Synthesis, crystal structure and thermal properties of catena-poly[[bis(4-methylpyridine)nickel(II)]-di-μ-thiocyanato], which shows an alternating all-trans and cis–cis–trans-coordination of the NiS2Np2Nt2 octahedra (p = 4-methylpyridine, t = thiocyanate)

In the crystal structure of the title compound, Ni(NCS)2(C6H7N)2 (C6H7N = 4-methylpyridine), the NiII cations are in an octahedral coordination and are linked by pairs of anionic ligands into corrugated chains in which the cations show alternating all-trans and cis–cis–trans coordination geometries. Upon heating, the title compound transforms into Ni(NCS)2(C6H7N), which is isotypic to its Cd analog as proven by a Rietveld refinement.

The title compound, [Ni(NCS) 2 (C 6 H 7 N) 2 ] n , was prepared by the reaction of Ni(NCS) 2 with 4-methylpyridine in water.Its asymmetric unit consists of two crystallographically independent Ni II cations, of which one is located on a twofold rotational axis whereas the second occupies a center of inversion, two independent thiocyanate anions and two independent 4-methylpyridine coligands in general positions.Each Ni II cation is octahedrally coordinated by two 4-methylpyridine coligands as well as two N-and two S-bonded thiocyanate anions.One of the cations shows an all-trans, the other a cis-cis-trans configuration.The metal centers are linked by pairs of �-1,3-bridging thiocyanate anions into [101] chains.X-ray powder diffraction shows that a pure crystalline phase has been obtained and thermogravimetry coupled to differential thermoanalysis reveals that the title compound loses half of the 4-methylpyridine coligands and transforms into Ni(NCS) 2 (C 6 H 7 N).Nearly pure samples of this compound can be obtained by thermal annealing and a Rietveld refinement demonstrated that it is isotypic to its recently reported Cd analog [Neumann et al., (2020).CrystEngComm. 22,[184][185][186][187][188][189][190][191][192][193][194] In its crystal structure, the metal cations are linked by one �-1,3(N,S)-and one �-1,3,3(N,S,S)-bridging thiocyanate anion into single chains that condense via the �-1,3,3(N,S,S)-bridging anionic ligands into double chains.

Chemical context
Coordination compounds based on transition-metal thiocyanates are an interesting class of compounds because they show an extremely large structural variability that can also lead to different polymorphs and isomers (Wo ¨hlert et al., 2013;Neumann et al., 2018;Jochim et al., 2020).This can be traced back to the fact that this anionic ligand shows many coordination modes (terminal, �-1, 1-and �-1,3-bridging) and that compared to, e.g.azides, the coordinating donor atoms are different.Moreover, in many cases an octahedral coordination of the metal center is observed and in such compounds the metal cations are usually linked by pairs of anionic ligands into chains.Chain compounds are usually formed with monocoordinating neutral coligands, whereas bridging coligands lead to the formation of layers.Five different isomeric configurations exist for such an octahedral coordination, including all-trans and all-cis and three different cis-cis-trans configurations (Fig. 1).In the majority of compounds, an alltrans configuration is observed but there are also examples of compounds in which the all-cis, cis-cis-trans, cis-trans-cis or trans-cis-cis configurations are present (see Database survey).
The structural variability is further increased if such configurations alternate and in this context we have reported on compounds with the composition Ni(NCS) 2 (4-chloropyridine) 2 [Cambridge Structural Database (CSD) refcodes UHUVIF and UHUVIF01; Jochim et al., 2018] and Co(NCS) 2 -(4-chloropyridine) 2 (GIQQIJ and GIQQIJ01;Bo ¨hme et al., 2020).For this compound, two different isomers were obtained, in both of which the Co II cations are octahedrally coordinated and linked into chains by pairs of thiocyanate anions.However, in one of these isomers an all-trans configuration is present, leading to the formation of linear chains, whereas in the second an alternating all-trans and cis-cis-trans configuration is observed, which leads to the formation of corrugated chains.Solvent-mediated conversion experiments show that the isomer with corrugated chains is more stable than that with linear chains, which is suprising because, as mentioned above, most compounds form linear chains (Jochim et al., 2018;Bo ¨hme et al., 2020).In this context, it is noted that not only the metal configuration can alternate, because we have prepared the first Co(NCS) 2 coordination polymer with a linear chain structure in which an alternating fivefold and sixfold coordination is present (WEKVUH; Bo ¨hme et al., 2022).
However, based on the results for the 4-chloropyridine compounds, we remembered the chloro-methyl exchange rule, which states that compounds with such ligands are very often structurally similar or even isotypic due to the fact that the van der Waals radius of a methyl group is similar to that of a chlorine atom (Desiraju & Sarma, 1986).Consequently, we assumed that similar isomers might be prepared with 4methylpyridine, C 6 H 7 N. Therefore, Ni(NCS) 2 was reacted with 4-methylpyridine, which leads to the formation of the title compound with the composition Ni(NCS) 2 (C 6 H 7 N) 2 , which is isotypic to the most stable isomer of Ni(NCS) 2 (4-chloropyridine) 2 and Co(NCS) 2 (4-chloropyridine) 2 , which are already reported in the literature (Jochim et al., 2018).We have not found any evidence for the formation of a metastable isomer with linear chains and 4-methylpyridine as ligand.
In this context it is mentioned that a compound with the stated composition Ni(NCS) 2 (C 6 H 7 N) 2 is already reported in the CSD (QQQGJV; Solaculu et al., 1974).This compound crystallizes in the orthorhombic space group I222 with the unusual value of Z = 18 formula units in the unit cell.Unfortunately, no atomic positions were presented and this entry is limited to unit-cell parameters, crystal system and space group.In fact, for this space group Z = 16 would be expected if two crystallographically independent formula units were present in the asymmetric unit, and in this case the volume for each non-hydrogen atom is calculated to be 23.2A ˚3, which is a relatively high value and might point to some solvent molecules being present.Therefore, from our point of view the existence of this crystalline form is at least questionable.
There is another entry in the CSD with this composition and for this structure atomic coordinates are available (ITMPNI; Lipowski & Andreetti, 1978).In this structure, each Ni II cation is octahedrally coordinated by 4-methylpyridine coligands, two terminal N-bonded thiocyanate anions as well as one Sand one N-bonding bridging thiocyanate anion.Each two Ni II cations are linked by pairs of �-1,3-bridging anionic ligands into dinuclear complexes.Because the chemical composition is

