Synthesis, crystal structure and thermal properties of bis(acetonitrile-κN)bis(3-bromopyridine-κN)bis(thiocyanato-κN)cobalt(II)

The crystal structure of the title compound consists of discrete octahedral complexes, which are linked by intermolecular C—H⋯S and C—H⋯N hydrogen bonds into chains.


Chemical context
Coordination compounds based on thiocyanate anions show a large structural variability, which to some extend can be traced back to the versatile coordination behavior of this ligand and this is surely one reason why, for example, many isomeric compounds are known (Jochim et al., 2020a;Bö hme et al., 2020;Neumann et al., 2018;Werner et al., 2015a). Moreover, in bridging thiocyanate anions a reasonable magnetic exchange is present, which can lead to compounds with a variety of magnetic properties (Palion-Gazda et al., 2015;Mekuimemba et al., 2018). In this context, compounds based on Co II cations are of special interest, not only because they can show antiferromagnetic or ferromagnetic ordering but as also for the strong magnetic anisotropy and slow relaxation of the magnetization indicative of single-chain-magnet behavior that can be observed (Bö hme et al., 2019;Switlicka et al., 2020;Werner et al., 2014;Shurdha et al., 2013;Prananto et al., 2017;Mautner et al., 2018). The latter is observable in linear chain compounds, in which the Co II cations are octahedrally coordinated in an all trans-configuration and linked by pairs of thiocyanate anions (Werner et al., 2015b;Mautner et al., 2018;Rams et al., 2020). This is the most common structural motif for such compounds, although corrugated chains or layers are also known (Chen et al., 2002;Wang et al., 2005;Jin et al., 2007;Shi et al., 2007;Yang et al., 2007;Suckert et al., 2016). All of these are reasons why we became interested in this class of compounds in order to study the influence of a chemical and a structural modification on their magnetic properties (Wö hlert et al., 2014;Neumann et al., 2019;Rams et al., 2017aRams et al., ,b, 2020Jochim et al., 2020b;Ceglarska et al., 2021).
In this context we have reported on the synthesis, crystal structures and magnetic properties of coordination compounds based on Co(NCS) 2 and 3-bromopyridine (C 5 H 4 BrN) as a ligand (Bö hme et al., 2022). During these investigations, we obtained discrete complexes with the composition Co(NCS) 2 (3-bromopyridine) 4 , Co(NCS) 2 (3bromopyridine) 2 (H 2 O) 2 and Co(NCS) 2 (3-bromopyridine) 2 (MeOH) 2 that lose the ligands stepwise upon heating, leading to the formation of compounds with the composition [(Co(NCS) 2 ) 2 (3-bromopyridine) 3 ] n and [Co(NCS) 2 (3-bromopyridine) 2 ] n , in which the Co cations are linked into chains by pairs of -1,3 bridging thiocyanate anions. In the latter compound, each of the Co II cations is octahedrally coordinated, whereas in the former an alternating octahedral and square-planar coordination is observed, which is very rare for thiocyanate compounds and had never been observed previously with Co(NCS) 2 . Later it was found that the compound with the mixed coordination can also be obtained from solution, which is impossible for the other compound with only an octahedral coordination. Unfortunately, the synthesis of the latter is difficult to achieve because all thermogravimetric curves are not well resolved and thermal annealing of the discrete complexes can lead to pure samples, but sometimes this is not the case. Therefore, we looked for other precursors that might show a similar reactivity and that might lead more easily to pure samples. In the course of these investigations, we obtained crystals of the title compound by the reaction of Co(NCS) 2 , 3-bromopyridine and acetonitrile. The CN stretching vibration of the thiocyanate anions is observed at 2066 cm À1 in the IR spectrum, which points to the presence of terminal N-bonded thiocyanate anions (Fig. S1). Single-crystal structure analysis proved that the structure consists of discrete complexes with the composition Co(NCS) 2 (3-bromopyridine) 2 (acetonitrile) 2 and a comparison of the experimental XRPD pattern with that calculated from single-crystal data reveals that a pure phase has been obtained ( Fig. S2). Therefore, this compound might be a suitable precursor for the synthesis of compounds, in which the Co II cations are linked by -1,3 bridging thiocyanate anions into chains.
Investigations using thermogravimetry and differential thermoanalysis (TG-DTA) show several mass losses, that are each accompanied with endothermic events in the DTA curve ( Fig. S3). The experimental mass loss is in excellent agreement with that, calculated for the removal of two acetonitrile ligands (Ám = 14.3%), whereas the values for the second and third mass loss roughly correspond to the emission of half a 3-bromopyridine ligand in each step (Ám = 13.8%). Therefore, one can assume that in the first TG step a compound with the composition Co(NCS) 2 (3-bromopyridine) 2 will form, which transforms into (Co(NCS) 2 ) 2 (3-bromopyridine) 3 upon further heating. XRPD investigations of the residue obtained after the first mass loss reveal the formation of a compound of poor crystallinity that cannot be identified. In contrast, a comparison of the powder pattern of the residue formed after the second TG step with that calculated for [(Co(NCS) 2 ) 2 (3bromopyridine) 3 ] n (Bö hme et al., 2022) retrieved from the literature proves that this chain compound has formed (Fig. S4).

