Syntheses and crystal structures of the five- and sixfold coordinated complexes diisoselenocyanatotris(2-methylpyridine N-oxide)cobalt(II) and diisoselenocyanatotetrakis(2-methylpyridine N-oxide)cobalt(II)

The crystal structures of the title compounds, which arose from the same reaction, consist of discrete complexes in which the cobalt cations are either in a CoN2O3 trigonal–bipyramidal or a CoN2O4 octahedral coordination.

The reaction of CoBr 2 , KNCSe and 2-methylpyridine N-oxide (C 6 H 7 NO) in ethanol leads to the formation of crystals of [Co(NCSe) 2 (C 6 H 7 NO) 3 ] (1) and [Co(NCSe) 2 (C 6 H 7 NO) 4 ] (2) from the same reaction mixture.The asymmetric unit of 1 is built up of one Co II cation, two NCSe À isoselenocyanate anions and three 2-methylpyridine N-oxide coligands, with all atoms located on general positions.The asymmetric unit of 2 consists of two cobalt cations, four isoselenocanate anions and eight 2-methylpyridine N-oxide coligands in general positions, because two crystallographically independent complexes are present.In compound 1, the Co II cations are fivefold coordinated to two terminally Nbonded anionic ligands and three 2-methylpyridine N-oxide coligands within a slightly distorted trigonal-bipyramidal coordination, forming discrete complexes with the O atoms occupying the equatorial sites.In compound 2, each of the two complexes is coordinated to two terminally N-bonded isoselenocyanate anions and four 2-methylpyridine N-oxide coligands within a slightly distorted cis-CoN 2 O 4 octahedral coordination geometry.In the crystal structures of 1 and 2, the complexes are linked by weak C-H� � �Se and C-H� � �O contacts.Powder X-ray diffraction reveals that neither of the two compounds were obtained as a pure crystalline phase.

Chemical context
Numerous crystal structures of transition-metal thiocyanate coordination compounds have been reported in the literature, which are of interest not only because of their versatile structural behavior but also for their magnetic properties.In contrast, much less is known about the corresponding selenocyanate coordination compounds.This might originate from the fact that such compounds are frequently less stable and very often more difficult to prepare, especially if the focus is on the synthesis of compounds with bridging anionic ligands, which are of interest because of their promising magnetic properties.In this context, compounds based on cobalt are of special interest because they can show versatile magnetic behaviors including ferromagnetic ordering or single-chain magnet behavior (Mautner et al., 2018;Rams et al., 2017 and2020).In this regard we have shown that, for example, exchange of seleno by thiocyanate anions leads to an increase of the energy barrier and the intrachain interactions (Neumann et al., 2019).
Concerning the structural behavior of cobalt thio-and selenocyanates, in most cases an octhaedral coordination geometry is observed, independent of the question whether discrete complexes or coordination polymers are considered.
In rarer cases, especially with strong donor ligands, a tetrahedral coordination is found, with many examples for thiocyanates (Mautner et al., 2018;Neumann et al., 2018), whereas for selenocyanate compounds with additional coligands, no example has been reported.There are only two compounds with the composition CoHg(NCSe) 4 (Cambridge Structural Database refcode MURQOH; Li et al., 2006) and [Co(NCSe) 4 ] 2À [(NH 4 ) 2 ] + (QQQBEY; Kergoat et al., 1970) that contain no additional coligands and in which the metal cations are either linked into chains or in which discrete complexes are formed.Finally, with Co(NCS) 2 , compounds with a fivefold coordination are rarer than with a fourfold coordination, and with selenocyanate only three examples are found.This includes two discrete complexes with tridentate ligands [DUCVEF (Hopa et al., 2020) and VONXUT (Solanki & Kumar, 2014)] and one tetranuclear complex (QIRYOI; Das et al., 2018).In this context we have reported the first Co(NCS) 2 compound, which consists of chains, in which an alternating five-and sixfold Co II coordination is observed (Bo ¨hme et al., 2022).
However, in most of our recent investigations we used pyridine derivatives as coligands, but recently we reported some compounds where we used pyridine N-oxide derivatives as coligands (Na ¨ther & Jess, 2023, 2024a,b).In the course of these investigations we obtained two different discrete complexes by the reaction of Co(NCS) 2 and 4-methylpyridine N-oxide (Na ¨ther & Jess, 2024a).In one of these complexes a trigonal-bipyramidal coordination and in the second complex an octahedral coordination is observed, which is surprising because Co(NCS) 2 compounds with a fivefold coordination and pyridine N-oxide coligands were unknown at this time.In further work we used 2-methylpyridine N-oxide as a coligand, which lead to the formation of Co(NCS) 2 (2-methylpyridine Noxide) 3 in which the Co II cations shows a trigonal-pyramidal coordination as was the case with 4-methylpyridine N-oxide as coligand (Na ¨ther & Jess, 2024c).Many experiments were performed but the corresponding octhaedral complex was not obtained.Based on these findings, we decided to try to prepare the corresponding compounds with Co(NCSe) 2 and in this context it is noted that no selenocyanate coordination compounds with pyridine N-oxide derivatives and transitionmetal cations are reported in the literature (see Database survey).Therefore, CoBr 2 , KNCSe and 2-methylpyridine N-oxide were reacted, which lead to the formation of two different crystals that were investigated by single-crystal X-ray diffraction.

