Observation of highly stable and symmetric lanthanide octa-boron inverse sandwich complexes

While boron forms a wide range of metal borides with important industrial applications, there has been relatively little attention devoted to lanthanide boride clusters. Here we report a joint photoelectron spectroscopy and quantum chemical study on two octa-boron di-lanthanide clusters, Ln₂B₈^- (Ln = La, Pr). We found that these clusters form highly stable inverse sandwich structures, [Ln-B₈-Ln]^-, with strong Ln and B₈ bonding via interactions between the Ln 5d orbitals and the delocalized σ and π orbitals on the B₈ ring. A (d-p)δ bond, involving the 5dδ and the antibonding π orbital of the B₈ ring, is observed to be important in the Ln-B₈ interactions. The highly symmetric inverse sandwich structures are overwhelmingly more stable than any other isomers. Upon electron detachment, the (d-p)δ orbitals become half-filled, giving rise to a triplet ground state for neutral La₂B₈ In addition to the two unpaired electrons in the (d-p)δ orbitals upon electron detachment, the neutral Pr₂B₈ complex also contains two unpaired 4f electrons on each Pr center. The six unpaired spins in Pr₂B₈ are ferromagnetically coupled to give rise to a septuplet ground state. The current work suggests that highly magnetic Ln…B₈…Ln inverse sandwiches or 1D Ln…B₈…Ln nanowires may be designed with novel electronic and magnetic properties.