Synthesis, crystal structure and thermal behavior of tetrakis(3-cyanopyridine N-oxide-κO)bis(thiocyanato-κN)cobalt(II), which shows strong pseudosymmetry

The crystal structure of the title compound consists of discrete complexes in which the CoII cations are octahedrally coordinated and show strong pseudosymmetry.


Chemical context
The synthesis of new coordination compounds is still an important field in chemistry.In most cases, such compounds are prepared in solution but there are alternatives, where they are prepared in the solid state using, for example, molecular milling (Stolar et al., 2017;Darwish et al., 2019), grinding (Adams et al., 2007) or molten-flux synthesis (Ho ¨ller et al., 2010;Scho ¨nfeld et al., 2012).In our own investigations, we frequently use thermal ligand removal of suitable precursor compounds for the solid-state synthesis of new coordination compounds that mostly consist of discrete complexes, in which the anionic ligands are only terminally bonded.Upon heating, these precursors frequently lose their neutral coligands in a stepwise fashion, forming intermediate compounds with condensed networks in which the metal cations are linked by the anionic ligands into one-, two-or three-dimensional networks.In the beginning, our interest focused on transitionmetal-halide coordination compounds (Na ¨ther et al., 2001;Na ¨ther & Jess, 2004), but in recent years we have used this approach for the synthesis of transition-metal thio-and selenocyanates because these anionic ligands mediate reasonable magnetic exchange, which allows the preparation of compounds that show versatile magnetic behavior (Palion-Gazda et al., 2015;Mekuimemba, et al., 2018).In this context, of special interest are compounds based on Co II in which the cations are linked by pairs of thio-or selenocyanate anions into chains, because they can show three-dimensional but especially one-dimensional magnetic ordering (Werner et al., 2014;Rams et al., 2020).The major advantage of our approach is the fact that the new compounds are obtained in quantitative yield and that frequently metastable polymorphs or isomers can be prepared that often are not available from solution (Werner et al., 2015).
In recent investigations, N-donor coligands have been used that mostly consist of pyridine derivatives (Rams et al., 2017), but to investigate the influence of the coligands on the magnetic anisotropy of Co II centers, we also used S-donor coligands, such as ethylenethiourea, that lead to a modified magnetic behavior (Jochim et al., 2020).In a continuation of this work, we became interested in O-donor coligands and we found that only very few Co(NCS) 2 compounds with bridging thiocyanate anions and such coligands have been reported in the literature (see Database survey).We also found that only in Co(NCS) 2 (THF) 2 the Co II cations are linked by pairs of bridging anionic ligands into linear chains (Cambridge Structural Database refcode QIKQUY; Shurdha et al., 2013).In this context, we became interested in pyridine N-oxide derivatives, for which two Co(NCS) 2 compounds with bridging thiocyanate anions are reported in the literature (see Database survey).In our first investigations, we used 3-cyanopyridine N-oxide (C 6 H 4 N 2 O) as a coligand, which is commercially available and for which no coordination compounds have been reported.However, independent of the amounts of Co(NCS) 2 and 3-cyanopyridine N-oxide in the synthesis, the same crystalline phase was always obtained.The CNstretching vibration of the cyano group is observed at 2241 cm À 1 in the IR spectrum, indicating that this group is not involved in the metal coordination (Fig. S1).The CNstretching vibration of the thiocyanate anion occurs at 2051 cm À 1 , which proves that the anionic ligand is only terminally coordinated (Fig. S1).To confirm all these assumptions, the new crystalline phase was characterized by single crystal X-ray diffraction (see below).

Structural commentary
The asymmetric unit of the title compound, Co(SCN) 2 (C 6 H 4 N 2 O) 4 , consists of one crystallographically independent Co II cation that is located on a center of inversion, as well as one independent thiocyanate anion and two independent 3-cyanopyridine N-oxide coligands in general positions (Fig. 1).The Co II cations therefore adopt trans-CoN 2 O 4 octahedral geometries (Fig. 1).Bond lengths and angles correspond to literature values and show that the octahedra are slightly distorted (Table 1).
If the structure is checked for higher symmetry using PLATON (Spek et al., 2020) or checkCIF, a pseudo-translation and space group I2/m is suggested with 100% fit.The structure can easily be refined in this space group and the refinement leads to reasonable reliability factors.The refinement in space group I2/m, however, leads to significantly higher residuals than in space group P2 1 /n [R(F) for 2829 reflections with F o > 4�(F o ) = 0.027 in P2 1 /n versus 0.033 for 1446 reflections with F o > 4�(F o ) in I2/m and wR(F 2 ) = 0.083 for all 2829 independent reflections (P2 1 /n) versus 0.092 for all 1446 reflections (I2/m)].In this context, it is noted that nearly all reflections violating the centering are observed.Moreover, from the refinement in I2/m it is obvious that significantly

Figure 1
The molecular structure of the title compound with labeling and displacement ellipsoids drawn at the 50% probability level.Symmetry code: (i) enlarged anisotropic displacement parameters are observed, which are much larger than expected for a measurement at 100 K, indicating too high symmetry (Fig. 2).For all these reasons, the crystal structure is presented in the monoclinic primitive space group P2 1 /n.

Supramolecular features
In the extended structure of the title compound, the complexes are arranged in columns that proceed along the crystallographic a-axis (Fig. 3).In this direction the translation leading to the pseudo-centering is also obvious.Several C-H� � �O, C-H� � �S and C-H� � �N contacts are observed between the complexes, but from the distances and angles it is obvious that they do not correspond to strong interactions (Table 2).

Thermoanalytical investigations
Based on the single-crystal data, an X-ray powder pattern was calculated and compared with the experimental pattern, which proves that the title compound was obtained as a pure phase (Fig. S2).Because in our synthetic investigations no further compounds were detected, it was checked whether a compound with a more condensed network is available by thermal ligand removal.Therefore, the title compound was investigated simultaneously by differential thermoanalysis and thermogravimetry under nitrogen.Upon heating, only one mass loss of 58.8% is observed until 400 � C that does not fit to a stepwise loss of the 3-cyanopyridine N-oxide ligands (calculated mass loss for each 3-cyanopyridine N-oxide ligand = 18.3%;Fig. S3).From the DTA curve, the onset of an endothermic event is visible, followed by a strong exothermic event at a peak temperature of 220 � C.This is an unusual observation, because in previous investigations using pyridine derivatives the ligand removal always proceeds in an endothermic reaction.Oxidation of the compound might be excluded because all measurements were performed in a nitrogen atmosphere and therefore one must assume that this ligand is thermally unstable and decomposes upon heating.In agreement with these observations, the residue obtained at 400 � C is amorphous against X-rays (Fig. S4).Molecular structure of the title compound refined in space group I2/m with labeling and displacement ellipsoids drawn at the 50% probability level.Symmetry codes: (i) À x, 1 À y, 1 À z, (ii) À x, y, 1 À z, (iii) x, 1 À y, z.

Figure 3
The packing of the title compound showing the arrangement of the discrete complexes along the crystallographic a-axis.
cations are fivefold coordinated by two terminally N-bonded thiocyanate anions, two 3-cyanopyridine N-oxide ligands and one water molecule, whereas the Mn and Cd compounds consist of dinuclear units, in which each metal cation is octahedrally coordinated by one water molecule, one terminal and two bridging thiocyanate anions and two 4-cyanopyridine Noxide ligands, and are linked into dinuclear units by pairs of �-1,3-bridging thiocyanate anions.Some compounds based on Co(NCS) 2 and pyridine N-oxide derivatives in which the Co II cations are linked by �-1,3bridging thiocyanate anions are also known.This include the two isotypic compounds (4-methylpyridine N-oxide)bisthiocyanate)cobalt(II) (MEQKOJ; Zhang et al., 2006b) (4-methoxypyridine N-oxide)bisthiocyanate)cobalt(II) (TERRAK; Zhang et al., 2006a), (4-methylpyridine N-oxide)(methanol) bisthiocyanate)cobalt(II) (REKBUF; Shi et al., 2006) and bis(4-nitropyridine N-oxide)bis(thiocyanate)cobalt(II) (TILHIG; Shi et al., 2007).In the first two compounds, the Co II cations are linked by pairs of thiocyanate anions into corrugated chains that are further connected into layers by �-1,1(O, O) bridging coligands.In the third compound with methanol, two Co II cations are linked by pairs of anionic ligands into dinuclear units and are further linked by pairs of �-1,1(O,O) bridging 4-nitropyridine N-oxide ligands.In the compound with the 4-nitro substituent, the cations are linked by pairs of bridging thiocyanate anions into chains that are corrugated because of the cis-cis-trans configuration at the Co II centers.

Synthesis:
Single crystals were obtained by the reaction of 0.25 mmol (43.5 mg) Co(SCN) 2 and 1 mmol (120 mg) 3-cyanopyridine Noxide in 1 ml of ethanol.The reaction mixture was stored overnight, which lead to the formation of yellow needle-like crystals.
For the preparation of larger amounts of a microcrystalline powder, the same amount of reactants were stirred in 2 ml of ethanol for 1 d.

Experimental details:
The PXRD 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 (� = 1.540598A ˚).
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.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 3.The hydrogen atoms were positioned with idealized geometry and were refined with U iso (H) = 1.2U eq (C) using a riding model.As mentioned in the Structural commentary, the compound shows strong pseudosymmetry because of a pseudo-lattice translation indicating a centering, but our investigations show that the structure is best described in the primitive space group P2 1 /n instead of I2/m.This is obvious in the reliability factors obtained by refinements in both space groups, but especially from the large components of the anisotropic displacement parameters if the structure is refined in the body-centered space group.Moreover, nearly all of the reflections that would violate the centering were observed.

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.
Symmetry code:

Table 3
Experimental details.