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 [PhL2CrIII-η1-O2][MX]2 (PhL = -OPh2SiOSiPh2O-, MX+ = [CoCl]+, [ZnBr]+, [ZnCl]+) starting from the CrII precursor complex [PhL2CrII]Li2(THF)4. The first strategy proceeds via the exchange of Li+ by [MX]+ through the addition of MX2 to [PhL2CrII]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 [PhL2CrIII-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 [PhL2CrIIICl][CoCl]2(THF)3. However, it provided access to the hetero-bimetallic complexes [PhL2CrIII-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 [PhL2CrIII-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 [RL2CrIII-η1-O2]M2 complexes.