Synthesis, crystal structure and thermal properties of a new polymorphic modification of diisothiocyanatotetrakis(4-methylpyridine)cobalt(II)

The crystal structure of title compound consists of discrete complexes in which the CoII cations are octahedrally coordinated to two terminally N-bonded thiocyanate anions and four 4-methylpyridine coligands and represents a new polymorphic modification of Co(NCS)2(4-methylpyridine)4, which is already reported in the literature.


Chemical context
Polymorphism is a widespread phenomenon and of equal importance in academic and industrial research.It is frequently found in organic compounds but there are also several examples where it is observed in coordination compounds (Moulton & Zaworotko, 2001;Braga & Grepioni, 2000;Tao et al., 2012).This is the case, for example, for coordination compounds based on thiocyanate anions, which we have been interested in for several years.The majority of polymorphic modifications in this class of compounds are observed for discrete complexes with terminally N-bonded ligands (Wo ¨hlert et al., 2013;Neumann et al., 2018a).In contrast, compounds with a bridging coordination of the anionic ligands typically form isomeric modifications (Mautner et al., 2018;Neumann et al., 2018b;Bo ¨hme et al., 2020;Jochim et al., 2018).Within this project, we are especially interested in compounds based on Co(NCS) 2 which, because if its high magneticotropy, shows a versatile magnetic behavior (Rams et al., 2017(Rams et al., , 2020)).In the course of these investigations, we became interested in 4-methylpyridine as coligand, with a special focus on Co(NCS) 2 compounds.
Several compounds based on Co(NCS) 2 have already been reported with this ligand, predominantly discrete complexes with a tetrahedral or an octahedral coordination, with most of them forming clathrates (see Database survey).As part of our synthetic work we have obtained crystals that were characterized by single-crystal X-ray diffraction.This proves that a discrete complex with the composition Co(NCS) 2 (4-methylpyridine) 4 was obtained.Based on these findings, a CSD search was performed, which revealed that the structure of a compound with this composition had already been reported by Solacolu and co-workers and Harris and co-workers [refcodes QQQGKG (Solacolu et al., 1974) and VERNUC (Harris et al., 2003)].The title compound crystallizes differently, which means that we have obtained a new polymorphic modification of this complex.

Structural commentary
The title compound, Co(NCS) 2 (4-methylpyridine) 4 , is isotypic to Ni(NCS) 2 (4-methylpyridine) 4 already reported in the literature (refcode ICMPNI01; Kerr &Williams, 1977 andSoldatov et al., 2004).Its asymmetric unit consists of one Co II cation, two thiocyanate anions and four 4-methylpyridine coligands that are located in general positions (Fig. 1).The metal cations are sixfold coordinated to two terminally Nbonded thiocyanate anions and four 4-methylpyridine coligands into discrete complexes.Bond lengths and angles are comparable to those in the polymorphic modification already reported in the literature (refcode VERNUC; Harris et al., 2003) and show that a slightly distorted octahedral coordination is present (Table 1).
The title compound represents a further polymorph of the modifications that have already been reported in the literature [refcodes QQQGKG (Solacolu et al., 1974) and VERNUC (Harris et al., 2003)], but it is noted that some contradictory results have been published.The modification reported by Harris and co-workers crystallizes in the tetragonal space group I4 1 /a with eight formula units in the unit cell and a unitcell volume of 6329.415A ˚3.The form reported by Solacolu and co-workers crystallizes in the space group I4 1 /a but with twelve formula units in the unit cell with a unit-cell volume of 6877.013A ˚3.However, in the same paper they also present a p-xylene clathrate crystallizing in the same space space group with a unit-cell volume of 6324.998A ˚3, which is very similar to that in the modification reported by Harris et al.It is therefore likely that the two unit-cell volumes were accidentally mixed up and that only one modification of Co(NCS) 2 (4-methyl-pyridine) 4 is reported.This is further supported by the fact that in the form reported by Solacula et al. with Z = 12, each non-hydrogen atom would need a volume of 16.4A ˚3, which seem to be much too low for such a complex.Unfortunately, no atomic coordinates are given for the ansolvate and the solvate reported by Solacula et al. and therefore those crystal structures cannot be compared with that of the form reported by Harris et al.
However, if the volume of each non hydrogen atom in the title compound (20.3A ˚3) is compared with that of the modification reported by Harris et al. (22.6 A ˚3), it is obvious that the title compound is much more densely packed, indicating that this modification represents the thermodynamically stable form, at least at 0 K.

