Gas phase fragmentation of adducts between dioxygen and closo-borate radical anions

Abstract

The icosahedral closo-borate anions, [closo-B12X12]2− (B12) and [1-H-closo-1-CB11X11]− (CB11) with X=H or halogen, are exceptionally stable weakly coordinating anions. Recent studies, which focused on the gas phase ion chemistry of this class of anions, revealed several unusual ion-molecule and unimolecular dissociation pathways. Herein, we demonstrate that collisional activation of highly iodinated closo-borate anions results in a loss of an iodine atom followed by a spontaneous addition of O2 present in a mass spectrometer background producing B12-O2 and CB11-O2 species. We examined the electronic structure of a representative O2 adduct using photoelectron spectroscopy and explored collision-induced dissociation (CID) pathways of a series of iodinated B12-O2 and CB11-O2 anions with different substituents. A systematic study of the gas phase dissociation pathways of the B12-O2 and CB11-O2 anions revealed several unusual dissociation pathways characteristic of this class of species. Abundant fragment ions are formed by neutral loss of BOX2 (X = boron bound substituent), Y2H (Y = boron bound halogen, if any H is present in the molecular ion) and BO2. Loss of BO2 was observed for CB11-O2 but not for B12-O2 anions. Substantial differences in stability of the corresponding fragments are responsible for the observed differences in dissociation of B12-O2 and CB11-O2 species. Furthermore, we show that in addition to O2 other molecules with a radical character (e.g. SCH3) can form adducts with closo-borate anions after iodine abstraction. The boron cage fragmentation observed for the O2 adducts is atypical for highly iodinated closo-borates and is attributed to the strength of BO bonds which dominate the thermochemistry of B12-O2 and CB11-O2 fragmentation reactions.