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 (LnB(8)(−), Ln = La, Pr, Tb, Tm, Yb) with the rare +I oxidation state. The global minimum of the LnB(8)(−) species changes from C(s) to C(7v) symmetry accompanied by an oxidation-state change from +III to +I from the early to late lanthanides. All the C(7v)-LnB(8)(−) clusters can be viewed as a monovalent Ln(I) coordinated by a η(8)-B(8)(2−) doubly aromatic ligand. The B(7)(3−), B(8)(2−), and B(9)(−) series of aromatic boron clusters are analogous to the classical aromatic hydrocarbon molecules, C(5)H(5)(−), C(6)H(6), and C(7)H(7)(+), 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.