Synthesis, crystal structure and thermal properties of the dinuclear complex bis(μ-4-methylpyridine N-oxide-κ2 O:O)bis[(methanol-κO)(4-methylpyridine N-oxide-κO)bis(thiocyanato-κN)cobalt(II)]

Reaction of Co(NCS)2 with 4-methylpyridine N-oxide in methanol leads to the formation of Co2(NCS)4(4-methylpyridine N-oxide)4(methanol)2, in which the CoII cations are linked by pairs of μ-1,1-bridging 4-methylpyridine N-oxide coligands into centrosymmetric dinuclear units.


Chemical context
The synthesis of new coordination compounds and polymers is still an important topic in inorganic chemistry because of their versatile structural behavior and their varied physical properties.One important part of these investigations includes the synthesis of compounds with paramagnetic metal cations to prepare materials with promising magnetic behavior.In several cases, the cations are linked by small-sized anionic ligands and in this regard, compounds based on thiocyanate anions are of interest because this anionic ligand can mediate magnetic exchange (Palion-Gazda et al., 2015;Mekuimemba et al., 2018;Shurdha et al., 2013;Rams et al., 2017Rams et al., , 2020)).Compared to cyanides or azides, this anionic ligand shows many more coordination modes and consequently a more pronounced structural variability, leading to metal thiocyanate substructures that consist of linear and corrugated chains or layered structures of different topology (Wo ¨hlert et al., 2013;Werner et al., 2015;Neumann et al. 2018;Bo ¨hme et al., 2020Bo ¨hme et al., , 2022)).However, most paramagnetic metal cations are not very chalcophilic and therefore, the N-terminal coordination mode frequently dominates over the various bridging modes.
However, in recent work we used pyridine N-oxide derivatives as coligands that can be terminally O-bonded or that can bridge two metal cations in the �-1,1(O,O) bridging mode, leading to an enhanced structural variability.In the beginning, we focused on Co(NCS) 2 compounds because, among other things, this cation is of special interest in terms of its magnetic properties (Murrie, 2010;Mautner et al., 2018a,b;Rams et al., 2017Rams et al., , 2020)).In the course of this project, we became interested in 4-methylpyridine N-oxide as a coligand.With this ligand, two compounds with the composition Co(NCS) 2 (4-methylpyridine N-oxide) (Refcode: MEQKOJ, Zhang et al., 2006a) and Co(NCS) 2 (4-methylpyridine N-oxide)(methanol) (Refcode: REKBUF; Shi et al., 2006a) have already been reported in the literature.In the first compound, the Co II cations octahedrally coordinate two N-and two S-bonding thiocyanate anions and two �-1,1(O,O)-bridging 4-methylpyridine N-oxide coligands, and are connected by pairs of bridging thiocyanate anions into corrugated chains.These chains are further linked into layers by �-1,1(O,O)-bridging 4-methylpyridine N-oxide coligands (Zhang et al., 2006a).In the second compound, the Co II cations sixfold coordinate two bridging and one terminal thiocyanate anion, two O atoms of two bridging 4-methylpyridine N-oxide ligands and one methanol molecule (Refcode: REKBUF; Shi et al., 2006a).The Co cations are linked by alternating pairs of �-1,3-bridging thiocyanate anions and �-1,1(O,O)-bridging 4-methylpyridine N-oxide coligands into chains.
In our own synthetic work, we have added two additional compounds with the composition Co(NCS) 2 (4-methylpyridine N-oxide) 3 and Co(NCS) 2 (4-methylpyridine N-oxide) 4 , that form discrete complexes with two different metal coordinations (Na ¨ther & Jess, 2024).In the latter compound, an octahedral coordination is observed, whereas the former shows a trigonal-bipyramidal coordination, which is relatively rare for Co II cations.Surprisingly, this compound can easily be prepared, whereas only a few crystals of the complex with a sixfold coordination were accidentally obtained.Much effort was made to prepare Co(NCS) 2 (4-methylpyridine N-oxide) 4 but without any success.In the course of these investigations, we always found additional reflections in some of the powder patterns of products prepared in methanol that do not correspond to the discrete complexes or to the coordination polymers mentioned above.Therefore, an additional crystalline phase based on Co(NCS) 2 and 4-methylpyridine N-oxide must exist.Based on these findings the synthesis conditions were varied, leading to the formation of a new crystalline phase that was characterized by single-crystal X-ray diffraction.This proves that a dinuclear complex with methanol was obtained, that is somehow structurally related to Co(NCS) 2 (4methylpyridine N-oxide)(methanol), which has already been reported in the literature (refcode REKBUF; Shi et al., 2006a).

