Dynamic Four-Membered Rings with 2π Aromaticity.

Achieving structural dynamism with aromatic systems has been challenging. We show that structural dynamism could be achieved with the 2π aromatic compounds. The structural preferences and dynamic behavior of the recently synthesized four-membered rings (4MRs) isoelectronic to cyclobutadiene dication (CBD) using density functional theory with energy correction at CCSD(T) and DLPNO-CCSD(T) with the def2-TZVPP basis set are explored. The isoelectronic replacement with triel and tetrel elements in the periodic table to CBD led to several 4MRs with the molecular formula A₂X₂; A corresponding to B, Al, and Ga, and X to Si and Ge. Structural dynamism is investigated using the different attached substituents. Among the different 4MRs, Si₂Al₂, with all of the attached substituents, shows the lowest transition barrier for structural changes. We find that the aromaticity of the transition state plays a significant role in determining the dynamic behavior. Molecular dynamics simulations at room temperature confirm the structural dynamism. The orbital correlation diagram is analyzed between the ground and the transition states. The structural isomers of (L^PhA)₂(XPh)₂, with L corresponding to N,N'-dimethyl(phenylamidinato) are investigated. Although Si₂X₂ compounds show more preference for cyclic isomers, Ge₂X₂ shows comparable stability for the cyclic and open cyclic/bicyclic structures.