Tetrakis(4-benzoylpyridine-κN)bis(isothiocyanato-κN)manganese(II)

In the crystal structure of the title compound, the MnII cations are octahedrally coordinated by two terminally N-bonded thiocyanate anions and four 4-benzoylpyridine coligands into discrete complexes, which are further linked into chains by intermolecular C—H⋯O hydrogen bonding.

The asymmetric unit of the title compound, [Mn(NCS) 2 (C 12 H 9 NO) 4 ], consists of one Mn II cation located on a centre of inversion, one thiocyanate anion and two 4-benzoylpyridine co-ligands. The Mn II cation is octahedrally coordinated by two terminally N-bonded anionic ligands and four N-bonded 4-benzoylpyridine co-ligands within a slightly distorted octahedron. Individual complexes are linked by intermolecular C-HÁ Á ÁO hydrogen-bonding interactions into chains running along the c-axis direction. Simultaneous thermogravimetry and differential scanning calorimetry measurements reveal a decomposition in two separate steps, in each of which two co-ligands are removed. The compound obtained after the first step has the composition [Mn(NCS) 2 (C 12 H 9 NO) 2 ] and is of unknown structure, before in the second step decomposition into [Mn(NCS) 2 ] is observed. Magnetic susceptibility measurements show the Mn II cations to be in the high-spin state, and that weak antiferromagnetic interactions between the complexes are present.

Chemical context
Thiocyanate anions are versatile ligands that, in combination with neutral organic co-ligands, can form coordination compounds and polymers of different dimensionality. The most common coordination modes include N-terminal and -1,3-bridging (Buckingham, 1994;Palion-Gazda et al., 2017;Mautner et al., 2017). The bridging mode is of special interest because magnetic exchange can be mediated by the anionic ligands (Palion-Gazda et al., 2015;Mekuimemba et al., 2018;Gonzá lez et al., 2012;Guillet et al., 2016). In this context, we have reported the syntheses, structures and magnetic properties of a number of compounds, in which transition metal cations such as Mn II , Fe II , Co II and Ni II are octahedrally coordinated by two neutral N-donor co-ligands and four thiocyanate anions and are linked into linear or corrugated chains by pairs of anionic ligands (Suckert et al., 2017a;Werner et al., 2015;Wö hlert et al., 2013Wö hlert et al., , 2014a. In the course of our project, we have also used 4-benzoylpyridine as coligand, leading to the formation of two isotypic chain compounds with general composition [M(NCS) 2 (4-benzoylpyridine) 2 ] (M = Co, Ni). In both compounds, dominating ferromagnetic interactions are observed but the Co II compound additionally shows a slow relaxation of the magnetization, indicating single-chain magnetism (Rams et al., 2017;Jochim et al., 2018). In contrast to most other compounds, in which all ligands are in the trans-position, in the 4-benzoylpyridine coordination polymers with Co II and Ni II , the central metal cation shows a cis-cis-trans coordination.
In this context, the question arose about which kind of metal coordination is observed for the corresponding Mn II compound, which is less chalcophilic compared to Co II and Ni II . Therefore, [Mn(NCS) 2 ] was reacted with 4-benzoylpyridine in different ratios and only crystals of a compound with composition [Mn(NCS) 2 (4-benzoylpyridine) 4 ] were obtained, as determined by single crystal X-ray diffraction. If the experimental X-ray powder pattern is compared with that calculated from single crystal data, it is obvious that a pure crystalline phase has been obtained (see Fig. S1 in the supporting information). In the IR spectrum, the asymmetric C N-stretching vibration is observed at 2054 cm À1 , which is in agreement with the presence of terminal N-bonded thiocyanate anions (Fig. S2). Magnetic susceptibility measurements in a field of 1 kOe show paramagnetic behaviour. From the temperature-independent susceptibility curve, it is obvious that dominating antiferromagnetic interactions are present, which is frequently observed for similar discrete complexes based on [Mn(NCS) 2 ]. The susceptibility curve was analysed using the Curie-Weiss law, leading to a magnetic moment of 6.0 m B , which is in good agreement with that expected for an Mn II cation in a high-spin configuration (Fig. S3). As previously shown, co-ligand-rich precursor complexes can be transformed into co-ligand-deficient compounds with more condensed thiocyanate networks by thermal decomposition (Neumann et al., 2018b). Therefore, the title compound was investigated by simultaneous thermogravimetry and differential thermoanalysis (TG-DTA). Upon heating, two mass loss steps are observed in the TG curve, accompanied by two endothermic events in the DTA curve (Fig. S4). The experimental mass loss in each step of 40.4 and 40.5% is in good agreement with that calculated for the removal of two 4-benzoylpyridine ligands in each step. When in a second TG measurement the residue formed after the first mass loss was isolated and investigated by X-ray powder diffraction, it became clear that the powder pattern was not related to those for [Co(NCS) 2 (4-benzoylpyridine) 2 ] and [Cd(NCS) 2 (4-benzoylpyridine) 2 ], indicating that a new crystalline phase had formed (Fig. S5). Indexing of the powder pattern failed, and therefore the structure of this compound remains unknown. However, the C N stretch observed in the IR spectrum of this residue is found at 2078 cm À1 , which is close to that in [Cd(NCS) 2 (4-benzoylpyridine) 2 ] (2088 cm À1 ), indicating the presence of -1,3-bridging anionic ligands (Fig. S6).

