Anion Dependent Redox Changes in Iron Bis-terdentate Nitroxide {NNO} Chelates

The reaction of [Fe^II(BF₄)₂]·6H₂O with the nitroxide radical, 4,4-dimethyl-2,2-di(2-pyridyl) oxazolidine-N-oxide (L^•), produces the mononuclear transition metal complex [Fe^II(L^•)₂](BF₄)₂ (1) which has been investigated using temperature dependent susceptibility, Mössbauer spectroscopy, electrochemistry, density functional theory (DFT) calculations, and X-ray structure analysis. Single crystal X-ray diffraction analysis and Mössbauer measurements reveal an octahedral low spin Fe²⁺ environment where the pyridyl donors from L^• coordinate equatorially while the oxygen containing the radical from L^• coordinates axially forming a linear O^•··Fe(II)··O^• arrangement. Magnetic susceptibility measurements show a strong radical−radical intramolecular antiferromagnetic interaction mediated by the diamagnetic Fe²⁺ center. This is supported by DFT calculations which show a mutual spatial overlap of 0.24 and a spin density population analysis which highlights the antiparallel spin alignment between the two ligands. Similarly the monocationic complex [Fe^III(L⁻)₂](BPh₄)·0.5H₂O (2) has been fully characterized with Fe-ligand and N−O bond length changes in the X-ray structure analysis, magnetic measurements revealing a Curie-like S = 1/2 ground state, electron paramagnetic resonance (EPR) spectra, DFT calculations, and electrochemistry measurements all consistent with assignment of Fe in the (III) state and both ligands in the L⁻ form. 2 is formed by a rare, reductively induced oxidation of the Fe center, and all physical data are self-consistent. The electrochemical studies were undertaken for both 1 and 2, thus allowing common Fe-ligand redox intermediates to be identified and the results interpreted in terms of square reaction schemes.