Structural, Mössbauer, and EPR investigations on two oxidation states of a five-coordinate, high-spin synthetic heme. Quantitative interpretation of zero-field parameters and large quadrupole splitting

The ferrous and ferric form of a ("picket-fence" porphyrinato)(acetato)iron complex, [Fe^II/^IIICH₃CO₂)(TPPivP)] ^-,0, were synthesized and characterized by UV-visible, ¹H NMR, EPR, and Mössbauer spectroscopy. The structure of the ferrous complex was determined by X-ray diffraction. Crystal data at -100°C: [Fe^II(CH₃CO₂)(TPpivP)][NaC222]·C ₆H₅Cl (C₉₀H₁₀₈N₁₀O₁₂· NaClFe); monoclinic; a = 18.040 (5), b = 21.521 (5), c = 22.605 (5) Å; β = 100.37 (5)°; Z = 4, D_calc = 1.259 g cm³; space group P2₁/n. The five-coordinate iron atom is bonded to four porphyrinato nitrogens 〈Fe-N_p〉 = 2.107 (14) Å and to an oxygen atom of the acetate ion (Fe-O_acetate = 2.034 (3) Å), placed inside the molecular cavity of the picket-fence porphyrin. Mössbauer spectra were recorded in the two oxidation states of the complex at temperatures varying from 1.5 to 200 K in fields of 0-6.21 T. The ferrous complex has a large quadrupole splitting, ΔE_Q = 4.25 mm s^-1, nearly independent of temperature. In the ferric species, the quadrupole splitting, ΔE_Q = 1.1 mm s^-1, is as normally found in ferric high-spin iron porphyrins. The spin-Hamiltonian analysis of the spectra yields the zero-field parameters D = -0.9 cm^-1 and E/D = 0.33 and the magnetic hyperfine parameters A_x,y = -17 T and A_z = -13.3 T in the ferrous complex (spin S = 2) and D = 7.5 cm^-1, E/D ≈ 0 and A_x,y,z = -20 T in the ferric species (S = 5/2). The values of the zero-field parameters of the ferric species are confirmed by EPR analysis; the g values are g_x = 1.960, g_y = 2.017, and g_z = 2.00. The zero-field splittings and effective g values in the ferric complex are interpreted in terms of a crystal-field model. Theoretical estimates of the quadrupole splitting and zero-field parameters in the ferrous complex are given on the basis of molecular-orbital calculations. The relation between the zero-field tensor (D) and electronic and X-ray structure in the ferrous species is discussed.