Control of Spin State by Ring Conformation of Iron(III) Porphyrins. A Novel Model for the Quantum-Mixed Intermediate Spin State of Ferric Cytochrome<i>c</i>‘ from Photosynthetic Bacteria

Abstract

(2,3,7,8,12,13,17,18-Octaethyl-5,10,15,20-tetraphenylporphinato)iron(III) chloride Fe III (OETPP)Cl and (2,3,7,8,12,13,17,18-octamethyl-5,10,15,20-tetraphenylporphinato)iron(III) chloride Fe III (OMTPP)Cl complexes have been synthesized and characterized by 1 H NMR and X-ray crystallography. Both molecules are severely nonplanar and assume saddle shapes in solid state. Variable-temperature 1 H NMR studies confirm that the conformational distortions are maintained in solution with Δ G ⧧ = 15.8 and 10.1 kcal/mol for ring inversion for Fe(OETPP)Cl and Fe(OMTPP)Cl, respectively. EPR ( g ⊥ = 5.2−5.3 at 77 K), magnetic moments (μ eff = 4.7−5.2 μ B at 300 K), and structural data (Fe−N p = 2.03 Å, Fe−Cl = 2.24−2.25 Å) all indicate that unlike high-spin Fe III (TPP)Cl and Fe III (OEP)Cl ( S = 5/2), Fe III (OETPP)Cl and Fe III (OMTPP)Cl these complexes are of the uncommon quantum-mixed S = 5/2, 3/2 intermediate-spin state. Saddle-shaped ring deformations lower the symmetries of the complexes into C 2v . Other than the nonaxial symmetric EPR spectra of both complexes, 1 H NMR spectrum of Fe III (OETPP)Cl shows large asymmetry to the methylene proton shifts. Certain cytochromes c ‘ from photosynthetic bacteria reported to be of similar quantum-mixed intermediate-spin and showed EPR signals of rhombic symmetry have been noted to be with saddle-shaped deformations. These anomalous spin states and electronic structure asymmetry are ascribed to the ring deformation of the porphyrin macrocycle.