Synthesis, crystal structure and properties of bis(isoselenocyanato-κN)tetrakis(4-methoxypyridine-κN)cobalt(II)

In the crystal structure of the title compound, the cobalt cations are sixfold coordinated by two terminal N-bonded selenocyanate anions and four 4-methoxypyridine coligands within a slightly distorted octahedral coordination.

Reaction of CoCl 2 Á6H 2 O with KNCSe and 4-methoxypyridine in water led to the formation of the title compound, [Co(NCSe) 2 (C 6 H 7 NO) 4 ] or Co(NCSe) 2 (4methoxypyridine) 2 , which was characterized by single-crystal X-ray diffraction. Its asymmetric unit consists of one crystallographically independent Co cation, two selenocyanate anions and four 4-methoxypyridine coligands in general positions. In the crystal structure, the cobalt cations are sixfold coordinated by two terminal N-bonded selenocyanate anions and four 4-methoxypyridine coligands within a slightly distorted octahedral coordination. Between the complexes, weak C-HÁ Á ÁSe interactions are found. IR spectroscopic investigations revealed that the CN stretching vibration of the anionic ligands is observed at 2068 cm À1 , which is in agreement with the presence of only terminally coordinated selenocyanate anions. PXRD measurements prove that a pure compound was obtained. Differential thermoanalysis coupled to thermogravimetry (DTA-TG) at different heating rates shows that the TG curves are poorly resolved. PXRD measurements of the residue obtained by a TG measurement prove that an amorphous compound was obtained.

Chemical context
Coordination compounds based on transition-metal thiocyanates show versatile structural behavior (Buckingham, 1994;Barnett et al., 2002;Werner et al., 2015) and promising magnetic properties, because this ligand is able to mediate reasonable magnetic exchange (Barasiń ski et al., 2010;Palion-Gazda et al., 2015;Mousavi et al., 2020). In this context, compounds based on Co(NCS) 2 are of special interest because they can show interesting magnetic behavior, such as, for example, slow relaxations of the magnetization, which is indicative of single-chain magnetism (Lescouë zec et al., 2005;Sun et al., 2010;Dhers et al., 2015). For the synthesis of such compounds, the Co II cations must be linked via the anionic ligands into mono-periodic or di-periodic networks. Compounds with di-periodicity are rare; the majority of compounds being mono-periodic, in which the Co II cations are octahedrally coordinated and linked into chains by pairs of anionic ligands (Guang et al., 2007;Shi et al., 2007;Shurdha et al., 2013;Prananto et al., 2017). If the chains are linear, ferromagnetic ordering (Werner et al., 2014) or single-chain magnet behavior (Mautner et al., 2018) is observed and if they are corrugated or exhibit an alternating Co coordination, the magnetic exchange is weak or completely suppressed (Dockum et al., 1983;Bö hme et al., 2020Bö hme et al., , 2022. All this is well investigated for Co(NCS) 2 compounds but not much is known for compounds based on Co(NCSe) 2 , because only two compounds with -1,3-bridging selenocyanate anions are reported in the literature (Boeckmann et al., 2011;Wö hlert et al., 2012;Neumann et al., 2019). First results indicate that they behave in a similar manner to their thiocyanate analogs and that the exchange of thio-by selenocyanate leads to an increase in the magnetic intrachain interactions (Neumann et al., 2019).
Unfortunately, the synthesis of compounds in which Co II cations are linked by selenocyanate anions into chains in solution is always difficult to achieve or even impossible, because Co II is not very chalcophilic and therefore, in most cases, compounds with terminally coordinated selenocyanate anions are obtained. To overcome this problem, we have developed an alternative approach for the synthesis of coordination networks based on thermal ligand removal of suitable precursor compounds that can be used for the synthesis of a wide range of materials including thio-and selenocyanates but also halide coordination compounds (Werner et al., 2014;Boeckmann et al., 2011;Nä ther & Jess, 2004). For thiocyanate compounds, the precursors usually consist of discrete complexes of the general formula Co(NCS) 2 (L) 4 (L = monocoordinating coligand), in which the Co II cations are octahedrally coordinated by two terminal N-bonded thiocyanate anions and four coligands. Upon heating, most of compounds of this type lose half of the ligands in the first mass loss and the octahedral metal coordination is retained by the sulfur atoms that were not involved in the metal coordination in the precursor, which enforces the formation of compounds with bridging anionic ligands.
In the course of our systematic work we became interested in the corresponding Co(NCSe) 2 compounds with 4-methoxypyridine as coligand, because its thiocyanate analog Co(NCS) 2 (4-methoxypyridine) 2 crystallizes in the desired chain structure and is well investigated. This compound shows a metamagnetic transition and single-chain relaxations and this was investigated on powders but also using single crystals Foltyn et al., 2022). The reaction of CoCl 2 Á6H 2 O, KSeCN with 4-methoxypyridine in water, however, always led to the formation of a compound with the composition Co(NCSe) 2 (4-methoxypyridine) 4 (see Synthesis and crystallization) for which the CN stretching vibration of the anionic ligand is observed at 2068 cm À1 , indicative for the presence of only terminally bonded selenocyanate anions (Fig. S1). Even if CoCl 2 Á6H 2 O and KSeCN were used in excess, no other crystalline phase was obtained. To identify this phase unambiguously, single crystals were grown and characterized by single-crystal X-ray diffraction.

