Syntheses and crystal structures of the ethanol, acetonitrile and diethyl ether Werner clathrates bis(isothiocyanato-κN)tetrakis(3-methylpyridine-κN)nickel(II)

The crystal structures of the title compounds consist of discrete octahedral complexes that are arranged in such a way that cavities are formed in which the solvate molecules are located.

The reaction of nickel(II)thiocyanate with 3-methylpyridine (3-picoline; C 6 H 7 N) in different solvents leads to the formation of crystals of bis(isothiocyanato-N)tetrakis(3-methylpyridine-N)nickel(II) as the ethanol disolvate, [Ni(NCS) 2 -(C 6 H 7 N) 4 ]Á2C 2 H 5 OH (1), the acetonitrile disolvate, [Ni(NCS) 2 (C 6 H 7 N) 4 ]Á-2CH 3 CN (2), and the diethyl ether monosolvate, [Ni(NCS) 2 (C 6 H 7 N) 4 ]ÁC 4 H 10 O (3). The crystal structures of these compounds consist of Ni II cations coordinated by two N-bonded thiocyanate anions and four 3-methylpyridine ligands to generate NiN 6 octahedra with the thiocyanate groups in a trans orientation. In compounds 1 and 2 these complexes are located on centers of inversion, whereas in compound 3, they occupy general positions. In the crystal structures, the complexes are packed in such a way that cavities are formed in which the solvent molecules are located. Compounds 1 and 2 are isotypic, which is not the case for compound 3. In compounds 1 and 2 the solvate molecules are disordered, whereas they are fully ordered in compound 3. Disorder is also observed for one of the 3-methylpyridine ligands in compound 2. Powder X-ray diffraction and IR measurements show that at room temperature all compounds decompose almost immediately into the same phase, as a result of the loss of the solvent molecules.

