Studies of the reactivity of N-heterocyclic carbenes with halogen and halide sources

Abstract

The reactivity of the N-heterocyclic carbene (NHC) 1,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene, IMes, with a series of halide sources, including 1,1,1,2,2,2-hexachloroethane, 1,2-dibromoethane and dibromine, has been assessed. These result in the formation of the imidazolium species 1,3-bis(2,4,6-trimethylphenyl)-2- chloroimidazolium chloride, [IMesCl][Cl], 1,3-bis(2,4,6-trimethylphenyl)imidazolium bromide, [IMesH][Br] and 1,3-bis(2,4,6-trimethylphenyl)-2-bromoimidazolium bromide, [IMesBr][Br]. Treatment of IMes with 2.0 equiv. of carbon tetrabromide, CBr₄ , in tetrahydrofuran or benzene yields the new NHC 1,3-bis(2,4,6- trimethylphenyl)-4,5-dibromoimidazol-2-ylidene, IMesBr₂ . IMesBr₂ is indefinitely stable in air and has been characterised by XRD. The molecular and supramolecular structures of compounds [IMesBr][Br]_3MeCN, [IMesH][Br], [IMesCl][Cl]•MeCN and [IMesCl][AlCl₄], the latter formed from the addition of aluminium trichloride to [IMesCl][Cl], are discussed.