Structural commentary
The asymmetric unit of the title compound, Co 2 (NCS) 4 (4methylpyridine N-oxide) 4 (methanol) 2 , consists of one cobalt cation, two thiocyanate anions, one methanol molecule and two 4-methylpyridine N-oxide coligands, all of them located in general positions.The Co cations sixfold coordinate two terminal N-bonding thiocyanate anions, one methanol molecule and one terminal as well as two �-1,1(O,O)-bridging 4-methylpyridine N-oxide coligands (Fig. 1).Bond lengths and angles are similar to those in related compounds (Shi et al., 2006a) and show that the octahedra are slightly distorted (Table 1).Each two cobalt cations are linked via two �-1,1(O, O)-bridging 4-methylpyridine N-oxide coligands into dinuclear units, with the Co 2 O 2 rings that are the central motif located on centers of inversion (Fig. 1).Similar Co 2 O 2 rings are also observed in the related compound Co(NCS) 2 (4-methylpyridine N-oxide)(methanol), in which the Co cations are additionally linked via alternating pairs of �-1,3-bridging thiocyanate anions and �-1,1(O,O)bridging 4-methylpyridine N-oxide coligands into chains (Shi et al., 2006a).

Supramolecular features
In the crystal structure of the title compound, the dinculear units are arranged in columns along the crystallographic a-axis direction (Fig. 2).Several C-H� � �S, one C-H� � �O and one C-H� � �N contacts are observed between the complexes, but only for some of them are the C-H� � �X angles close to linearity and the H� � �X distances relatively short, indicating a significant interaction (Fig. 2, Table 2).
With Cu(II), one compound with the composition Cu(NCS) 2 -(4-methylpyridine N-oxide) is reported in which the Cu(II) cations are octahedrally coordinated by two N and three S-bonding thiocyanate anions and one terminal O-coordinating 4-methylpyridine N-oxide) coligand (refcode TEB-TAW; Shi et al., 2006d).The Cu(II) cations are connected into linear chains by pairs of bridging thiocyanate anions, that are further linked via Cu 2 S 2 rings into double chains.

Additional investigations
The title compound was also investigated by powder X-ray diffraction.Comparison of the experimental pattern with that calculated from single-crystal data reveals that this compound is of low crystallinity and that only a poor powder pattern can be obtained (Fig. 3).The low signal-to-noise ratio originates from the fact that only relatively large crystals were obtained, that could not be crushed into smaller crystals because in this case the compound started to decompose.However, it is obvious that no pure crystalline phase was obtained.In this context, it is noted that in those cases where different batches were investigated, the powder patterns always showed some differences.However, comparison of the experimental pattern with those calculated for the title compound and for Co(NCS) 2 (4-methylpyridine N-oxide) compounds retrieved from the literature indicate that the title compound is contaminated with a small amount of the discrete complex Co(NCS) 2 (4methylpyridine N-oxide) 3 (Na ¨ther & Jess, 2024).In fact, this is difficult to prove because the powder pattern was measured at room temperature, whereas the patterns calculated for the literature compounds are based in part on structure determinations at lower temperatures.
However, measurements with thermogravimetry and differential thermoanalysis (TG-DTA) show three mass losses, of which the first is accompanied by an endothermic and the second by a strong exothermic signal in the DTA curve (Fig. 4).The first mass loss of 6.4% is a bit lower than that calculated for the removal of the methanol molecules (7.5%), whereas the sum of the second and third mass losses is slightly higher than expected for the removal of all 4-methylpyridine N-oxide coligands (51.2%).However, the strong exothermic signal points to a decomposition of the coligands, as is usually observed for pyridine N-oxide derivatives (Na ¨ther & Jess, 2023, 2024).To characterize the compound formed after the first mass loss, it was isolated in a second TG run and investigated by PXRD.The powder pattern proves that a new crystalline phase of low crystallinity had been obtained that obviously contains a large amount of amorphous content (Figure S1).If the experimental pattern of the residue is compared with that calculated for Co(NCS) 2 (4-methylpyridine N-oxide) reported in the literature (Refcode: MEQKOJ, Zhang et al., 2006a), it is obvious that this compound has formed by methanol removal.

Synthesis:
The title compound was prepared by the reaction of 0.5 mmol (87 mg) of Co(SCN) 2 and 1 mmol (109 mg) of 4-methylpyridine N oxide in 1 mL of methanol.The reaction mixture was stored overnight, leading to the formation of violet-colored crystals that were always contaminated with Co(NCS) 2 (4-methylpyridine N-oxide) 3 (Na ¨ther & Jess, 2024).

Experimental details:
The data collection for single-crystal structure analysis was performed using an XtaLAB Synergy, Dualflex, HyPix diffractometer from Rigaku with Cu K� radiation.The PXRD measurements were either performed with the single-crystal diffractometer mentioned above (Fig. S1) or with a Stoe Transmission Powder Diffraction System STADI P (Fig. 3) equipped with a MYTHEN 1K detector and a Johansson-type Ge(111) monochromator using Cu K� 1 radiation (� = 1.540598A ˚). 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.

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.

Figure 1
Figure 1The molecular structure of the title compound with atom labelling and displacement ellipsoids drawn at the 50% probability level.The disorder of the H atoms of one of the methyl groups is shown with full and open bonds.[Symmetry code: (i) À x + 1, À y, À z + 1.]

Figure 2
Figure 2 Crystal structure of the title compound in a view along the crystallographic a axis.Intermolecular C-H� � �S and O-H� � �O hydrogen bonding is shown as dashed lines

Table 3
Experimental details.
Computer programs: CrysAlis PRO