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.