Small molecule activation: SbF₃ auto-ionization supported by transfer and mesoionic NHC rearrangement

The reactivity of antimony trifluoride, SbF₃, in non-polar aprotic solvents has been unexplored due to its negligible solubility. The reaction of the [SbF₃(tmen)] complex (2) with the N-heterocyclic carbene, 1,3-bis(2,6-diisopropylphenyl)imidazole-2-ylidene, (L^Dipp), probably leads to the formation of a neutral [(L^Dipp)SbF₃] intermediate (3) in non-polar solvents. Mild heat treatment of the reaction mixture unexpectedly activates the SbF₃ molecules to undergo the auto-ionization reaction. In this process, one of the (L^Dipp) ligands undergoes rearrangement into the mesoionic carbene (MIC) and further coordinates to the cation to form [(L^Dipp)₂SbF₂]⁺[SbF₄]^-, product (4). Studies have confirmed that N,N,N',N'-tetramethylethane-1,2-diamine (tmen) present in the solution is not involved in the conversion mechanism and serves solely as an auxiliary ligand to increase the solubility of SbF₃. The compound (4) shows a rare pnictogen(iii) halide auto-ionization process with the first mesoionic NHC rearrangement on any metal/metalloid fluoride compound and the first reported reaction system where a N-heterocyclic carbene ligand facilitates the auto-ionization process on a fluorido substrate.