Small molecule activation: SbF3 auto-ionization supported by transfer and mesoionic NHC rearrangementElectronic supplementary information (ESI) available: NMR and Raman spectra. CCDC 1454012, 1454013, 1523950 and 1523951. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c6dt04909e

The reactivity of antimony trifluoride, SbF 3 , in non-polar aprotic solvents has been unexplored due to its negligible solubility. The reaction of the [SbF 3 (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 )SbF 3 ] intermediate ( 3 ) in non-polar solvents. Mild heat treatment of the reaction mixture unexpectedly activates the SbF 3 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 ) 2 SbF 2 ] + [SbF 4 ] − , 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 SbF 3 . 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. Product [(L Dipp ) 2 SbF 2 ] + [SbF 4 ] − is the first example of mesoionic carbene rearrangement on any metal/metalloid fluorido substrate and the first example of a NHC auto-ionization product on any fluorido substrate reported to date.