Synthesis, crystal structure and thermal properties of poly[bis[μ2-3-(aminomethyl)pyridine]bis(thiocyanato)cobalt(II)]

In the crystal structure of the title compound the CoII cations are octahedrally coordinated by two terminal N-bonded thiocyanate anions and four 3-(aminomethyl)pyridine coligands, of which two are coordinated through the pyridine N atom and two through the amino N atom. The cations are linked by the coligands into layers, that are further connected into a three-dimensional network by intermolecular N—H⋯S hydrogen bonding.


Chemical context
Coordination compounds based on thiocyanate anions show a variety of structures, that can be traced back to the versatile coordination behavior of this ligand (Buckingham, 1994, Wö hlert et al., 2014Werner et al., 2015a). Even if the majority of compounds contain only terminal N-bonded ligands, there is a large number of compounds in which the metal cations are linked by these anionic ligands into networks of different dimensionality (Ðaković et al., 2010;Kozísková et al., 1990;Kabešová et al., 1990;Prananto et al., 2017;Suckert et al., 2016;Wellm et al., 2018). In those cases where the metal cations are octahedrally coordinated, different isomers can additionally be found, in which the metal cations are either all-trans or ciscis-trans coordinated Rams et al., 2017). Which kind of compound is observed depends among other things on the nature of the metal cation, because the synthesis of compounds with bridging anionic ligands is easier with chalcophilic cations such as, for example, Cd II , whereas less chalcophilic metal cations such as Mn II , Fe II and especially Co II and Ni II in several cases lead to the formation of compounds with terminal N-bonded thiocyanate anions. This is of importance because this anionic ligand is able to mediate substantial magnetic exchange (Bassey et al., 2020;Mekuimemba et al., 2018;Palion-Gazda et al., 2015;Mousavi et al., 2020), which can lead to compounds that show a variety of magnetic properties. Co II is of special importance because of its high magnetic anisotropy (Mautner et al., 2018;Neumann et al., 2019). This led to a renewed interest into compounds in which the metal cations are linked by these anionic ligands into chains or layers and an increasing number ISSN 2056-9890 have been reported in the literature over the last decade (Jin et al., 2007;Shi et al., 2006;Mautner et al., 2018).
In our own investigations we are especially interested in transition-metal thiocyanate coordination polymers based on cobalt in which the metal cations are linked by -1,3-bridging anionic ligands into chains, because these compounds can show single-chain magnet (SCM) behavior. These are compounds in which the spins are ferromagnetically aligned along a chain with strong magnetic exchange within the chain and only weak interchain interactions to prevent 3D ordering (Sun et al., 2010;Miyasaka et al., 2005). In the course of this project we have prepared a large number of compounds with the general composition M(NCS) 2 (L) 2 where L represents a pyridine derivative substituted at the 4-position (Werner et al., 2015b;Rams et al., 2017Rams et al., , 2020. In principle, SCM behavior can also be observed in 2D compounds if the ferromagnetic chains are linked into layers by bridging ligands that do not mediate strong magnetic exchange. Therefore, we became interested in 3-(aminomethyl)pyridine as it can coordinate to metal cations via the pyridine and the amino N atom and for which no cobalt(II) thiocyanate compounds had been reported. Therefore, we reacted Co(NCS) 2 with 3-(aminomethyl)pyridine in different molar ratios, which always led to the formation of crystalline powders with the composition Co(NCS) 2 (3-(aminomethylpyridine) 2 (see Synthesis and crystallization). This composition indicated that either the organic coligand does not bridge neighboring metal centers or that only terminal-coordinated thiocyanate anions are present. IR spectroscopic measurements reveal that the CN stretching vibration of the anionic ligand is observed at 2077cm À1 , which points to the presence of terminal N-bonded anionic ligands (Fig. S1). To prove these assumptions, single crystals were grown and characterized by single-crystal X-ray diffraction, which proves that this crystalline phase is isotypic to the corresponding Cd compound already reported in the literature, in which the Cd II or Co II cations are linked into layers by the 3-(aminomethyl)pyridine ligands (see Structural commentary). Comparison of the experimental X-ray powder pattern with that calculated from the single-crystal data proves that a pure crystalline phase has been obtained (Fig. S2). For the more chalcophilic Cd II cations another compound with the composition Cd(NCS) 2 (3-(aminomethylpyridine) is known, in which the Cd II cations are linked by bridging anionic ligands. With Co(NCS) 2 we found no access to this compound in solution and, therefore, we tried to prepare a 3-(aminomethyl)pyridine-deficient phase by thermal ligand removal from the title compound. Therefore, the title compound was investigated by thermogravimetry coupled to differential scanning calorimetry (TG-DSC). Upon heating at a rate of 8 C min À1 the compound starts to decompose at about 215 C and upon further heating a steady mass loss with no discrete decomposition events is observed (Fig. S3). To increase the resolution a second TG-DSC measurement with 1 C min À1 was performed, which does not improve the resolution significantly ( Fig. S4). Based on these measurements, there is no indication for the formation of another currently unknown 3-(aminomethyl)pyridine-deficient compound.

