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

Abstract

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.

NMR Studies of Paramagnetic Metallocarbaporphyrinoids

“…[7,[81][82][83] This unique spectroscopic feature can be related to the extensive ruffling of the macrocycle, [82,[84][85][86] equatorial C-coordination, and/or axial pyridine coordination. [7] A conformational rearrangement process was detected which involves two structures differentiated by macrocyclic ruffling.…”

Hemoglobina extracelular gigante de Glossoscolex paulistus: um extraordinário sistema supramolecular hemoproteico

“…Recentemente, uma vasta literatura vem sendo publicada abordando as conformações do anel porfirínico, as relações com a configuração de spin eletrônico do centro férrico, suas implicações físico-químicas como reatividade e potencial redox e repercussões espectroscópicas nas várias técnicas empregadas. 137,146,[154][155][156][157][158] Dentre estas técnicas, vale mencionar o emprego amplamente difundido da ressonância magnética nuclear (RMN), fazendo uso, no caso do centro férrico (Fe(III)), do acoplamento hiperfino para sondar a conformação e as circunvizinhanças do anel porfirínico. O emprego desta técnica espectroscópica para avaliar a interação hiperfina abrange os núcleos dos ligantes axiais, os núcleos do anel porfirínico e também a interação com o spin nuclear dos núcleos pertencentes aos resíduos de aminoácidos formadores dos limites da cavidade do bolsão hidrofóbico do heme.…”

Proton NMR Study of Low‐Spin meso‐Unsubstituted β‐Substituted Alkyl Iron Porphyrins: Remarkable Influence of Peripheral Substitution on Spin Density

“…The former studies generally indicated that the electronic ground state was (d xy ) 2 (d xz ,d yz ) 3 for these complexes. However, more recently, it was shown that the bis(cyanido) complexes of meso-substituted Fe III porphyrins having alkyl substituents of increasing size [17] and other iron(III) porphyrins having bulky substituents in the meso position [18][19][20][21] show a large shift in the pyrrole protons towards the diamagnetic region, which is indicative of a large contribution of the unusual (d xz d yz ) 4 2 ] appear at very high magnetic field (δ = -10 to -20 ppm). This large upfield shift of the β-pyrrole protons indicates the existence of large spin densities on the β-pyrrole carbons, which is induced by the interactions between porphyrin 3e g and iron d π orbitals.…”