ConspectusMetallacycles, 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.

金属杂环是将至少一个碳中心替换为金属原子的碳环化合物衍生物，在有机合成中一直是一种强大的工具。尽管主族、过渡和锕系金属的金属杂环已得到广泛研究，但包含稀土元素（Sc, Y 和镧系元素）的金属杂环由于合成挑战而仍然难以捉摸。然而，这些元素的电正性以及由此产生的 RE-C 键极化，加上金属杂环内的内在协同效应，赋予了稀土金属杂环独特的性质和丰富但尚未充分开发的反应化学特性。在这篇文章中，我们介绍了五元环状稀土金属杂环的发展与应用。 在过去十年里，我们成功地使用两种关键合成策略合成了各种五元全碳稀土金属杂环：（i）通过金属交换制备稀土金属环戊二烯和螺状金属环戊二烯，并提供不同的配位环境来影响它们的反应性；（ii）通过金属交换和还原过程，实现了螺状金属环戊烯和2-丁烯四烷基负离子（BTA）桥连双核稀土金属杂环戊烯的合成。这些金属环结构代表了含有稀土元素的第一类此类环状结构。我们对这些金属杂环的研究揭示了独特的反应性和新的反应模式。由于稀土金属杂环的高度内在活性和多反应位点，它们不仅能够有效地活化小分子，而且对于某些小分子还表现出独特的作用方式。例如，RE金属环戊二烯与碳二亚胺的反应展示了多样化的插入/重排化学，受到诸如碳二亚胺当量数和溶剂选择等因素的影响。稀土金属杂环介导的白色磷[3 + 1]裂解展示了一种与主族和过渡金属类似物观察到的不同激活模式。此外，在RE中心发现交叉卡宾阴离子偶联和RE-金属介导的苯开环元反应，引入了新的反应模式，表明通过合理设计，稀土金属可以表现出类似于或甚至超越过渡金属的性质。这些反应模式进一步推动了RE金属杂环在合成化学中的应用开发。 此外，还发现了一些这些稀土金属杂环的新颖特性，这源自其独特的几何和电子结构。结构分析和理论计算揭示了BTA桥连双核稀土金属环戊烯的非平面芳香性，将非平面芳香性的概念扩展到了碳-RE金属杂环领域。此外，得益于丁二烯二负离子和BTA配体的氧化还原能力，BTA桥连双核稀土金属杂环的基于配体的氧化还原化学表现出多样且高效的多电子转移过程，突显了这些金属杂环在氧化还原化学中的潜力。 对稀土金属杂环的研究，包括它们的构建、表征、性质、反应性和合成应用，极大地丰富了f区元素金属杂环领域。我们希望这篇文章能够激发更多关于新的有机金属试剂和金属环介导转化的探索，推动稀土化学领域的持续进步。