Structural commentary
The asymmetric unit of the title compound consists of one crystallographically independent Co cation that is located on a center of inversion, as well as one thiocyanate anion, one 3-bromopyridine and one acetonitrile ligand in general positions ( Fig. 1). The methyl H atoms of the acetonitrile ligands are disordered over two orientations rotated by about 120 and were refined using a split model. The Co II cations are octahedrally coordinated by two symmetry-related 3-bromopyridine and two acetonitrile ligands as well as two terminal N-bonded thiocyanate anions into discrete complexes (Fig. 1). Bond lengths and angles correspond to literature values and from the bonding angles it is obvious that the octahedra are moderately distorted ( Fig. 1 and Table 1). This is in agreement with the values for the octahedral angle variance of 10.02 2 and the mean octahedral quadratic elongation of 1.0037, calculated using the method of Robinson et al. (1971). The sixmembered rings of the 3-bromopyridine ligands coordinating to the Co II cations are coplanar by symmetry.

Supramolecular features
In the extended structure of the title compound, the discrete complexes are linked by C-HÁ Á ÁS and C-HÁ Á ÁBr inter-actions into a three-dimensional network ( Fig. 2 and Table 2). One C-HÁ Á ÁS angle is close to linear, whereas the other C-HÁ Á ÁS and C-HÁ Á ÁBr angles are much less than 180 , indicating only weak interactions (Table 2). There are additional C-HÁ Á ÁN contacts but their bonding angles are very far from linear ( Table 2). The discrete complexes are arranged in stacks that propagate along the crystallographic b-axis direction (Fig. 2). Within these stacks, neighboring pyridine rings are nearly coplanar with an angle between their mean planes of 10.8 (1) and a distance between the centroids of the rings of 4.037 (1) Å , indicating very weakstacking interactions (Fig. 3). Crystal structure of the title compound with view along the crystallographic b-axis and intermolecular C-HÁ Á ÁS and C-HÁ Á ÁBr hydrogen bonding shown as dashed lines. Please note that the methyl H atoms of the acetonitrile ligand are disordered. Table 2 Hydrogen-bond geometry (Å , ). Symmetry codes: (i) Àx þ 1; Ày þ 1; Àz þ 1; (ii)

Figure 3
Crystal structure of the title compound showing the orientation of neighboring 3-bromopyridine rings. The distance between the centroids of the six-membered rings and the angles between the ring planes are shown. Please note that the methyl H atoms of the acetonitrile ligand are disordered. coligands as well as one terminal and two -1,3 bridging thiocyanate anions (Handy et al., 2017). In CuNCS(3-bromopyridine), the copper(I) cations are tetrahedrally coordinated and linked into layers by -1,3 bridging thiocyanate anions (Miller et al., 2011). In the second Cu II compound (CuNCS) 2 (3-bromopyridine) 4 , the copper cations are also tetrahedrally coordinated but linked into chains by -1,3 bridging thiocyanate anions (Nicholas, et al., 2017). With Ni(NCS) 2 and 3-bromopyridine several compounds are reported, including the discrete complexes with octahedrally coordinated Ni II cations Ni(NCS) 2 (3-bromopyridine) 4 , Ni(NCS) 2 (3-bromopyridine) 2 (H 2 O) 2 and Ni(NCS) 2 (3-bromopyridine) 2 (CH 3 OH) 2 (Krebs et al., 2021). Also included is a compound with acetonitrile with the composition Ni(NCS) 2 (3-bromopyridine) 2 ÁCH 3 CN, but in this structure the Ni II cations are linked into corrugated chains by -1,3 bridging thiocyanate anions that are connected via intermolecular hydrogen bonding into a three-dimensional network that contains channels in which acetonitrile solvate molecules are embedded (Krebs et al., 2021). Finally, when the discrete complexes are heated, a transformation into Ni(NCS) 2 (3-bromopyridine) 2 is observed, in which the Ni II cations are octahedrally coordinated and linked into chains by pairs of -1,3 bridging thiocyanate anions (Krebs et al., 2021). The latter compound is isotypic to its Co II analog reported recently (Bö hme et al., 2022).
With diamagnetic cations, four structures are reported. The compounds M(NCS) 2 (3-bromopyridine) 4 (M = Zn, Cd) are isotypic and consist of discrete complexes in which the metal cations are octahedrally coordinated by four 3-bromopyridine ligands and two terminal N-bonded thiocyanate anions (Wö hlert et al., 2013). Upon heating, half of the 3-bromopyridine ligands are removed and a transformation into compounds of the composition M(NCS) 2 (3-bromopyridine) 2 (M = Zn, Cd) is observed (Wö hlert et al., 2013). The Zn compounds consist of tetrahedral complexes, whereas in the Cd compound the cations are linked by pairs of anionic ligands into chains.

Synthesis and crystallization
Co(NCS) 2 and 3-bromopyridine were purchased from Merck. All chemicals were used without further purification. After storing 0.5 mmol of Co(NCS) 2 (87.5 mg) and 0.5 mmol of 3-bromopyridine (48.8 ml) in 2.0 ml of acetonitrile for 3 d at room temperature, light-red single crystals of the title compound suitable for single-crystal X-ray analysis were obtained. The IR spectrum was measured using an ATI Mattson Genesis Series FTIR Spectrometer, control software WINFIRST, from ATI Mattson. The XRPD measurement was performed with Cu K 1 radiation ( = 1.540598 Å ) using a Stoe Transmission Powder Diffraction System (STADI P) equipped with a MYTHEN 1K detector and a Johansson-type Ge(111) monochromator. 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
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) = 1.2U eq (C) (1.5 for methyl H atoms) using a riding model. The H atoms of the methyl group of the acetonitrile ligand are disordered in two orientations and were refined in ratio 50:50 using a split model. Crystal data, data collection and structure refinement details are summarized in Table 3.

Bis(acetonitrile-κN)bis(3-bromopyridine-κN)bis(thiocyanato-κN)cobalt(II)
Crystal data  (10) 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.