Crystal Structure of a Six-coordinated (2,3,7,8,12,13,17,18-Octaethylporphyrinato)iron(III) Complex with Two 4-Methylpyridine N-Oxides

The six-coordinated iron(III) porphyrin complexes with the neutral oxygen as axial ligands has attracted much attention of chemists and biochemists who are working to reveal the functions and catalytic processes of heme proteins as well as their model complexes.1 Recently, we reported that the sixcoordinated iron(III) porphyrin complexes with suitably substituted pyridine N-oxides as axial ligands exhibit a quite rare spin-crossover phenomena between high-spin (S = 5/2) and low-spin (S = 1/2) states, if the ligand field of substituted pyridine N-oxides as axial ligands is stronger.2 This is one of the examples that show the importance of model studies to gain deeper insight into enzymatic mechanisms in naturally occurring heme proteins. In this work, we report on the crystal structure of a six-coordinated (2,3,7,8,12,13,17,18-octaethylporphyrinato)iron(III), [Fe(OEP)(4-MePyNO)2]BF4, which have eight ethyl groups at the β-pyrrole position and two 4-methylpyridine N-oxides at the axial position. Its chemical structure is depicted in Fig. 1. The general synthetic procedure was carried out according to previous literature.2,3 The precursor iron(III) porphyrin complex, Fe(OEP)Cl, was purchased from Sigma-Aldrich. The complex was converted to tetrafluoroborate complexes Fe(OEP)BF4 by the reaction with AgBF4 (4 eq.) in a THF solution. The crude solid thus obtained was purified by recrystallization from CH2Cl2, THF, and hexane. [Fe(OEP)(4-MePyNO)2]BF4 was prepared by the addition of the axial ligands 4-methylpyridine N-oxides (13.3 mg, 122 μmol) into a CH2Cl2 solution of Fe(OEP)BF4 (20.5 mg, 30.4 μmol). An analytically pure solid sample was obtained by recrystallization from a CH2Cl2/hexane mixed solvent solution, which was identified by 1H NMR, UV-Vis, and ESI-TOF-mass, as described in Supporting Information. Crystals suitable for X-ray structure analysis were obtained from CH2Cl2/toluene/hexane mixed solvent solutions for [Fe(OEP)(4-MePyNO)2]BF4. An X-ray measurement of the single crystal was performed with a Rigaku VariMax Saturn-724 (1.2 kW Mo rotating anode) at 100 K. A single crystal for X-ray analysis was obtained by the diffusion method. X-ray diffraction data were processed using CrysAlisPro, followed by the CrystalStructure Ver. 4.2. 1 crystallographic software package. The structure was solved from the processed data using SIR 2011,4 and refined on F2 by full-matrix least-squares using the SHELXL-2014/7 program package.5 All calculations were performed using the CrystalStructure Ver. 4.2. 1 crystallographic software package. Crystal data and details concerning the data collection are given in Table 1. All nonhydrogen atoms were refined anisotropically. Hydrogen atoms were located in the calculated positions and refined by using riding models. There were two independent porphyrin molecules in an asymmetric unit cell. The axial ligand of 4-MePyNO and the counter anion of BF4 were disordered over two positions with an occupancy ratio of 0.5021(16):0.4979(16). The final structure was validated by using the Platon CIF check. 2017 © The Japan Society for Analytical Chemistry