Structural commentary
Single-crystal structure determination proved that the title compound, Co(NCSe) 2 (4-methoxypyridine) 4 , consists of discrete complexes in which the Co cations are sixfold coordinated to four 4-methoxypyridine coligands and two terminal selenocyanate anions that coordinate via the N atom of the anionic ligand to the metal center (Fig. 1). The asymmetric unit consists of one Co II cation, two selenocyanate anions and four 4-methoxypyridine ligands in general positions. From the bond lengths and angles, it is obvious that the octahedra are slightly distorted (Table 1). This is also obvious from the angle variance of 1.77 and the quadratic elongation of 1.00 calculated using the method of Robinson (Robinson et al., 1971).
The title compound is isotypic to M(NCS) 2 (4-methoxypyridine) (M = Co, Fe, Ni) already described in the literature (Mautner et al., 2018;Jochim et al., 2018). In this context, it is noted that Ni(NCS) 2 (4-methoxypyridine) crystallizes in two polymorphic modifications, of which the form (orthorhombic, space group Pccn) that is not isotypic to the title compound and M(NCS) 2 (4-methoxypyridine (M = Co, Fe) represents the Crystal structure of the title compound with labeling and displacement ellipsoids drawn at the 50% probability level. Table 1 Selected geometric parameters (Å , ).
90.76 (9) thermodynamically stable phase at room temperature (Jochim et al., 2018). However, from this experimental observation one cannot conclude that the title compound is metastable at room temperature and that a second form must exist. It is also noted that the thiocyanate analogs with manganese and cadmium crystallize in a third form (monoclinic, space group C2/c) and that the Cd(NCS) 2 compound also shows dimorphism and additionally crystallizes in a fourth form (tetragonal, space group P4 1 ), which shows the pronounced structural variability for such simple complexes (Jochim et al., 2019). Finally, it is noted that we have not found any evidence that the title compound crystallizes in a further crystalline form.

Supramolecular features
In the crystal structure, the discrete complexes are arranged in an irregular manner ( Fig. 2) There are a number of intermolecular C-HÁ Á ÁO and C-HÁ Á ÁSe contacts, but for most of them the C-HÁ Á ÁX (X = O, Se) angle is far from linear and the HÁ Á ÁX distances are too large for any significant interaction ( Table 2). Some of C-HÁ Á ÁSe contacts exhibit angles larger than 150 , which might point to some interaction ( Fig. 2 and Table 2).

