In recent years, it has become clear that the presence of redox‐inactive Lewis acidic metal ions can decisively influence the reactivity of metal–dioxygen moieties that are formed in the course of O2 activation, in molecular complexes, and metalloenzymes. Superoxide species are often formed as the primary intermediates but they are mostly too unstable for a thorough investigation. We report here a series of chromium(III) superoxide complexes [L2Cr]M2O2(THF)y (L=−OSiPh2OSiPh2O−, M+=Li+, Na+, K+ and y=4, 5), which could be accessed, studied spectroscopically and partly crystallized at low temperatures. They only differ in the two incorporated Lewis acidic alkali metal counterions (M+) and it could thus be shown that the nature of M+ determines considerably its interaction with the superoxide ligand. This interaction, in turn, has a significant influence on the stability and reactivity of these complexes towards substrates with OH groups. Furthermore, we show that stability and reactivity are also highly solvent dependent (THF versus nitriles), as donor solvents coordinate to the alkali metal ions and thus also influence their interaction with the superoxide moiety. Altogether, these results provide a comprehensive and detailed picture concerning the correlation between spectroscopic properties, structure, and behavior of such superoxides, that may be exemplary for other systems.

O2 activation at a chromium­(II) siloxide complex in propionitrile leads to a chromium­(III) complex with an end-on bound superoxide ligand, while the reaction in tetrahydrofuran leads to a side-on peroxo chromium­(IV) compound. The superoxide reacts faster with (2,2,6,6-tetramethylpiperidin-1-yl)­oxyl hydroxylamine while the peroxide, unlike the superoxide, proved capable of deformylating aldehydes. The system was found to represent a unique case, where even a switching between the two structures can be achieved via the solvent; its ability to coordinate at the position trans to the O2 ligand is decisive, as supported by density functional theory studies. Altogether, the results show that subtle changes can determine for an initially formed metal-dioxygen adduct, whether it exists as a superoxide or a peroxide, which thus merits consideration in discussions on mechanisms and possible reaction routes.

Multimetallic complexes have recently seen increased attention as next-generation chargec arriers forn onaqueous redox flow batteries. Herein, we report the electrochemical performance of am olecular iron-molybdenum oxido complex, {[(Me 3 TACN)Fe][m-(MoO 4 k 3 O,O',O")]} 4 (Fe 4 Mo 4 O 16 ). In symmetric battery charging schematics, Fe 4 Mo 4 O 16 facilitates reversible two-electron storage with coulombic efficiencies > 99 %o ver1 00 cycles (5 days) with no molecular decomposition and minimal capacity fade.Energy efficiency throughout cycling remainedh igh (~82 %), as ar esult of the rapid electron-transfer kinetics observed for each of the complex'sf our redox events. We also report the synthesiso ft he analogouss ynthetic frameworks featuring tungstate vertices or bridging-sulfide moieties, revealing key observations relevant to structure-function relationships and design criteria for theset ypes of heterometallic ensembles.

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.

The isolation and identification of intermediates formed in the course of the activation of dioxygen at transition metal centers reveals important mechanistic insights concerning such processes. We previously reported the reaction of the dinuclear Cr II complex [L 2 Cr 2 (MeCN) 2 ] [Li(MeCN)] 2 (L = PhSi(OSiPh 2 O À) 3) (5) with dioxygen, which resulted in the formation of the Cr IV oxo complex [L 2 Cr 2 O 2 ] [Li(THF) 2 ] 2 (6), as the final room temperature stable product. Here we now report the isolation and characterization of an intermediate en route to 6, namely the dinuclear Cr III superoxo complex [L 2 Cr 2 (O 2) 2 ][Li(MeCN)] 2 (7). 7 is the first example of a structurally characterized dinuclear Cr III superoxo complex with two independent side-on bound superoxo ligands. Reactivity studies outline the capability of this superoxo complex to activate weak OÀ H bonds.