Electrophilic dodecaborate fragment ions B12X11- are among the most reactive anions known and are capable of converting inert reagents to charged compounds through covalent bonding. In this study, we investigate the reactivity of B12X11- (X = Br, I) with a series of hydrocarbons in two environments: (1) in the gas phase, using ion-molecule reactions and collision-induced dissociation (CID), and (2) at vacuum/solid interfaces, via fragment ion deposition onto surface layers of reagents. In the gas phase, n-alkyl groups of hydrocarbons are bound to B12X11- via the substitution of a proton by the electrophilic vacant boron atom. However, hydrocarbons with double and triple bonds are bound directly to B12X11-, resulting in strongly bound adducts with characteristic fragmentation behavior. Aromatic compounds can bind to the vacant boron atom in B12X11- by donating electron density from the aromatic unit, forming a quasi-tetrahedrally coordinated carbon atom, and the adduct fragments upon CID to regenerate B12X11- ions. Contrary to the gas-phase results, B12X11- reacts with all hydrocarbons at layer interfaces via proton substitution, regardless of the degree of saturation. Computational investigations rationalize that the thermochemically most favorable geometry of the B12X11- ─hydrocarbon adduct can often explain their observed fragmentation behavior in the gas phase. However, in the condensed phase, low-lying transition states allow rearrangement into the proton substitution binding mode, which is favorable due to entropic effects (e.g., proton dissipation in the layer). These results emphasize the possible differences in the formation of products of reactive ions in the gas phase and at interfaces, contributing to the selectivity control in reactive ion deposition experiments and providing a foundation to apply the "universal binder" B12X11- in analytical and preparative mass spectrometry for charge tagging of nonpolar organic molecules.
Rohdenburg et al. (Thu,) studied this question.