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

Emile L. Bominaar, Xiao Qi Ding, Abbasher Gismelseed, Eckhard Bill, Heiner Winkler, Alfred X. Trautwein, Habib Nasri, Jean Fischer, Raymond Weiss

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Abstract

The ferrous and ferric form of a ("picket-fence" porphyrinato)(acetato)iron complex, [FeII/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: [FeII(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, Dcalc = 1.259 g cm3; space group P21/n. The five-coordinate iron atom is bonded to four porphyrinato nitrogens 〈Fe-Np〉 = 2.107 (14) Å and to an oxygen atom of the acetate ion (Fe-Oacetate = 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, ΔEQ = 4.25 mm s-1, nearly independent of temperature. In the ferric species, the quadrupole splitting, ΔEQ = 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 Ax,y = -17 T and Az = -13.3 T in the ferrous complex (spin S = 2) and D = 7.5 cm-1, E/D ≈ 0 and Ax,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 gx = 1.960, gy = 2.017, and gz = 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.

Original languageEnglish
Pages (from-to)1845-1854
Number of pages10
JournalInorganic Chemistry
Volume31
Issue number10
Publication statusPublished - 1992

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ASJC Scopus subject areas

  • Inorganic Chemistry

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