Probing the Electronic Structure of [BH] Dianion Encapsulated by an Octamethylcalix[4]pyrrole Molecule.

Despite being an important -borate in condensed phase boron chemistry, isolated [BH] is electronically unstable and has never been detected in the gas phase. Herein, we report a successful capture of this fleeting species through binding with an octamethylcalix[4]pyrrole (omC4P) molecule to form a stable gaseous omC4P·[BH] complex and its characterizations utilizing negative ion photoelectron spectroscopy (NIPES). The recorded NIPE spectrum, contributed by both omC4P and [BH], is deconvoluted by subtracting the omC4P contribution to yield a [BH] spectrum. The obtained [BH] spectrum consists of four major bands spanning the electron binding energy (EBE) range from 1 to 5 eV, with the EBE gaps matching excellently with the energy intervals of computed high-lying occupied molecular orbitals of the [BH] dianion. This study showcases a generic method to utilize omC4P to capture unstable multiply charged anions in the gas phase for experimental determination of their electronic structures.