Enchanting Realm of Five-Membered Rare-Earth Metallacycles

Metallacycles, derivatives of carbocyclic compounds wherein a metal atom replaces at least one carbon center, have been a constant powerhouse in organic synthesis. While metallacycles of main-group, transition, and actinide metals have been extensively studied, those incorporating rare-earth (RE) elements (Sc, Y, and lanthanides) have remained elusive primarily due to synthetic challenges. Nevertheless, the electropositive character of these elements and the resulting polarization of RE–C bonds, along with the intrinsic synergistic effects within metallacycles, endow RE metallacycles with unique properties and a rich, yet largely untapped, reaction chemistry. In this Account, we present the development and applications of five-membered RE metallacycles.

Over the past decade, we have successfully synthesized a variety of five-membered all-carbon rare-earth metallacycles using two key synthetic strategies: (i) transmetalation, which has been employed to prepare RE metallacyclopentadienes and spiro-metallacyclopentadienes, which, featuring various ligand systems, provide distinct coordination environments around the RE center, significantly influencing their reactivity, and (ii) transmetalation and reduction, enabling the synthesis of RE spiro-metallacyclopentenes and 2-butene tetraanion (BTA)-bridged dinuclear RE metallacyclopentenes. The reduction process proceeds via either self-disproportionation of metallacyclopentadienes or reduction by divalent RE centers or alkali metals. These metallacycles represent the first instances of such RE-containing metallacyclic ring structures.

Our investigations into these metallacycles have uncovered unique reactivities and new reaction modes. The high intrinsic reactivity and multiple reactive sites of rare-earth metallacycles enable them not only to activate small molecules efficiently but also to exhibit distinct activation modes for some small molecules. For instance, reactions of RE metallacyclopentadienes with carbodiimides showcase diverse insertion/rearrangement chemistry, influenced by various factors such as number of equivalents of carbodiimide and the solvent choice. The RE metallacyclopentadiene-mediated [3 + 1] fragmentation of white phosphorus demonstrates an activation mode markedly different from that observed with main-group and transition metal analogs. Moreover, the discovery of cross-carbanion coupling at RE centers and RE-metal-mediated ring-opening metathesis of benzene introduces new reaction modes, demonstrating that, with rational design, RE metals can exhibit properties similar to or even surpassing those of transition metals. These reaction modes have further led to the development of applications for RE metallacycles in synthetic chemistry.

Additionally, some novel properties of these rare-earth metallacycles have been uncovered, stemming from their unique geometric and electronic structures. Structural analysis and theoretical calculations have revealed the nonplanar aromaticity of BTA-bridged dinuclear RE metallacyclopentenes, extending the concept of nonplanar aromaticity into the chemistry of carbon–RE metallacycles. Furthermore, benefiting from the redox capabilities of butadiene dianion and BTA ligands, the ligand-based redox chemistry of BTA-bridged dinuclear RE metallacyclopentenes demonstrates diverse and efficient multielectron transfer processes, highlighting the potential of these metallacycles for redox chemistry.

The studies of rare-earth metallacycles, encompassing their construction, characterization, properties, reactivity, and synthetic applications, have greatly enriched the field of f-block metallacycles. We hope that this Account will inspire further exploration into the synthesis of new organometallic reagents and metallacycle-mediated transformations, fueling continued progress in rare-earth chemistry.