Crystal structure of bis(4-methoxypyridine-κN)(meso-5,10,15,20-tetraphenylporphyrinato-κ4 N,N′,N′′,N′′′)iron(III) perchlorate

In the crystal structure of the title compound, the FeIII ions are octahedrally coordinated by four N atoms of a porphyrin moiety as well as two 4-methoxypyridine ligands into discrete complexes that are charge-balanced by perchlorate anions.

Crystal structure of bis(4-methoxypyridine-jN)- (meso-5,10,15,20-tetraphenylporphyrinatoj 4  In the crystal structure of the title compound, [Fe(C 44 H 28 N 4 )(C 6 H 7 NO) 2 ]ClO 4 , the Fe III ions are coordinated in an octahedral fashion by four N atoms of the porphyrin moiety and two N atoms of two 4-methoxypyridine ligands into discrete complexes that are located on inversion centers. Charge-balance is achieved by perchlorate anions that are disordered around twofold rotation axes. In the crystal structure, the discrete cationic complexes and the perchlorate anions are arranged into layers with weak C-HÁ Á ÁO interactions between the cations and the anions. The porphyrin moieties of neighboring layers show a herringbone-like arrangement.

Chemical context
Porphyrins are of great interest for a number of different applications in medicine and nature Shankar et al., 2018;Dommaschk et al., 2015). For example, metal porphyrins show spin crossover (SCO), which is the key step in a number of enzymatic reactions, e.g. catalysts in selective CH activation (cytochrome P450) (Konishi et al., 1992;Momenteau et al., 1983), hydrogen peroxide decomposition (catalases) (Maté et al., 2001) and a number of other biologically important processes (Collman et al., 1995;Gunter et al., 1994;Morgan & Dolphin, 1987). The spin state and electronic configuration of ferrous porphyrins are dependent on temperature, pressure, light or axial ligands. ISSN 2056-9890 Iron(III) porphyrins can exist in high-spin (S = 5 2 ), intermediate-spin (S = 3 2 ), admixed-spin (S = 3 2 , 5 2 ) and low-spin (S = 1 2 ) states of iron (Scheidt, 2000;Ikezaki et al., 2009;Nakamura, 2006;Shankar et al., 2018). Most of the anionic ligands such as chloride, hydroxide and azide lead to the formation of complexes in the high-spin state, whereas weak ligands like ClO 4 À and SbF 6 À usually give the complexes in an admixedspin state (Scheidt, 2000). However, six-coordinate complexes with strong axial ligands tend to be in the low-spin state (Scheidt, 2000). In our ongoing investigations on SCO compounds based on iron porphyrins, we became interested in the complex bis(4-methoxypyridine-N) (meso-5,10,15,20tetraphenylporphyrinato-4 N,N 0 ,N 00 ,N 000 iron(III) perchlorate, which was synthesized and characterized by high-resolution mass spectroscopy (Shankar et al., 2018). Preliminary investigations indicate that the complex is in the low-spin state but unfortunately no single crystals were obtained. In the course of subsequent investigations, we we able to obtain crystals by the layering technique starting from the Fe III tetraphenylporphyrin perchlorate complexes and using 4-methoxypyridine dissolved in dichloromethane as the lower and n-heptane as the upper layer. These crystals were identified by single crystal X-ray diffraction, which confirmed that crystals of the title compound were obtained.

Structural commentary
The crystal structure of the title compound consists of discrete complexes which lie on inversion centers. The Fe III ions are sixfold coordinated by four N atoms of the porphyrin moiety and two N atoms of two 4-methoxypyridine ligands in an octahedral coordination environment (Fig. 1). The Fe-N bond lengths to the porphyrin atoms of 1.9989 (13) Å and to the pyridine N atoms of 2.0002 (13) Å are nearly identical and the iron cations are located exactly in the plane of the coordinating porphyrin N atoms ( Table 1). The Fe-N bond lengths to the two axial 4-methoxypyridine ligands at 2.0 Å are typical for low-spin complexes (Geiger et al., 1985;Scheidt & Geiger, 1979), whereas high-spin complexes have a significant longer bond length of about 2.2 Å (Geiger et al., , 1985. The N-Fe-N bond angles within the equatorial porphyrin plane range between 88.56 (5) and 91.44 (5) , whereas that to the axial ligands are 180 because of symmetry, which proves that the octahedra are slightly distorted ( Table 1). The six-membered ring planes of the two coordinating 4-methoxypyridine ligands are eclipsed and rotated relative to the Fe-N bonds of the Fe III -porphyrin moiety (Fig. 2). Two of the four phenyl rings are nearly perpendicular to the porphyrin ring planes with a dihedral angle of 87.82 (5) , whereas the other two rings are rotated out of this plane by 63.64 (5) . The positive charge of the Fe IIIporphyrin moiety is compensated by one perchlorate anion that is disordered around a twofold rotation axis.

Figure 1
Molecular structure of the title compound with displacement ellipsoids drawn at the 50% probability level. Atoms with the suffix A are generated by the symmetry operation (1 À x, 1 À y, 1 À z). Table 1 Selected geometric parameters (Å , ).

Figure 2
Molecular structure of the title compound viewed onto the porphyrin plane.

Supramolecular features
In the crystal structure, the Fe-porphyrin cations and the perchlorate anions are each arranged in layers that are located parallel to the ab plane (Fig. 3). These layers are connected to the perchlorate anions by weak C-HÁ Á ÁO contacts (Table 2). For one of these contacts, the C-HÁ Á ÁO angle is close to linearity, indicating weak intermolecular hydrogen bonding ( Fig. 3 and Table 2). The porphyrin units of neighboring layers exhibit a herringbone-like arrangement (Fig. 4).

Synthesis and crystallization
Fe III tetraphenylporphyrin perchlorate (FeTPPClO 4 ) was synthesized as previously reported (Shankar et al., 2018). The layering technique was used for crystallization. The lower layer was dichloromethane with 50 mL 4-methoxypyridine and n-heptane was used for the upper antisolvent.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 3. The C-H hydrogen atoms were positioned with idealized geometries (C-H = 0.95-0.98 Å ; methyl H atoms allowed to rotate but not to tip) and were refined isotropically using a riding model with U iso (H) = 1.2U eq (C) or 1.5U eq (C-methyl). The perchlorate anion is disordered around a twofold rotation axis that passes through O1 and thus, disordered because of symmetry.  Crystal packing of the title compound viewed along the a axis. Table 2 Hydrogen-bond geometry (Å , ).   SHELXL2014 (Sheldrick, 2015); molecular graphics: XP (Sheldrick, 2008) and DIAMOND (Brandenburg, 2014); software used to prepare material for publication: publCIF (Westrip, 2010). 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.