Monovalent lanthanide(I) in borozene complexes.

Lanthanide (Ln) elements are generally found in the oxidation state +II or +III, and a few examples of +IV and +V compounds have also been reported. In contrast, monovalent Ln(+I) complexes remain scarce. Here we combine photoelectron spectroscopy and theoretical calculations to study Ln-doped octa-boron clusters (LnB8-, Ln = La, Pr, Tb, Tm, Yb) with the rare +I oxidation state. The global minimum of the LnB8- species changes from Cs to C7v symmetry accompanied by an oxidation-state change from +III to +I from the early to late lanthanides. All the C7v-LnB8- clusters can be viewed as a monovalent Ln(I) coordinated by a η8-B82- doubly aromatic ligand. The B73-, B82-, and B9- series of aromatic boron clusters are analogous to the classical aromatic hydrocarbon molecules, C5H5-, C6H6, and C7H7+, respectively, with similar trends of size and charge state and they are named collectively as "borozenes". Lanthanides with variable oxidation states and magnetic properties may be formed with different borozenes.