meso Substituent Effects on the Geometric and Electronic Structures of High-Spin and Low-Spin Iron (III) Complexes of Mono-meso-Substituted Octaethylporphyrins

Introduction of a single meso substituent into ClFe^III(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 ClFe^III(meso-Ph-OEP), ClFe^III(meso-n-Bu-OEP), ClFe^III(meso-MeO-OEP), ClFe^III(meso-Cl-OEP), ClFe^III(meso-NC-OEP), ClFe^III(meso-HC(O)-OEP), and ClFe^III(meso-O₂N-OEP) have been isolated and characterized by their UV/vis and paramagnetically shifted ¹H NMR spectra. The structures of both ClFe^III(meso-Ph-OEP) and ClFe^III(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 ClFe^III(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 C_s molecular symmetry. In pyridine solution the high-spin six-coordinate complexes {(py)-ClFe^III(meso-R-OEP)} form. In methanol solution in the presence of excess potassium cyanide, the low-spin six-coordinate complexes K[(NC)₂Fe^III(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 (d_xz,d_yz)⁴(d_xy)¹ ground electronic state as the meso substituent becomes more electron donating.