An integrated approach to the mid-spin state (S=3/2) in six-coordinate iron(III) chiroporphryrins

An intermediate-spin state very close to the mid-spin state (S = 3/2) can b e stabilized in a ferric porphyrin by an integrated approach which combines the favorable effects of a weak axial field strength and of a small macroc ycle hole. Axial ligand exchange by reaction of chloroiron(III)tetramethylc hiroporphyrin [(TMCP)FeCl] with silver perchlorate in ethanol-chloroform le ads to ethanol-ligated ferric chiroporphyrins. Two distinct crystalline pro ducts containing a bisethanol complex [[(TMCP)Fe-III(EtOH)(2)]ClO4] and thr ee variants of a mixed ethanol-water complex [[(TMCP)Fe-III(EtOH)(H2O)]ClO4 ] have been structurally characterized in the solid state. The small hole o f the ruffled chiroporphyrin and the weak axial oxygen ligation result in s trongly tetragonally distorted complexes. The sh-coordinate species exhibit long axial Fe-O bond distances (2.173(5)-2.272(4) Angstrom) and the shorte st equatorial Fe-N(av) distances (1.950(5)-1.978(7) Angstrom) found as yet in a ferric porphyrin, reflecting a singly occupied d(z)(2) orbital and a l argely depopulated d(x2-y2) orbital. An intriguing case of bond-stretch iso merism is seen for the axial Fe-O bonds in two crystallographically indepen dent mixed ethanol-water species, and it is accounted for by their distinct intra- and intermolecular hydrogen-bond arrays. The Mossbauer spectrum (de lta = 0.35(1) mm s(-1) and Delta E-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 (mu(eff) = 3.8 mu(B) in the range 50-300 K) strongly supports a mid-spin state (S = 3/2). The EPR s pectrum at 80 K (g(perpendicular to) approximate to 4.0, g(parallel to) app roximate to 2.00) is consistent with a nearly pure mid-spin state ((4)A(2)) With little rhombic distortion. The H-1 NMR spectra in CDCl3-EtOH exhibit upfield-shifted resonances for the pyrrole protons (delta approximate to -3 0 ppm) which are consistent with the depopulated iron d(x2-y2) orbital. Sol ution equilibria with water and various alcohols, and the spin state of the corresponding species, are discussed on the basis of the NMR data. The bis ethanol and ethanol-water species are potential models of unknown hemoprote in ligation states such as Tyr(OH)/Tyr(OH) or Tyr(OH)/H2O that could be obt ained by site-directed mutagenesis.