Dynamic Structural Fluxionality in a Planar Tetracoordinate Carbon Cluster Stabilized by Boron Ligands

The design of boron-based molecular rotors stems from boron-carbon binary clusters containing multiple planar hypercoordinate carbons (phCs, such as C₂B₈). However, the design of boron-coordinated phCs is challenging due to boron's tendency to occupy hypercoordinate centers more than carbon. Although this challenge has been addressed, the designed clusters of interest have not exhibited dynamic fluxionality similar to that of the initial C₂B₈. To address this issue, we report a σ/π doubly aromatic CB₂H₅ ⁺ cluster, the first global minimum containing a boron-coordinated planar tetracoordinate carbon atom with dynamic fluxionality. Dynamics simulations show that two ligand H atoms exhibit alternate rotation, resulting in an intriguing dynamic fluxionality in this cluster. Electronic structure analysis reveals the flexible bonding positions of the ligand H atoms because they do not participate in π delocalized bonding nor bond to any other non-carbon atom, highlighting this rotational fluxionality. Unprecedentedly, the fluxional process involves not only the usual conversion of the number of bonding atoms, but also the type of bonding (3c π bonds ↔4c σ bonds), which is an uncommon fluxional mechanism. The cluster represents an effort to apply phC species to molecular machines.