The smallest 4f-metalla-aromatic molecule of cyclo-PrB₂ ^- with Pr-B multiple bonds

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 PrB₂ ^- based on a joint photoelectron spectroscopy and quantum chemical investigation. Global minimum structural searches reveal that PrB₂ ^- has a C _2v triangular structure with a paramagnetic triplet ³B₂ electronic ground state, which can be viewed as featuring a trivalent Pr(III,f²) and B₂ ^4-. Chemical bonding analyses show that this cyclo-PrB₂ ^- 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 B₂ ^4- tetraanion by the high-lying 5d orbitals of the 4f-elements, completing the isoelectronic B₂ ^4-, C₂ ^2-, N₂, and O₂ ²⁺ series.