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

摘要

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, UV-vis, ¹H 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 E1/2 between −0.9 to −1.2 V versus Fc⁺/Fc. Treatment of >3 with 0.5% Na/Hg amalgam gave the mono-reduced complex [Fe(LN3S)(DMAP)]⁰ (>4), which was characterized by X-ray crystallography, UV-vis, EPR (g = [2.155, 2.057, 2.038]) and Mössbauer (δ = 0.33 mm s⁻¹; ΔEQ = 2.04 mm s⁻¹) spectroscopies. 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 (Stotal = ½) 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 mono-reduced >4 appears to react with O2 to give a mixture of S- and Fe-oxygenates. The nickel(II) complex >5 does not react with O2, and even when the mono-reduced nickel complex is produced, it appears to undergo only outer-sphere oxidation with O2.