Dioxygen Activation by Siloxide Complexes of Chromium(II) and Chromium(IV)

The reaction of a tripodal trisilanol with n-butyllithium and CrCl2 results in a dinuclear Cr-II complex (1), which is capable of cleaving O-2 to yield in a unique complex (2) with an asymmetric diamond core composed of two Cr-IV=O units. Magnetic susceptibility data reveal significant exchange coupling of Cr-II (S = 2) in 1 and large zero-field splitting for Cr-IV (S = 1) in 2 owing to strong spin-orbit coupling of the ground state. The Cr-IV=O compound can also be generated using PhIO, and evidence was gathered that although it is the stable product isolated after excessive O-2 treatment, it further activates O-2 to yield an intermediate species that oxidizes THF or Me-THF. By extensive O-18 labeling studies we were able to show, that in the course of this process O-18(2) exchanges its label with siloxide O atoms of the ligand via terminal oxido ligands.