Crystal structure of diethanolbis(thiocyanato)bis(urotropine)cobalt(II) and tetraethanolbis(thiocyanato)cobalt(II)–urotropine (1/2)

The crystal structure of both compounds consists of discrete complexes with terminal N-bonded thiocyanate anions in which the Co cations are sixfold coordinated within slightly distorted octahedra, which are linked by intermolecular hydrogen bonding, in one compound via additional urotropine solvate molecules, into three-dimensional networks.


Chemical context
Thiocyanate anions are versatile ligands that exhibit a variety of coordination modes, leading to rich structural chemistry (Nä ther et al., 2013). For less chalcophilic metal cations such as Mn II , Fe II , Co II or Ni II , most compounds contain terminal Nbonded thiocyanate anions, whereas for chalcophilic metal cations such as for example Cd II , the -1,3-bridging mode is preferred. Therefore, the synthesis of bridging compounds with the former cations is sometimes difficult to achieve, which is a pity, because such compounds are of interest due to their magnetic properties (Mautner et al., 2018;Mekuimemba et al., 2018;Mousavi et al., 2020;Palion-Gazda et al., 2015;Suckert et al., 2016). This is especially the case with cobalt, which frequently exhibits interesting behavior due to its large magnetic anisotropy, so we and others have been studying such compounds for several years (Shi et al., 2006;Jin et al., 2007;Wellm et al., 2020;Prananto et al., 2017). Within this project we are interested for example in the influence of the co-ligand on the magnetic anisotropy and the magnetic behavior of compounds, in which the cations are linked by thiocyanate anions into chains Rams et al., 2020;Werner et al., 2014Werner et al., , 2015.
In the course of our systematic work, we became interested in urotropine as a co-ligand. Therefore, we reacted Co(NCS) 2 with urotropine in acetonitrile, which leads to the formation of a compound with the composition [Co(NCS) 2 (H 2 O) 2 (urotropine) 2 ]Á(urotropine) 2 (MeCN) 2 consisting of discrete ISSN 2056-9890 complexes, which are linked by urotropine and acetonitrile solvate molecules into a hydrogen-bonded network (Krebs et al., 2021). In principle, the formation of discrete solvato complexes would be no problem because in several cases such complexes can be transformed by thermal decomposition into the desired compounds with a bridging coordination of the anionic ligands (Nä ther et al., 2013), but XRPD measurements proved that this crystalline phase was not obtained pure.
In further work, we used ethanol as a solvent leading to the formation of two different crystals in the same batch that were characterized by single-crystal X-ray diffraction. The crystals in this batch were crushed and investigated by XRPD. Comparison of the experimental pattern with that calculated for 1 and 2 reveal that only 1 can be detected together with at least one additional and unknown crystalline phase. The reason for this observation is unclear, but it might be that 2 is unstable and transforms into a new phase on grinding.

Structural commentary
The asymmetric unit of 1, Co(NCS) 2 (urotropine) 2 (EtOH) 2 , consists of one crystallographically independent Co cation, located on a center of inversion, as well as one thiocyanate anion, one urotropine ligand and one ethanol molecule occupying general positions (Fig. 1). In 2, [Co(NCS) 2 )(EtOH) 4 ]Á(urotropine) 2 , the asymmetric unit contains one cobalt cation on position of site symmetry 222 (Wyckoff position c), one thiocyanate anion that is located on a twofold rotation axis and one urotropine molecule on an inversion axis (Fig. 2). The Co-N distance to the thiocyanate anions in 1 is slightly shorter than in 2, whereas the Co-O bond length to the ethanol ligand is longer (compare Tables 1  and 2). The former can be traced back to the fact that in 2 the Co cation is exclusively coordinated by ethanol, whereas in 1 this cation is additionally coordinated by a urotropine ligand, which is a stronger donor than ethanol, transferring additional charge to the Co center. This leads to a strengthening of the Co-N thiocyanate bond and therefore this bond length is shorter. This is also supported by previous investigations when discrete complexes with an N 6 (four N atoms of N-donor co-ligands) or N 4 O 2 (two N-donor co-ligands and two e.g. water molecules) coordination were compared. For N 4 O 2 coordination, the CN stretching vibration of the thiocyanate anions is significantly shifted to higher values, which indicates that the C-N bond becomes stronger, leading to a weakening of the Crystal structure of compound 1 with labeling and displacement ellipsoids drawn at the 50% probability level. Symmetry codes for the generation of equivalent atoms: (i) Àx + 1, Ày + 1, Àz + 1.
Co-N bond . The angles around the Co cations deviate from the ideal octahedral values, which shows that the octahedra are slightly distorted (see supporting information). The octahedron in 2 is more distorted than in 1, which is obvious from the octahedral angle variance (1.8138 for 1 and 8.1624 for 2) and the mean octahedral quadratic elongation (1.0062 for 1 and 1.0023 for 2) calculated by the method of Robinson et al. (1971).

