Synthesis, crystal structure and thermal properties of dibromidobis(2-methylpyridine N-oxide-κO)cobalt(II)

The crystal structure of the title compound consists of discrete tetrahedral complexes that are linked by weak intermolecular hydrogen bonding. Upon heating, melting before decomposition is observed.


Chemical context
Numerous transition-metal halide coordination compounds have been reported in the literature (Peng et al., 2010).Most of these compounds are characterized by metal halide substructures such as, for example, mono-and dinuclear units, chains or layers, that can be further linked by bridging ligands into 1-, 2-and 3-D networks (Peng et al., 2010;Na ¨ther et al., 2007).We are especially interested in the thermal properties of such compounds because we have found that compounds with a high ratio between the metal halide and the ligands lose their ligands stepwise upon heating and transform into new compounds that usually show condensed metal-halide substructures (Na ¨ther et al., 2001(Na ¨ther et al., , 2002;;Na ¨ther & Jess, 2004).
In this context, we have recently reported a new dinuclear complex with the composition [(CoBr 2 ) 2 (2-methylpyridine N-oxide) 4 ]•n-butanol in which the Co II cations are fivefold coordinated by two bromide anions and one terminal as well as two bridging 2-methylpyridine N-oxide ligands and linked into dinuclear units by two symmetry-related �-1,1(O,O) 2-methylpyridine N-oxide coligands (Na ¨ther & Jess, 2023).The n-butanol solvate molecules can be removed by thermogravimetry, leading to the formation of a crystalline compound with the composition [CoBr 2 (2-methylpyridine N-oxide) 2 ], for which the powder pattern is completely different from that of the pristine compound.We also found that the butanol molecules have already been lost upon storage at roomtemperature, leading to the same crystalline phase as that obtained by thermal ligand removal.Moreover, the new crystalline phase shows two endothermic events before decomposition, which points to an interesting thermal behavior.Unfortunately, we were not able to solve its structure from PXRD data.Therefore, in the present work we performed a large number of crystallization experiments.The crystals obtained were characterized by single-crystal X-ray diffraction.The analysis proves that a new compound with the composition [CoBr 2 (2-methylpyridine N-oxide) 2 ] was obtained, consisting of discrete complexes for which the calculated powder pattern is identical to that of the phase obtained by butanol removal from the dinuclear complex mentioned above.Larger amounts of a crystalline powder are easily available and comparison of the experimental powder pattern with that calculated from single crystal data proves that the title compound was obtained as a pure phase (Fig. 1), which allowed a detailed investigation of the thermal properties of the title compound to be undertaken.

Structural commentary
The asymmetric unit of the title compound, [CoBr 2 (2methylpyridine N-oxide) 2 ] (1), consists of one Co II cation, two bromide anions and two 2-methylpyridine N-oxide coligands that are located in general positions (Fig. 2).Compound 1 forms discrete complexes in which the Co II cations are fourfold coordinated by two bromide anions and two neutral 2-methylpyridine N-oxide coligands (Fig. 2).Bond lengths and angles correspond to literature values and show that the tetrahedra are slightly distorted (Table 1).It is noted that the two known discrete tetrahedral complexes [CuCl 2 (2-methylpyridine N-oxide) 2 ] and [ZnCl 2 (2-methylpyridine N-oxide) 2 ] (refcodes QQQBVY and QQQBXY; Kidd et al., 1967) are not isotypic to the title compound.For the latter compound, this is surprising because there are many examples in the literature where tetrahedral Co II and Zn II complexes are isotypic.On the other hand, there are very few examples reported in the literature where the thermodynamic relations between such complexes were fully investigated.It has been found, for example, that for two isotypic complexes the Co complex is thermodynamically stable at room temperature, whereas the isotypic Zn complex is metastable (Neumann et al., 2018(Neumann et al., , 2019)).

Supramolecular features
In the crystal structure of compound 1, a number of intermolecular C-H� � �O and C-H� � �Br contacts are observed, but most of the contacts show angles far from linearity, indicating that these correspond to very weak interactions (Table 2).However, a few of them exhibit distances and angles research communications Acta Cryst. (2024). E80, 152-156   Na ¨ther and Jess � [CoBr 2 (C 6 H 7 NO) 2 ] 153   Figure 1 Experimental (top) and calculated (bottom) powder patterns for the title compound.

