Oxoiron(IV) complexes as synthons for the assembly of heterobimetallic centers such as the Fe/Mn active site of Class Ic ribonucleotide reductases

Nonheme oxoiron(IV) complexes can serve as synthons for generating heterobimetallic oxo-bridged dimetal complexes by reaction with divalent metal complexes. The formation of Fe ^III –O–Cr ^III and Fe ^III –O–Mn ^III complexes is described herein. The latter complexes may serve as models for the Fe ^III –X–Mn ^III active sites of an emerging class of Fe/Mn enzymes represented by the Class 1c ribonucleotide reductase from Chlamydia trachomatis and the R2-like ligand-binding oxidase (R2lox) found in Mycobacterium tuberculosis. These synthetic complexes have been characterized by UV–Vis, resonance Raman, and X-ray absorption spectroscopy, as well as electrospray mass spectrometry. The Fe ^III –O–Cr ^III complexes exhibit a three-band UV–Vis pattern that differs from the simpler features associated with Fe ^III –O–Fe ^III complexes. The positions of these features are modulated by the nature of the supporting polydentate ligand on the iron center, and their bands intensify dramatically in two examples upon the binding of an axial cyanate or thiocyanate ligand trans to the oxo bridge. In contrast, the Fe ^III –O–Mn ^III complexes resemble Fe ^III –O–Fe ^III complexes more closely. Resonance Raman characterization of the Fe ^III –O–M ^III complexes reveals an ¹⁸ O-sensitive vibration in the range of 760–890 cm ⁻¹ . This feature has been assigned to the asymmetric Fe ^III –O–M ^III stretching mode and correlates reasonably with the Fe–O bond distance determined by EXAFS analysis. The likely binding of an acetate as a bridging ligand to the Fe ^III –O–Mn ^III complex 12 lays the foundation for further efforts to model the heterobimetallic active sites of Fe/Mn enzymes.