Crystal structure of octakis(4-methoxypyridinium) bis(4-methoxypyridine-κN)tetrakis(thiocyanato-κN)ferrate(III) bis[(4-methoxypyridine-κN)pentakis(thiocyanato-κN)ferrate(III)] hexakis(thiocyanato-κN)ferrate(III) with iron in three different octahedral coordination environments

The crystal structure of the title compound consists of three different negatively charged discrete octahedral iron(III) complexes, that are charge-balanced by 4-methoxypyridinium cations.

The crystal structure of the title salt, (C 6 H 8 NO) 8 [Fe(NCS) 4 (C 6 H 7 NO) 2 ]-[Fe(NCS) 5 (C 6 H 7 NO)] 2 [Fe(NCS) 6 ], comprises three negatively charged octahedral Fe III complexes with different coordination environments in which the Fe III atoms are coordinated by a different number of thiocyanate anions and 4-methoxypyridine ligands. Charge balance is achieved by 4-methoxypyridinium cations. The asymmetric unit consists of three Fe III cations, one of which is located on a centre of inversion, one on a twofold rotation axis and one in a general position, and ten thiocyanate anions, two 4-methoxypyridine ligands and 4-methoxypyridinium cations (one of which is disordered over two sets of sites). Beside to Coulombic interactions between organic cations and the ferrate(III) anions, weak N-HÁ Á ÁS hydrogen-bonding interactions involving the pyridinium N-H groups of the cations and the thiocyanate S atoms of the complex anions are mainly responsible for the cohesion of the crystal structure.

Chemical context
Recently, the synthesis of new coordination compounds based on paramagnetic metal cations has become increasingly interesting. In particular, compounds in which the paramagnetic metal cations are linked by small-sized anionic ligands that can mediate magnetic exchange are of special importance. For example, this can be achieved by thio-or selenocyanate anions that are able to coordinate to a central metal cation in different ways (Palion-Gazda et al., 2015;Guillet et al., 2016;Prananto et al., 2017). Most of the reported compounds contain terminally N-bonded thiocyanate ligands, whereas compounds with these ligands in a bridging mode are relatively rare. Nevertheless, the latter can be obtained by thermal decomposition of precursor complexes with terminal anionic ligands, as we have recently shown. With monodentate co-ligands, such as simple pyridine derivatives substituted in the 4-position, we were able to synthesize a number of compounds (predominantly including divalent cobalt or nickel), in which the metal cations are linked by pairs of anionic ligands into chains (Rams et al., 2017a,b;Wö hlert et al., 2012;Werner et al., 2015). In this context, divalent iron compounds are also of interest, but are scarce in comparison to divalent cobalt or nickel compounds because they are more difficult to synthesize in solution due to the poor oxidation ISSN 2056-9890 stability of Fe II . Therefore, we attempted to prepare either a coordination polymer with planned composition [Fe(NCS) 2 (4methoxypyridine) 2 ] n or a discrete complex with composition [Fe(NCS) 2 (4-methoxypyridine) 4 ], which on thermal annealing might be transformed into the desired coordination polymer. 4-Methoxypyridine was selected because this ligand exhibits a strong donor substituent in the 4-position in comparison to the pyridine or 1,2-bis(4-pyridyl)ethylene ligands we have already investigated (Boeckmann & Nä ther, 2012;Wö hlert et al., 2013). In the course of these investigations, we accidently obtained crystals of the title compound, (C 6 H 8

Structural commentary
The asymmetric unit of the title compound comprises three iron(III) cations, of which one is located on a centre of inversion (Fe3), one on a twofold rotation axis (Fe1) and one in a general position (Fe2), as well as ten thiocyanate anions, two 4-methoxypyridine ligands and four 4-methoxypyridinium cations, one of which is disordered over two sets of sites.
The three Fe III cations form discrete anionic complexes that are charge-balanced by the 4-methoxypyridinium cations. For each of the cations, the N-H hydrogen atom was clearly located, indicating an oxidation state of +III for iron. Each of the three Fe III cations shows a different octahedral coordin-ation environment. Fe1 is coordinated by two pairs of symmetry-related terminal-N-bonding thiocyanate anions defining the equatorial plane of the octahedron, whereas the two axial positions are occupied by the N atoms of two symmetry-related 4-methoxypyridine ligands (Fig. 1). The Fe1-N distances to the anionic ligands are similar and significantly shorter than those to the neutral 4-methoxypyridine co-ligands (Table 1). Fe2 is coordinated by five crystallographically independent N-bonding thiocyanate anions and by one 4-methoxypyridine ligand that occupies one of the axial positions (Fig. 1). The Fe2-N bond lengths are comparable to those of Fe1, except that of an equatorial thiocyanate anion (N4) that is somewhat elongated. Interestingly, the distance to the N7 atom of the thiocyanate anion that is trans to the 4-methoxypridine ligand is comparable to the other short Fe-N distances (Table 1). Fe3 is octahedrally coordinated by three pairs of N-bonding thiocyanate anions related by a centre of inversion (Fig. 1). The Fe-N distances scatter over a wider range between 2.030 (2) and 2.075 (2) Å (Table 1) Symmetry codes: (i) Àx þ 1; y; Àz þ 3 2 ; (ii) Àx þ 1; Ày; Àz þ 1.

