MISSING HEME SPIN STATE AND A MODEL FOR CYTOCHROME C' - MIXED S = (3/2, 5/2) INTERMEDIATE SPIN FERRIC PORPHYRIN - PERCHLORATO(MESO-TETRAPHENYLPORPHINATO)IRON(III)

The uncommon d5 intermediate spin state is found in a series of mfjo-tetraphenylporphyrinatoiron(lll) complexes of general formula Fe(Y)(TPP) where Y is a so-called weak ligand (C1O4, BF4, PF6, SbF6, and CF3SO3). Synthesis is achieved by AgY metathesis with FeCl(TPP). The spin state is characterized by magnetic moments in the range μeff = 4.5-5.3 μB at 300 K. Detailed studies on the perchlorato derivative further characterize the spin state with a somewhat curved Curie-Weiss plot (4-300 K), Mössbauer data (ΔEq = 3.5, δ = 0.38 mm s-1 at 4.2 K), solid-state ESR (g⊥ = 4.75), and a single-crystal X-ray structure on the 0.5w-xylene solvate of Fe(OClO3)(TPP). Crystal data follow: monoclinic, a = 14.736 (3) Å, b = 15.519 (3) Å, c = 17.506 (3) Å, β = 95.17 (1)°; space group P21/n;Z = 4; pcaled = 1.378, pObsd = 1.379 g/cm3. The perchlorato ligand is monodentate with an unusually short Fe-O distance (2.029 (4) Å). The average Fe-N bond distance is 2.001 (5) Å with the iron(lll) atom displaced by an intermediate amount (0.3 Å) from the porphyrin plane. These dimensions are significantly shorter than those of high-spin five-coordinate ferric porphyrins and are consistent with depopulation of the dx2-y2 orbital. The solid-state g⊥ = 4.75 value together with the magnetic moment, which is greatly in excess of the S = 3/2 spin-only value, is interpreted in terms of a quantum mechanically mixed S = 3/2, 5/2 state remarkably similar to that of certain low-temperature cytochromes c'. Investigation of the solution behavior of Fe(OClC3)(TPP) by NMR, ESR, Mössbauer, and visible spectroscopy, however, suggests that a predominantly high-spin species exists in the solution phase. The relevance of axial ligand field strength changes to hemoprotein structure and spin state is discussed with particular reference to the histidine of the cytochromes c'. That a perchlorate ligand causes an intermediate spin complex while chloride, bromide, alkoxide, azide, etc., form high-spin ferric porphyrin complexes leads to the conclusion that perchlorate can be a weaker field ligand than previously supposed, despite relatively good binding. Qualitative crystal field arguments to rationalize the spin state require an interesting compensating dependence of the equatorial ligand field of the porphyrin upon that of the axial ligand. Copyright © 1979, American Chemical Society. All rights reserved.