STRUCTURAL, MOSSBAUER, AND EPR INVESTIGATIONS ON 2 OXIDATION-STATES OF A 5-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/III)(CH3CO2)(TPpivP)]-,0, were synthesized and characterized by UV-visible, H-1 NMR, EPR, and Mossbauer spectroscopy. The structure of the ferrous complex was determined by X-ray diffraction. Crystal data at -100-degrees-C: [Fe(II)(CH3CO2)(TPpivP)][Na subset-of C222].C6H5Cl(C90H108N10O12NaClFe); monoclinic; a = 18.040 (5), b = 21.521 (5), c = 22.605 (5) angstrom; beta = 100.37 (5)-degrees; Z = 4, D(calc) = 1.259 g cm3; space group P2(1)/n. The five-coordinate iron atom is bonded to four porphyrinato nitrogens [Fe-N(p)] = 2.107 (14) angstrom and to an oxygen atom of the acetate ion (Fe-O(acetate) = 2.034 (3) angstrom), placed inside the molecular cavity of the picket-fence porphyrin. Mossbauer spectra were recorded in the two oxidation states of thc 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, DELTA-E(Q) = 4.25 mm s-1, nearly independent of temperature. In the ferric species, the quadrupole splitting, DELTA-E(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 almost-equal-to 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.