Routes to Heterotrinuclear Metal Siloxide Complexes for Cooperative Activation of O₂

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 [^PhL₂Cr^III-η¹-O₂][MX]₂ (^PhL = ^-OPh₂SiOSiPh₂O^-, MX⁺ = [CoCl]⁺, [ZnBr]⁺, [ZnCl]⁺) starting from the Cr^II precursor complex [^PhL₂Cr^II]Li₂(THF)₄. The first strategy proceeds via the exchange of Li⁺ by [MX]⁺ through the addition of MX₂ to [^PhL₂Cr^II]Li₂(THF)₄ before the reaction with dioxygen, whereas in the second approach a salt metathesis reaction is undertaken after O₂ activation by adding MX₂ to [^PhL₂Cr^III-η¹-O₂]Li₂(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 [^PhL₂Cr^IIICl][CoCl]₂(THF)₃. However, it provided access to the hetero-bimetallic complexes [^PhL₂Cr^III-η¹-O₂][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 [^PhL₂Cr^III-η¹-O₂][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 O₂ activating compounds and provide the basis for further tuning of the reactivity of [^RL₂Cr^III-η¹-O₂]M₂ complexes.