Wydział Chemii

Jacek Wojaczyński

Lechosław Latos-Grażyński

Witold Hrycyk

Ewa Pacholska

Krystyna Rachlewicz

Ludmiła Szterenberg

1996

Inorganic Chemistry

35

6861-6872

10.1021/ic960772p

Naukowa

Angielski

Artykuł

The NMR spectra of a series of β-substituted iron(III) tetraphenylporphyrin (2-X-TPP) complexes have been studied to elucidate the relationship between the electron donating/withdrawing properties of the 2-substituent and the ¹H NMR spectral pattern. The electronic nature of the substituent has been significantly varied and covered the −0.6 to 0.8 Hammett constant range. Both high-spin and low-spin complexes of the general formula (2-X-TPP)Fe^IIICl and [(2-X-TPP)Fe^III(CN)₂]^- have been investigated. The ¹H NMR data for the following substituents (X) have been reported:  py⁺, NO₂, CN, CH₃, BzO (C₆H₅COO), H, D, Br, Cl, CH₃, NH₂, NH₃⁺, NHCH₃, OH, and O^-. The ¹H NMR resonances for low-spin dicyano complexes have been completely assigned by a combination of two-dimensional COSY and NOESY experiments. In the case of selected high-spin complexes, the 3-H resonance has been identified by the selective deuteration of all but the 3-H position. The pattern of unambiguously assigned seven pyrrole resonances reflects the asymmetry imposed by 2-substitution and has been used as an unique ¹H NMR spectroscopic probe to map the spin density distribution. The pyrrole isotropic shifts of [(2-X-TPP)Fe^III(CN)₂]^- are dominated by the contact term. In order to quantify the substituent effect, the dependence of isotropic shift of all low-spin pyrrole resonances and 3-H high-spin pyrrole resonance versus Hammett constants has been studied. The electronic effect is strongly localized at the β-substituted pyrrole. The major change of the isotropic shift has also been noted for only one of two adjacent pyrrole rings, i.e., at 7-H and 8-H positions. These neighboring protons, located on a single pyrrole ring, experienced opposite shift changes when electron withdrawing/donating properties were modified. Two other pyrrole rings for all investigated derivatives revealed considerably smaller, substituent related, isotropic shift changes. A long-range secondary isotopic shift has been observed for [(2-D-TPP)Fe^III(CN)₂]^-. The effect is consistent with a general spin density distribution mechanism due to β-substitution. A fairly good correlation between the 3-H isotropic shift of (2-X-TPP)Fe^IIICl and the Hammett constant has been found as well. The observed contact shift pattern of [(2-X-TPP)Fe^III(CN)₂]^- reflects spin π delocalization into the highest filled MO equivalent to the unsubstituted porphyrin 3e(π) orbital. To account for the substituent contribution, the semiquantitative Fenske−Hall LCAO method has been used to determine the molecular orbitals involved in the spin density delocalization. For low-spin complexes, ¹³C pyrrole resonances of carbons bearing a proton have been identified by means of a ¹H−¹³C HMQC experiment. The reversed order of ¹³C resonance patterns as compared to their ¹H NMR counterparts has been determined, e.g., the largest isotropic shift of 3-H has been accompanied by the smallest measured ¹³C isotropic shift. Analysis of the isotropic shifts in (2-X-TPP)Fe^IIICl and [(2-X-TPP)Fe^III(CN)₂]^- suggests that the observed regularities of the electronic structure modification due to the β-substitution should apply to iron(III) natural porphyrin or geoporphyrin complexes.

Iron, Pyrroles, Substituents, Quantum mechanics, Resonance structures

https://doi.org/10.1021/ic960772p

https://www.acs.org/content/acs/en.html