Aromaticity, an old but still fantastic topic, has long attracted considerable interest of chemists. Generally, π aromaticity is described by π-electron delocalization in closed circuits of unsaturated compounds whereas σ-electron delocalization in saturated rings leads to σ aromaticity. Interestingly, our recent study shows that σ aromaticity can be dominating in an unsaturated three-membered ring (3MR) of cyclopropaosmapentalene. An interesting question is raised: Can the σ aromaticity, which is dominant in the unsaturated 3MR, be extended to other cyclopropametallapentalenes? If so, how could the metal centers, ligands, and substituents affect the σ aromaticity? Here, we report a thorough theoretical study on these issues. The nucleus-independent chemical shift calculations and the anisotropy of the current-induced density plots reveal the dominant σ aromaticity in these unsaturated 3MRs. In addition, our calculations show that substituents on the 3MRs have significant effects on the σ aromaticity, whereas the ligand effect is particularly small.
Aromaticity is one of the most fundamental and fascinating chemical topics, attracting both experimental and theoretical chemists owing to its many manifestations. Both σ- and π-aromaticity can be classified depending on the character of the cyclic electron delocalization. In general, σ-aromaticity stabilizes fully saturated rings with σ-electron delocalization whereas the traditional π-aromaticity describes the π-conjugation in fully unsaturated rings. Here, we demonstrate a strong correlation between nucleus-independent chemical shift (NICS) values and extra cyclic resonance energies (ECREs), which are used to evaluate the σ-aromaticity in an unsaturated three-membered ring (3MR) of cyclopropene, which were computed by molecular orbital (MO) theory and valence bond (VB) theory, respectively. Further study shows that the fully unsaturated ring in methylenecyclopropene and its metallic analogy is σ-aromatic. Our findings revolutionize the fundamental knowledge of the concept of σ-aromaticity, thus opening an avenue to design σ-aromaticity in other fully unsaturated systems, which are traditionally reserved as the domain of π-aromaticity.
Isolation of the simplest 4π three-membered heterocycles (1H-azirine, oxirene, thiirene, and selenirene) remains a big challenge due to their π-antiaromaticity and significant ring strain. Here we demonstrate that the incorporation of a transition-metal fragment could stabilize the antiaromatic selenirene and pentalene frameworks simultaneously by density functional theory (DFT) calculations. Experimental verification leads to the Se-containing metallapolycycles, osmapentaloselenirenes, with remarkable thermal stability. The osmaselenirene unit in the metallapolycycle is determined to be the first example of σ-aromaticity dominating in an unsaturated Se-containing ring. Our results not only highlight a remarkable stabilization by the transition-metal but also widen the scope of σ-aromaticity in unsaturated rings, which is traditionally reserved for the domain of π-aromaticity.
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