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

Abstract

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) A) and the shortest equatorial Fe-N(av) distances (1.950(5)-1.978(7) A) found as yet in a ferric porphyrin, reflecting a singly occupied dz2 orbital and a largely depopulated dx2-y2 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 (delta = 0.35(1) mm s-1 and delta EQ = 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 spectrum at 80 K (g perpendicular approximately 4.0, g parallel approximately 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 (delta approximately -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 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.

Recent developments in the 13 C NMR spectroscopic analysis of paramagnetic hemes and heme proteins

“…Ferrihemes possessing the S=/2, (d xy ) 2 (d xz , d yz ) 2 (d z 2 ) 1 spin state have been shown to exhibit complicated distortions from planarity [85,98,99], which suggests they might exist in solution as a complex mixture of interconverting conformers with similar energies. Nonplanar hexacoordinated ferrihemes possessing a pure S=3/2 spin state also exhibit a unique pattern of 13 C NMR shifts [86] with very large downfield β shifts (≈1,000 ppm), large downfield C α shifts (≈600 ppm), and large upfield C m shifts (≈-300 ppm) (Fig.…”

Enantiomer Resolution of Intrinsically Chiral C21‐Alkylated N‐Confused Porphyrin Complexes

“…[1][2][3][4] There are several strategies for designing chiral porphyrins involving the introduction of chiral substituents onto the macrocyclic periphery of the predefined macrocycle, or the annulation of building blocks bearing such substituents; [2][3][4][5][6][7][8][9][10] this often yields systems of defined configuration of the chiral macrocycles which do not require enantiomer separation. [11] Another approach relies on the modification of prochiral systems such as etioporphyrin I, protoporphyrin IX, or mono-b-substituted meso-aryl porphyrins, leading to a differentiation of the macrocycles faces, and thus to planar chirality.…”

Characterization of Three Members of the Electron‐Transfer Series [Fe(pda)2]n (n=2−, 1−, 0) by Spectroscopy and Density Functional Theoretical Calculations [pda=Redox Non‐innocent Derivatives of N,N′‐Bis(pentafluorophenyl)‐o‐phenylenediamide(2−, 1.−, 0)]

“…Ferric species of intermediate-spin are more common for five-coordinate square-pyramidal [6,14] and six-coordinate distorted octahedral species of C 4v symmetry. [30] According to simple considerations from crystal field (CF) theory, the valence contribution to the EFG at an intermediate-spin Fe III center should vanish with a resulting quadrupole splitting close to zero. [31] In contrast, we observe DE Q = 4.45 mm s .…”