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 FeIII(OETPP)Cl and (2,3,7,8,12,13,17,18-octamethyl-5,10,15,20-tetraphenylporphinato)iron(III) chloride FeIII(OMTPP)Cl complexes have been synthesized and characterized by 1H NMR and X-ray crystallography. Both molecules are severely nonplanar and assume saddle shapes in solid state. Variable-temperature 1H NMR studies confirm that the conformational distortions are maintained in solution with ΔG⧧ = 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 (μeff = 4.7−5.2 μB at 300 K), and structural data (Fe−Np = 2.03 Å, Fe−Cl = 2.24−2.25 Å) all indicate that unlike high-spin FeIII(TPP)Cl and FeIII(OEP)Cl (S = 5/2), FeIII(OETPP)Cl and FeIII(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 C2v. Other than the nonaxial symmetric EPR spectra of both complexes, 1H NMR spectrum of FeIII(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.