Control of Spin State by Ring Conformation of Iron(III) Porphyrins. A Novel Model for the Quantum-Mixed Intermediate Spin State of Ferric Cytochromec‘ 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 A, Fe−Cl = 2.24−2.25 A) 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 E...