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

In the crystal structure of the title compound, the FeII cations are octahedrally coordinated into discrete complexes that are linked into layers via intermolecular O—H⋯O hydrogen bonding.

In the crystal structure of the title compound, [Fe(NCS) 2 (C 12 H 9 NO) 2 (CH 4 O) 2 ], the Fe II cations are octahedrally coordinated by two N atoms of 4-benzoylpyridine ligands, two N atoms of two terminal isothiocyanate anions and two methanol molecules into discrete complexes that are located on centres of inversion. These complexes are linked via intermolecular O-HÁ Á ÁO hydrogen bonds between the methanol O-H H atoms and the carbonyl O atoms of the 4-benzoylpyridine ligands, forming layers parallel to (101). Powder X-ray diffraction proved that a pure sample was obtained but that this compound is unstable and transforms into an unknown crystalline phase within several weeks. However, the solvent molecules can be removed by heating in a thermobalance, which for the aged sample as well as the title compound leads to the formation of a compound with the composition Fe(NCS) 2 (4-benzoylpyridine) 2 , which exhibits a powder pattern that is similar to that of Mn(NCS) 2 (4-benzoylpyridine) 2 .

Chemical context
The synthesis of new coordination compounds is still an important topic in modern coordination chemistry. In most cases, such compounds are prepared in solution but there are alternative routes such as, for example, molecular milling or synthesis in molten flux (Braga et al., 2005(Braga et al., , 2006James et al., 2012;Hö ller et al., 2010;Schö nfeld et al., 2012). In this context, thermal annealing is also of interest, especially for precursors that contain volatile ligands. This approach has been proven to be particularly useful for the synthesis of thiocyanate coordination polymers, in which the metal cations are linked by the anionic ligands, because such compounds are frequently difficult to prepare in solution because their counterparts with terminally N-bonded anionic ligands are usually more stable (Nä ther et al., 2013). This is one of the reasons why we became interested in this class of compounds several years ago. In most cases, our precursors consist of discrete complexes, in which the metal cations are octahedrally coordinated by two terminal N-bonded thiocyanate anions and four pyridinebased co-ligands. If the described compounds are heated, the co-ligands are removed in a stepwise manner, which enforces the formation of compounds with bridging anions because the octahedral coordination is usually retained (Nä ther et al., 2013) might be traced back to the fact that this anionic ligand exhibits a large structural variety (Mautner et al., 2016a(Mautner et al., ,b, 2018. Following this approach, in most cases compounds are obtained in which the metal cations are octahedrally coordinated by two N-and two S-bonding thiocyanate anions as well as two coligands, all of them in trans-positions, and are linked into chains by pairs of -1,3-bridging anions (Neumann et al., 2019;Rams et al., 2020;Mautner et al., 2018). However, in some cases a cis-cis-trans coordination is observed, which can lead to the formation of linear but also to corrugated chains (Jochim et al., 2018;Neumann et al., 2020). This is the case for e.g. [M(NCS) 2 (4-benzoylpyridine) 2 ] n (M = Co, Ni), in which the two N and two S atoms of the bridging anionic ligands are in cis-positions, whereas the two apical 4benzoylpyridine ligands are trans-coordinating (Rams et al., 2017;Jochim et al., 2018). This is in contrast to the corresponding Cd compound, in which the Cd II cations shows the usual all-trans coordination (Neumann et al., 2018b). In this context, we became interested in the corresponding compounds based on Mn II and Fe II . However, the compounds with bridging anionic ligands are not available from solution and therefore, we tried to prepare them by thermal decomposition of discrete complexes with the composition M(NCS) 2 (4-benzoylpyridine) 4 (M = Mn and Fe; Wellm et al., 2018Wellm et al., , 2019a. Unfortunately, the X-ray powder pattern of the residues are of low quality and don't seem to be isotypic to the Co II , Ni II or Cd II phases. Therefore, we looked for a precursor that consists of two different coligands, one of which is more volatile and we found that, with methanol as solvent, crystals with the composition Fe(NCS) 2 (4-benzoylpyridine) 2 -(CH 3 OH) 2 can be obtained. Comparison of the experimental XRPD patterns with that calculated from single-crystal data proves that a pure phase has been obtained ( Fig. 1). We also have found that on storage over weeks, the title compound transforms into a new crystalline phase that exhibits a powder pattern completely different from that of the title compound (see Figure S1 in the supporting information). Compared to the title compound, the CN stretching vibration of the thiocyanate anions is shifted from 2050 cm À1 to 2074 cm À1 , and from thermogravimetric measurements it is indicated that about half of the methanol molecules are removed ( Figure  S2). If the solvent is removed completely from this crystalline phase, the CN stretching vibration shifts to 2084 cm À1 and a powder pattern is observed that cannot be indexed and that is different from those calculated for the known phases of [M(NCS) 2 (4-benzoylpyridine) 2 ] n (M = Co, Ni, Cd; Figure S1). However, if the title compound is heated in a thermobalance, two mass losses are observed that are in reasonable agreement with that calculated for the removal of the methanol molecules in the first and the remaining 4-benzoylpyridine ligands in the second step (calculated: 10.6% and 60.5%). If the residue formed after methanol removal is investigated by XRPD, it is obvious that the same crystalline phase has been obtained that will form if the discrete complex Fe(NCS) 2 (4-benzoylpyridine) 4 is thermally decomposed (Figures S1 and S2; Wellm, & Nä ther, 2019a). There are some similarities to the pattern of the residue obtained by thermal decomposition of Mn(NCS) 2 (4-benzoylpyridine) 4 , but it is different from those calculated for [M(NCS) 2 (4-benzoylpyridine) 2 ] n (M = Co, Ni, Cd; Figure S1).  one Fe II cation located on a centre of inversion (Fig. 2). The Fe II cation is octahedrally coordinated by two thiocyanate anions, two methanol and two 4-benzoylpyridine ligands, with each pair of the same ligand in the trans-position. The Fe-N bond length to the 4-benzoylpyridine ligand [2.2270 (12) Å ] is longer than that to the thiocyanate anion [2.0823 (15) Å ] ( Table 1). From the bond angles around the metal centers as well as the value for the angle variance (0.93) and the quadratic elongation (1.002) calculated by a procedure published by Robinson et al. (1971), it is obvious that the octahedra are slightly distorted. The 4-benzoylpyridine ligands are not coplanar as demonstrated by the values of the dihedral angles between the pyridine ring (N11/C11-C15) and the carbonyl group (C13/C16/C17/O11) of 47.9 (1) and between the carbonyl group (C13/C16/ C17/O11) and the phenyl ring (C17-C22) of 16.6 (1) .