Figure 2
Crystal structure of the title compound with labeling and displacement ellipsoids drawn at the 50% probability level.
that point to intermolecular hydrogen bonding and, if they are considered as significant interactions, the discrete complexes are connected into chains propagating along the a-axis direction (Fig. 3 and Table 2).

Thermoanalytical investigations
As mentioned above, recent investigations of the dinuclear complex tetrabromo-tetrakis(2-methylpyridine N-oxide)dicobalt(II) butanol solvate with thermogravimetry and differential thermoanalysis (TG-DTA) showed an endothermic signal after butanol removal where the sample mass did not change (Na ¨ther & Jess, 2023).Because it is the title complex that formed after solvent removal, its thermal properties were investigated in more detail using TG-DTA and DSC measurements (differential scanning calorimetry) as well as thermomicroscopy.
Upon heating, one poorly resolved mass loss is observed in the TG curve, which is accompanied by a strong exothermic event in the DTA curve at 278 � C. The latter signal points to a decomposition of the 2-methylpyridine N-oxide ligand (Fig. S1).More importantly, before the first mass loss, two endothermic events at 109 and 155 � C are observed in the DTA curve, which show that the overall thermal behavior is more complex.Therefore, DSC heating and cooling curves were measured, where two endothermic signals were observed upon heating (Fig. S2).Upon cooling, no exothermic signal was observed, which proves that the second endothermic event is irreversible.In contrast, if the title compound is measured up to 120 � C and cooled down, an exothermic event is visible, which shows that this process is in principle reversible (Fig. 4).The same observations were made in the second heating and cooling run.However, the enthalpy of these events continuously decreases, which means that this event is not entirely reversible.
The residues obtained at 120 and 180 � C in the DSC measurements were investigated by powder X-ray diffraction (PXRD), which showed that the residue formed after the first endothermic event corresponds to the title complex (Fig. S3).
No PXRD pattern could be measured for the residue formed after the second endothermic event because it adhered to the bottom of the crucible, indicative of melting.To investigate this in more detail, thermomicroscopic measurements were performed, which show melting at about 164 � C (Fig. S5).This is in agreement with other tetrahedral Co but also Zn complexes, which melt upon heating (Neumann et al., 2018(Neumann et al., , 2019)).
Finally, to investigate the origin of the first reversible endothermic event at 109 � C, single-crystal measurements were performed between 23 and 167 � C. Surprisingly, there are no structural changes and all data sets could be refined perfectly in space group P2 1 2 1 2 1 .The crystal decomposes upon further heating.The reason for this thermal event is therefore still unknown.

Database survey
A search of the CSD (version 5.43, last update March 2023; Groom et al., 2016) using CONQUEST (Bruno et al., 2002) reveals that no crystal structures of cobalt halide compounds with 2-methylpyridine N-oxide have been reported.As mentioned above, one compound with the composition [(CoBr 2 ) 2 (2-methylpyridine N-oxide) 4 ]•n-butanol was published recently (Na ¨ther & Jess, 2023) but does not yet appear as a hit.
Synthesis: 109 mg CoBr 2 (0.5mmol) and 218 mg 2-picoline N-oxide (2 mmol) were stirred for 1 d in n-butanol at room temperature.The precipitate was filtered off and dried in air.Single crystals were obtained using the same conditions but without stirring.N.B.When stoichiometric amounts were used, in some batches a very small amount of the known compound [CoBr 2 ] 2 (2-methylpyridine N-oxide) 4 •n-butanol was found (Na ¨ther & Jess, 2023).The IR spectrum of the title compound is shown in Fig. S5.

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 ˚). 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.Differential scanning calorimetry measurements were performed with a DSC from Mettler Toledo in Al pans under nitrogen atmosphere with 10 � C min À 1 .The IR spectra were measured using an ATI Mattson Genesis Series FTIR Spectrometer, control software: WINFIRST, from ATI Mattson.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 3. C-bound hydrogen atoms were positioned with idealized geometry (methyl H atoms allowed to rotate but not to tip) and were refined isotropically with U ı ˜so (H) = 1.2 U eq (C) (1.5 for methyl hydrogen atoms) using a riding model.One reflection (outlier) was removed using the OMIT command.

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 3
Figure 3 Crystal structure of the title compound viewed along the a-axis.Intermolecular C-H� � �O and C-H� � �Br hydrogen bonds are shown as dashed lines.

Table 3
Experimental details.