Covalently Bound Tetracoordinated Organoborons as Superhalogens: A Combined Negative Ion Photoelectron Spectroscopy and Theoretical Study

Molecular species with electron affinities (EAs) larger than that of the chlorine atom (3.6131 eV) are superhalogens. The corresponding negative ions, namely, superhalogen anions, are intrinsically very stable with high electron binding energies (EBEs) and widely exist as building blocks of bulk materials and ionic liquids. The most common superhalogen anions proposed and experimentally confirmed to date are either ionic salts or compact inorganic species. Herein, we report a new class of superhalogen species, a series of tetracoordinated organoboron anions [BL₄]⁻ (L = phenyl (<b>1</b>), 4-fluorophenyl (<b>2</b>), 1-imidazolyl (<b>3</b>), L₄ = H­(pyrazolyl)₃ (<b>4</b>)) with bulky organic ligands covalently bound to the central B atom. Negative ion photoelectron spectroscopy (NIPES) reveals that all of these anions possess EBEs higher than that of Cl^– with the adiabatic/vertical detachment energy (ADE/VDE) of 4.44/4.8 (<b>1</b>), 4.78/5.2 (<b>2</b>), 5.08/5.4 (<b>3</b>), and 4.59/4.9 eV (<b>4</b>), respectively. First-principles calculations confirmed high EBEs of [BL₄]⁻ and predicted that these anions are thermodynamically stable against fragmentation. The unraveled superhalogen nature of these species provides a molecular basis to explain the wide-ranging applications of tetraphenylborate (TPB) (<b>1</b>) and trispyrazolylborate (Tp) (<b>4</b>) in many areas spanning from industrial waste treatment to soft material synthesis and organometallic chemistry