Low-Spin Iron(III) Chiroporphyrins: (1)H NMR Studies of Cyanide and Substituted Imidazole Coordination

Low-spin complexes of iron(III) chiroporphyrin, obtained from (1R)-cis-caronaldehyde acid methyl ester and pyrrole as the atropisomer, with R-imidazoles and cyanide have been studied by means of 1D and 2D (1)H NMR spectroscopy. A complete spectral assignment of resonances has been done on the basis of observed scalar, NOE, and EXSY correlations in 2D COSY and NOESY experiments. The chemical shift of beta-H pyrrole resonances have been used as a sensitive probe of electronic state of iron(III) metal ion. Cyanide anion coordination both in methanol and methylene dichloride results in formation of bis(cyanide) low-spin complexes with the rare (d(xz)(),d(yz)())(4) (d(xy)())(1) electronic ground state, revealed by pyrrole beta-H resonances at 11.12 and 10.56 ppm at 293 K, whereas imidazole and 1-methylimidazole produce the bis-ligated complexes with the (d(xy)())(2)(d(xz)(),d(yz)())(3) ground state. In case of sterically hindered imidazole derivatives, i.e., 2-methylimidazole and 1,2-dimethylimidazole, two rotational isomers have been observed at 293 K. Both electronic configurations contribute to the ground state of metal ion for the latter. The steric bulkiness of 2-methylimidazole (or 1,2-dimethylimidazole) is required to freeze a favorable configuration, even at room temperature, providing the perpendicular orientation of two imidazole planes which seems to be instrumental in the stabilization of the rare (d(xz)()d(yz)())(4)(d(xy)())(1) electronic state.