Control of spin state by ring conformation of iron(III) porphyrins. A novel model for the quantum-mixed intermediate spin state of ferric cytochrome c' from photosynthetic bacteria

(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 H-1 NMR and X-ray crystallography. Both molecules are severely nonplanar and assume saddle shapes in solid state. Variable-temperature H-1 NMR studies confirm that the conformational distortions are maintained in solution with Delta G(double dagger) = 15.8 and 10.1 kcal/mol for ring inversion for Fe(OETPP)Cl and Fe(OMTPP)Cl, respectively. EPR (g(perpendicular to) = 5.2-5.3 at 77 K), magnetic moments (mu(eff) = 4.7-5.2 mu(B) at 300 K), and structural data (Fe-N-p = 2.03 Angstrom, Fe-Cl = 2.24-2.25 Angstrom) all indicate that unlike high-spin Fe-III(TPP)Cl and Fe-III(OEP)Cl (S = 5/2), Fe-III(OETPP)CI 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-2 upsilon. Other than the nonaxial symmetric EPR spectra of both complexes, H-1 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.