The smallest 4f-metalla-aromatic molecule of cyclo-PrB(2)(−) 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 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.