Crystal structures of two Co(NCS)2 urotropine coordination compounds with different Co coordinations

In the crystal structure of the title compounds, discrete complexes with different Co coordinations are observed, which are linked by intermolecular hydrogen bonding into three-dimensional networks.


Chemical context
Recently, we reported the crystal structure of two new coordination compounds with the composition [Co(NCS) 2 (urotropine) 2 (ethanol) 2 ] and [Co(NCS) 2 (ethanol) 4 ](urotropine) 2 (Krebs et al., 2022). Both compounds consist of discrete complexes, in which the cobalt cations are octahedrally coordinated by two terminal N-bonded thiocyanate anions and by four ethanol and two ethanol and two urotropine ligands, respectively. These investigations were performed to prepare precursors that on thermal decomposition transform into coordination polymers in which the cobalt cations are linked by -1,3 bridging thiocyanate anions into chains or layers (Nä ther et al., 2013). Several such compounds have been ISSN 2056-9890 reported in the literature and they are of interest because they show ferromagnetic or antiferromagnetic ordering or a slow relaxation of the magnetization, which is indicative for singlechain magnetism Shi et al., 2006;Jin et al., 2007;Jochim et al., 2020;Prananto et al., 2017;Mautner et al., 2018;Rams et al., 2020;Ceglarska et al., 2021;Werner et al., 2014Werner et al., , 2015Suckert et al., 2016;Wellm et al., 2020). In this context, urotropine as a coligand was of interest because this ligand is able to form networks (Czubacka et al., 2012;Li et al., 2012), is magnetically silent and one compound with cadmium had already been reported in which the metal cations are linked by the anionic ligands into chains (Bai et al., 2009).
However, for the preparation of the two compounds mentioned above, cobalt thiocyanate was reacted with urotropine in ethanol and X-ray powder measurements show that none of these compounds can be prepared as a pure crystalline phase. Either the desired compounds were obtained as the minor phase or the experimental powder patterns were completely different from the calculated one. These investigations indicate that additional compounds are present and that the desired compounds are not very stable and transform in solution. Therefore, additional crystallization experiments were performed, which lead to the formation of single crystals of two new compounds that were identified by single crystal X-ray diffraction. Even these compounds contain ethanol as a ligand but in one compound one coordination site is simultaneously occupied by ethanol and water, which might originate from some residual water in the solvent used in the synthesis, whereas in the second compound the cobalt cations are coordinated by only one urotropine and three ethanol ligands. All this indicates that, for this system, different species are in equilibrium in solution and some phase crystallizes, presumably by kinetic control, which means that the synthesis is difficult to control.

Structural commentary
The asymmetric unit of compound 1 consists of two crystallographically independent Co cations that are located on centres of inversion as well as two thiocyanate anions, four urotropine ligands, three ethanol and one water molecule that occupy general positions (Fig. 1). One of the cobalt cations (Co1) is sixfold coordinated to two terminal N-bonded thiocyanate anions, two urotropine ligands and two ethanol molecules into discrete complexes (Fig. 1, top left). The methyl carbon atom of these ethanol molecules is disordered in two positions and was refined using a split model. The second cobalt cation is also sixfold coordinated, forming discrete complexes, to two terminal N-bonded thiocyanate anions, two urotropine ligands and two oxygen atoms, but the latter positions are mixed occupied by water and ethanol in a ratio of 8:2, leading to an overall composition for 1 of [Co(NCS) 2 (urotropine) 2 (ethanol) 1.2 (H 2 O) 0.8 Á1.6ethanolÁ-4urotropine. In the case where it is occupied by water, an ethanol molecule is hydrogen bonded to this water molecule; if it is occupied by ethanol, this ethanol solvate molecule is not present (Fig. 1, top right). The position of the disordered O atoms of the water and ethanol molecule was resolved and all O-H H atoms were clearly located in the difference map and refined isotropically with reasonable displacement parameters, using restraints for the O-H distances (see Refinement). The Co-N bond lengths to the thiocanate anions are similar in both complexes, which is also valid for the bond length to the urotropine ligands (Table 1). In contrast, the Co-O bond length to the water molecule is shorter than those to the ethanol molecules (Table 1), even if there might be some uncertainty in the distances because of the disorder.
The asymmetric unit of compound 2 consists of one crystallographically independent cobalt cation, one urotropine ligand and three ethanol molecules, all of them located in general positions (Fig. 2). In this compound the cobalt cations are sixfold coordinated to two terminal N-bonded thiocyanate anions, one urotropine ligand and three ethanol molecules. The Co-N and Co-O bond lengths are comparable to those in compound 1 and to similar ethanol complexes retrieved from the literature (Krebs et al., 2021a, Table 2). From the angles around the Co cations, it is obvious that in all compounds the octahedra are slightly distorted (see supporting information). It is noted that compound 2 completes the series of Co(NCS) 2 -urotropine compounds with ethanol as an additional ligand, because in this compound the cobalt cations are coordinated to one urotropine and three ethanol ligands, whereas in the other compounds reported recently the cobalt cations are either coordinated to two urotropine and two ethanol ligands or to four ethanol ligands (Krebs et al., 2021a).

