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 insight into designing new boride nanomaterials. Inspired by the recently discovered mono-deck inverse sandwich D9h La2B9 (1A1') (1) and based on extensive first-principles theory calculations, we predict herein the possibile existence of a series of bi-deck inverse sandwich di-lanthanide boron complexes including D9d La2[B18] (3A1g) (2), D9d La2[B18]2 (1A1g) (3), and C2h La2[B2@B18] (1Ag) (4) which all contain a tubular Bn 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 Bn bi-deck 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 characterizations.