Figure 2
Crystal structure of the title compound with labeling and displacement ellipsoids drawn at the 50% probability level.Symmetry codes (i) identical to that of the title compound, it might be denoted as an isomer.

Structural commentary
The asymmetric unit of the title compound, Ni(NCS) 2 -(C 6 H 7 N) 2 , consists of two crystallographically independent Ni II cations, two crystallographically independent thiocyanate anions and two crystallographically independent 4-methylpyridine coligands (Fig. 2).Whereas the anionic and neutral ligands occupy general positions, one of the Ni II cations ( Ni1) is located on a crystallographic twofold rotation axis, and the second Ni II cation (Ni2) occupies a center of inversion.Each of the Ni II cations is sixfold coordinated by two N-bonding and two S-bonding �-1,3-bridging thiocyanate anions as well as two 4-methylpyridine coligands.Ni2 shows an all-trans configuration whereas Ni1 is in a cis-cis-trans arrangement with the S-bonding thiocyanate anions and the 4-methylpyridine coligands in cis and the N-bonding thiocyanate in trans positions (Fig. 3).For the Ni II cation that shows an alltrans coordination, the Ni-S bond lengths are a bit shorter, whereas the Ni-N distances to the coligands are slightly longer compared to the cation in the cis-cis-trans configuration (Table 1).The metal cations are linked by pairs of �-1,3bridging anionic ligands into chains that, because of the alternating all-trans and cis-cis-trans configurations of the metal ions, are corrugated (Fig. 3).This compound is isotypic to the thermodynamically stable isomer of Ni(NCS) 2 (4-chloropyridine) 2 (UHUVIF01; Bo ¨hme et al., 2020), which indicates that the title compound is ther-modynamically stable.This is further supported by the fact that in all of our synthetic work we never found hints that a further isomer could be prepared.

Supramolecular features
In the crystal structure of the title compound, the chains propagate along [101] with each chain surrounded by six neighboring chains (Fig. 4).There are no significant intermolecular C-H� � �N or C-H� � �S contacts and there are also no hints of any �-� stacking interactions.

Figure 4
Crystal structure of the title compound with view along [101].