Thermal properties
Based on the single-crystal structural data, a powder pattern was calculated and compared with the experimental pattern, which shows that a pure crystalline phase was obtained (Fig. 3).
To investigate if a crystalline ligand deficient phase with the composition Co(NCS) 2 (4-methoxypyridine) 4 can be obtained, measurements using differential thermal analysis and thermogravimetry with 8 C min À1 were performed. Upon heating, the TG curve shows two poorly resolved mass losses at about 160 and 250 C that are accompanied with endothermic events in the DTA curve (Fig. 4) Crystal structure of the title compound viewed along the crystallographic a-axis direction. C-HÁ Á ÁSe interactions are shown as pink dashed lines. Table 2 Hydrogen-bond geometry (Å , ).

Figure 3
Experimental (top) and calculated PXRD pattern (bottom) of the title compound.

Figure 4
DTG, TG and DTA curves for the title compound measured at 8 C min À1 in a nitrogen atmosphere. first event consists of two different thermal events that cannot be successfully resolved. Nevertheless, the experimental mass loss was calculated for all three events, which shows that the first mass loss is in reasonable agreement with that calculated for the removal of one 4-methoxypyridine ligand (Ám = À15.5%), whereas the second mass loss points to the removal of two additional 4-methoxypyridine ligands (Fig. 4). This would indicate that in the first step a compound with the composition Co(NCSe) 2 (4-methoxypyridine) 3 is formed, which transforms into Co(NCSe) 2 (4-methoxypyridine) upon further heating. Compounds with such a ratio between the metal salt and neutral coligands are known for thiocyanate coordination compounds, but are very rare for selenocyanates. One compound with the composition Ni(NCSe) 2 [N,N 0 -bis(3aminopropyl)methylamine] 2 is found in which each Ni cation is octahedrally coordinated by three N atoms of one (3aminopropyl)methylamine ligand plus two bridging and two terminal selenocyanate anions (Vicente et al., 1993). Two of the Ni cations are linked by pairs of -1,3-bridging anionic ligands into dinculear units. At first glance, the Ni:coligand ratio seems to be different but one (3-aminopropyl)methylamine ligand replaces three monocoordinating ligands. For a ratio of 1:1 between M(NCS) 2 and coligand, no examples can be found with selenocyanate anions but a few examples with thiocyanate are reported in the literature, including Ni(NCS) 2 (4-aminopyridine), in which Ni(NCS) 2 double chains are observed (Neumann et al., 2018).
To increase the resolution, measurements at different heating rates were performed, but the TG curves look similar and are still poorly resolved (Fig. S2). However, to investigate if different crystalline phases can be prepared, the residues obtained at different temperatures were isolated and investigated by PXRD, which proved that they are amorphous, and in Fig. S3 one of these patterns is shown as a representative. We also tried to anneal samples of the title compound at constant temperatures but always obtained amorphous intermediates. Therefore, no more efforts were made. . The Cu compound forms discrete complexes with a square-planar coordination, while the Co compounds consist of isomers forming discrete tetrahedral complexes as well as a chain compound with an octahedral coordination, which is also the case for the Co and Cd compounds.

Synthesis and crystallization
CoCl 2 Á6H 2 O and KSeCN were purchased from Aldrich and 4-methoxypyridine was purchased from Alfa Aesar.
Synthesis: Larger amounts of a microcrystalline powder were obtained by the reaction of 0.15 mmol (35.7 mg) of CoCl 2 Á6H 2 O with 0.30 mmol (43.3 mg) of KSeCN and 0.60 mmol (60.8 mL) of 4-methoxypyridine in 1 ml of demineralized water. The mixture was stirred for 2 d at room temperature, the lightpink-colored precipitate was filtered off and washed with a very small amount of water. Single crystals were obtained by slow evaporation of the solvent from the filtrate. It is noted that the same compound is obtained if CoCl 2 Á6H 2 O and 4-methoxypyridine are used in a 1:1 ratio.
Experimental details: 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.

Bis(isoselenocyanato-κN)tetrakis(4-methoxypyridine-κN)cobalt(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.