Oxoiron(IV) units are often implicated as intermediates in the catalytic cycles of non-heme iron oxygenases and oxidases. The most reactive synthetic analogues of these intermediates are supported by tetradentate tripodal ligands with N-methylbenzimidazole or quinoline donors, but their instability precludes structural characterization. Herein we report crystal structures of two [FeIV(O)(L)]2+ complexes supported by pentadentate ligands incorporating these heterocycles, which show longer average Fe–N distances than the complex with only pyridine donors. These longer distances correlate linearly with log k2′ values for O- and H-atom transfer rates, suggesting that weakening the ligand field increases the electrophilicity of the Fe=O center. The sterically bulkier quinoline donors are also found to tilt the Fe=O unit away from a linear N-Fe=O arrangement by 10°.
Bibliographical noteFunding Information:
This work was supported by grants from the U.S. National Science Foundation (CHE-1665391 to L.Q. and CHE-1654060 to Y.G.). The National High Magnetic Field Laboratory is supported by U.S. National Science Foundation through Cooperative Agreement DMR-1644779 and the State of Florida. The Bruker-AXS D8 Venture Diffractometer was purchased through a grant from NSF/MRI 1229400 and the University of Minnesota. We thank Prof. Steven Kass for the use of their FTIR spectrometer and the reviewers for their helpful suggestions.
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- hydrogen atom transfer (HAT)
- non-heme iron complexes
- oxoiron(IV) complexes
- two-state reactivity