Synthesis, crystal structure and thermal properties of tetrakis(3-methylpyridine-κN)bis(thiocyanato-κN)nickel(II)

In the crystal structure of the title compound, the nickel cations are octahedrally coordinated by two terminal N-bonded thiocyanate anions and four 3-methylpyridine ligands.


Chemical context
Thiocyanate anions are versatile ligands that can coordinate in many different ways to metal cations. The most common coordination is the terminal mode, in which these anionic ligands are only connected via the N or S atom, while the latter is only rarely observed. For several reasons, the -1,3 bridging coordination is more interesting and can lead to the formation of chains or layers (Nä ther et al., 2013). There are also a few compounds with more condensed thiocyanate networks that can form if these anionic ligands take up, for example, the -1,3,3 (N,S,S) bridging mode (Nä ther et al., 2013).
We have been interested in this class of compounds for several years targeting, for example, compounds that show interesting magnetic properties (Suckert et al., 2016;Werner et al., 2014Werner et al., , 2015a. In most cases, the neutral coligands used by us and others comprise pyridine derivatives and many such compounds have been reported in the literature (Mautner et al., 2018;Bö hme et al., 2020;Rams et al., 2020). If less chalcophilic metal cations such as Mn II , Fe II , Co II or Ni II are used, compounds with the composition M(NCS) 2 (L) 4 (M = Mn, Fe, Co, Ni and L = pyridine derivative) are frequently obtained, in which the metal cations are octahedrally coordinated by two terminal N-bonded thiocyanate anions and four coligands. Many of them have already been reported in the literature. If such compounds are heated, in several cases two of the coligands are removed, leading to a transformation to coliganddeficient compounds, in which the metal cations are linked by the anionic ligands and this is the reason why we are also interested in such discrete complexes (Nä ther et al., 2013).
Throughout these investigations, we became interested in Ni compounds with 3-methylpyridine as coligand for which some complexes have already been reported in the literature. However, all of these compounds consist of octahedral discrete complexes and the majority forms solvates with the composition Ni(NCS) 2 (3-methylpyridine) 4 ÁX with X = bis(trichloromethane) (LAYLOM; Pang et al., 1992), which crystallizes in space group P1, bis(dichloromethane) (LAYLIG; Pang et al., 1992), which crystallizes in space group C2/c, mono-tetrachloromethane, mono-dibromo-dichloromethane, mono-dichloromethane and mono-2,2-dichloropropane clathrates (JICMIR, LAYLAY, LAYLUS and LAYLEC;Pang et al., 1990Pang et al., , 1992 as well as mono-trichloromethane (CIVJEW and CIFJEW01; Nassimbeni et al., 1984Nassimbeni et al., , 1986, all of which crystallize in the orthorhombic space group Fddd. Surprisingly, for unknown reasons, the crystal structure of the ansolvate is unknown. What is common to all of the solvates mentioned above is the fact that they contain non-polar solvents, which cannot coordinate to metal cations. We used solvents with donor atoms able to coordinate when attempting to prepare compounds with the composition Ni(NCS) 2 (3methylpyridine) 2 (solvent) 2 . Upon heating, these should lose their two solvent molecules, transforming into compounds with a bridging coordination. Surprisingly, even in this case, octahedral complexes with the composition Ni(NCS) 2 (3methylpyridine) 4 ÁX (X = acetonitrile, ethanol, diethyl ether) were obtained . We have found that these solvates are unstable and have lost their solvents already at room temperature. X-ray powder diffraction (XRPD) proves that, independent of the crystal structure of the precursor, the same crystalline phase is always obtained (Fig. 1) which, according to IR spectroscopic data, bears only terminal Nbonded anionic ligands. Unfortunately no single crystals were obtained by this procedure, which means that the crystal structure of the ansolvate remained unknown. Starting from these observations, we tried to prepare crystals of the ansolvate using a variety of solvents and we eventually obtained crystals with the desired composition from H 2 O. The CN stretching vibration of the anions in the crystals is observed at 2072 cm À1 , indicating the presence of terminal thiocyanate anions (Fig. S1). Single crystal X-ray diffraction proves that the hitherto missing ansolvate has formed and XRPD investigations reveal the formation of a phase-pure sample (Fig. S2). Comparison of the experimental powder pattern obtained by solvent removal from the acetonitrile, ethanol and diethyl ether solvates with that calculated for the ansolvate proves that all of these crystalline phases are identical (Fig. 1). TG-DTA measurements show that the title compound decomposes in three steps, which are all accompanied by an endothermic event in the DTA curve (Fig. S3). The calculated mass loss per coligand amounts to 17.0%, which means that the first step (33.3%) is in reasonable agreement with the loss of two ligands and the second (15.7%) and third (14.9%) step with the loss of one ligand each, indicating the formation of additional compounds.

