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Synthesis and Ligand Non-Innocence of Thiolate-Ligated (N4S) Iron(II) and Nickel(II) Bis(imino)pyridine Complexes

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journal contribution
posted on 16.09.2013, 00:00 by Leland R. Widger, Yunbo Jiang, Maxime A. Siegler, Devesh Kumar, Reza Latifi, Sam P. de Visser, Guy N. L. Jameson, David P. Goldberg
The known iron­(II) complex [FeII(LN3S)­(OTf)] (1) was used as starting material to prepare the new biomimetic (N4S­(thiolate)) iron­(II) complexes [FeII(LN3S)­(py)]­(OTf) (2) and [FeII(LN3S)­(DMAP)]­(OTf) (3), where LN3S is a tetradentate bis­(imino)­pyridine (BIP) derivative with a covalently tethered phenylthiolate donor. These complexes were characterized by X-ray crystallography, ultraviolet–visible (UV-vis) spectroscopic analysis, 1H nuclear magnetic resonance (NMR), and Mössbauer spectroscopy, as well as electrochemistry. A nickel­(II) analogue, [NiII(LN3S)]­(BF4) (5), was also synthesized and characterized by structural and spectroscopic methods. Cyclic voltammetric studies showed 13 and 5 undergo a single reduction process with E1/2 between −0.9 V to −1.2 V versus Fc+/Fc. Treatment of 3 with 0.5% Na/Hg amalgam gave the monoreduced complex [Fe­(LN3S)­(DMAP)]0 (4), which was characterized by X-ray crystallography, UV-vis spectroscopic analysis, electron paramagnetic resonance (EPR) spectroscopy (g = [2.155, 2.057, 2.038]), and Mössbauer (δ = 0.33 mm s–1; ΔEQ = 2.04 mm s–1) spectroscopy. Computational methods (DFT) were employed to model complexes 35. The combined experimental and computational studies show that 13 are 5-coordinate, high-spin (S = 2) FeII complexes, whereas 4 is best described as a 5-coordinate, intermediate-spin (S = 1) FeII complex antiferromagnetically coupled to a ligand radical. This unique electronic configuration leads to an overall doublet spin (Stotal = 1/2) ground state. Complexes 2 and 3 are shown to react with O2 to give S-oxygenated products, as previously reported for 1. In contrast, the monoreduced 4 appears to react with O2 to give a mixture of sulfur oxygenates and iron oxygenates. The nickel­(II) complex 5 does not react with O2, and even when the monoreduced nickel complex is produced, it appears to undergo only outer-sphere oxidation with O2.