Activation of Dioxygen via Neodymium-Alkali Metal Clusters

The discovery of new dioxygen binding and activation modes is of paramount importance in biological and synthetic systems. Herein, we describe two rare earth-alkali metal clusters displaying unusual reactivity with dioxygen. We isolate a trans-end-on peroxo dineodymium tetrapotassium species, (LH₄Nd)₂(trans-μ-η¹:η¹-O₂)K₄(thf)₄ (5), from reduction reactions of LH₅Nd (1) and K[LH₄Nd] (2) employing KC₈ in dry O₂. Cluster 5 contains the first end-on peroxo coordination to an f-block metal. Surprisingly, when the weaker reductant sodium naphthalenide (Na[C₁₀H₈]) is used, we isolate the cluster (LH₄Nd)₂(μ₆-O)Na₄(thf)₄ (6), indicating dioxygen's O-O bond has been cleaved. The typically weak π-backbonding interaction of 4f-block elements to stabilize the end-on binding mode of O₂ is realized in 5 through a Lewis acid-assisted support of the peroxo species. Cleaving of the O-O bond in 6 is attributed to an increased Lewis acid effect rather than a larger chemical potential driving force since |E_red(KC₈)| > |E_red ([C₁₀H₈]^-|. Lanthanide oxos are highly reactive; however, the oxo reactivity in 6 is tamed by dimerization and protection with four equatorial closely associated Na ions. This work demonstrates a synergistic effect between the rare earth and alkali metals in the binding and activation of dioxygen and provides a novel route to examine lanthanide peroxo/oxo chemistry.