Crystal structure, synthesis and thermal properties of tetrakis(4-benzoylpyridine-κN)bis(isothiocyanato-κN)iron(II)

In the crystal structure of the title compound, the FeII ions are ocathedrally coordinated, forming discrete complexes that are linked into chains by intermolecular C—H⋯O interactions.


Chemical context
Coordination compounds based on thio-or selenocyanate anions have attracted much interest in recent years because of their luminescence behavior and their versatile magnetic properties (Mekuimemba et al., 2018;Palion-Gazda et al., 2015Mautner et al., 2016a,b;Nä ther et al., 2013). For the latter, compounds are of special interest in which paramagnetic transition-metal cations are linked by the anionic ligands into 1D or 2D coordination polymers. Some of them show single-chain-magnet behavior 2014a;Mautner et al., 2018), others are ferromagnets (Suckert et al., 2016) and in a few cases the critical temperature can be tuned by mixed-crystal formation (Neumann et al., 2018a(Neumann et al., , 2019. However, in most cases compounds are obtained from solution in which the anionic ligands are only terminally Nbonded, which frequently leads to the formation of discrete complexes (Mautner et al., 2015(Mautner et al., , 2017. These compounds can be transformed into coordination polymers by thermal decomposition, in which some of the co-ligands are irreversibly removed , leading to the formation of polymorphic or isomeric modifications (Wö hlert et al., 2014b). In several cases Mn II , Fe II , Co II , Ni II and Cd II compounds behave similarly but in others, different modifications are obtained depending on the actual metal cation. This is the case e.g. for thiocyanate complexes with 4-benzoylpyridine as co-ligand. The discrete complexes with the composition M(NCS) 2 (4-benzoylpyridine) 4 (M = Co and Ni) transform into isotypic chain compounds with the composition [M(NCS) 2 (4-benzoylpyridine) 2 ] n , whereas both the Mn and Cd compounds each form a different crystalline ISSN 2056-9890 phase (Neumann et al., 2018b;. Therefore, we became interested in the corresponding complex with Fe II to check if this compound could also transform into a 4-benzoylpyridine-deficient phase and if this phase would be isotypic to that with Mn II , Co II or Cd II . The synthesis of the title compound can easily be achieved by the reaction of Fe(Cl) 2 Á4H 2 O and K(SCN) 2 with 4-benzoylpyridine, leading to the formation of phase pure samples (see Figure S1 in the supporting information). Upon heating, two mass losses are observed, of which the first one is in agreement with that expected for the removal of half of the 4-benzoylpyridine co-ligands ( Figure S2). If the residue formed after the first thermogravimetric step is investigated by XRPD, it is obvious that a crystalline phase is formed ( Figure S3) that is not isotypic to [M(NCS) 2 (4-benzoylpyridine) 2 ] n (M = Co, Cd) but very similar to that of Mn(NCS) 2 (4-benzoylpyridine) 2 (Rams et al., 2017;Neumann et al., 2018b;. Additionally, IR spectra show that the residue exhibits bridging -1,3-coordinating thiocyanate anions, in contrast to the terminal thiocyanate anions of the title compound ( Figure  S4). Unfortunately, as is the case for the Mn II compound, the powder pattern cannot be indexed and no single crystals can be obtained. Therefore, the structure of this compound is still unknown.

Figure 1
View of a discrete complex with the atom labeling and displacement ellipsoids drawn at the 50% probability level. Symmetry code: (i) Àx, Ày + 1, Àz + 1.
H11, H31, H15 and H35 that might contribute to the stabilization of the complexes (Table 2).

Supramolecular features
The discrete complexes are connected by relatively weak C-HÁ Á ÁO hydrogen bonds between the C-H hydrogen atoms and the atom O21(Àx, 1 À y, 2 À z) of a symmetry-related 4-benzoylpyridine ligand, forming 12-membered rings that are located on centres of inversion ( Fig. 2 and Table 2). Atom O21 acts as acceptor for two hydrogen bonds from C15-H15 and C35-H35; thus each complex is connected by four hydrogen bonds to two additional symmetry-equivalent complexes, leading to the formation of chains that extend along the c-axis direction (Figs. 2 and 3 and Table 2). There are no further directed interactions observed between the chains (Fig. 3).

Database survey
There are several crystal structures reported in the Cambridge Structure Database (Version 5.40, last update February 2018; Groom et al., 2016) that consist of transition-metal cations, thiocyanate anions and 4-benzoylpyrine. In most of these compounds, the metal cations are octahedrally coordinated. Three of them are coordination polymers in which the cations are connected by pairs of -1,3-coordinating thiocyanate anions, with the 4-benzoylpyridine ligands being perpendicular to the elongation axis of the chain (Neumann et al., 2018b;Rams et al., 2017;Jochim et al., 2018). The other octahedral compounds are either discrete complexes with only 4-benzoylpyridine as neutral co-ligand, isotypic to the title compound and of the general composition M(NCS) 2 (4benzoylpyridine) 4 (M = Co II , Ni II , Mn II , Zn II and Cd II ; Drew et al., 1985;Soliman et al., 2014;Neumann et al., 2018b), or solvates that are built up of two terminally N-bonded thiocyanates, two 4-benzoylpyridine ligands and acetonitrile (Suckert et al., 2017b) or methanol as solvent (Suckert et al., 2017a;Wellm & Nä ther, 2019). Additionally, there is a quadratic planar Cu II complex (Bai et al., 2011) and a tetrahedral Zn II complex (Neumann et al., 2018b) in which the metal cation is coordinated by two terminally Nbonded thiocyanates and two 4-benzoylpyridine ligands.

Synthesis and crystallization
Fe(Cl) 2 Á4H 2 O and K(SCN) 2 were purchased from Merck and 4-benzoylpyridine was purchased from Alfa Aesar.

Synthesis:
Crystals of the title compound suitable for single crystal X-ray diffraction were obtained within three days by the reaction of 59.6 mg Fe(Cl) 2 Á4H 2 O (0.3 mmol) and 58.3 mg (0.6 mmol) K(SCN) 2 with 27.5 mg 4-benzoylpyridine (0.15 mmol) in ethanol (1.5 mL), followed by slow evaporation of the solvent. Crystal packing of the title compound viewed along the crystallographic a axis with intermolecular C-HÁ Á ÁO hydrogen bonds (Table 2) shown as dashed lines.

Experimental details:
Differential thermal analysis-thermogravimetric (DTA-TG) measurements were performed in a dynamic nitrogen atmosphere in Al 2 O 3 crucibles using an STA PT1600 thermobalance from Linseis. The XRPD measurements were performed by using a Stoe transmission powder diffraction system (STADI P) with Cu K radiation that was equipped with a linear, position-sensitive MYTHEN detector from Stoe & Cie. The IR data were measured using a Bruker Alpha-P ATR-IR spectrometer.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 3. Hydrogen atoms were positioned with idealized geometry (C-H = 0.95 Å ) and were refined using a riding model with U iso (H) = 1.2U eq (C).

Tetrakis(4-benzoylpyridine-κN)bis(isothiocyanato-κN)iron(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.