Reductive Activation of O2 by Non-Heme Iron(II) Benzilate Complexes of N4 Ligands: Effect of Ligand Topology on the Reactivity of O2‑Derived Oxidant

A series of iron­(II) benzilate complexes (1–7) with general formula [(L)­FeII(benzilate)]+ have been isolated and characterized to study the effect of supporting ligand (L) on the reactivity of metal-based oxidant generated in the reaction with dioxygen. Five tripodal N4 ligands (tris­(2-pyridylmethyl)­amine (TPA in 1), tris­(6-methyl-2-pyridylmethyl)­amine (6-Me3-TPA in 2), N1,N1-dimethyl-N2,N2-bis­(2-pyridylmethyl)­ethane-1,2-diamine (iso-BPMEN in 3), N1,N1-dimethyl-N2,N2-bis­(6-methyl-2-pyridylmethyl)­ethane-1,2-diamine (6-Me2-iso-BPMEN in 4), and tris­(2-benzimidazolylmethyl)­amine (TBimA in 7)) along with two linear tetradentate amine ligands (N1,N2-dimethyl-N1,N2-bis­(2-pyridylmethyl)­ethane-1,2-diamine (BPMEN in 5) and N1,N2-dimethyl-N1,N2-bis­(6-methyl-2-pyridylmethyl)­ethane-1,2-diamine (6-Me2-BPMEN in 6)) were employed in the study. Single-crystal X-ray structural studies reveal that each of the complex cations of 1–3 and 5 contains a mononuclear six-coordinate iron­(II) center coordinated by a monoanionic benzilate, whereas complex 7 contains a mononuclear five-coordinate iron­(II) center. Benzilate binds to the iron center in a monodentate fashion via one of the carboxylate oxygens in 1 and 7, but it coordinates in a bidentate chelating mode through carboxylate oxygen and neutral hydroxy oxygen in 2, 3, and 5. All of the iron­(II) complexes react with dioxygen to exhibit quantitative decarboxylation of benzilic acid to benzophenone. In the decarboxylation pathway, dioxygen becomes reduced on the iron center and the resulting iron–oxygen oxidant shows versatile reactivity. The oxidants are nucleophilic in nature and oxidize sulfide to sulfoxide and sulfone. Furthermore, complexes 2 and 4–6 react with alkenes to produce cis-diols in moderate yields with the incorporation of both the oxygen atoms of dioxygen. The oxygen atoms of the nucleophilic oxidants do not exchange with water. On the basis of interception studies, nucleophilic iron­(II) hydroperoxides are proposed to generate in situ in the reaction pathways. The difference in reactivity of the complexes toward external substrates could be attributed to the geometry of the O2-derived iron–oxygen oxidant. DFT calculations suggest that, among all possible geometries and spin states, high-spin side-on iron­(II) hydroperoxides are energetically favorable for the complexes of 6-Me3-TPA, 6-Me2-iso-BPMEN, BPMEN, and 6-Me2-BPMEN ligands, while high spin end-on iron­(II) hydroperoxides are favorable for the complexes of TPA, iso-BPMEN, and TBimA ligands.

本研究合成并表征了一系列通式为[(L)FeII(benzilate)]+的二苯乙醇酸铁(II)配合物（iron(II) benzilate complexes）1~7，旨在探究辅助配体（L）对与氧气（dioxygen）反应生成的金属基氧化剂反应活性的影响。研究中共使用7种配体，包括5种三足N4配体：三(2-吡啶甲基)胺（tris(2-pyridylmethyl)amine, TPA，对应配合物1）、三(6-甲基-2-吡啶甲基)胺（tris(6-methyl-2-pyridylmethyl)amine, 6-Me3-TPA，对应配合物2）、N1,N1-二甲基-N2,N2-双(2-吡啶甲基)乙二胺（N1,N1-dimethyl-N2,N2-bis(2-pyridylmethyl)ethane-1,2-diamine, iso-BPMEN，对应配合物3）、N1,N1-二甲基-N2,N2-双(6-甲基-2-吡啶甲基)乙二胺（N1,N1-dimethyl-N2,N2-bis(6-methyl-2-pyridylmethyl)ethane-1,2-diamine, 6-Me2-iso-BPMEN，对应配合物4）以及三(2-苯并咪唑甲基)胺（tris(2-benzimidazolylmethyl)amine, TBimA，对应配合物7）；另外还包含2种线性四齿胺配体：N1,N2-二甲基-N1,N2-双(2-吡啶甲基)乙二胺（N1,N2-dimethyl-N1,N2-bis(2-pyridylmethyl)ethane-1,2-diamine, BPMEN，对应配合物5）以及N1,N2-二甲基-N1,N2-双(6-甲基-2-吡啶甲基)乙二胺（N1,N2-dimethyl-N1,N2-bis(6-methyl-2-pyridylmethyl)ethane-1,2-diamine, 6-Me2-BPMEN，对应配合物6）。单晶X射线结构表征结果显示，配合物1~3、5的阳离子均含单核六配位铁(II)中心，配位单元包含单阴离子苯乙醇酸根；而配合物7的阳离子为单核五配位铁(II)中心。在配合物1和7中，苯乙醇酸根通过其中一个羧酸氧原子以单齿模式与铁中心配位；在配合物2、3和5中，苯乙醇酸根则通过羧酸氧原子与中性羟基氧原子以双齿螯合模式配位。所有上述二苯乙醇酸铁(II)配合物均可与氧气反应，实现苯乙醇酸的定量脱羧，生成二苯甲酮。在该脱羧路径中，氧气在铁中心被还原，所生成的铁-氧氧化剂展现出多样反应活性：该氧化剂具有亲核性，可将硫化物氧化为亚砜（sulfoxide）和砜（sulfone）。此外，配合物2、4~6可与烯烃反应，以中等产率生成顺式二醇（cis-diols），且反应过程中氧气的两个氧原子均被掺入产物中。该亲核氧化剂的氧原子不会与水发生交换。通过捕获实验研究，我们提出该反应路径中原位生成了亲核性铁(II)氢过氧化物。不同配合物对外来底物的反应活性差异可归因于O2衍生的铁-氧氧化剂的几何构型差异。密度泛函理论（DFT, Density Functional Theory）计算结果表明，在所有可能的几何构型与自旋态中，对于搭载6-Me3-TPA、6-Me2-iso-BPMEN、BPMEN及6-Me2-BPMEN配体的配合物，高自旋侧式配位铁(II)氢过氧化物在能量上更为有利；而对于搭载TPA、iso-BPMEN及TBimA配体的配合物，高自旋端基配位铁(II)氢过氧化物更为稳定。