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Di- and Trinuclear Zinc and Cobalt Complexes and Their Reactivity towards Dioxygen



A new ligand L with two adjacent pyrazole–pyridine moieties, which are linked by a CHO unit was developed and utilised to synthesise three multinuclear metal complexes. The reaction of HL with NEt3 and zinc(II) halides or cobalt(II) chloride leads to complexes of the type [M2L2]2+ (M = Co, Zn). Temperature-dependent magnetic measurement of [Co2L2](PF6)2 (I) showed a ferromagnetic coupling of the Co ions. Subsequently, the reactivity of these complexes with O2 was investigated. Complex I reacts with O2 to form a cobalt(II) complex in which the ligand is oxygenated at the central carbon atom, that is, a gem-diol is formed. As [Zn2L2](PF6)2 (II) does not react with O2, the Co ions mediate the oxygenation of the ligand in [Co2L2]2+. The reaction of Zn(OAc)2 with HL under inert conditions leads to the formation of the trinuclear complex [Zn3L(OAc)5] (III), which decomposes in air. Further investigation showed that O2 causes ligand oxidation in the central carbon position, which leads to the corresponding ketone upon decomplexation. The primary decomposition products are presumably gem-diolate zinc species. These results point to different reaction mechanisms for the activation of dioxygen by I and III.

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