Figure 3
Crystal structure of compound 1 with a view of a chain formed by intermolecular O-HÁ Á ÁN hydrogen bonding along the crystallographic a-axis. Intermolecular hydrogen bonding is shown as dashed lines.

Figure 4
Crystal structure of compound 1 with a view along the crystallographic aaxis with intermolecular C-HÁ Á ÁS hydrogen bonding shown as dashed lines.

Figure 5
In the crystal structure of 2, each complex is linked to neighboring complexes via intermolecular O-HÁ Á ÁN hydrogen bonds between the four O-H hydrogen atoms of one complex and the N atoms of the urotropine molecules of four neighboring complexes to form a three-dimensional network ( Fig. 5 and Table 4). From the HÁ Á ÁN distance and the O-HÁ Á ÁN angle it is obvious that this corresponds to a strong interaction. In contrast to 1, no C-HÁ Á ÁS hydrogen bonding is observed and the additional C-HÁ Á ÁN contact represents a weak interaction (Table 4).

Database survey
The Cambridge structure Database (CSD version 5.42, last update November 2020; Groom et al., 2016) et al., 1999, MOTNIS02;Chakraborty et al., 2006, MOTNIS03;Lu et al., 2010), that also form discrete complexes with terminal N-bonded thiocyanate anions. The structure of these compounds is somehow related to that in 1 and 2 with the major difference being that the ethanol is replaced by water. Discrete complexes have also been reported with other transition-metal thiocyanates including, for example, nickel (Refcode: XILROA; Bai et al., 2007, XILROA01;Lu et al., 2010) and zinc (Refcode: SIMXIY; Kruszynski & Swiatkowski, 2018), but none of them contains ethanol as a co-ligand. The latter structure with the composition [Zn(NCS) 2 (urotropine) 2 -(H 2 O) 2 ]Á[Zn(NCS) 2 (H 2 O) 4 ]Á2H 2 O contains two different complexes, one of them similar to 1 and the second similar to 2 with the difference that the EtOH is exchanged by water.
Finally, it is noted that with cadmium and mercury a crystal structure with urotropine is reported in which the Cd cations are linked by pairs of thiocyanate anions into chains, which are further linked by the urotropine ligand (Refcode: DOZZOI; Bai et al., 2009 and DIJSIY;Mak & Wu, 1985). The formation of such a compound can be traced back to the fact that cadmium and mercury are much more chalcophilic than cobalt. There is one additional structure with cadmium similar to that mentioned above. In this structure, the cadmium cations are linked by pairs of thiocyanate anions into chains that are either connected by two EtOH molecules or urotro-

Synthesis and crystallization
Synthesis Co(NCS) 2 and urotropine were purchased from Merck. All chemicals were used without further purification. Single crystals of 1 and 2 were obtained by reacting 0.15 mmol of Co(NCS) 2 (26.3 mg) with 0.6 mmol of urotropine (84.1 mg) in 1 mL of ethanol after one day.

Experimental details
The data collection for single crystal structure analysis was performed using a Rigaku XtaLAB Synergy Dualflex kappadiffractometer equipped with HyPix hybrid photon counting HPC detector, using Cu-K radiation from a PhotonJet microfocus X-ray source.
The PXRD measurements were performed with Cu K 1 radiation ( = 1.540598 Å ) using a Stoe Transmission Powder Diffraction System (STADI P) equipped with a MYTHEN 1K detector and a Johansson-type Ge(111) monochromator.

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 positioned with idealized geometry (methyl H atoms allowed to rotate but not to tip) and were refined isotropically with U ĩso (H) = 1.2U eq (C) (1.5 for methyl H atoms). 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. The crystal of 1 was twinned by non-merohedry and therefore, a twin refinement using data in HKLF-5 format was performed where all equivalents were merged [BASF parameter = 0.309 (1)

Bis(ethanol-κO)bis(thiocyanato-κN)bis(urotropine-κN)cobalt(II) (1)
Crystal data 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. Refinement. Refined as a 2-component twin.

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.