Structural commentary
The asymmetric unit of the title compound, Ni(NCS) 2 (3methylpyridine) 4 , consists of one Ni II cation, two thiocyanate anions and four 3-methylpyridine coligands that occupy general positions. One of the 3-methylpyridine coligands is disordered and was refined using a split model (Fig. 2). In the crystal structure of the title compound, the nickel cations are sixfold coordinated by two terminal N-bonded thiocyanate anions and four 3-methylpyridine coligands and from the bond lengths and angles it is obvious that the octahedra are slightly distorted (Table 1). This can also be seen from the octahedral angle variance (with a value of 11.2355 2 ) and the mean octahedral quadratic elongation (with a value of 1.0042) determined by the method of Robinson et al. (1971).

Supramolecular features
In the crystal structure of the title compound, the discrete complexes are arranged into layers that are located in the ab plane ( Fig. 3: top). These layers are separated from neighbouring layers by pairs of 3-methylpyridine ligands that show a butterfly-like arrangement. There are no indications forstacking or intermolecular hydrogen bonding. There are only C-HÁ Á ÁN and C-HÁ Á ÁS contacts, but from the distances and angles it is obvious that these are not significant interactions. The arrangement of the complexes in the title compound is similar to that in the solvates Ni(NCS) 2 (3-methylpyridine) 4 Áethanol and the isotypic compound Ni(NCS) 2 (3methylpyridine) 4 Áacetonitrile , indicating some structural relationship (Fig. 3). However, the third solvate, Ni(NCS) 2 (3-methylpyridine) 4 Ádiethyl ether  is not isotypic to the ethanol and acetonitrile solvates, yet also transforms into the title compound upon solvent removal. Even in this compound, a similar arrangement of the complexes is formed, which strongly suggests that the same crystalline ansolvate phase is particularly stable.

Figure 2
Crystal structure of the title compound with atom labeling and displacement ellipsoids drawn at the 50% probability level using XP in SHELX-PC (Sheldrick, 1996). The disorder of one of the 3-methylpyridine ligands is shown as full and open bonds.

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 XRPD measurements were performed with a Stoe Transmission Powder Diffraction System (STADI P) equipped with a MYTHEN 1K detector and a Johansson-type Ge(111) monochromator using Cu K 1 radiation ( = 1.540598 Å ).
The IR spectra were measured using an ATI Mattson Genesis Series FTIR Spectrometer, control software: WINFIRST, from ATI Mattson.
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 thermobalance from Linseis. The instrument was calibrated using standard reference materials.

Refinement
All crystals are of poor quality and merohedrally twinned with at least two componenents that are difficult to separate as is obvious from a view along the b* direction (Fig. S4). Therefore, a twin refinement using data in HKLF-5 format was performed, leading to a BASF parameter of 0.457 (5). Refinement using anisotropic displacement parameters leads to relatively large components of the anisotropic displacement parameters, indicating static or dynamic disordering. For one of the four crystallographically independent 3-methylpyridine coligands, the disorder was resolved and this ligand was refined using a split model with restraints. The C-bound H atoms were positioned with idealized geometry (methyl H atoms allowed to rotate but not to tip) and were refined isotropically with U iso (H) = 1.5U eq (C) for methyl H atoms and with U iso (H) = 1.2 U eq (C) for all other H atoms using a riding model. Crystal data, data collection and structure refinement details are summarized in Table 2

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. Refinement. Refined as a two-component twin.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )
x y z U iso */U eq Occ. (