An Integrated Approach to the Mid-Spin State (S = 3/2) in Six-Coordinate Iron(III) Chiroporphyrins

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 a weak axial field strength and of a small macrocycle hole. Axial ligand exchange by reaction of chloroiron(III)tetramethylchiroporphyrin [(TMCP)FeCl] with silver perchlorate in ethanol−chloroform leads to ethanol-ligated ferric chiroporphyrins. Two distinct crystalline products containing 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 the ruffled 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 equatorial Fe−N(av) distances (1.950(5)−1.978(7) Å) found as yet in a ferric porphyrin, reflecting a singly occupied d z 2 orbital and a largely depopulated d x 2 - y 2 orbital. An intriguing case of bond-stretch isomerism is seen for the axial Fe−O bonds in two crystallographically independent mixed ethanol−water species, and it is accounted for by their distinct intra- and intermolecular hydrogen-bond arrays. The Mössbauer spectrum (δ = 0.35(1) mm s-1 and ΔE Q = 3.79(1) mm s-1 at 77 K) indicates a strong tetragonal distortion around the ferric ion, in agreement with the structural data. The value of the magnetic moment (μ eff = 3.8 μ B in the range 50−300 K) strongly supports a mid-spin state (S = 3/2). The EPR spectrum at 80 K (g ⊥ ≈ 4.0, g ∥ ≈ 2.00) is consistent with a nearly pure mid-spin state (4A2) with little rhombic distortion. The 1H NMR spectra in CDCl3−EtOH exhibit upfield-shifted resonances for the pyrrole protons (δ ≈ −30 ppm) which are consistent with the depopulated iron d x 2 - y 2 orbital. Solution equilibria with water and various alcohols, and the spin state of the corresponding species, are discussed on the basis of the NMR data. The bisethanol and ethanol−water species are potential models of unknown hemoprotein ligation states such as Tyr(OH)/Tyr(OH) or Tyr(OH)/H2O that could be obtained by site-directed mutagenesis.