Supramolecular features
In the crystal of the title compound, the discrete complex molecules are linked by intermolecular O-HÁ Á ÁO hydrogen bonds between the hydroxyl H atom of the methanol ligand and the carbonyl oxygen atom of a 4-benzoylpyridine ligand of a neighbouring complex ( Table 2). Each of the complexes are linked to four symmetry-equivalent complexes into layers parallel to (101) (Fig. 3). Between these layers, no pronounced intermolecular interactions are observed (Fig. 4).

Figure 3
Crystal packing of the title compound viewed along (101) with intermolecular O-HÁ Á ÁO hydrogen bonding shown as dashed lines.

Figure 4
Crystal packing of the title compound viewed along the crystallographic b axis with intermolecular O-HÁ Á ÁO hydrogen bonding shown as dashed lines.

Figure 2
View of a discrete complex with atom labelling and displacement ellipsoids drawn at the 50% probability level. Symmetry code: (i) Àx, Ày, Àz + 1.

Synthesis and crystallization
FeCl 2 Á4 H 2 O and KSCN 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 by the reaction of 59.6 mg FeCl 2 Á4H 2 O (0.3 mmol) and 58.3 mg of KSCN (0.6 mmol) with 27.5 mg of 4-benzoylpyridine (0.15 mmol) in methanol (1.5 mL) within a few days.
For the synthesis of larger amounts of a polycrystalline powder, 398 mg of FeCl 2 Á4H 2 O (2 mmol) and 396 mg of KSCN (4 mmol) were stirred in methanol (2 mL) for 16 h and the precipitating KCl was filtered off and washed two times with methanol (0.5 mL). 366 mg of (2 mmol) 4-benzoylpyridine were added and this reaction mixture was stirred for four days. The product was filtered off and directly analysed, because it proved to be unstable at room temperature if stored for a longer time.
Experimental details: Differential thermoanalysis and thermogravimetry (DTA-TG) was performed in a dynamic nitrogen atmosphere in Al 2 O 3 crucibles using an STA PT1600 thermobalance from Linseis. The XRPD measurements were performed using a Stoe Transmission Powder Diffraction System (STADI P) with Cu K radiation that was equipped with a linear positionsensitive 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. The C-H H atoms were positioned with idealized geometry and were refined with fixed isotropic displacement parameters U iso (H) = 1.2 U eq (C) for aromatic and U iso (H) = 1.5 U eq (C) for methyl H atoms using a riding model. The O-H H atom was located in a difference map, its bond length was set to an ideal value of 0.84 Å and finally, it was refined with U iso (H) = 1.5 U eq (O) using a riding model. Crystal structure, synthesis and thermal properties of bis(4-benzoylpyridine-

Bis(4-benzoylpyridine-κN)bis(isothiocyanato-κN)bis(methanol-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.