<i>meso</i> Substituent Effects on the Geometric and Electronic Structures of High-Spin and Low-Spin Iron(III) Complexes of Mono-<i>m</i><i>eso</i>-Substituted Octaethylporphyrins

Introduction of a single meso substituent into ClFeIII(OEP) or K[(NC)2Fe(OEP)] results in significant changes in the geometric and/or spectroscopic properties of these complexes. The mono-meso-substituted iron(III) complexes ClFeIII(meso-Ph-OEP), ClFeIII(meso-n-Bu-OEP), ClFeIII(meso-MeO-OEP), ClFeIII(meso-Cl-OEP), ClFeIII(meso-NC-OEP), ClFeIII(meso-HC(O)-OEP), and ClFeIII(meso-O2N-OEP) have been isolated and characterized by their UV/vis and paramagnetically shifted 1H NMR spectra. The structures of both ClFeIII(meso-Ph-OEP) and ClFeIII(meso-NC-OEP) have been determined by X-ray crystallography. Both molecules have five-coordinate structures typical for high-spin (S = 5/2) iron(III) complexes. However, the porphyrins themselves no longer have the domed shape seen in ClFeIII(OEP), and the N4 coordination environment possesses a slight rectangular distortion. These high-spin, mono-meso-substituted iron(III) complexes display 1H NMR spectra in chloroform-d solution which indicate that the conformational changes seen in the solid-state structures are altered by normal molecular motion to produce spectra consistent with Cs molecular symmetry. In pyridine solution the high-spin six-coordinate complexes {(py)ClFeIII(meso-R-OEP)} form. In methanol solution in the presence of excess potassium cyanide, the low-spin six-coordinate complexes K[(NC)2FeIII(meso-R-OEP)] form. The 1H NMR spectra of these show that electron-donating substituents produce an upfield relocation of the meso-proton chemical shifts. This relocation is interpreted in terms of increased contribution from the less common (dxz,dyz)4(dxy)1 ground electronic state as the meso substituent becomes more electron donating.