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

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 1–3 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 3–5. The combined experimental and computational studies show that 1–3 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.已知的二价铁配合物[FeII(LN3S)(OTf)]（1）作为起始原料，用于制备新型仿生（N4S-硫醇盐）二价铁配合物[FeII(LN3S)(py)](OTf)（2）与[FeII(LN3S)(DMAP)](OTf)（3），其中LN3S为带有共价连接苯硫醇盐给体的四齿双(亚胺)吡啶（BIP）衍生物。上述配合物通过X射线晶体学、紫外-可见（UV-vis）光谱分析、1H核磁共振（NMR）、穆斯堡尔（Mössbauer）光谱以及电化学方法完成表征。一种二价镍类似物[NiII(LN3S)](BF4)（5）也被合成，并通过结构与光谱方法进行了表征。循环伏安研究表明，1~3与5均存在单还原过程，其半波电位E1/2相对于二茂铁鎓/二茂铁（Fc+/Fc）介于-0.9 V至-1.2 V之间。以0.5%钠汞齐处理配合物3，得到单还原配合物[Fe(LN3S)(DMAP)]0（4），该产物通过X射线晶体学、紫外-可见光谱分析、电子顺磁共振（EPR）光谱（g = [2.155, 2.057, 2.038]）以及穆斯堡尔光谱（δ = 0.33 mm s–1；ΔEQ = 2.04 mm s–1）完成表征。采用计算方法（密度泛函理论，DFT）对配合物3~5进行建模。结合实验与计算研究结果表明，1~3均为五配位高自旋（S=2）二价铁配合物，而配合物4则可最优描述为五配位中间自旋（S=1）的二价铁配合物，且与配体自由基发生反铁磁耦合。这种独特的电子构型使其整体呈现二重态自旋（Stotal = 1/2）基态。研究显示，配合物2与3可与氧气反应生成S位氧化产物，这与此前报道的配合物1的反应行为一致。与之相反，单还原产物4与氧气反应则生成硫氧化物与铁氧化物的混合物。二价镍配合物5不与氧气发生反应，即便生成单还原镍配合物，其与氧气也仅发生外层球氧化反应。