Ueda Kaho, Sato Wataru, Yanagisawa Sachiko, Kubo Minoru, Hada Masahiko, Fujii Hiroshi
Department of Chemistry, Biology, and Environmental Science, Graduate School of Humanities and Sciences, Nara Women's University, Nara 630-8506, Japan.
Graduate School of Science, University of Hyogo, Ako, Hyogo 678-1297, Japan.
J Inorg Biochem. 2024 Jun;255:112544. doi: 10.1016/j.jinorgbio.2024.112544. Epub 2024 Mar 29.
Resonance Raman (rR) spectroscopy has been applied to study the nature of the iron-oxo (Fe=O) moiety of oxoiron(IV) porphyrin π-cation radical complex (CompI). While the axial ligand effect on the nature of the Fe=O moiety has been studied with rR spectroscopy, the porphyrin ligand effect has not been studied well. Here, we investigated the porphyrin ligand effect on the Fe=O moiety with rR spectroscopy. The porphyrin ligand effect was modulated by the electron-withdrawing effect of the porphyrin substituent at the meso-position. This study shows that the frequency of the Fe=O stretching band, ν(Fe=O), hardly change even when the electron-withdrawing effect of the porphyrin substituent changes. This result is further supported by theoretical calculation of CompI. The natural atomic charge analysis reveals that the oxo and axial ligands work to buffer the electron-withdrawing effect of the porphyrin substituent. The electron-withdrawing porphyrin substituent shifts an electron population from the ferryl iron to the porphyrin, but the decreased electron population on the ferryl iron is compensated by the shift of the electron population from the oxo ligand and the axial ligand. The shift of the electron population makes the Fe-axial ligand bond length short, but the Fe=O bond length unchanged, resulting in the invariable ν(Fe=O) frequency.
共振拉曼（rR）光谱已被用于研究氧合铁（IV）卟啉π-阳离子自由基配合物（化合物I）的铁-氧（Fe=O）部分的性质。虽然已经通过rR光谱研究了轴向配体对Fe=O部分性质的影响，但卟啉配体的影响尚未得到充分研究。在此，我们用rR光谱研究了卟啉配体对Fe=O部分的影响。卟啉配体的影响通过中位卟啉取代基的吸电子效应来调节。这项研究表明，即使卟啉取代基的吸电子效应发生变化，Fe=O伸缩带的频率ν(Fe=O)也几乎不变。化合物I的理论计算进一步支持了这一结果。自然原子电荷分析表明，氧和轴向配体起到缓冲卟啉取代基吸电子效应的作用。吸电子的卟啉取代基将电子云从高价铁转移到卟啉上，但高价铁上减少的电子云通过氧配体和轴向配体的电子云转移得到补偿。电子云的转移使Fe-轴向配体键长缩短，但Fe=O键长不变，导致ν(Fe=O)频率不变。
