Structure of the Deoxymyoglobin Model [Fe(TPP)(2-MeHIm)] Reveals Unusual Porphyrin Core Distortions

The preparation and characterization of the deoxymyoglobin model (2-methylimidazole)(tetraphenylporphinato)iron(II) is described. The preparation and crystallization from chlorobenzene leads to a new crystalline phase that has
been structurally characterized. The complex is the most ordered example of a deoxymyoglobin model yet
characterized. The X-ray structure determination reveals a number of distortions both in the iron coordination
group and in the porphyrin core that result from the steric bulk of the axial ligand. Some of these distortions have
been noted previously in related species; however, the demonstration of porphyrin core distortions and an asymmetry
in the Fe−Np bond distances are new observations. These may have functional significance for this important type
of heme protein coordination group. The new structure emphasizes that high-spin iron(II) porphyrinate derivatives
display substantial structural pliability with significant variations in iron atom displacements, porphyrin core hole
size, and axial and equatorial Fe−N bond lengths. The new complex has also been characterized by zero-field and
applied field magnetic Mössbauer spectroscopy. Mössbauer parameters are characteristic for high-spin iron, although
they also reveal an extremely rhombic site for iron(II). Crystal data at 130 K for [Fe(TPP)(2-MeHIm)]·1.5C6H5Cl: 
a = 12.334(3) Å, b = 13.515(6) Å, c = 14.241(7) Å, α = 70.62(3)°, β = 88.29(2)°, γ = 88.24(3)°, triclinic,
space group, P1̄, V = 2238(2) Å3, Z = 2.