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

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 [ ^Ph L ₂ Cr ^III -η ¹ -O ₂ ][MX] ₂ ( ^Ph L = ^- OPh ₂ SiOSiPh ₂ O ^- , MX ⁺ = [CoCl] ⁺ , [ZnBr] ⁺ , [ZnCl] ⁺ ) starting from the Cr ^II precursor complex [ ^Ph L ₂ Cr ^II ]Li ₂ (THF) ₄ . The first strategy proceeds via the exchange of Li ⁺ by [MX] ⁺ through the addition of MX ₂ to [ ^Ph L ₂ Cr ^II ]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 MX ₂ to [ ^Ph L ₂ Cr ^III -η ¹ -O ₂ ]Li ₂ (THF) ₄ . The first strategy is not applicable in the case of redox-active metal ions, such as Fe ²⁺ or Co ²⁺ , as it leads to the oxidation of the central chromium ion, as exemplified with the isolation of [ ^Ph L ₂ Cr ^III Cl][CoCl] ₂ (THF) ₃ . However, it provided access to the hetero-bimetallic complexes [ ^Ph L ₂ Cr ^III -η 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 [ ^Ph L ₂ Cr ^III -η 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 [ ^R L ₂ Cr ^III -η ¹ -O ₂ ]M ₂ complexes.