The Influence of Alkali Metal Ions on the Stability and Reactivity of Chromium(III) Superoxide Moieties Spanned by Siloxide Ligands

Abstract

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.

A Polysiloxide Complex with two Chromium(III) η2‐Superoxo Moieties

“…X‐ray analysis led to the structure of [L 2 Cr 2 (O 2 ) 2 ][Li(MeCN)] 2 ( 7 ) revealing that each chromium center is binding one O 2 molecule in a side‐on fashion (Figure 2). The O−O bond distance of 1.346(9) Å is in good agreement with values previously observed for chromium siloxide complexes with end‐on bound superoxo ligands (1.334(7), 1.324(7) and 1.334(4) Å) [5,8] …”

“…Both the position of the bands and the isotopic shift of Δ( 16 O− 18 O)=51 cm −1 are typical for chromium bound superoxo ligands (Figure 3). [4a,5,8–9] From these results it can be concluded that the reaction of 5 with dioxygen involves the formation of the dinuclear Cr III superoxo complex 7 as an intermediate (Scheme 3). The formation of such a complex is very uncommon, as typically upon activation of the first O 2 molecule at one metal center of a dinuclear complex the superoxo entity thus formed immediately reacts/interacts with the second metal center in close proximity.…”

“…As Cr III superoxo complexes are known to activate weak O−H bonds, [4c,8,10] we employed TEMPO‐H as a substrate to test the reactivity of 7 . The reaction mixture was analyzed by means of EPR spectroscopy, which revealed the formation of a nitroxide radical confirming that 7 is capable of abstracting a hydrogen atom from TEMPO‐H (Figure S1).…”

“…The OÀ O bond distance of 1.346(9) Å is in good agreement with values previously observed for chromium siloxide complexes with end-on bound superoxo ligands (1.334(7), 1.324(7) and 1.334(4) Å). [5,8] The assignment as a superoxo ligand was corroborated by Resonance Raman measurements: After bubbling 16 O 2 into a DCM solution of 5 a new band at 1100 cm À 1 appeared, which was shifted to 1049 cm À 1 when 18 O 2 was employed. Both the position of the bands and the isotopic shift of Δ( 16 OÀ 18 O) = 51 cm À 1 are typical for chromium bound superoxo ligands (Figure 3).…”

High‐spin square planar iron(II) alkali metal siloxide complexes – influence of the alkali metal and reactivity towards O2 and NO

“…Bearing in mind recent results, which we had obtained with analogous complexes containing chromium(II) as central metal ion, [L 2 Cr][M(Solv) 2 ] 2 (M=Li, Na, K), [12] the O 2 reactivity of [L 2 Fe][M(Solv) 2 ] 2 shifted into our focus, as there is a relation between chromium and iron: chromium(II) in contact with O 2 often forms intermediates comparable to those formed by corresponding iron(II) compounds, however, the Cr/O 2 adducts are often somewhat more stable and can thus be investigated in more detail, thus providing an idea on the proceedings in case of the corresponding iron analogues. [13] For the complexes (M=Li, Na, K) in contact with O 2 we had been able to show that chromium(III) superoxides are formed and that their stability and reactivity is dependent on the alkali metal ions M + , as these were found to interact with the superoxide ligand.…”