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

Abstract

The known iron­(II) complex [Fe^II(LN₃S)­(OTf)] (<b>1</b>) was used as starting material to prepare the new biomimetic (N₄S­(thiolate)) iron­(II) complexes [Fe^II(LN₃S)­(py)]­(OTf) (<b>2</b>) and [Fe^II(LN₃S)­(DMAP)]­(OTf) (<b>3</b>), 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, [Ni^II(LN₃S)]­(BF₄) (<b>5</b>), was also synthesized and characterized by structural and spectroscopic methods. Cyclic voltammetric studies showed <b>1</b>–<b>3</b> and <b>5</b> undergo a single reduction process with <i>E</i>_1/2 between −0.9 V to −1.2 V versus Fc⁺/Fc. Treatment of <b>3</b> with 0.5% Na/Hg amalgam gave the monoreduced complex [Fe­(LN₃S)­(DMAP)]⁰ (<b>4</b>), which was characterized by X-ray crystallography, UV-vis spectroscopic analysis, electron paramagnetic resonance (EPR) spectroscopy (<i>g =</i> [2.155, 2.057, 2.038]), and Mössbauer (δ = 0.33 mm s^–1; Δ<i>E</i>_Q = 2.04 mm s^–1) spectroscopy. Computational methods (DFT) were employed to model complexes <b>3</b>–<b>5</b>. The combined experimental and computational studies show that <b>1</b>–<b>3</b> are 5-coordinate, high-spin (<i>S</i> = 2) Fe^II complexes, whereas <b>4</b> is best described as a 5-coordinate, intermediate-spin (<i>S</i> = 1) Fe^II complex antiferromagnetically coupled to a ligand radical. This unique electronic configuration leads to an overall doublet spin (<i>S</i>_total = 1/2) ground state. Complexes <b>2</b> and <b>3</b> are shown to react with O₂ to give S-oxygenated products, as previously reported for <b>1</b>. In contrast, the monoreduced <b>4</b> appears to react with O₂ to give a mixture of sulfur oxygenates and iron oxygenates. The nickel­(II) complex <b>5</b> does not react with O₂, and even when the monoreduced nickel complex is produced, it appears to undergo only outer-sphere oxidation with O₂