Abstract
The concept of metalla-aromaticity proposed by Thorn-Hoffmann (Nouv. J. Chim. 1979, 3, 39) has been expanded to organometallic molecules of transition metals that have more than one independent electron-delocalized system. Lanthanides, with highly contracted 4f atomic orbitals, are rarely found in multiply aromatic systems. Here we report the discovery of a doubly aromatic triatomic lanthanide-boron molecule PrB(2) (-) based on a joint photoelectron spectroscopy and quantum chemical investigation. Global minimum structural searches reveal that PrB(2) (-) has a C (2v) triangular structure with a paramagnetic triplet (3)B(2) electronic ground state, which can be viewed as featuring a trivalent Pr(III,f(2)) and B(2) (4-). Chemical bonding analyses show that this cyclo-PrB(2) (-) species is the smallest 4f-metalla-aromatic system exhibiting σ and π double aromaticity and multiple Pr-B bonding characters. It also sheds light on the formation of the rare B(2) (4-) tetraanion by the high-lying 5d orbitals of the 4f-elements, completing the isoelectronic B(2) (4-), C(2) (2-), N(2), and O(2) (2+) series.