Chemical context
The synthesis and structural characterization of new compounds is still an important topic in coordination chemistry, because some of them might have the potential for future applications such as magnetic behavior. In this context, coordination compounds in which the cations are linked by small-sized anionic ligands into networks of different dimensionality are of special interest. Therefore, many compounds based on, for example, cyanide or azide ligands have been reported in the literature. Magnetic exchange can also be mediated by thiocyanate anions and this is one reason why we and others have been interested in this class of compounds for many years (Mautner et al., 2018. Regarding this, compounds are of interest in which the paramagnetic metal cations are linked by thiocyanate anions into chains or layers (Werner et al., 2014(Werner et al., , 2015aSuckert et al., 2016). In contrast to azides or cyanides, the synthesis of thiocyanates with bridging coordination is more difficult to achieve, because metal cations such as Mn II , Fe II , Co II and Ni II are less chalcophilic and therefore prefer a terminal N coordination. Nevertheless, a large number of compounds with -1,3-bridging thiocyanate anions have been reported in recent years (Mautner et al., 2018 andWerner et al., 2015a,b).
In our own investigations, we are particularly interested in the influence of the neutral co-ligand on the chemical reactivity, the crystal structure and the magnetic properties of thiocyanate coordination polymers of 3d metal cations. In most cases, we used pyridine derivatives that are substituted in the 4-position as co-ligands, but recently we also became interested in such ligands where the substitutent is located in the 3-position, including 3-methylpyridine (also called 3-picoline), C 6 H 7 N. With Co(NCS) 2 , two discrete complexes with the composition Co(NCS) 2 (C 6 H 7 N) 4 (refcodes EYAROM and EYAROM01; Boeckmann et al., 2011 andMałecki et al., 2012) and Co(NCS) 2 (C 6 H 7 N) 2 (H 2 O) 2 (EYAREC; Boeckmann et al., 2011) are deposited in the Cambridge Structural Database, in which the cobalt cations are octahedrally coordinated by two terminal N-bonded thiocyanate anions and four 3-methylpyridine in the former compound and two 3-methylpyridine and two water ligands in the latter. Upon heating, these complexes lose half of their coligands and transform into Co(NCS) 2 (C 6 H 7 N) 2 (EYARIG; Boeckmann et al., 2011) before a decomposition into Co(NCS) 2 is observed. Surprisingly, in contrast to most other compounds with pyridine derivatives substituted in the 4-position where chains or layers are formed, in this compound the Co II cations are tetrahedrally coordinated by two terminal N-bonded thiocyanate anions and two 3-methylpyridine co-ligands, forming discrete complexes.
Most compounds with 3-methylpyridine as co-ligand are reported with Ni(NCS) 2 , but surprisingly in none of them are the Ni II cations linked by the thiocyanate anions. This includes, for example, Ni(NCS) 2 (C 6 H 7 N) 2 (H 2 O) 2 (MEGCEH; Tan et al., 2006), which is isotypic to its cobalt analog. Moreover, a number of compounds consist of discrete complexes with the general composition Ni(NCS) 2 (C 6 H 7 N) 4 in which the Ni II cations are octahedrally coordinated by two terminal N-bonded thiocyanate anions as well as by four 3-methylpyridine co-ligands. In all of these compounds, the discrete complexes are packed in such a way that cavities are formed, in which additional solvate molecules are embedded. Altogether, three different structure types are observed. The mono-dichloromethane (Laylus, Pang et al., 1992), mono-trichloromethane (CIVJEW and CIFJEW01; Nassimbeni et al., 1984Nassimbeni et al., , 1986, mono-tetrachloromethane, mono-dibromodichloromethane and mono-2,2-dichloropropane clathrates (JICMIR, LAYLAY and LAYLEC;Pang et al., 1990Pang et al., , 1992 crystallize in the orthorhombic space group Fddd. If two molecules of trichloromethane are incorporated, the clathrate crystallizes with triclinic symmetry in space group P1 (LAYLOM; Pang et al., 1992) and the bis(dichloromethane) clathrate crystallizes in the monoclinic space group C2/c (LAYLIG; Pang et al., 1992). It is noted that the two latter unit cells are crystallographically unrelated. The formation of these clathrates for such simple nickel complexes is surprising because this is not observed in practically all other complexes with Ni(NCS) 2 and pyridine derivatives as co-ligands. However, it might be traced back to the fact that all of these solvents are non-polar and cannot coordinate to Ni II cations to form, for example, solvato octahedral complexes with the composition Ni(NCS) 2 (C 6 H 7 N) 2 (L) 2 (L = co-ligand).
Based on these assumptions, we tried to prepare additional compounds based on Ni(NCS) 2 and 3-methylpyridine as coligand, for which we used diethyl ether, ethanol and acetonitrile as solvents. All of them can coordinate to Ni II cations, which might lead to solvato complexes that afterwards might be transformed into the desired compounds with a bridging coordination by thermal decomposition. On the other hand, they are not very strong donor ligands, which means that compounds with a bridging coordination of the anionic ligands might form directly. With all three solvents, suitable crystals were obtained, which were characterized by single-crystal X-ray diffraction. Structure analysis reveals that even in this case, clathrates with the composition Ni(NCS) 2 (C 6 H 7 N) 4 Á 2 ethanol (1), Ni(NCS) 2 (C 6 H 7 N) 4 Á 2 acetonitrile (2) and Ni(NCS) 2 (C 6 H 7 N) 4 Á diethyl ether (3) have formed, which crystallize in two different structure types, with compounds 1 and 2 isotypic to the bis(dichloromethane) clathrate reported by Pang et al. (1992). Unfortunately, all of these compounds lose their solvents almost immediately at room temperature and X-ray powder diffraction shows that the same crystalline phase is obtained (Fig. S1 in the supporting information). In their IR spectra, the CN stretching vibration is observed at 2074 cm À1 , indicating that the anionic ligands are still terminally N-bonded (Fig. S2). Therefore, one can assume that a solvent-free compound with the composition Ni(NCS) 2 (C 6 H 7 N) 4 has formed, that still consists of discrete complexes and for which the crystal structure is unknown.

Structural commentary
The asymmetric units of Ni(NCS) 2 (C 6 H 7 N) 4 Á 2 ethanol (1) and Ni(NCS) 2 (C 6 H 7 N) 4 Á 2 acetonitrile (2) consist of half of an Ni II cation that is located on a center of inversion, one thiocyanate anion and two 3-methylpyridine ligands as well as one ethanol (1) and one acetonitrile (2) solvate molecules in general positions ( Figs. 1 and 2). The asymmetric unit in Ni(NCS) 2 (C 6 H 7 N) 4 Á diethyl ether (3) consists of one Ni II cation, two thiocyanate anions, four 3-methylpyridine ligands and one diethyl ether solvate molecule that occupy general positions (Fig. 3). In compounds 1 and 2, the solvate molecules are disordered and were refined using a split model (see Refinement), whereas in compound 3 they are fully ordered. The ethanol and acetonitrile solvates 1 and 2 crystallize in the monoclinic C-centered space group C2/c and are isotypic to the bis(dichloromethane) clathrate reported by Pang et al. (1992). Compound 3 crystallizes in space group P2 1 /n and its structure type is different from that of the solvates of Ni(NCS) 2 (C 6 H 7 N) 4 already reported in the literature (see Chemical Context).
In all three compounds the nickel(II) cations are octahedrally coordinated by two terminal N-bonded thiocyanate anions and four 3-methylpyridine co-ligands, forming discrete complexes. In compound 1 and 2 the discrete complexes are located on centers of inversion, whereas in compound 3 the complexes are located in general positions. The Ni-N bond lengths are comparable in all three compounds (Tables 1-3) and from the bonding angles, it is obvious that all octahedra are slightly distorted (see supporting information). This is reflected in the octahedral angle variance and the mean octahedral quadratic elongation calculated by the method of Robinson et al. (1971), which amount to 0.0857 2 and 1.0004, respectively, for compound 1, 0.3299 2 and 1.0006 for compound 2 and 1.0694 2 and 1.0010 for compound 3.

