Fresh Impetus in the Chemistry of Calcium Peroxides

Redox-inactive metal ions are essential in modulating the reactivity of various oxygen-containing metal complexes and metalloenzymes, including photosystem II (PSII). The heart of this unique membrane-protein complex comprises the Mn₄CaO₅ cluster, in which the Ca²⁺ ion acts as a critical cofactor in the splitting of water in PSII. However, there is still a lack of studies involving Ca-based reactive oxygen species (ROS) systems, and the exact nature of the interaction between the Ca²⁺ center and ROS in PSII still generates intense debate. Here, harnessing a novel Ca-TEMPO complex supported by the β-diketiminate ligand to control the activation of O₂, we report the isolation and structural characterization of hitherto elusive Ca peroxides, a homometallic Ca hydroperoxide and a heterometallic Ca/K peroxide. Our studies indicate that the presence of K⁺ cations is a key factor controlling the outcome of the oxygenation reaction of the model Ca-TEMPO complex. Combining experimental observations with computational investigations, we also propose a mechanistic rationalization for the reaction outcomes. The designed approach demonstrates metal-TEMPO complexes as a versatile platform for O₂ activation and advances the understanding of Ca/ROS systems.