Structural commentary
The asymmetric unit of the title compound, Co(NCS) 2 (C 6 H 8 N 2 ) 2 , consists of one Co II cation that is located on a center of inversion as well as one thiocyanate anion and one 3-(aminomethyl)pyridine coligand in general positions (Fig. 1). The Co II cations are sixfold coordinated by two symmetry-equivalent terminal N-bonded anionic ligands as well as four symmetry-equivalent 3-(aminomethyl)pyridine coligands, of which two are coordinated through the pyridine N atom and two through the amino N atom to the cations, with each pair of identical atoms in the trans position to each other Figure 1 Crystal structure of the title compound with labeling and displacement ellipsoids drawn at the 50% probability level. Symmetry codes: A = Àx, Ày + 1, Àz, B = 1 2 À x, À 1 2 À y, 1 2 À z, C = À 1 2 + x, 3 2 À y, À 1 2 + z. Color code: Co (red), N (blue) and S (orange). (Fig. 1). The Co-N bond lengths to the amino N atom are significantly shorter than those to the pyridine N atoms, indicating that this is the stronger interaction ( Table 1). The bond angles around the Co II centers deviate by less than 1.95 (6) from the ideal values, which indicates that the octahedra are only slightly distorted (Table 1). This is also obvious from the octahedral angle variance of 1.6 and the mean octahedral quadratic elongation of 1.001 calculated using the method of Robinson (Robinson et al., 1971). The Co cations are linked by bridging 3-(aminomethyl)pyridine ligands into layers that are parallel to the bc plane (Fig. 2). These layers are constructed of large rings that consist of four Co II cations and four 3-(aminomethyl)pyridine coligands (Fig. 2).

Supramolecular features
The Co(NCS) 2 layers are arranged in stacks that elongate along the crystallographic a-axis direction (Fig. 2). The layers are linked into a three-dimensional network by intermolecular N-HÁ Á ÁS hydrogen bonding between the thiocyanate S atoms and the amino H atoms, in which the S atoms act as acceptors for two of these hydrogen bonds ( Fig. 3 and Table 2). The N-HÁ Á ÁS angles are close to linear, which indicates that this is a strong interaction. There are additional C-HÁ Á ÁS and C-HÁ Á ÁN intra-and intermolecular interactions, but their geometrical parameters indicate that these are not strong interactions (Table 2). Crystal structure of the title compound viewed along the crystallographic a axis. The H atoms are omitted for clarity. (i) Àx þ 1 2 ; y À 1 2 ; Àz þ 1 2 ; (ii) Àx; Ày þ 1; Àz; (iii) x À 1 2 ; Ày þ 3 2 ; z À 1 2 .

Figure 3
Crystal structure of the title compound viewed along the crystallographic b axis. Intermolecular N-HÁ Á ÁS hydrogen bonds are shown as dashed lines.
which additional 3-(aminomethyl)pyridine solvate molecules are embedded. The same report also describes M(NCS) 2 [3-(aminomethyl)pyridine] 2 [M = Cd (QEKZEO), Zn (QEKYUD)], which is isotypic to the title compound. Finally, two compounds with the composition M(NCS) 2 [3-(aminomethyl)pyridine] [M = Cd (QEKZIS), Zn (QEKZAK)] are reported. The Zn compound consists of dimers, in which each two Zn II cations are linked by each two 3-(aminomethyl)pyridine ligands. In contrast, in the crystal structure of the Cd compound, the Cd II cations are linked into chains by the 3-(aminomethyl)pyridine ligands that are further connected into layers by -1,3-bridging thiocyanate anions. This compound is the only one which shows an cis-cis-trans coordination of the metal cations.

Experimental details
Elemental analysis was performed using a EURO EA elemental analyzer fabricated by EURO VECTOR Instruments. The IR spectrum was measured using an ATI Mattson Genesis Series FTIR Spectrometer, control software: WINFIRST, from ATI Mattson. The PXRD measurement was performed with Cu K 1 radiation ( = 1.540598 Å ) using a Stoe Transmission Powder Diffraction System (STADI P) that is equipped with a MYTHEN 1K detector and a Johanssontype Ge(111) monochromator. Thermogravimetry and differential scanning calorimetry (TG-DSC) measurements were performed in a dynamic nitrogen atmosphere in Al 2 O 3 crucibles using a STA-PT 1600 thermobalance from Linseis. The instrument was calibrated using standard reference materials.
Synthesis 3-(Aminomethyl)pyridine and Co(NCS) 2 were purchased from Alfa Aesar. All chemicals were used without further purification. Single crystals were obtained by reacting 1 mmol Co(NCS) 2 (175.1 mg) with 0.2 mmol 3-(aminomethyl)pyridine (216.3 mg) in 3 mL of ethanol. After approximately one week blue-colored crystals were obtained, which were suitable for single crystal X-ray analysis. For the synthesis of crystalline powders the same amounts of reactants were stirred in 1 mL of ethanol for 3 d. The blue-colored precipitate was filtered and dried in air. Yield: 70%. Elemental analysis calculated for C 14 H 16 N 6 CoS 2 (391.4 g mol À1 ) C 42.96%, H 4.12%, N 21.47%, S 16.39%, found: C 42.82%, H 4.01%, N 21.32%, S 16.29%.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 3. All non-hydrogen atoms were refined anisotropically. C-H and N-H H atoms were located in difference maps but positioned with idealized geometry and refined isotropically with U iso (H) = 1.2U eq (C) (1.5 for amino H atoms) using a riding model.

Poly[bis[µ 2 -3-(aminomethyl)pyridine]bis(thiocyanato)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.