Oxygen versus Sulfur Coordination in Cobalt Superoxo Complexes: Spectroscopic Properties, O 2 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 (Me 3 TACN)(S 2 SiMe 2 ) ( 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 O 2 reversibly in a range of solvents at low temperatures. A cobalt(III)-superoxo complex, Co III (O 2 · - )(Me 3 TACN)((OSi 2Ph ) 2 O) ( 2 ), was generated, and was analyzed by UV-vis, EPR, and resonance Raman spectroscopies. Unlike its sulfur-ligated analogue, complex 2 can thermally release O 2 to regenerate 1 . Vibrational assignments for selective 18 O isotopic labeling of both O 2 and disiloxide ligands in 2 are consistent with a 6-coordinate, Co(η 1 -O 2 · - )("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(O 2 )(Me 3 TACN)(S 2 SiMe 2 ), indicating that the differences in the electronic structure for O versus S ligation do not have a large impact on H-atom abstraction reactivity.