Unwrapping the Dodecaborane Core: Structure, Electronic Properties, and Chemical Reactivity Across the Complete...

Successively stripping the exohedral substituents from the stable closo-dodecaborate anion [B₁₂I₁₂]^2- results in structural transformations of the icosahedral B₁₂ into a (quasi-)planar unsubstituted B₁₂ unit. Previous studies have revealed that [B₁₂I₁₁]^- to [B₁₂I₈]^- ions contain a closed B₁₂ unit, while [B₁₂I₇]^- is the first ion in the series with an opened B₁₂ unit. Further transitions in geometry, electronic structure, and chemical reactivity across the whole range of fragments [B₁₂I_n]^- (n = 11-1) remain elusive. Herein, we report a systematic investigation to explore the chemical properties of these intermediate structures by using negative ion photoelectron spectroscopy (NIPES), ion mobility spectrometry (IMS), gas-phase ion-molecule reactions, and quantum chemical calculations. [B₁₂I_n]^- ions can be categorized into three groups: (1) very reactive ions with intact (quasi-)icosahedral B₁₂ cages (n = 11-8), (2) less reactive ions with open structures (n = 7-4), and (3) almost unreactive ions with (quasi-)planar structures (n = 3-1). Preparative mass spectrometry shows that ions with the unsaturated B₁₂ core (category 2) tend to form smaller, fully substituted closo-borate anions [B_mX_m]^2- (m = 6-11 with X = I, H, OH) on surfaces. In contrast, ions in category 3 cannot be found on the surface and apparently decay into volatile products. This research provides fundamental insights into the physical and chemical properties of B₁₂ units depending on their substitution level and paves the way for the rational design of boron-rich compounds by using unconventional [B₁₂I_n]^- building blocks.