Supramolecular features
In the crystal structure of the title compound, the discrete complexes are arranged in columns that propagate along the Figure 1 Crystal structure of the title compound with atom labeling and displacement ellipsoids drawn at the 50% probability level.

Table 1
Selected geometric parameters (A ˚, � ).2) it is unlikely that these are significant interactions.
In contrast, the form reported by Harris et al., exhibits three-dimensional pores (Fig. 3), which might be responsible for the low density of this modification.Moreover, because most clathrates are isotypic to the form reported by Harris et al., it is possible that these solvates lose their solvent molecules and transform into the ansolvate, presumably without collapse of the overall structure.

Additional investigations
The experimental X-ray powder pattern of the title compound was compared with that calculated from single-crystal data; this proves that a pure crystalline phase has been obtained (Fig. 4).
The title compound was also investigated by thermogravimetry and differential thermoanalysis (TG-DTA) measurements.Upon heating, several mass losses are  observed, which are accompanied by endothermic events in the DTA curve (Fig. 4).From the DTG curve, it is obvious that all mass losses are poorly resolved (Fig. 5).The experimental mass loss of the first and second step is in rough agreement with that calculated for the removal of one 4-methylpyridine ligand in each step (�m calc.= 17.0%).Upon further heating, the remaining 4-methylpyridine ligands are removed and the Co(NCS) 2 formed as an intermediate decomposes.

Synthesis
Co(NCS) 2 (99.9%) and 4-methylpyridine (98%) were purchased from Sigma Aldrich.Single crystals of the title compound suitable for structure determination were obtained by dissolving 0.25 mmol (43.8 mg) of Co(NCS) 2 in 7 mL of demineralized water.To this solution, 1.00 mmol (97.3 ml) of 4-methylpyridine were added and the reaction mixture was heated to 413 K for 15 min in a closed vial.Afterwards, it was cooled to 363 K and stored at this temperature overnight, leading to the formation of violet-colored crystals.Larger amounts of a crystalline powder were prepared by stirring 0.50 mmol (87.6 mg) of Co(NCS) 2 and 2.00 mmol (194.6 ml) of 4-methylpyridine in 2 mL of demineralized water for 3 d at room-temperature.The violet-colored powder was filtered off and dried in air.

Experimental details
The X-ray powder pattern was measured using a Stoe Transmission Powder Diffraction System (STADI P) equipped with a linear, position-sensitive MYTHEN 1K detector from Stoe & Cie. Thermogravimetry and differential thermoanalysis (TG-DTA) measurements were performed in a dynamic nitrogen atmosphere in Al 2 O 3 crucibles with 8 � C min À 1 using a STA-PT 1000 thermobalance from Linseis.The TG-DTA instrument was calibrated using standard references materials.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 3. Hydrogen atoms were positioned with idealized geometry (methyl H atoms allowed to rotate   and not to tip) and were refined with U ı ˜so (H) = 1.2U eq (C) (1.5 for methyl H atoms) using a riding model.

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 axis direction (Fig.2).A number of C-H� � �N and C-H� � �S contacts are observed between the complexes, but from the H� � �N and H� � �S distances and the C-H� � �N and C-H� � �S angles (Table

Figure 2
Figure 2Crystal structure of the title compound in a view along the crystallographic b-axis direction.

Figure 5
Figure 5 DTG, TG and DTG curves for the title compound.The mass loss is given in % and the peak temperature in � C.

Figure 4
Figure 4Experimental (top) and calculated (bottom) X-ray powder patterns of the title compound.

Table 3
Experimental details.