Synthesis, X-Ray Crystallographic Characterization, and Electronic Structure Studies of a Di-Azide Iron(III) Complex: Implications for the Azide Adducts of Iron(III) Superoxide Dismutase

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• 作者：Laurie E. Grove ; Jason K. Hallman ; Joseph P. Emerson ; Jason A. Halfen ; Thomas C. Brunold
• 刊名：Inorganic Chemistry
• 出版年：2008
• 出版时间：July 7, 2008
• 年：2008
• 卷：47
• 期：13
• 页码：5762 - 5774
• 全文大小：1864K
• 年卷期：v.47,no.13(July 7, 2008)
• ISSN：1520-510X

文摘

We have synthesized and characterized, using X-ray crystallographic, spectroscopic, and computational techniques, a six-coordinate diazide Fe³⁺ complex, LFe(N₃)₂ (where L is the tetradentate ligand 7-diisopropyl-1,4,7-triazacyclononane-1-acetic acid), that serves as a model of the azide adducts of Fe³⁺ superoxide dismutase (Fe³⁺SOD). While previous spectroscopic studies revealed that two distinct azide-bound Fe³⁺SOD species can be obtained at cryogenic temperatures depending on protein and azide concentrations, the number of azide ligands coordinated to the Fe³⁺ ion in each species has been the subject of some controversy. In the case of LFe(N₃)₂, the electronic absorption and magnetic circular dichroism spectra are dominated by two broad features centered at 21 500 cm⁻¹ (ε ≈ 4000 M⁻¹ cm⁻¹) and ~30 300 cm⁻¹ (ε ≈ 7400 M⁻¹ cm⁻¹) attributed to N₃⁻ → Fe³⁺ charge transfer (CT) transitions. A normal coordinate analysis of resonance Raman (RR) data obtained for LFe(N₃)₂ indicates that the vibrational features at 363 and 403 cm⁻¹ correspond to the Fe−N₃ stretching modes (ν_Fe−N3) associated with the two different azide ligands and yields Fe−N₃ force constants of 1.170 and 1.275 mdyne/Å, respectively. RR excitation profile data obtained with laser excitation between 16 000 and 22 000 cm⁻¹ reveal that the ν_Fe−N3 modes at 363 and 403 cm⁻¹ are preferentially enhanced upon excitation in resonance with the N₃⁻ → Fe³⁺ CT transitions at lower and higher energies, respectively. Consistent with this result, density functional theory electronic structure calculations predict a larger stabilization of the molecular orbitals of the more strongly bound azide due to increased σ-symmetry orbital overlap with the Fe 3d orbitals, thus yielding higher N₃⁻ → Fe³⁺ CT transition energies. Comparison of our data obtained for LFe(N₃)₂ with those reported previously for the two azide adducts of Fe³⁺SOD provides compelling evidence that a single azide is coordinated to the Fe³⁺ center in each protein species.