Crystal structure of bis(4-benzoylpyridine-κN)bis(methanol-κO)bis(thiocyanato-κN)nickel(II) methanol monosolvate

In the crystal structure of the title compound, the NiII cations are octahedrally coordinated by two terminally N-bonded thiocyanate anions, two methanol molecules and two 4-benzoylpyridine coligands into discrete complexes that are linked by intermolecular C—H⋯S, C—H⋯O and O—H⋯O hydrogen-bonding interactions into a three-dimensional network with channels in which the non-coordinating methanol solvate molecules are located.

Unfortunately, for most paramagnetic transition metal cations, the bridging modes are energetically less favored and thus, compounds with a terminal coordination are usually obtained. Nevertheless, we have found an alternative approach to overcome this problem by transformation of the latter compounds through thermal annealing into the desired compounds that have bridging anions. For the alternative ISSN 2056-9890 synthesis of such coordination polymers with bridging anionic ligands, a precursor consisting of a discrete complex can be used in which the metal cations are coordinated by two terminal N-bonded thiocyanate anions and four co-ligands that, in our cases, consist of pyridine derivatives. Upon controlled heating, two of the four co-ligands can be removed. Frequently, this treatment yields the desired compounds with bridging coordination as intermediates, which can easily be investigated by thermogravimetry. In some cases, no discrete decomposition steps are observed because all co-ligands are removed in one step. Under these circumstances, alternatives are required that are based on precursor complexes comprising only two of the pyridine derivatives as ligands and two coordinating and volatile molecules such as water or methanol. The ligand molecules are emitted in a discrete step (also observable in a thermogravimetrical measurement), which directly produces the desired compounds in quantitative yield. It is also noted that this approach often leads to the formation of polymorphs or isomers that are different from the compounds obtained from solution (Werner et al., 2015;Jochim et al., 2018).
In this context we have reported two isotypic compounds with chain-structures that have the general composition M(NCS) 2 (4-benzoylpyridine) 2 where M = Co, Ni (Rams et al., 2017;Jochim et al., 2018). Here the metal cations are linked into linear chains with a cis-cis-trans coordination, in contrast to most other compounds with similar linear chains where all ligands are in trans positions. This is somewhat surprising because Cd(NCS) 2 (4-benzoylpyridine) 2 also forms linear chains with an all-trans coordination of the cations (Neumann et al., 2018a). Therefore, our intention was to test if a different isomer with, for example, Ni can be prepared by thermal annealing. A complex with composition Ni(NCS) 2 (4benzoylpyridine) 4 has already been reported in the literature. It decomposes in several steps, but only the intermediate after complete removal of 4-benzoylpyridine was examined (Soliman et al., 2014). We have synthesized this compound again and investigated its thermal properties. The residue formed after removal of half of the 4-benzoylpyiridine ligands is of poor crystallinity and does not correspond to a pure phase. Therefore, we searched for a more promising precursor; during these investigations, crystals of the title compound were obtained and characterized by single crystal X-ray diffraction. X-ray powder diffraction revealed that the compound directly isolated from the reaction mixture is a nearly pure phase but always contaminated with a very small amount of Ni(NCS) 2 (4-benzoylpyridine) 4 (see Fig. S1 in the supporting information). More importantly, if the title compound is filtered off, it decomposes very quickly into an unknown crystalline phase that does not correspond to that of Ni(NCS) 2 (4-benzoylpyridine) 4 already reported in the literature. However, this sample is still contaminated with Ni(NCS) 2 (4-benzoylpyridine) 4 , and any attempt to completely index its powder pattern failed (Fig. S2 in the supporting information).

Structural commentary
The crystal structure of the title compound consists of discrete complexes in which the Ni II cations are sixfold coordinated by two crystallographically independent thiocyanate anions, two methanol molecules and two 4-benzoylpyridine ligands (Fig. 1). The Ni-N bond lengths to the anionic ligands of 2.009 (3) and 2.034 (3) Å are shorter than those to the 4-benzoylpyridine ligands [2.092 (2) and 2.104 (2) Å ; the Ni-O distances to the methanol ligands are longer again at 2.108 (2) and 2.154 (2) Å . The coordination sphere around Ni II can be described as a slightly distorted octahedron. This is also obvious from the angle variance and the quadratic elon-gation, which were calculated to be 4.7 and 1.022 (Robinson et al., 1971). The 4-benzoylpyridine ligand is not planar. The dihedral angle between the pyridine ring (N11, C11-C15) and the carbonyl plane (C13, C16, C17, O11) amounts to 56.86 (16) and that between the phenyl ring (C17-C22) and the carbonyl group (C13, C16, C17, O11) to 12.49 (17) . The second ligand has corresponding values of 48.61 (17) between the pyridine ring (N31, C31-C35) and the carbonyl group (C33, C36, C37, O31) and 16.69 (18) between the phenyl ring (C37-C42) and the carbonyl group (C33, C36, C37, O31). There is a short intramolecular contact between one of the aromatic hydrogen atoms (H35) and one of the thiocyanate N atoms (N1); however, the corresponding C-HÁ Á ÁN angle deviates strongly from linearity, indicating only a weak interaction (Table 1).

