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, Ln(2)B(8)(-) (Ln = La, Pr). We found that these clusters form highly stable inverse sandwich structures, [Ln-B-8-Ln](-), with strong Ln and B-8 bonding via interactions between the Ln 5d orbitals and the delocalized sigma and pi orbitals on the B-8 ring. A (d-p)delta bond, involving the 5d delta and the antibonding pi orbital of the B-8 ring, is observed to be important in the Ln-B-8 interactions. The highly symmetric inverse sandwich structures are overwhelmingly more stable than any other isomers. Upon electron detachment, the (d-p)delta 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)delta 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...B-8...Ln inverse sandwiches or 1D Ln...B-8...Ln nanowires may be designed with novel electronic and magnetic properties.