Access to cationic polyhedral carboranes via dynamic cage surgery with N-heterocyclic carbenes

Abstract

Polyhedral boranes and heteroboranes appear almost exclusively as neutral or anionic species, while the cationic ones are protonated at exoskeletal heteroatoms or they are instable. Here we report the reactivity of 10-vertex closo-dicarbadecaboranes with one or two equivalents of N-heterocyclic carbene to 10-vertex nido mono- and/or bis-carbene adducts, respectively. These complexes easily undergo a reaction with HCl to give cages of stable and water soluble 10-vertex nido-type cations with protonation in the form of a BHB bridge or 10-vertex closo-type cations containing one carbene ligand when originating from closo-1,10-dicarbadecaborane. The reaction of a 10-vertex nido mono-carbene adduct with phosphorus trichloride gives nido-11-vertex 2-phospha-7,8-dicarbaundecaborane, which undergoes an oxidation of the phosphorus atom to P = O, while the product of a bis-carbene adduct reaction is best described as a distorted C₂B₆H₈ fragment bridged by the (BH)₂PCl₂⁺ moiety.

Fig. 1. Reactivity of 10-vertex closo-dicarbadecaboranes.

All the ten-vertex carboranes show different reactivity towards the carbene and subsequently hydrogen chloride or phosphorus trichloride. closo-1,2-dicarbadecaborane is referred to as ortho (o, red trace); closo-1,6-dicarbadecaborane is referred to as meta (m, blue race), closo-1,10-dicarbadecaborane is referred to as para (p, green trace). .

Fig. 2. The molecular structures of o-1 (left), m-2 (centre) and p-2 (right).

ORTEP-type plots, 40% probability level. The 2,6-diisopropylphenyl groups are displayed as wireframes for clarity.

Fig. 3. The molecular structures of o-2a (left), m-2a (centre) and p-2a (right).

ORTEP-type plots, 40% probability level. Cl⁻ or HCl₂⁻ anions and dichlormethane solvates are omitted for clarity. The 2,6-diisopropylphenyl groups are displayed as wireframes for clarity.

Fig. 4. Computed (B3LYP/cc-pVTZ) ESP surface for p-2a.

The colour range of the ESP in kcal/mol. The negative surface is located on HCl₂^-, the areas with the most positive ESP are the CH vertexes of the carborane cage and CH = CH fragment of the NHC.

Fig. 5. The molecular structures of o-1a (left), o-1b (centre) and o-2b (right).

ORTEP-type plots, 40% probability level. Chloride anion and dichlormethane solvates are omitted for clarity in o-2b. The 2,6-diisopropylphenyl groups are displayed as wireframes for clarity.