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

Significance Lanthanide borides constitute an important class of materials with wide industrial applications, but clusters of lanthanide borides have been rarely investigated. It is of great interest to study these nanosystems, which may provide molecular-level understanding of the emergence of new properties and provide insight into designing new boride materials. We have produced lanthanide boride clusters and probed their electronic structure and chemical bonding. Two Ln2B8− clusters are presented, and they are found to possess inverse sandwich structures. The neutral Ln2B8 complexes are found to possess D8h symmetry with strong Ln–B8 bonding. A unique (d–p)δ bond is found to be important for the Ln–B8–Ln interactions. The Ln2B8 inverse sandwich complexes broaden the structural chemistry of the lanthanide elements and provide insights into bonding in lanthanide boride materials. 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.