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.