Starting a subnanoscale tank tread: dynamic fluxionality of boron-based B10Ca alloy cluster

Alloying an elongated B10 cluster with Ca is shown to give rise to a dynamically fluxional B10Ca cluster, the latter behaving like a tank tread at the subnanoscale. Computer global search identifies the B10Ca C2 (1A) global-minimum structure, which is chiral in nature and retains the quasi-planar moiety of bare B10 cluster with Ca capped at one side, forming a half-sandwich. The rotation barrier of B10Ca cluster is reduced with respect to B10 by one order of magnitude, down to 1 kcal mol−1 at the PBE0/6-311+G* level, which demonstrates structural fluxionality at 600 K and beyond via molecular dynamics simulations. Structurewise, the Ca alloying in B10Ca cluster generates rhombic defect holes, preactivating the species and making it flexible against deformation. Chemical bonding analyses indicate that the B10Ca cluster is a charge-transfer [B10]2−[Ca]2+ complex, being doubly π/σ aromatic with the 6π and 10σ electron-counting. Such a pattern offers ideal π/σ delocalization and facilitates fluxionality. In contrast, bare B10 cluster has conflicting aromaticity with 6π and 8σ electrons, which is nonfluxional with a barrier of 12 kcal mol−1. Double π/σ aromaticity versus conflicting aromaticity is a key mechanism that distinguishes between fluxional B10Ca and nonfluxional B10 clusters, offering a compelling example that the concept of aromaticity (and double aromaticity) can be exploited to design dynamically fluxional nanosystems.