Models for the Active Site of Oxygen-Binding Hemoproteins. Dioxygen Binding Properties and the Structures of (2-Methylimidazole)-meso-tetra(α,α,α,α-o-Pivalamidophenyl)porphyrinatoiron(II)-Ethanol and Its Dioxygen Adduct

When crystals of (2-methylimidazole)-mcro-tetra(α,α,α,α-o-pivalamidophenyl)porphyrinatoiron(II)-ethanol, Fe(TpivPP)(2-MeIm)-EtOH, are exposed to dioxygen, the crystals of the resultant dioxygen adduct are still suitable for diffraction studies. The direct, precise determination of the stereochemical changes accompanying oxygenation of an iron(ll)- (porphyrinato)(base) complex has been carried out using conventional X-ray diffraction methods. The structures have been refined by full-matrix, least-squares methods, using 4176 and 5183 reflections for the deoxy and oxy complexes, respectively, to R indices on F² of 0.162 and 0.120. For the portion of data where F₀² > 3σ(F₀²) the respective indices on F are 0.086 and 0.083. Crystal data for the deoxy compound follow: space group C⁶_2h-C2/c, Z = 4, molecular symmetry C2, a = 18.871 (11) Å, b = 19.425 (13) Å, c = 18.434 (11) Å, β = 91.48 (3)°, V = 6755.0 Å3. The oxy complex is nearly isomorphic with Z = 4 in space group C2/c with a cell of dimensions a = 18.864 (5) Å, b = 19.451 (5) Å, c = 18.287 (5) Å, β = 91.45 (2)°, V = 6707.0 Å3. Some selected parameters for the coordination spheres, with those in square brackets pertaining to the dioxygen adduct, follow: Fe-N_porph = 2.068 (5), 2.075 (5) [1.997 (4), 1.995 (4)] Å; Fe-N_1m = 2.095 (6) [2.107 (4)] Å. The iron atom is displaced 0.399 [0.086] Å from the least-squares plane of the porphinato nitrogen atoms toward the imidazole ligand. The Fe-O separation is 1.898 (7) Å. The average O-O separation is 1.22 (2) Å and the Fe-O-O angle is 129 (1)°, In the presence of ethanol the deoxy complex binds dioxygen reversibly, noncooperatively, and with lower affinity than when the sample is desolvated- in the latter case dioxygen uptake has been found to be cooperative. The structure and properties of these possible models for T-state deoxy- and oxyhemoglobin are correlated and then compared with the 1-methylimidazole analogue. The sterically active 2-methyl substituent appears to perturb the Fe-O bond but not the Fe-N|m bond.