Synthesis, crystal structure and reactivity of bis(μ-2-methylpyridine N-oxide-κ2 O:O)bis[dibromido(2-methylpyridine N-oxide-κO)cobalt(II)] butanol monosolvate

The crystal structure of the title compound consists of dinuclear complexes, in which the CoII cations are fivefold coordinated and linked by centrosymmetric pairs of μ-1,1(O,O)-bridging 2-methylpyridine N-oxide coligands.


Chemical context
Transition-metal halide coordination compounds show a large structural variability because the halide anions can act as terminal or bridging ligands (Peng et al., 2010).This can lead to the formation of metal-halide substructures of different dimensionality, like, e.g.mono-and dinuclear units, chains, double chains or layered compounds, that can be further connected by the use of bridging coligands (Peng et al., 2010 andNa ¨ther et al., 2007).In general the dimensionality of the network predominantly depends on the ratio between the transition metal halide and the coligand.Compounds with a large ratio usually show a low dimensionality and form discrete units, whereas the dimensionality of the metal halide substructure increases with decreasing amount of the coligands (Na ¨ther et al., 2001;Na ¨ther and Jess, 2001).Even if in the majority of cases such compounds were prepared in solution, we have found that upon heating, the coligand-rich compounds lose their ligands stepwise, which leads to the formation of compounds with higher dimensionality (Na ¨ther et al., 2001;Na ¨ther & Jess, 2004).In the beginning, this approach was used for the preparation of Cu I compounds (Na ¨ther et al., 2001(Na ¨ther et al., , 2002)), but later it was expanded to compounds with twofold positively charged cations, because even such compounds show a variety of structures of different dimensionality (Na ¨ther et al., 2007).In this context, it is noted that this thermal ligand removal can also be used for the synthesis of compounds with different anions such as, for example, thio-or selenocyanates (Werner et al., 2015;Rams et al., 2020).
In recent work, we exclusively used N-donor coligands that in most cases consist of pyridine derivatives.Therefore, the question arose whether this method could also be expanded to other coligands and in this context we became interested in pyridine N-oxide derivatives, because in contrast to pyridine derivatives they can act as terminal but also as bridging ligands.In this context, it would also be of interest if they show a similar thermal reactivity to that of the pyridine analogs.It is also noted that some transition-metal halide compounds with pyridine N-oxide derivatives have already been reported in the literature.In the course of our systematic work we used 2-methylpyridine N-oxide as ligand, for which some transitionmetal halide compounds have already been reported in the literature.Compounds based on cobalt(II) are not reported, which also might be of interest in terms of magnetic proper-ties.In the first experiments we reacted CoBr 2 with 2-methylpyridine in different solvents and from n-butanol we obtained blue-colored crystals that were identified by singlecrystal structure analysis.

Structural commentary
The asymmetric unit of the title compound, [CoBr 2 ] 2 (2methylpyridine N-oxide) 4 •n-butanol, consists of one Co II cation as well as two bromide anions and two 2-methylpyridine N-oxide coligands in general positions (Fig. 1) and one n-butanol molecule that is located on a center of inversion and is therefore disordered due to symmetry (Fig. 2).This disorder remains constant if the refinement is performed in the space group P1 (see Refinement).The Co II cations are fivefold coordinated by two bromide anions as well as one terminal and two bridging 2-methylpyridine N-oxide coligands.From the bond lengths and angles it is obvious that an irregular Co coordination is present, that is in between that of a trigonal bipyramid and a tetragonal pyramid (Table 1).Each of the two Co II cations is linked by two �-1,1(O,O) 2-methylpyridine N-oxide coligands into dinuclear units that are located on centers of inversion (Fig. 1).The distance between the two Co II cations within the four-membered Co 2 O 2 rings amounts to 3.4196 (7) A ˚and the rings are planar.
In this context, it is noted that a compound with the composition [CuCl 2 ] 2 (4-methylpyridine N-oxide) 4 is reported, which shows a structure that is analogous to that of the title Crystal structure of the dinuclear unit in the title compound with labeling and displacement ellipsoids drawn at the 50% probability level.Symmetry code: (i) À x + 1, À y + 1, À z + 1.

Figure 2
Crystal structure of the disordered butanol molecule in the title compound with labeling and displacement ellipsoids drawn at the 50% probability level.Symmetry code: (ii) À x + 1, À y, À z.The disorder is shown with full and open bonds.

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 them show angles far from linearity, indicating that these correspond to very weak interactions (Table 2).However, a few of them show distances and angles that point to intermolecular hydrogen bonding and if they are considered as significant interactions, the discrete complexes are connected into chains that propagate along the crystallographic a-axis direction (Fig. 3 and Table 2).The n-butanol molecules are located between these chains and are linked via O-H� � �Br hydrogen bonding to the chains.Because they are disordered around a center of inversion, in the middle of Fig. 3 it appears that they interconnect to neighboring chains, but in fact they are always arbitrarily connected to only one of these chains (Fig. 3).