Supramolecular features
In the crystal structure of the title compound, extensive hydrogen bonding is observed (Table 3). The discrete complex around Co1 is linked to two urotropine solvate molecules via intermolecular O-HÁ Á ÁN hydrogen bonding ( Fig. 3 and Table 3). For the Co2 complex, two different surroundings are observed. In the case where this cation is coordinated to water, this water molecule is hydrogen bonded to two urotropine ligands and two ethanol molecules (Fig. 4,top and  Crystal structure of compound 2 with labelling and displacement ellipsoids drawn at the 50% probability level.

Figure 7
Crystal structure of compound 2 with a view along the crystallographic a axis and intermolecular O-HÁ Á ÁN hydrogen bonding shown as dashed lines.

Figure 6
Crystal structure of compound 1 with a view along the crystallographic b axis and intermolecular hydrogen bonding shown as dashed lines.
In the crystal structure of compound 2, the discrete complexes are linked by strong intermolecular O-HÁ Á ÁN hydrogen bonding between two of the three O-H hydrogen atoms of the ethanol ligands and two urotropine N atoms into layers that are parallel to the bc plane ( Fig. 7 and Table 4). These layers are further linked by intermolecular O-HÁ Á ÁS and C-HÁ Á ÁS hydrogen bonding into a three-dimensional network ( Table 4). Some of the O-HÁ Á ÁS and C-HÁ Á ÁS angles are close to linearity, indicating that these are relatively strong interactions (Table 4).

Database survey
In the Cambridge Structure Database (CSD version 5.42, last update November 2020; Groom et al., 2016) there are already several structures reported that contain cobalt thiocyanate and urotropine as a ligand, but only one of them contains additional ethanol (Krebs et al., 2021a). Most of them contain water as a ligand or solvate molecule. In of two crystallographically independent discrete complexes in which the cobalt cations are coordinated by two terminal Nbonded thiocyanate anions and four water or two water and two urotropine ligands with additional water as solvate molecules. There is also one complex with water and methanol as ligands with the composition [Co(NCS) 2 (urotropine)-(CH 3 OH) 2 (H 2 O)] (Refcode: POFGAT; Shang et al., 2008), in which the cobalt cations are octahedrally coordinated by the N atoms of two thiocyanate anions, two methanol, one water and one urotropine ligand. Moreover, a compound with the composition [Co(NCS) 2 (urotropine) 2 (CH 3 CN) 2 ] that also consists of discrete complexes has been reported (Krebs et al., 2021). It is noted that even with other metal cations only discrete complexes are reported, such as, for example, with nickel

Synthesis and crystallization
Synthesis Co(NCS) 2 and urotropine were purchased from Merck. All chemicals were used without further purification.  Crystals of compound 1 suitable for single-crystal X-ray diffraction were obtained after one day by the reaction of 0.15 mmol of Co(NCS) 2 (26.3 mg) with 0.60 mmol of urotropine (84.1 mg) in 1.0 mL of ethanol at room temperature. The reaction of 0.15 mmol of Co(NCS) 2 (26.3 mg) with 0.15 mmol of urotropine (21.0 mg) in 2.0 mL of ethanol at room temperature led to the formation of single crystals of compound 2.
The data collection for single-crystal structure analysis was performed using an XtaLAB Synergy, Dualflex, HyPix diffractometer from Rigaku with Cu K radiation.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 5. All non-hydrogen atoms were refined anisotropically. The C-H hydrogen atoms were located in the difference map but 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 H atoms) using a riding model. The O-H hydrogen atoms were located in the difference map and were refined with restraints for the O-H distance (DFIX) and varying isotropic displacement parameters in compound 1, whereas in compound 2 they were positioned with idealized geometry allowed to rotate but not to tip and were refined isotropically with U iso (H) = 1.5U eq (O) using a riding model. In compound 1, the methyl group of the EtOH molecule coordinated to Co1 is disordered and was refined using a split model. In this compound, Co2 is either coordinated to water or to EtOH. In this case the O atoms occupy nearly the same crystallographic positions but finally both O atoms can be refined separately with anisotropic displacement parameters. In the case where Co2 is coordinated to water, it is hydrogen bonded to one EtOH solvate molecule. If Co2 is coordinated to EtOH, the position of the EtOH solvate molecule cannot be occupied. Therefore, the site occupation factor (sof) of the EtOH solvate molecule must be identical to that of the coordinated water molecule. In the beginning the sof was refined, leading to values close to 0.8 for the water and 0.2 for the coordinated EtOH molecule but in the final refinements it was fixed at 0.8 and 0.2. The H-atom positions of both, water and EtOH, were clearly located and were refined with restraints and varying isotropic displacement parameters. This leads to comparable and reasonable values for the O-H distances as well as for the isotropic displacement parameters of the O-H hydrogen atoms.  (Rigaku OD, 2021); cell refinement: CrysAlis PRO (Rigaku OD, 2021); data reduction: CrysAlis PRO (Rigaku OD, 2021). Program(s) used to solve structure: SHELXT2014/5 (Sheldrick, 2015a) for (1); SHELXT2014/4 (Sheldrick, 2015a) for (2). For both structures, program(s) used to refine structure: SHELXL2016/6 (Sheldrick, 2015b). Molecular graphics: DIAMOND (Brandenburg & Putz, 1999) for (1); OLEX2 (Dolomanov et al., 2009) for (2). Software used to prepare material for publication: publCIF (Westrip, 2010) for (1); OLEX2 (Dolomanov et al., 2009) for (2).  (10) 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.