Additional investigations
Powder X-ray diffraction measurements demonstrate that the title compound was obtained as a pure phase (Fig. 5).
The title compound was also investigated by thermogravimetry and differential thermoanalysis (TG-DTA) measurements.Upon heating, several mass losses are observed that are accompanied by endothermic events in the DTA curve (Fig. S1).From the DTG curve it is obvious that all mass losses are poorly resolved (Fig. S1).The experimental mass loss of the first and second steps is in rough agreement with that calculated for the removal of one 4-methylpyridine ligand in each step (�m calc.= 17.0%), indicating that a more 4-methylpyridine-deficient compound with the composition Ni(NCS) 2 (4-methylpyridine) has formed.
Lowering the heating rate did not lead to better resolved curves and, therefore, isolation of this intermediate seems to be impossible.It was also not possible to prepare this phase from solution, even if an excess of Ni II was used in the synthesis.Therefore, samples of the title compound were annealed for different times at different temperatures below the decomposition temperature observed in the TG-DTA measurements, until no reflections of the pristine compound 1 were present.In this case, a well-defined crystalline phase was obtained, for which the CN stretching vibrations of the anionic ligands are observed at 2118, 2141 and 2196 cm À 1 , indicating that a more complex thiocyanate network has formed (Fig. S2).Unfortunately, indexing of this pattern did not lead to a reasonable unit cell but we reported the crystal structure of a compound with the composition Cd(NCS) 2 (C 6 H 7 N) a few years ago (Neumann et al., 2020).Because Cd II cations are much more chalcophilic than Ni II cations, such compounds can easily be prepared and crystallized from solution.Based on the crystallographic data of Cd(NCS) 2 (C 6 H 7 N), a Rietveld refinement was performed for the residue obtained by thermal decomposition of the title compound, which proves that the Ni compound is isotypic and that this sample is contaminated with a small amount of Ni(NCS) 2 [6.1 (3) wt.%], which might originate from a slightly too long tempering of the title compound (Fig. S3).In the crystal structure of Ni(NCS)  linked by one �-1,3(N,S)-and one �-1,3,3(N,S,S)-bridging thiocyanate anion into single chains that condense via the �-1,3,3(N,S,S)-bridging anionic ligands into double chains.The single chains are linked by each two S atoms sharing common edges, forming Ni 2 S 2 rings.
Finally, it is noted that for the isotypic compounds with nickel and cobalt and 4-chloropyridine, no ligand-deficient compounds were detected and the reason for this difference in the thermal reactivity for isotypic compounds is not clear.
6.00 mmol (1471 mg) of NiCl 2 •6H 2 O, 12.00 mmol (1160 mg) of KNCS and 12.00 mmol (1160 ml) of 4-methylpyridine were stirred in 5 ml of demineralized water at room temperature for 1 day.The precipitate was filtered off, washed with demineralized water and dried in air.Single crystals of the title compound were obtained using the same molar ratio of the reactants without stirring.Elemental analysis for C 14 H 14 N 4 NiS 2 : calculated C 46. 57, H 3.91, N 15.51; S 17.76, found C 46.78, H 4.08, N 15.72 S 17.94.The purity was also shown by powder X-ray diffraction (see Fig. S4).An IR spectrum of the title compound can be found in Fig. S4.

Experimental details
Elemental analysis was performed with a vario MICRO cube from Elementar Analysensysteme GmbH.IR spectra were recorded at room temperature on a Bruker Vertex70 FT-IR spectrometer using a broadband spectral range extension VERTEX FM for full mid and far IR.Thermogravimetry and differential thermoanalysis (TG-DTA) measurements were performed in a dynamic nitrogen atmosphere in Al 2 O 3 crucibles with an 8 � C min À 1 heating rate using a STA-PT 1000 thermobalance from Linseis.The TG-DTA instrument was calibrated using standard reference materials.X-ray powder diffraction experiments were performed using a Stoe STADI P transmission powder diffractometer with Cu K�1 radiation (� = 1.540598A ˚), a Johann-type Ge(111) monochromator and a MYTHEN 1K detector from Dectris.The structure refinement was carried out using TOPAS Academic version 6.0 (Coelho, 2018).For the Rietveld refinement, the structure of Cd(NCS) 2 (C 6 H 7 N) was used as starting model with Cd replaced by Ni.All ligands were set up as rigid bodies with bond lengths taken from the literature, including split positions for four carbon atoms to model rotational disorder of the ligand.The positions of the cation and all ligands were subject to unconstrained refinement, which quickly converged to a convincing R Bragg of 1.2%.

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. Fractional

Figure 1
Figure 1 View of the different geometric isomers for an octahedral MA 2 B 2 C 2 coordination with the corresponding notation and idealized point group.The N atoms of the neutral co-ligands are shown in red.

Figure 3
Figure 3Crystal structure of the title compound with view of a chain and labeling of the Ni II cations.

Figure 5
Figure 5Experimental (top) and calculated (bottom) X-ray powder pattern of the title compound.

Figure 6
Figure 6Crystal structure of Ni(NCS) 2 (C 6 H 7 N) obtained by thermal decomposition of the title compound.

Table 2
Experimental details.