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

The known iron­(II) complex [Fe^II(LN₃S)­(OTf)] (1) was used as starting material to prepare the new biomimetic (N₄S­(thiolate)) iron­(II) complexes [Fe^II(LN₃S)­(py)]­(OTf) (2) and [Fe^II(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, [Ni^II(LN₃S)]­(BF₄) (5), was also synthesized and characterized by structural and spectroscopic methods. Cyclic voltammetric studies showed 1–3 and 5 undergo a single reduction process with E_1/2 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; ΔE_Q = 2.04 mm s^–1) spectroscopy. Computational methods (DFT) were employed to model complexes 3–5. The combined experimental and computational studies show that 1–3 are 5-coordinate, high-spin (S = 2) Fe^II complexes, whereas 4 is best described as a 5-coordinate, intermediate-spin (S = 1) Fe^II complex antiferromagnetically coupled to a ligand radical. This unique electronic configuration leads to an overall doublet spin (S_total = 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.