Adaptive Aromaticity in Metallasilapentalynes

Cyclic molecules with 4n + 2 or 4n electrons are aromatic in the lowest singlet state (S0) or the lowest triplet state (T1) according to Hückel and Baird’s rules. Thus, the design of aromatic species in both the S0 and T1 states (termed as adaptive aromaticity) is particularly challenging. In this work, we demonstrate that metallasilapentalynes show adaptive aromaticity supported by structural, magnetic, and electronic indices, in sharp contrast to metallapentalynes, which exhibit aromaticity in the S0 state only. The sharp difference in aromaticity could be caused by the distinctive excitation pattern. Specifically, the T1 state in metallasilapentalyne is formed by the excitation from the π to σ* molecular orbital, whereas it is excited from π to π* molecular orbital in metallapentalyne. In addition, a series of metallasilapentalynes with adaptive aromaticity are predicted by tuning the ligands. Our findings enrich the family of adaptive aromatics, inviting experimental chemists’ examination.