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

• Published in: Inorganic chemistry Vol. 47; no. 13; pp. 5762 - 5774
• Main Authors: Grove, Laurie E, Hallman, Jason K, Emerson, Joseph P, Halfen, Jason A, Brunold, Thomas C
• Format: Journal Article
• Language: English
• Published: United States American Chemical Society 07.07.2008
• Subjects: Adducts; Azides (inorganic); Azides - chemistry; Constants; Crystallography; Crystallography, X-Ray; Electrons; Ferric Compounds - chemical synthesis; Ferric Compounds - chemistry; Ligands; Molecular Structure; Spectrum Analysis, Raman; Stretching; Superoxide dismutase; Superoxide Dismutase - chemistry; X-rays
• ISSN: 0020-1669; 1520-510X; 1520-510X
• DOI: 10.1021/ic800073t

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 (νFe−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 νFe−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.