Unwrapping the Dodecaborane Core: Structure, Electronic Properties, and Chemical Reactivity Across the Complete [B₁₂I_n]− Series (n = 11–1)

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.