Metallapentalene-Fused Metallacyclopropenes: Synthesis, Reactivity and σ-Aromaticity

Aromatic compounds are fundamental to synthetic chemistry, biomedicine, and materials science. Among them, metallaaromatics─which incorporate at least one transition metal into an aromatic framework─represent a distinctive class that has attracted substantial interest. The concept of aromaticity, traditionally rooted in π-electron delocalization, has been expanded by σ-aromaticity─first proposed by Dewar in 1979 and subsequently established in all-metal clusters. Over the past decade, our group has developed a family of metal-centered conjugated polycyclic metallacycles termed carbolong complexes in which a carbon chain coordinates to a metal center via a minimum of three metal-carbon bonds. These complexes exhibit remarkable stability towards water, oxygen, light, and heat, owing to polydentate chelation and aromatic stabilization. This Account summarizes our systematic investigations into metallapentalene-fused metallacyclopropene derivatives─a unique subclass that integrates a highly strained three-membered ring into a robust aromatic scaffold. We describe our journey from the establishment of carbon-chain-based frameworks via one-pot multiyne cyclizations to more elaborate carbolong skeletons through controlled [2+1] cycloadditions with isocyanides and zinc carbenoids. Building on this carbon-rich platform, we extended the chemistry to peripheral p-block elements, incorporating heteroatoms from groups 15, 16, and 17 into the metallacyclopropene core to yield metalla-azirines, -phosphirenes, -chalcogenirenium, and -halirenium ions. The fused metallacyclopropenes serve as powerful synthetic intermediates, participating in formal [3+1], [3+2], and [3+3] cycloadditions to construct complex polycyclic architectures. The resulting materials exhibit broad near-infrared absorption and high photothermal conversion efficiency and are promising for smart materials, drug delivery, and cancer phototherapy. Complementary computational studies have elucidated their electronic structures. We established σ-aromaticity as a key stabilizing factor that counterbalances ring strain─validating a long-debated concept and providing a design principle for stabilizing transient intermediates in transition-metal heterocycles. Systematic analyses of heteroatom-doped congeners reveal clear periodic trends: the magnitude of σ-aromaticity─and consequently the overall stability─is finely tuned by the electronic nature and size of the incorporated heteroatom. Ring strain energy calculations show that appropriate substitution can significantly alleviate strain, reflecting the agreement between computation and experiment. Collectively, this work not only advances the fundamental understanding of metallacyclic aromaticity but also paves the way for the rational design of functional metallaaromatic materials with tailored properties.