Structural, Magnetic, and Dynamic Characterization of the (d xz ,d yz ) 4 (d xy ) 1 Ground-State Low-Spin Iron(III) Tetraphenylporphyrinate Complex [(p-TTP)Fe(2,6-XylylNC) 2 ]CF 3 SO 3 Ge ´ rard Simonneaux,* ,² Volker Schu 1 nemann, ‡ Christophe Morice, ² Laurence Carel, ² Loı 1 c Toupet, § Heiner Winkler, ‡ Alfred X. Trautwein,* ,‡ and F. Ann Walker* ,⊥ Contribution from the Laboratoire de Chimie Organome ´ tallique et Biologique, UMR CNRS 6509, UniVersite ´ de Rennes 1, Campus de Beaulieu, 35042 Rennes Cedex, France, Institut fu ¨ r Physik, Medizinische UniVersita ¨ t zu Lu ¨ beck, 160 Ratzeburger Allee, D-23538 Lu ¨ beck, Germany, Groupe de Physique Cristalline, UA CNRS 040804, UniVersite ´ de Rennes 1, Campus de Beaulieu, 35042 Rennes Cedex, France, and Department of Chemistry, UniVersity of Arizona, Tucson, Arizona 85721-0041 ReceiVed NoVember 30, 1999 Abstract: The synthesis and characterization of the trifluoromethanesulfonate salt of bis(2,6-xylyl isocyanide)- tetrakis(p-tolyl)porphyrinatoiron(III), [(p-TTP)Fe(2,6-xylylNC) 2 ]CF 3 SO 3 (1), is reported. The crystal structure shows that the porphyrinate ring is strongly ruffled. The equatorial Fe-N bond distances average to 1.961(7) Å for 1, which is quite short for low-spin iron(III) porphyrinate derivatives. Two additional complexes, having TPP (2) and m-TTP (3) as the porphyrinates, were also synthesized and studied by NMR, EPR, and Mo ¨ssbauer spectroscopy. All physical properties are consistent with a low-spin iron(III) porphyrinate with the less-common ground-state configuration (d xz ,d yz ) 4 (d xy ) 1 . The 1 H NMR chemical shift of the pyrrole protons at 297 K is +10.7 ppm for 1. The EPR spectrum of 1 in solution is axial, with g ⊥ ) 2.15 and g || ) 1.94, ∑g 2 ) 13.0, while in the solid state g ⊥ ) 2.2 and g || ) 1.94, ∑g 2 ) 13.4. The Mo ¨ssbauer spectrum of 1 at 190 K has an isomer shift of 0.14 mm/s and quadrupole splitting of 1.81 mm/s. Magnetic Mo ¨ssbauer spectra analyzed in the intermediate spin-spin relaxation regime by the dynamic line-shape formalism of Blume and Clauser confirm this electron configuration and yield large negative quadrupole splittings, ∆E Q )-1.8 to -2.0 mm/s for the three complexes. To our knowledge, this is the first case in which the Mo ¨ ssbauer spectra of low-spin ferriheme systems have been analyzed in terms of the effect of intermediate rates of spin fluctuations on the appearance of the spectra. Analysis of the temperature dependence of the quadrupole splitting, ∆E Q , for 2 yielded a different estimate of the energy separation between the (d xz ,d yz ) 4 (d xy ) 1 ground state and an excited state than did the temperature dependence of the NMR isotropic shifts. It is postulated that the excited state is actually the planar transition state between the two ruffled conformations of the porphyrinate that are related by “inversion”. To explain the temperature dependence of both NMR isotropic shifts and Mo ¨ssbauer quadupole splittings, the planar transition state must have the (d xy ) 2 (d xz ,d yz ) 3 electron configuration. The energy barrier appears to be smaller in homogeneous solution than in the solid state and is considerably lower than that predicted for the (d xz ,d yz ) 3 (d xy ) 2 excited electronic state of the ruffled conformation on the basis of the EPR g values. Introduction Investigations of the structural and spectroscopic properties of Fe(III) porphyrinates have been useful in understanding both structural and electronic properties of the bis-histidine- coordinated cytochromes b 5 1 and c 3 , 2,3 as well as the membrane- bound cytochromes b with bis(histidine) coordination 4,5 includ- ing the two b cytochromes of mitochondrial “complex III” (also known as ubiquinone-cytochrome c oxidoreductase) and chlo- roplast cytochrome b 6 . EPR spectroscopy, 6 and in some cases Mo ¨ ssbauer spectroscopy, 7 of well-defined low-spin heme model compounds with high basicity pyridines, imidazoles, or cyanides as the axial ligands has shown that their “large g max ” EPR signal is indicative of the (d xy ) 2 (d xz ,d yz ) 3 electronic ground state and near degeneracy of d xz and d yz . For complexes with planar axial ligands this correlates with perpendicular alignment of these ligands, 7-10 while rhombic EPR signals are correlated with parallel alignment of planar axial ligands. Tetraphenylporphinatoiron(III) bis(4-cyanopyridine)perchlo- rate, [(TPP)Fe(4-CNPy) 2 ]ClO 4 , 11 for which the axial ligands ² Laboratoire de Chimie Organome ´tallique et Biologique, Universite ´ de Rennes 1. ‡ Medizinische Universita ¨t zu Lu ¨beck. § Groupe de Physique Cristalline, UA CNRS 040804, Universite ´ de Rennes 1. ⊥ University of Arizona. (1) Mathews, F. S.; Czerwinski, E. W.; Argos. P. 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