Anion dependent redox changes in iron bis-terdentate nitroxide {NNO} chelates

The reaction of [Fe-II(BF4)(2)]center dot 6H(2)O with the nitroxide radical, 4,4-dimethyl-2,2-di(2-pyridyl) oxazolidine-N-oxide (L-center dot) produces the mononuclear transition metal complex [Fe-II(L-center dot)(2)](BF4)(2) (1) which has been investigated using temperature dependent susceptibility, Mossbauer spectroscopy, electrochemistry, density functional theory (DFT) calculations, and X-ray structure analysis. Single crystal X-ray diffraction analysis and Mossbauer measurements reveal an octahedral low spin Fe2+ environment where the pyridyl donors from L-center dot coordinate equatorially while the oxygen containing the radical from L-center dot coordinates axially forming a linear O-center dot center dot center dot Fe(II)center dot center dot O-center dot 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)center dot 0.5H(2)O (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.