Figure 1
View of the three different coordination spheres of the Fe III cations in the title compound. Displacement ellipsoids are drawn at the 50% probability level. [Symmetry codes: (i) 1 À x, y, 3 2 À z; (ii) 1 À x, Ày, 1 À z.] octahedra (Robinson et al., 1971), was calculated for each of the discrete complexes. The greatest value of hocti 2 is found for Fe1 ( hocti 2 = 8.89) followed by Fe2 ( hocti 2 = 2.34) and Fe3 ( hocti 2 = 0.28). Thus for Fe1, the bond angles deviate more from the ideal values compared to Fe2 and Fe3, with the latter showing the smallest distortion from an ideal octahedron.
It is noted that a number of discrete anionic complexes based, for example, on Mn II or Fe II thiocyanates, are reported in which the metal cations are four-, five-, or sixfold coordinated by anionic and additional neutral co-ligands. What makes the title compound so special is the fact that its crystal structure contains three different coordination spheres for iron in one crystal structure, suggesting a snapshot of the species that might be present in equillibrium in solution. Therefore it is not surprising that pure samples were not obtained under the given conditions. X-ray powder diffraction revealed that for all batches, large amounts of additional crystalline phases were present that could not be identified (see Fig. S1 in the Supporting information).
The negative charges of the anionic complexes in the title compound (-1 for Fe1, 2Â À2 for Fe2 and À3 for Fe3) are compensated by eight 4-methoxypyridinium cations, of which each two are pairwise related by symmetry (Fig. 2).

Supramolecular features
The discrete anionic complexes are linked with the cations through weak intermolecular N-HÁ Á ÁS hydrogen bonds between the pyridinium hydrogen atoms and the thiocyanate sulfur atoms (Fig. 3, Table 2). The complex containing Fe3 is additionally involved in weak C aromatic -HÁ Á ÁN hydrogen bonding. Other short contacts indicate further weak C aromatic -HÁ Á ÁS and C methyl -HÁ Á ÁS hydrogen bonds, respectively, connecting the cations and anionic complexes into a three-dimensional network.

Database survey
In the Cambridge Structure Database (Version 5.38, last update 2017; Groom et al., 2016) only one structure containing both 4-methoxypyridine and thiocyanate ligands is reported. It consists of discrete complexes with ruthenium(II) as the central cation coordinated by two thiocyanate anions and four 4-methoxypyridine molecules (Cadranel et al., 2016). The structures of several ferrate complexes are deposited where Fe II or Fe III cations are present. With Fe II , this includes ((C 2 H 5 ) 4 N) 4 [Fe(NCS) 6 ] (Krautscheid & Gerber, 1999) Hill, 2012). Several complexes in which the Fe III cation is octahedrally coordinated by six thiocyanate anions are also known, like in (C 4 H 12 N) 3 [Fe(SCN) 6 ]Á4H 2 O (Addison et al., 2005), or in [Ru(phen) 3 ](NCS)[Fe(NCS) 4 ]ÁH 2 O (phen: 1,10-phenanthroline), in which it is tetrahedrally coordinated (Ghazzali et al., 2008). Moreover, with pyridine as ligand and pyridinium as cation, two structures are reported with a coordination identical to those in the title compound. In the structure of (C 5 H 6 N) 2 [Fe(SCN) 5 (C 5 H 5 N)]ÁC 5 H 5 N, the Fe III cations are octahedrally coordinated by five thiocyanate anions and one pyridine ligand (Wood et al., 2015). In the structure of (C 5 H 6 N)[Fe(SCN) 4 (C 5 H 5 N) 2 ] the Fe III cations are coordinated by two neutral pyridine ligands and four thiocyanate anions (Shylin et al., 2013). However, structures in which three different coordination spheres are simultaneously present like in the title compound have not been reported to date. View of the four crystallographically independent 4-methoxypyridinium cations. Displacement ellipsoids are drawn at the 50% probability level. The disorder of one of the cations is shown with solid (major component) and open (minor component) bonds.

Synthesis and crystallization
Iron(II) chloride tetrahydrate was obtained from Sigma Aldrich, potassium thiocyanate from Fluka and 4-methoxypyridine from TCI. No further purification was carried out. 49.7 mg iron(II) chloride tetrahydrate (0.25 mmol) and 48.6 mg potassium thiocyanate (0.50 mmol) were reacted with 50.8 ml 4-methoxypyridine (0.50 mmol) in 2.0 ml water at room temperature. After stirring the mixture for three hours, the resulting powder was filtered off and the filtrate was let to evaporate slowly at room temperature. After several weeks single crystals suitable for single crystal X-ray analysis were obtained. The synthesis of larger and pure amounts of the title compound was not successful because in all batches additional crystalline phases were present ( Supplementary Fig. S1).

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 3. The C-H and N-H hydrogen atoms were located in a difference-Fourier map but were positioned with idealized geometry (methyl H atoms were allowed to rotate but not to tip), and refined with U iso (H) = 1.2U eq (C or N) (1.5 for methyl H atoms) using a riding model with C aromatic -H = 0.95 Å , C methyl -H = 0.98 Å and N-H = 0.88 Å . One of the four crystallographically independent 4-methoxypyridinium cations is disordered over two sets of sites and was refined with a split model using restraints. The sites with minor occupation (occupancy 0.22) were refined with isotropic displacement parameters, the sites of the major component with anisotropic displacement parameters.   SHELXL2014 (Sheldrick, 2015); molecular graphics: XP (Sheldrick, 2008) and DIAMOND (Brandenburg, 2014); software used to prepare material for publication: publCIF (Westrip, 2010).