Structural, Moessbauer, 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

Abstract

P2,/n. The five-coordinate iron atom is bonded to four porphyrinato nitrogens (Fe-N,) = 2.107 (14) A and to an oxygen atom of the acetate ion (FA,, = 2.034 (3) A), placed inside the molecular cavity of the picket-fence porphyrin. Mhsbauer 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, AEQ = 4.25 mm s-I, nearly independent of temperature. In the ferric species, the quadrupole splitting, AEQ = 1.1 mm s-', 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-' and E/D = 0.33 and the magnetic hyperfine parameters A,, = -17 T and A, = -13.3 T in the ferrous complex (spin S = 2) and D = 7.5 cm-I, E/D = 0 and Ax,,= = -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, = 1.960, g, = 2.017, and g, = 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. Parallel to the discovery of the unusually large quadrupole splitting, PE - 4 mm s-', in the ferrous state of the prosthetic group term4 p460 of the multiheme enzyme hydroxylamine oxidoreductase from the bacterium Nitrosomonas europeae,'V2 comparably large splittings were observed in a number of synthetic five-coordinate porphyrins of the formula [Fe(X)(Porph)]-, where X is an anionic oxygen- or nitrogen-donor ligand or a halide ion.38 In order to gain more insight into the electronic structure of the iron ions in these complexes, field- and temperature-dependent MBssbauer measurements have been performed in the synthetic phenolato porphyrinato complex [ Fe(OC6H,)(TPP)]- by Lang et a1.* For the derivation of a set of zero-field parameters consistent with both the susceptibility and MBssbauer data in the phenolate complex, these authors introduced weak antiferromagnetic exchange coupling between the paramagnetic iron centers.8 Strong correlations between the sign and magnitude of the zero-field parameter D and the value of the exchange-coupling constant were found in the simulations of the magnetic susceptibility data. However, the spin-Hamiltonian parameters inferred from the Mbsbauer spectra are rather insensitive to the weak intermolecular couplings. The value of the zero-field parameter D obtained by Lang et al. is small and negative (see Table I). The principal components of the magnetic hyperfine tensor (A) in the complex are negative, which is characteristic of a dominant Fermi contact interaction. Furthermore, the A tensor shows anisotropy, lAll < IA, 1, originating from spin-dipolar interaction. The anisotropy ob the A tensor and the positive sign of the main component of the electric-field-gradient (EFG) tensor indicate that the iron(I1) ion in the phenolate complex possesses a half-filled subshell of spin-parallel electrons plus an excessn electron of opposite spin in an oblate orbital of d, ty~,~*,' The weak temperature dependence of the quadrupole sphttmg in the range 1-300 K shows that the ground orbital state is energetically well-separated from excited orbital states by energy gaps typically 1 order