Molecular structures and mixed spin states of chloroiron(III) complexes of the 2,3-diethyl-(detpp), 2,3,7,8-tetraethyl-(cis-tetpp), 2,3,12,13-tetraethyl-(trans-tetpp) and 2,3,7,8,12,13-hexaethyl-(hetpp) 5,10,15,20-tetraphenylporphyrin complexes

The chloroiron(III) complexes of the partially peripherally crowded 2,3-diethyl-2,3,12,13-tetraethyl-, 2,3,7,8-tetraethyl-, and 2,3,7,8,12,13-hexaethyl-5,10,15,20-tetraphenylporphyrins have been synthesised and their X-ray structures have been determined. The porphyrins present in these molecules are non-planar and assume asymmetric predominately saddle shapes. They are also slightly ruffled and domed according to an analysis of the out-of-plane distortions performed by using normal-coordinate structural decomposition (NSD). The saddle deformations, dominant in these chloroiron(III) complexes, are larger than those observed in all the cytochromes c’, whose structures were analysed by this method. Despite the large saddle-shaped distortions of the porphyrins present in these species, the quantum mechanical S = 3/2 spin admixtures into the S = 5/2 high-spin state (QMS state) observed are small. The EPR spectra of these Cβ-ethyl-substituted tetraphenylporphyrin complexes indicate S = 3/2 admixtures of 0% in Fe(detpp)Cl (1), 0.75% in Fe(trans-tetpp)Cl (2), 1.20% in Fe(cis-tetpp)Cl (3) and 2.75% in Fe(hetpp)Cl (4). The large saddle distortions of the porphyrins present in these compounds and the small S = 5/2,3/2 spin admixtures found indicate that the saddle distortions alone are probably not sufficient to cause the QMS states observed in several ferricytochromes c’ isolated from photosynthetic bacteria and in plant peroxidases.