Probing the Hyperconjugative Aromaticity of Cyclopentadiene and Pyrroliums Containing Group 7 Transition Metal Substituents

Aromaticity and hyperconjugation are two fundamental concepts in chemistry. Combining them together led to the proposal of the concept of hyperconjugative aromaticity by Mulliken in 1939. Now, it has been attracting considerable attention from both theoretical and experimental chemists. Recently, the concept of hyperconjugative aromaticity has been extended from main-group substituents to transition metal systems including groups 9, 10, and 11 transition metal substituents. Here, we report the hyperconjugative aromaticity in cyclopentadienes and pyrroliums containing group 7 transition metal substituents through density functional theory (DFT) calculations. It is found that the metal–metal bonding interaction can significantly reduce the aromaticity in cyclopentadienes, whereas the stronger σ-donor ligand, bridged hydride, and carbonyls can enhance aromaticity. All these findings expand the scope of the concept of hyperconjugative aromaticity, enriching aromatic organometallic chemistry.

芳香性与超共轭效应是化学领域的两个核心基础概念。二者结合后，穆利肯（Mulliken）于1939年提出了超共轭芳香性（hyperconjugative aromaticity）这一概念。时至今日，该概念已受到理论与实验化学家的广泛关注。近年来，超共轭芳香性的概念已从主族取代基拓展至包括第9、10、11族过渡金属取代基在内的过渡金属体系。本研究通过密度泛函理论（density functional theory, DFT）计算，报道了含第7族过渡金属取代基的环戊二烯与吡咯鎓离子中的超共轭芳香性。研究发现，金属-金属成键相互作用可显著降低环戊二烯的芳香性，而更强的σ给体配体、桥联氢化物与羰基则可提升芳香性。上述研究成果拓展了超共轭芳香性概念的应用范围，丰富了芳香性有机金属化学的研究内涵。