Thermoanalytical and powder X-ray powder investigations
Comparison of the experimental powder pattern of the title compound with that calculated from single-crystal data using structural data obtained at room temperature proves that a pure crystalline phase has been obtained (Fig. 4).
To investigate the thermal properties of the title compound including solvent removal, measurements using simultaneous differential thermoanalysis and thermogravimetry (DTA-TG) were performed.Upon heating, two mass losses are observed that are accompanied by endothermic events in the DTA curve (Fig. S1).From the DTG curve, it is obvious that the first mass loss is well resolved, which is not the case for the second mass loss.Moreover, the sample mass decreases continuously upon further heating, with no distinct step that points to the formation of a further compound (Fig. S1).The experimental mass loss of 8.9% in the first mass loss is in rough agreement with that calculated for the removal of the butanol molecules (�m calc = À 7.8%), indicating the formation of a new compound with the composition CoBr 2 (2-methylpyridine   N-oxide) 2 .It is noted that after the formation of the new intermediate compound there is an endothermic event where the sample mass does not change, indicating that the overall reaction is more complex.PXRD investigations of the residue obtained after the first mass loss prove that a highly crystalline and completely different phase has been obtained (please compare Fig. 1 and  S2) and IR investigations reveal significant differences, indicating that the Co coordination has changed (Figs.S3 and S4).
Finally, from the TG curve it is obvious that the first mass loss starts at very low temperature, indicating that the compound had already decomposed at room temperature (Fig. S1).Therefore, a freshly prepared batch of the title compound was stored for 60 h at room temperature and afterwards was investigated by PXRD, which proved that a transformation into the new crystalline phase obtained by solvent removal at elevated temperatures is obtained (Fig. S5).

Database survey
No crystal structures of cobalt halide compounds with methylpyridine N-oxide are reported in the CSD (version 5.43, last update March 2023; Groom et al., 2016) but some compounds with other transition-metal cations are known.
One compound with the composition MnCl 2 (2-methylpyridine N-oxide)(H 2 O) is also reported (refcode VEJMAB; Kang et al., 2017).In this compound, the Mn II cations are octahedrally coordinated by one terminal chloride anion, one terminal water molecule as well as two bridging chloride anions and two bridging 2-methylpyridine N-oxide coligands.The cations are linked by pairs of alternating �-1,1(O,O)bridging 2-methylpyridine N-oxide coligands and each of the two �(1,1) chloride anions into linear chains.
With 3-methylpyridine N-oxide and 4-methylpyridine N-oxide, no cobalt halide compounds are known but one compound with an essentially identical structure is reported with CuCl 2 and 4-methylpyridine N-oxide; this is mentioned in the Structural commentary (refcode CMPYUC;Johnson & Watson, 1971a).
An IR spectrum of the title compound can be found in Fig. S4.
Finally, it is noted that because of the disorder of the nbutanol molecule we also tried to prepare a compound with 1,4-butanediol instead of butanol, which should occupy the same position as that of the n-butanol molecule, but microcrystalline powders were always obtained that showed a powder pattern identical to that of the residues obtained by solvent removal from the title compound.

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.
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.
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.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 3.The C-H hydrogen 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 hydrogen atoms) using a riding model.
As already mentioned, the n-butanol molecule is disordered around a center of inversion, which is located exactly in the middle of the central C-C bond.Therefore, the generation of the symmetry-equivalent terminal atom formally lead to a molecule with a six-membered chain.However, the assignment of oxygen to the terminal atom lead to a much too high anisotropic displacement parameter, which decreased to a reasonable value if the site occupation is reduced to 0.5.After anisotropic refinement, only one electron-density peak is observed close to the O atom, which can clearly be assigned to the missing O-H hydrogen atom.For the C-O bond lengths, a restraint was used because otherwise a too long bond length was obtained.This presumably can be traced back to some disordering, because of the superposition of n-butanol molecules that are connect to different chains, which is also reflected in slightly enhanced components of the anisotropic displacement parameters of the C atoms of these molecules.
Finally it is noted that the disorder remains constant if the refinement is performed in the space group P1 and that no super structure reflections are visible that might point to a larger unit cell.(Sheldrick, 2015b), DIAMOND (Brandenburg & Putz, 1999) and publCIF (Westrip, 2010).

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.
Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å Figure 1

Figure 3
Figure 3Crystal structure of compound 1 viewed along the crystallographic a-axis.Intermolecular C-H� � �Br and C-H� � �O hydrogen bonding is shown as dashed lines.Please note that the n-butanol molecule is disordered around centers of inversion.For the n-butanol molecules between the chains the disorder is not removed, whereas for the n-butanol molecules left and right from the chains each one O atom is arbitrarily removed.

Figure 4
Figure 4Experimental (top) and calculated powder pattern (bottom) for the title compound.

Table 3
Experimental details.