Structural commentary
The asymmetric unit of compound 1, Co(NCSe) 2 (C 6 H 7 NO) 3 , consists of one crystallographically independent Co II cation, two independent isoselenocyanate anions and three independent 2-methylpyridine N-oxide coligands that are located in general positions (Fig. 1).The metal cations are fivefold coordinated to two terminally N-bonded isoselenocyanate anions and three 2-methylpyridine N-oxide coligands, forming discrete complexes.The coordination around the metal centers can be described as a slightly distorted trigonal bipyramid with the anionic ligands in the axial and the coligands in the equatorial positions (Fig. 1 and Table 1).The Co-N bond lengths of the two independent isoselenocyanate anions are significantly different (Table 1).As expected, the thiocyanate C-N-Co bond angles are close to linearity, whereas the N-O-Co angles are close to 120 � , because only one of the two lone pairs of the oxygen atom is involved in metal coordination (Table 1).Finally, it is noted that compound 1 is isotypic to the corresponding thiocyanate complex Co(NCS) 2 (C 6 H 7 NO) 3 , recently reported in the literature (Na ¨ther & Jess, 2024c).

Figure 1
The molecular structure of compound 1 with displacement ellipsoids drawn at the 50% probability level.In compound 2, Co(NCS) 2 (C 6 H 7 NO) 4 , two crystallographically independent complexes are present, in which each cobalt cation and each of the two crystallographically independent isoselenocyanate anions and each of the four independent 2-methylpyridine N-oxide coligands are in general positions (Fig. 2).In both of the complexes the metal cations are sixfold coordinated to two terminally N-bonded isocyanate anions and four 2-methylpyridine N-oxide ligands within slightly distorted octahedra (Fig. 2 and Table 2).Bond lengths and angles are comparable in both independent complexes.The Co-N distances of the isoselenocyanate anions and especially of the 2-methylpyridine N-oxide coligands in compound 2 are significantly longer than in compound 1, which can be traced back to the higher coordination number of the metal ion.As in compound 1, the C-N-Co angles are more or less linear, whereas the N-O-Co angles tend to be 120 � (Table 2).

Supramolecular features
In the crystal structure of compound 1 the complexes are linked by a relatively short C-H� � �Se contact of 3.00 A ˚into chains, which are connected into double chains by a slightly longer C-H� � �Se contact of 3.09 A ˚(Fig. 3 and Table 3).Within these chains, weak C-H� � �O contacts are observed (Fig. 3 and Table 3).These double chains propagate along the crystallographic c-axis direction and no directional intermolecular interactions are observed between them (Fig. 4).
In compound 2 the two crystallographically independent complexes are linked by one relatively short C-H� � �Se contact of 2.87 A ˚with an C-H� � �Se angle close to linearity The molecular structure of compound 2 with displacement ellipsoids drawn at the 50% probability level.
The crystal structure of compound 1 with view of a double chain that propagates along the crystallographic a-axis direction: C-H� � �Se and C-H� � �O contacts are shown with dashed lines.
into dimeric units (Fig. 5 and Table 4).There are numerous additional C-H� � �Se and C-H� � �O contacts with angles far from linearity that connect the dimeric units into a threedimensional network (Table 4).Within this network, the complexes the dimeric units are arranged into columns that stack along the crystallographic a-axis direction (Fig. 6).

Database survey
A search in the CSD (version 5.43, last update March 2023; Groom et al., 2016) using CONQUEST (Bruno et al., 2002) reveal that no transition-metal selenocyanate coordination compounds with pyridine N-oxide derivatives are reported.
There is only one compound with 4,4 0 -bipyridine N,N 0 -dioxide , in which the cobalt cations are octahedrally coordinated to two selenocyanate anions, two water molecules and two O atoms of the 4,4 0 -bipyridine N,N 0 -dioxide coligands and are linked into chains by the bridging 4,4 0 -bipyridine N,N 0dioxide ligands (ROLJEI; Jana et al., 2007).
It is also noted that with Co(NCS) 2 and pyridine N-oxide derivatives no structures with a fivefold coordination are reported in the CSD.There is only one recent example, which is Co(NCS) 2 (2-methylpyridine N-oxide) 3 , already mentioned in the Chemical context section (Na ¨ther & Jess, 2024c).

Synthesis and crystallization
CoBr 2 (99%), KNCSe (98.5%) and 2-methylpyridine N-oxide (96%) were purchased from Sigma Aldrich.0.25 mmol (54.7 mg) of CoBr 2 , 0.5 mmol of KNCSe (72.0 mg) and 4 mmol (436.4 mg) of 2-methylpyridine N-oxide in 1 ml of ethanol were reacted for 3 d at room temperature, which led to the formation of crystals of compound 1 (pink needles) and 2 (pink blocks) in the same batch.Reacting the components in the stoichiometric ratios given by the formulae were also tried but pure crystalline phases were never obtained: powder X-ray diffraction shows that they are always contaminated with additional unknown crystalline phases and with KBr, which is also a product of this reaction.

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.Hydrogen-bond geometry (Å, º)

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 4
Figure 4The crystal structure of compound 1 viewed along the crystallographic caxis direction: C-H� � �Se and C-H� � �O contacts are shown with dashed lines.

Figure 5
Figure 5View of the dimeric unit in compound 2 with C-H� � �Se contacts shown with dashed lines.

Figure 6
Figure 6 Crystal structure of compound 2 viewed along the crystallographic a-axis direction.For clarity only the two C-H� � �Se and C-H� � �O contacts between the dimeric units are shown with dashed lines.

Table 5
Experimental details.