Spontaneous Reduction of Mononuclear High-Spin Iron(III) Complexes to Mononuclear Low-Spin Iron(II) Complexes in Aqueous Media and Nuclease Activity via Self-Activation

Mononuclear high-spin [Fe-III(Pyimpy)Cl-3]center dot 2CH(2)Cl(2) (1 center dot 2CH(2)Cl(2)) and [Fe-III(Me-Pyimpy)Cl-3] (2), as well as low-spin Fe-II(Pyimpy)(2)](ClO4)(2) (3) and [FeII(Me-Pyimpy)(2)](ClO4)(2) (4) complexes of tridentate ligands Pyimpy and Me-Pyimpy have been synthesized and characterized by analytical techniques, spectral, and X-ray structural analyses. We observed an important type of conversion and associated spontaneous reduction of mono-chelated high-spin Fe-III (1 center dot 2CH(2)Cl(2) and 2) complexes to low-spin bis-chelated Fe-II complexes 3 and 4, respectively. This process has been explored in detail by UV/Vis, fluorescence, and H-1 NMR spectroscopic measurements. The high positive potentials observed in electrochemical studies suggested a better stabilization of Fe-II centers in 3 and 4. Theoretical studies by density functional theory (DFT) calculations supported an increased stabilization for 3 in polar solvents. Self-activated nuclease activity of complexes 1 center dot 2CH(2)Cl(2) and 2 during their spontaneous reduction was examined for the first time and the mechanism of nuclease activity was investigated.