STRUCTURAL MODEL FOR HEME IN HIGH-SPIN FERRIC HEMOPROTEINS - IRON ATOM CENTERING, PORPHINATO CORE EXPANSION, AND MOLECULAR STEREOCHEMISTRY OF HIGH-SPIN DIAQUO(MESO-TETRAPHENYLPORPHINATO)IRON(III) PERCHLORATE

The synthesis and characterization of a six-coordinate high-spin ferric porphyrin, diaquo(meso-tetra-phenylporphinato)iron(III) perchlorate ([Fe(TPP)(OH2)2]-C1O4) is described. Magnetic, Mössbauer, ESR, and structural characterization support a high-spin state (S = 5/2) assignment. Most importantly, the structural characterization of the title compound demonstrates that the large high-spin iron(III) atom is centered in the heme plane, in contrast to the commonly expected out-of-plane displacement of the iron(III) atom based on the structures of a number of other high-spin ferric porphyrins. In [Fe(TPP)(OH2)2]+, the large size of the high-spin iron(III) atom is accommodated by a radial expansion of the porphinato core with no displacement of the iron atom. The average Fe-N distance in the cen-trosymmetric molecule is 2.045 (8) Å, corresponding to an increase in the radius of the central hole of the porphinato ligand of ~0.055 Å relative to low-spin ferric derivatives. The axial Fe-water bond distance is 2.095 (2) Å. Crystal data for [Fe(TPP)(OH2)2]C1O4-2THF: α = 16.812 (2) k,b = 12.850 (2) Å, c = 21.346 (2) Å, orthorhombic, space group Pbcn, Z = 4, Pcalcd = 1-38 g/cm3, pobsd = 1.39 g/cm3. Porphinato core expansion and concomitant centering or near centering of the iron(III) atom in the heme plane are likely to occur in a number of high-spin ferric hemoproteins, including derivatives with thermal spin equilibria. The stereochemical features of [Fe(TPP)(OH2)2]+ provide a model for the geometry of heme in aquomethemoglobin. The data also suggest that considerable caution must be used when heme stereochemistry (coordination number) is assigned solely on magnetic properties. © 1979, American Chemical Society. All rights reserved.