Robert D. Orosz

Characterization of the admixed intermediate-spin complex bis(3-cyanopyridine)(octaethylporphinato)iron(III) perchlorate

The preparation of bis(3-cyanopyridine)(octaethylporphinato)iron(III) perchlorate is described. The molecular structure has been determined by an X-ray crystal structure determination. The centrosymmetric complex has coplanar axial pyridine ligands that are close to eclipsing an FeN p bond (φ=4°).The average equatorial bond distance is 2.012(3) A. The axial Fe-N(Py) bond distance is 2.269(6) A. The complex has been further characterized by EPR and Mossbauer spectroscopy and temperature dependent magnetic susceptibility measurements between 1.8 and 300 K. The complex has an axial EPR spectrum with g ⊥ =4.28 and g∥=1.97. The Mossbauer spectrum in zero field at 77 K has an isomer shift of 0.38 mm/s and a quadrupole split doublet of 2.61 mm/s. The Mossbauer has also been studied in an applied magnetic field (6 T) at 4.2 K and fitted with a crystal field model. The structure and spin state of this compound serve to emphasize an emerging pattern in the relationship of axial ligand orientation (φ angles) and sopin state. Crystal data: [Fe(OEP)(3-CNPy) 2 ]ClO 4 ·3CHCl 3 : a =10.463(7), b =24.805(26), c = 11.661(4) A, and β= 109.60(6)°, monoclinic, space group P2 1 /m, V =2851.0 A3, Z =2, observed data = 3118, R 1 =0.099, R 2 =0.102.

Comparative study of the optical and magnetic circular dichroism spectra of bis-thioether and -imidazole complexes of iron(III) tetraphenyl- and octaethyl-porphyrin. Models of haem co-ordination in bacterioferritins

The absorption and low-temperature (4.2 K) magnetic circular dichroism (MCD) spectra of the low-spin iron(III) complexes [Fe(oep)(tht)2]+, [Fe(tpp)(tht)2]+, [Fe(oep)(Him)2]+ and [Fe(tpp)(Him)2]+, where oep is 2,3,7,8,12,13,17,18-octaethylporphyrinate, tpp is 5,10,15,20-tetraphenylporphyrinate, tht is tetrahydrothiophene and Him is imidazole, have been compared to provide spectra of well defined bis-co-ordinated iron(III) porphyrin models for the diagnosis of axial haem ligation in proteins. Comparison of the shapes of the porphyrin-to-FeIII charge-transfer (c.t.) bands and the magnitudes of the MCD C terms in the near-infrared region show that the electronic spectra of low-spin iron(III) complexes of oep, and hence of protoporphyrin IX (3,7,12,17-tetramethyl-8,13-divinylporphyrin-2,18-dipropanoic acid), have only one transition, namely, a1u(π)→ eg(d) whereas the spectra of complexes of tpp contain two one-electron transitions a1u, a2u(π)→ eg(d). Substitution of a phenyl group for a hydrogen atom at the porphyrin methine bridge causes a large energy shift of the a2u→ eg(d) transition but leaves the other one unaffected. On changing the axial ligation of [Fe(oep)]+ from bis(imidazole) to bis(tht) the energy of the a1u(π)→ eg(d) c.t. band shifts from 6330 (1580) to 4700 cm–1(2130 nm). The energy shift is twice that observed when bis(imidazole)ligation is changed to imidazole–thioether ligation showing that the shifts of the c.t. bands obey an approximate additivity rule. The EPR and MCD spectra of the complex [Fe(oep)(tht)2]+ match well the spectra of the haem group in the bacterial iron storage protein, bacterioferritin, and hence provide strong support for the assignment of bis(methionine) co-ordination of the haem in this protein, a ligation scheme not previously reported for haem in any protein.