Routes to Heterotrinuclear Metal Siloxide Complexes for Cooperative Activation of O-2

摘要

The assembly of heterometallic complexes capable of activating dioxygen is synthetically challenging. Here, we report two different approaches for the preparation of heterometallic superoxide complexes [(L2CrIII)-L-Ph-eta(1)-O-2][MX](2) (L-Ph = -OPh2SiOSiPh2O-, MX+ = [CoCl](+), [ZnBr](+), [ZnCl](+)) starting from the CrII precursor complex [(L2CrII)-L-Ph]Li-2(THF)(4). The first strategy proceeds via the exchange of Li+ by [MX](+) through the addition of MX2 to [(L2CrII)-L-Ph]Li-2(THF)(4) before the reaction with dioxygen, whereas in the second approach a salt metathesis reaction is undertaken after O-2 activation by adding MX2 to [(L2CrIII)-L-Ph-eta(1)-O-2]Li-2(THF)(4). The first strategy is not applicable in the case of redox-active metal ions, such as Fe2+ or Co2+, as it leads to the oxidation of the central chromium ion, as exemplified with the isolation of [(L2CrCl)-L-Ph-Cl-III][CoCl](2)(THF)(3). However, it provided access to the hetero-bimetallic complexes [(L2CrIII)-L-Ph-eta(1)-O-2][MX](2) ([MX](+) = [ZnBr](+), [ZnCl](+)) with redox-inactive flanking metals incorporated. The second strategy can be applied not only for redox-inactive but also for redox-active metal ions and led to the formation of chromium(III) superoxide complexes [(L2CrIII)-L-Ph-eta(1)-O-2][MX](2) (MX+ = [ZnCl](+), [ZnBr](+), [CoCl](+)). The results of stability and reactivity studies (employing TEMPO-H and phenols as substrates) as well as a comparison with the alkali metal series (M+ = Li+, Na+, K+) confirmed that although the stability is dependent on the Lewis acidity of the counterions M and the number of solvent molecules coordinated to those, the reactivity is strongly dependent on the accessibility of the superoxide moiety. Consequently, replacement of Li+ by XZn+ in the superoxides leads to more stable complexes, which at the same time behave more reactive toward O-H groups. Hence, the approaches presented here broaden the scope of accessible heterometallic O-2 activating compounds and provide the basis for further tuning of the reactivity of [(L2CrIII)-L-R-eta(1)-O-2]M-2 complexes.