PNNL

Abstract

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 B120/– 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 [B12In]– (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. [B12In]– 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). While clusters with a high and low level of substitution form the enthalpically favored isomers, we observed a mixture of non-energetic but entropically favored isomers with characteristic reactivity for n=7-4. Preparative mass spectrometry shows that ions with the unsaturated B12 core (category 2) tend to form smaller, fully substituted closo-borate anions [BmXm]2– (m = 6–11 with X=I, H, OH) on surfaces. In contrast, ions in the 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 [B12In]– building blocks.