Oxygen versus Sulfur Coordination in Cobalt Superoxo Complexes: Spectroscopic Properties, O₂ Binding, and H-Atom Abstraction Reactivity

A five-coordinate, disiloxide-ligated cobalt(II) (S = 3/2) complex (1) was prepared as an oxygen-ligated analogue to the previously reported silanedithiolate-ligated Co^II(Me₃TACN)(S₂SiMe₂) (J. Am. Chem. Soc., 2019, 141, 3641-3653). The structural and spectroscopic properties of 1 were analyzed by single-crystal X-ray diffraction, electron paramagnetic resonance (EPR), and NMR spectroscopies. The reactivity of 1 with dioxygen was examined, and it was shown to bind O₂ reversibly in a range of solvents at low temperatures. A cobalt(III)-superoxo complex, Co^III(O₂^·-)(Me₃TACN)((OSi^2Ph)₂O) (2), was generated, and was analyzed by UV-vis, EPR, and resonance Raman spectroscopies. Unlike its sulfur-ligated analogue, complex 2 can thermally release O₂ to regenerate 1. Vibrational assignments for selective ¹⁸O isotopic labeling of both O₂ and disiloxide ligands in 2 are consistent with a 6-coordinate, Co(η¹-O₂^·-)(“end-on”) complex. Complex 2 reacts with the O-H bond of 4-methoxy-2,2,6,6-tetramethylpiperidin-1-ol (4-MeO-TEMPOH) via H-atom abstraction with a rate of 0.58(2) M^-1 s^-1 at −105 °C, but it is unable to oxidize phenol substrates. This bracketed reactivity suggests that the O-H bond being formed in the putative Co^III(OOH) product has a relatively weak O-H bond strength (BDFE ∼66-74 kcal mol^-1). These thermodynamic and kinetic parameters are similar to those seen for the sulfur-ligated Co(O₂)(Me₃TACN)(S₂SiMe₂), indicating that the differences in the electronic structure for O versus S ligation do not have a large impact on H-atom abstraction reactivity.