Predicting lanthanide boride inverse sandwich tubular molecular rotors with the smallest core-shell structure

Lanthanide-boron binary clusters possess interesting structures and bonding which may provide insights into designing new boride nanomaterials. Inspired by the recently discovered mono-decker inverse sandwich D_9h La₂B₉^- (¹A'₁) (1) and based on the extensive first-principles theory calculations, we predict herein the possible existence of a series of bi-decker inverse sandwich di-lanthanide boron complexes including D_9d La₂[B₁₈] (³A_1g) (2), D_9d La₂[B₁₈]^2- (¹A_1g) (3), and C_2h La₂[B₂@B₁₈] (¹A_g) (4) which all contain a tubular B_n ligand (n = 18, 20) sandwiched by two La atoms at the two ends. In these novel clusters, La₂[B₂@B₁₈] (4) as a tubular molecular rotor with the smallest core-shell structure reported to date in boron-based nanoclusters possesses a B₂-bar rotating constantly and almost freely inside the B₁₈ tube around it at room temperature. Detailed bonding analyses indicate that these complexes are stablized by effective (d-p)σ, (d-p)π, and (d-p)δ coordination interactions between the La centers and B_n bi-decker ligand. Six multi-center fluxional σ-bonds between the B₂-core and B₁₈ tube in La₂[B₂@B₁₈] (4) are found to be responsible for its unique fluxional behaviors. The IR and Raman spectra of the concerned species are simulated to facilitate their experimental characterization.