Supramolecular features
The crystal structure of the title compound is dominated by extensive intermolecular classical and non-classical hydrogenbonding interactions of medium-to-weak strengths (Table 1). Discrete complexes are linked by intermolecular O-HÁ Á ÁS hydrogen bonds into chains extending parallel to [010] (Fig. 2, top). Within such a chain, the complexes are related by the 2 1screw axis, resulting in a helical arrangement (Fig. 2, bottom). These chains are further linked by pairs of centrosymmetric C-HÁ Á ÁS hydrogen bonds into layers extending parallel to (101) (Fig. 3). Adjacent layers are linked into a three-dimensional network by C-HÁ Á ÁO hydrogen bonding between a hydrogen atom (H34) of one of the phenyl rings and the carbonyl O atom (O11) of a neighboring 4-benzoylpyridine ligand (Fig. 4). Within this network channels are formed in which the non-coordinating methanol molecules are embedded (Fig. 4). The solvent molecules are linked by O-HÁ Á ÁO hydrogen bonding and act both as a donor (O3) to a neighbouring carbonyl O atom (O11) and as an acceptor for a hydroxyl group (O1) of a methanol ligand (Fig. 4).  Table 1 Hydrogen-bond geometry (Å , ). Symmetry codes: (i) Àx þ 1; Ày þ 1; Àz; (ii) x À 1; y; z; (iii) Àx þ 3 2 ; y þ 1 2 ; Àz þ 1 2 .

Figure 2
Crystal structure of the title compound in a view along (top) and perpendicular (bottom) to the hydrogen-bonded chains. Intermolecular O-HÁ Á ÁS hydrogen bonding is shown as dashed lines.  (Drew et al., 1985;Soliman et al., 2014;Neumann et al., 2018b). There are also compounds where the metal cations are fourfold coordinated by the two N-bonded terminal thiocyanate anions and two 4-benzoylpyridine co-ligands, forming either a tetrahedral (Zn II complex) or a square-planar (Cu II complex) coordination sphere (Neumann et al., 2018a;Bai et al., 2011). The last group consists of octahedrally coordinated Co II cations that either contain two acetonitrile (Suckert et al., 2017b) or two methanol molecules (Suckert et al., 2017c) as coordinating solvent molecules.

Synthesis and crystallization
Ba(SCN) 2 Á3H 2 O and 4-benzoylpyridine were purchased from Alfa Aesar. NiSO 4 Á6H 2 O was purchased from Merck. All solvents and reactants were used without further purification. Ni(NCS) 2 was prepared by the reaction of equimolar amounts of NiSO 4 Á6H 2 O and Ba(NCS) 2 Á3H 2 O in water. The resulting white precipitate of BaSO 4 was filtered off, and the solvent was evaporated from the filtrate. The product was dried at room temperature. Crystals of the title compound suitable for single-crystal X-ray diffraction were obtained within a few days by the reaction of 52.5 mg Ni(NCS) 2 (0.30 mmol) with 27.5 mg 4-benzoylpyridine (0.15 mmol) in methanol (1.5 ml) at 354 K using culture tubes.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2. The hydrogen atoms were positioned with idealized geometry (C-H = 0.95-0.98 Å ; methyl H atoms in part were allowed to rotate but not to tip) and were refined with U iso (H) = 1.2U eq (C) (1.5 for methyl H atoms) using a riding model. The OH hydrogen atoms were located in a difference-Fourier map; their bond lengths were set to ideal values and finally they were refined with U iso (H) = 1.5U eq (O) using a riding model.  Computer programs: X-AREA (Stoe, 2008), XP in SHELXTL and SHELXS97 (Sheldrick, 2008), SHELXL2014/7 (Sheldrick, 2015), DIAMOND (Brandenburg, 1999) and publCIF (Westrip, 2010).

Bis(4-benzoylpyridine-κN)bis(methanol-κO)bis(thiocyanato-κN)nickel(II) methanol monosolvate
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.