Figure 3
The molecular structure of compound 3 with labeling and displacement ellipsoids drawn at the 50% probability level.

Supramolecular features
In the crystal structures, the Ni(NCS) 2 (C 6 H 7 N) 4 complexes are packed in such a way that cavities are formed, in which the solvate molecules are embedded (Figs. 4 and 5). In compound 1, both ethanol molecules are linked to the complex by O-HÁ Á ÁS hydrogen bonding between the hydroxyl hydrogen atom of the ethanol molecule and the thiocyanate S atom (Fig. 4). The HÁ Á ÁS distance amounts to 2.464 (4) Å and the O-HÁ Á ÁS angle to 172 (2) , which indicates that this is a strong interaction (Table 4). There is one additional intermolecular contact between a pyridine H atom and the ethanol O atom, but the distance and geometry of this contact shows that this should be only a very weak interaction (Table 4). In the isotypic compound 2, no pronounced intermolecular interactions are observed and the packing seems to be dominated by van der Waals interactions. This is similar in the diethyl ether solvate 3, where the complexes are arranged in stacks along the c-axis direction (Fig. 5). For all compounds, the void spaces occupied by the solvate molecules were calculated, leading to values of 221 Å 3 (6.5% of the unit-cell volume) for 1, 162 Å 3 (4.8%) for 2 and 165 Å 3 (5.1%) for 3. The higher value for compound 1 might be traced back to the intermolecular hydrogen bonding.

Database survey
Several thiocyanate compounds with transition metal cations and 3-methylpyridine as co-ligand are reported in the Cambridge Structure Database CSD (version 5.43, last update November 2021; Groom et al., 2016), including the Co and Ni compounds mentioned above. With Cd(NCS) 2 , one compound with the composition Cd(NCS) 2 (C 6 H 7 N) 2 (FIYGUP; Taniguchi et al., 1987) is reported, in which the Cd II cations are octahedrally coordinated and linked by pairs of thiocyanate anions into chains. With copper, discrete complexes with the composition Cu(NCS) 2 (C 6 H 7 N) 2 (ABOTET; Handy et al., 2017) and Cu(NCS) 2 (C 6 H 7 N) 3 (VEPBAT; Kabešová & Kožíšková , 1989) are reported. There is also one chain compound with the composition Cu(NCS) 2 (C 6 H 7 N) 2 (CUHBEM; Healy et al., 1984), in which the copper cations are tetrahedrally coordinated.

Figure 4
Crystal structure of compound 1 as a representative with view along the crystallographic b-axis and intermolecular O-HÁ Á ÁS hydrogen bonds shown as dashed lines.

Figure 5
Crystal structure of compound 3 with view along the crystallographic caxis.
Ni(NCS) 2 (C 6 H 7 N) 4 Á diethylether (3): In a mixture of diethyl ether and H 2 O, 0.25 mmol of Ni(NCS) 2 (43.7 mg) and 2.5 mmol of 3-methylpyridine (243 ml) were added. Single crystals in the form of light-purple blocks were obtained after heating the reaction mixture to 353 K and storing it at this temperature for two days.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 5. The C-bound H atoms were positioned with idealized geometry (methyl H atoms allowed to rotate but not to tip) and were refined isotropically with U iso (H) = 1.2 U eq (C) (1.5 for methyl H atoms) using a riding model.

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 2 )
x y z U iso */U eq Occ.

Bis(isothiocyanato-κN)tetrakis(3-methylpyridine-κN)\ nickel(II) acetonitrile disolvate (2)
Crystal data where P = (F o 2 + 2F c 2 )/3 (Δ/σ) max = 0.001 Δρ max = 0.80 e Å −3 Δρ min = −0.46 e Å −3 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.

Bis(isothiocyanato-κN)tetrakis(3-methylpyridine-κN)\ nickel(II) diethyl ether monosolvate (3)
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.