| Abstract
| - An intermediate-spin state very close to the mid-spin state (S = 3/2) can be stabilized in a ferric porphyrin by anintegrated approach which combines the favorable effects of a weak axial field strength and of a small macrocyclehole. Axial ligand exchange by reaction of chloroiron(III)tetramethylchiroporphyrin [(TMCP)FeCl] with silverperchlorate in ethanol−chloroform leads to ethanol-ligated ferric chiroporphyrins. Two distinct crystalline productscontaining a bisethanol complex [[(TMCP)FeIII(EtOH)2]ClO4] and three variants of a mixed ethanol−water complex[[(TMCP)FeIII(EtOH)(H2O)]ClO4] have been structurally characterized in the solid state. The small hole of theruffled chiroporphyrin and the weak axial oxygen ligation result in strongly tetragonally distorted complexes.The six-coordinate species exhibit long axial Fe−O bond distances (2.173(5)−2.272(4) Å) and the shortest equatorialFe−N(av) distances (1.950(5)−1.978(7) Å) found as yet in a ferric porphyrin, reflecting a singly occupied dz2orbital and a largely depopulated dx2-y2 orbital. An intriguing case of bond-stretch isomerism is seen for the axialFe−O bonds in two crystallographically independent mixed ethanol−water species, and it is accounted for bytheir distinct intra- and intermolecular hydrogen-bond arrays. The Mössbauer spectrum (δ = 0.35(1) mm s-1 andΔEQ = 3.79(1) mm s-1 at 77 K) indicates a strong tetragonal distortion around the ferric ion, in agreement withthe structural data. The value of the magnetic moment (μeff = 3.8 μB in the range 50−300 K) strongly supportsa mid-spin state (S = 3/2). The EPR spectrum at 80 K (g⊥ ≈ 4.0, g∥ ≈ 2.00) is consistent with a nearly puremid-spin state (4A2) with little rhombic distortion. The 1H NMR spectra in CDCl3−EtOH exhibit upfield-shiftedresonances for the pyrrole protons (δ ≈ −30 ppm) which are consistent with the depopulated iron dx2-y2 orbital.Solution equilibria with water and various alcohols, and the spin state of the corresponding species, are discussedon the basis of the NMR data. The bisethanol and ethanol−water species are potential models of unknownhemoprotein ligation states such as Tyr(OH)/Tyr(OH) or Tyr(OH)/H2O that could be obtained by site-directedmutagenesis.
- An intermediate-spin state very close to the mid-spin state (S = 3/2) can be stabilized in a ferric porphyrin by an integrated approach which combines the favorable effects of weak axial ligation (ethanol and water molecules) and of strong equatorial field strength (ruffled chiroporphyrin).
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