(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
C
2v
. 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.