Octahedral Non-Heme Oxo and Non-Oxo Fe(IV) Complexes: An Experimental/Theoretical Comparison

Electron-transfer series are described for three ferric complexes of the pentadentate ligand 4,8,11-trimethyl-1,4,8,11-tetraazacyclotetradecane-1-acetate (Me3cyclam−acetate) with axial chloride, fluoride, and azide ligands. These complexes can all be reduced coulometrically to their Fe(II) analogs and oxidized reversibly to the corresponding Fe(IV) species. The Fe(II), Fe(III), and Fe(IV) species have been studied spectroscopically and their UV−vis, Mössbauer, EPR, and IR spectra are presented. The fluoro species [(Me3cyclam−acetate)FeF]n+ (n = 0, 1, 2) have been studied computationally using density functional theory (DFT), and the electronic structure of the Fe(IV) dication [(Me3cyclam−acetate)FeF]2+ is compared with that of the isoelectronic Fe(IV) oxo cation [(Me3cyclam−acetate)FeO]+; the different properties of the two species are mainly due to the significantly covalent FeO π bonds in the latter.