Structure and mixed spin state of the chloroiron(III) complex of 2,3,7,8,12,13,17,18-octaphenyl-5,10,15,20-tetraphenylporphyrin, Fe(dpp)Cl

The chloroiron(III) complex Fe(dpp)Cl (1) of the peripherally crowded 2,3,7,8,12,13,17,18-octaphenyl-5,10,15,20-tetraphenylporphyrin or dodecaphenylporphyrin (dpp), has been synthesized and characterized by X-ray crystallography, normal-coordinate structural decomposition analysis (NSD), EPR and Mössbauer spectroscopy and magnetic susceptibility measurements. The dpp ligand present in the crystal structure of this molecule is predominantly saddle-shaped. It is also ruffled and slightly domed according to an analysis of the out-of-plane distortions performed by using NSD. This saddle deformation is significantly smaller than those observed in the chloroiron(III) complexes of the β-pyrrole octamethyl and octaethyl substituted tetraphenylporphyrins, Fe(omtpp)Cl and in the two crystalline forms known for Fe(oetpp)Cl (2, 2′). It is similar to that observed in the chloroiron(III) complex of the β-pyrrole hexaethyltetraphenylporphyrin, Fe(hetpp)Cl (6), in which the porphyrin ligand contains only three 3,4-diethyl substituted pyrrole rings. However, this saddle deformation present in 1 is still larger than those observed in all the cytochromes c′ and plant peroxidases whose heme structures were analyzed by NSD. The EPR spectrum of 1 shows that the contribution of the S=3/2 spin state to the S=5/2,3/2 quantum-mechanically spin-admixed state (QMS) is 6.75%. The Mössbauer spectrum obtained at room temperature and the magnetic susceptibility measured between 2 and 293 K of a powder sample of 1 are compatible with such a small S=5/2,3/2 spin admixture. However, although the saddle displacement (sad) of 2.75 Å found in Fe(dpp)Cl by NSD is slightly smaller than that of 2.90 Å observed in Fe(hetpp)Cl (6) or much smaller than that of 3.40 Å found in Fe(oetpp)Cl (2′), the S=3/2 contribution of 6.75% is larger than those observed in the other two compounds of 2.75 and 4.25%, respectively. This relatively large sad displacement of 2.75 Å observed in the porphyrin of 1 together with the small S=5/2,3/2 spin admixture of 6.75% indicates that the saddle distortion alone is probably not sufficient to cause the QMS states observed in several ferricytochromes c′ isolated from photosynthetic bacteria and in some plant peroxidases. The slightly larger S=5/2,3/2 spin admixture found in 1 (6.75%) relative to that of 2.75% observed in 6 could be related to the larger ruffling of the dpp ring of 1 relative to that of the hetpp ring present in 6. It may be that saddling along with other factors such as specific combinations of non-planar deformations, specific axial ligands and ligand geometries are necessary to realize a substantial S=5/2,3/2 spin admixture.