the metal cations are coordinated by two N-bonding thiocyanate anions and two S-bonding tetramethylthiourea molecules. Several compounds with transition-metal cations and tetramethylthiourea are reported in the CSD, of which two contain nickel cations. Both consist of discrete binuclear complexes in which the metal cations are connected by thiolate ligands. These complexes contain either two Ni II cations with a square-planar coordination geometry (Ito et al., 2009) or one Ni II and one Fe II cation with square-pyramidal and octahedral coordination geometries (Ohki et al., 2008), respectively. Several Ni(NCS) 2 compounds with other thiourea derivatives are also found, including polymeric compounds such as [Ni(NCS) 2 (ethylenethiourea) 2 ] n (Nardelli et al., 1966b) and discrete complexes like [Ni(NCS) 2 (N,N 0 -diethylthiourea) 4 ] (Amzel et al., 1969), but only one of those contains nickel cations with a squareplanar coordination geometry (Leovac et al., 1995).

Synthesis and crystallization
General Ni(NCS) 2 was synthesized using a procedure described in Jochim et al. (2018). The reagents NiSO 4 Á6H 2 O and Ba(NCS) 2 Á3H 2 O, which were used for this, were obtained from Merck and Alfa Aesar, respectively.

Figure 2
Crystal structure of the title compound with view of the two different chains formed by intermolecular C-HÁ Á ÁS hydrogen bonding between S1 and H13C (top) and between S1 and H12B (bottom). Intermolecular C-HÁ Á ÁS hydrogen bonding is shown as dashed lines. 132.3 mg) were reacted in 0.5 mL of n-butanol for one day, after which the residue was filtered off. Elemental analysis calculated for C 12 H 24 N 6 NiS 4 (439.32 g mol À1 ) C 32. 81, H 5.51, N 19.13, S 29.20, found: C 32.77, H 5.42, N 19.07, S 29.18 Experimental details Elemental analysis was performed using an 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 XRPD measurements were performed with Cu K 1 radiation ( = 1.540598 Å ) using a Stoe Transmission Powder Diffraction System (STADI P) that is equipped with a MYTHEN 1K detector and a Johansson-type Ge(111) monochromator. DTA-TG measurements were performed in a dynamic nitrogen atmosphere (5 NL h À1 ) in Al 2 O 3 crucibles using a STA-PT 1000 thermobalance from Linseis. The instrument was calibrated using standard reference materials.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 3. All non-hydrogen atoms were refined with anisotropic displacement parameters. The Cbound H atoms were positioned with idealized geometry (C-H = 0.98 Å ) allowing them to rotate, but not to tip and refined isotropically with U iso (H) = 1.5U eq (C).

Bis(tetramethylthiourea-κS)bis(thiocyanato-κN)nickel(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.
Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )