Functionalization of Saturated and Unsaturated Hydrocarbons with Electrophilic Anions [B ₁₂ X ₁₁ ] ⁻ (X = Br, I) in the Gas Phase and at Interfaces

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.