A Nonheme Thiolate-Ligated Cobalt Superoxo Complex: Synthesis and Spectroscopic Characterization, Computational Studies, and Hydrogen Atom Abstraction Reactivity

The synthesis and characterization of a Co­(II) dithiolato complex Co­(Me3TACN)­(S2SiMe2) (1) are reported. Reaction of 1 with O2 generates a rare thiolate-ligated cobalt–superoxo species Co­(O2)­(Me3TACN)­(S2SiMe2) (2) that was characterized spectroscopically and structurally by resonance Raman, EPR, and X-ray absorption spectroscopies as well as density functional theory. Metal–superoxo species are proposed to S-oxygenate metal-bound thiolate donors in nonheme thiol dioxygenases, but 2 does not lead to S-oxygenation of the intramolecular thiolate donors and does not react with exogenous sulfur donors. However, complex 2 is capable of oxidizing the O–H bonds of 2,2,6,6-tetramethylpiperidin-1-ol derivatives via H atom abstraction. Complementary proton-coupled electron-transfer reactivity is seen for 2 with separated proton/reductant pairs. The reactivity studies indicate that 2 can abstract H atoms from weak X–H bonds with bond dissociation free energy (BDFE) ≤ 70 kcal mol–1. DFT calculations predict that the putative Co­(OOH) product has an O–H BDFE = 67 kcal mol–1, which matches the observed pattern of reactivity seen for 2. These data provide new information regarding the selectivity of S-oxygenation versus H atom abstraction in thiolate-ligated nonheme metalloenzymes that react with O2.本文报道了一种二价钴二硫醇盐配合物Co(Me3TACN)(S2SiMe2)(1)的合成与表征。1与氧气(O₂)的反应生成一种罕见的硫醇盐配位钴超氧物种Co(O₂)(Me3TACN)(S2SiMe2)(2)，本文通过共振拉曼光谱、电子顺磁共振(Electron Paramagnetic Resonance, EPR)、X射线吸收光谱以及密度泛函理论(Density Functional Theory, DFT)对其进行了光谱与结构表征。此前研究认为，非血红素硫醇双加氧酶中的金属超氧物种可对金属配位的硫醇盐供体进行S-氧合反应，但物种2并不会使分子内硫醇盐供体发生S-氧合，也不与外源性硫供体发生反应。然而，配合物2可通过氢原子攫取过程氧化2,2,6,6-四甲基哌啶-1-醇衍生物的O-H键。配合物2与分离的质子/还原剂对反应时，展现出互补的质子耦合电子转移反应活性。反应性研究表明，配合物2可从键解离自由能(Bond Dissociation Free Energy, BDFE)≤70 kcal mol⁻¹的弱X-H键中攫取氢原子。密度泛函理论计算表明，推测生成的Co(OOH)产物的O-H键解离自由能为67 kcal mol⁻¹，这与配合物2所展现出的反应活性模式相符。上述数据为与氧气反应的硫醇盐配位非血红素金属酶中，S-氧合与氢原子攫取的选择性问题提供了新的认知。