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

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.

In comparison to their neutral or anionic counterparts, examples of cationic boron clusters remain scarce. Here, the authors prepare a variety of cationic polyhedral boranes by reacting closo-10-vertex carboranes with N-heterocyclic carbenes; the resulting open-cage cationic nido- arachno- or closo- derivatives are water soluble, which may enable unprecedented applications.

The in Situ NHC-Palladium Catalyzed Selective Activation of B(3)-H or B(6)-H Bonds of o-Carboranes for Hydroboration of Alkynes: An Efficient Approach to Alkenyl-o-carboranes

An in situ Pd-NHC catalyzed selective B(3,6)-H activation for hydroboration of internal alkynes has been accomplished under mild conditions. This work offers a facile approach for the synthesis of alkenyl-o-carboranes and has important reference for selective functionalization of B(3,6)-H bonds.

Investigation of Thiaborane closo- nido Conversion Pathways Promoted by N-Heterocyclic Carbenes

The 12-X- closo-SB₁₁H₁₀ (X = H or I) thiaboranes react with one or two molar equivalents of various N-heterocyclic carbenes (NHCs) to give the deprotonated 12-vertex species of [12-X-SB₁₁H₉·NHC]^-[NHC-H]⁺composition as kinetic products. The use of one molar equivalent of a sterically more hindered NHC reactant leads to the formation of 12-X-SB₁₁H₁₀·NHC adducts with a heavily distorted cage and the nido electron count. Further reaction of 12-I-SB₁₁H₁₀·NHC to deboronated 12-X-SB₁₀H₉·NHC proceeds in acetone to complete the closo- nido reaction pathway under the thermodynamic control. The structures of all compounds have been investigated by NMR spectroscopy and diffraction techniques. The results are supported by theoretical methods.