Structural commentary
In the crystal structure of the title compound, the Mn II cations are located on centers of inversion, whereas the unique thiocyanate anion and the two crystallographically independent 4-benzoylpyridine co-ligands occupy general positions. The Mn II cation is ocahedrally coordinated by two N-bonded  Table 1 Selected geometric parameters (Å , ).

Figure 1
View of a discrete complex with the atom labelling and displacement ellipsoids drawn at the 50% probability level. [Symmetry code: (i) Àx, Ày + 1, Àz + 1.] terminal thiocyanate anions and four neutral N-bonded 4-benzoylpyridine ligands. The Mn-N bond lengths are considerably shorter for the anionic ligand [2.1658 (15) Å ] than those for the neutral co-ligands [2.3200 (14) and 2.3232 (14) Å ; Fig. 1 and Table 1]. The bond lengths and angles reveal a slight distortion of the MnN 6 octahedron (Table 1), which is also obvious from the angle variance of 4.8 and the quadratic elongation of 1.022 (Robinson et al., 1971). Neither the pyridine nor the phenyl rings of the two 4-benzoylpyridine ligands are coplanar with the carbonyl planes. In the first ligand, the phenyl plane (C17-C22) is inclined at an angle of 23.08 (11) to the plane of the carbonyl group (O11,C13,C16,C17) and to the pyridine plane (N11,C11-C15) by 37.33 (10) . Corresponding values for the second co-ligand are 24.07 (11) between the carbonyl plane (O21,C33,C36,C37) and the phenyl ring (C37-C42) and 36.58 (10) for the pyridine ring (N31,C31-C35). There are weak intramolecular C-HÁ Á ÁN interactions between some of the aromatic hydrogen atoms and the thiocyanate N atoms, which might contribute to the stabilization of the conformation of the complex (Table 2).

Supramolecular features
In the crystal structure of the title compound, discrete complexes are linked by intermolecular C-HÁ Á ÁO hydrogenbonding interactions between the carbonyl O atom and the two hydrogen atoms H15 and H35. Each complex forms four such hydrogen bonds to neighbouring complexes, leading to the formation of chains that elongate in the direction of the c axis (Figs. 2 and 3, Table 2). Between the chains no distinct intermolecular interactions apart from van der Waals interactions are observed (Fig. 3).

Database survey
In the Cambridge Structure Database (Version 5.39, last update Aug 2018; Groom et al., 2016), there are ten structures of coordination compounds reported that are comprised of 4-benzoylpyridine ligands, thiocyanate anions and different transition metal cations. Firstly, there are two complexes in which the cations are coordinated each by two terminal Nbonded thiocyanate anions and two 4-benzoylpyridine ligands to form a square-planar complex with Cu II (Bai et al., 2011) and a tetrahedral complex with Zn II (Neumann et al., 2018a). There are also two complexes with coordinating solvate ligands, in which the Co II cation is octahedrally coordinated by two terminal N-bonded thiocyanate anions, two 4-benzoylpyridine ligands and either two methanol (Suckert et al., 2017a), or two acetonitrile molecules (Suckert et al., 2017b). As mentioned above, there is also a chain compound with composition [Co(NCS) 2 (4-benzoylpyridine) 2 ] in which the Co II cations are linked by pairs of -1,3-coordinating thiocyanate anions (Rams et al., 2017). It is also noted that two additional chain compounds with Cd II and Ni II are reported in literature (Jochim et al., 2018;Neumann et al., 2018a). Finally, there are one Ni II (Soliman et al., 2014), one Co II (Drew et al.,1985), one Zn and one Cd compound (Neumann et al., 2018a) that are isotypic with the title complex.

Synthesis and crystallization
Ba(SCN) 2 Á3H 2 O and 4-benzoylpyridine were purchased from Alfa Aesar. Mn(SO 4 )Á4H 2 O was purchased from Merck. All solvents and reactants were used without further purification. Mn(NCS) 2 was prepared by the reaction of equimolar amounts of MnSO 4 Á4H 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 finally dried at room-temperature.

Tetrakis(4-benzoylpyridine-κN)bis(isothiocyanato-κN)manganese(II)
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.