N-Heterocyclic carbene adducts of the heavier group 15 tribromides. Normal to abnormal isomerism and bromide ion abstraction

Abstract

The reactivity of the heavier group 15 tribromides, SbBr₃ and BiBr₃, towards 1,3-bis(2,6-diisopropylphenyl)-imidazol-2-ylidene (IPr) is described. These reactions quantitatively afford Lewis acid–base adducts, (IPr)EBr₃ (E = Sb 1; Bi 2), which readily react with AlBr₃ yielding cationic species [(IPr)EBr₂]⁺ (E = Sb 3; Bi 4). Under thermal treatment, the N-heterocyclic carbene ligands in 1 and 2 will readily isomerise to afford the abnormally-bonded (or mesoionic) complexes (aIPr)EBr₃ (E = Sb 5; Bi 6). As with 1 and 2, bromide abstraction from such compounds readily affords the cationic complexes [(aIPr)EBr₂]⁺ (E = Sb 7; Bi 8). Finally, in an effort to elucidate the isomerisation process which allows for the conversion of 1 and 2 to the abnormally bonded systems (compounds 5 and 6), compound 1 was reacted with a further equivalent of IPr to afford the cationic species [(aIPr)₂SbBr₂]⁺ (9). This strongly suggests that the normal to abnormal isomerisation of the N-heterocylic carbene ligands in compounds 1 and 2 is mediated by the presence of free IPr. Compound [9]Br can be used to access the dicationic species [(aIPr)₂SbBr]²⁺ (10), which we have identified spectroscopically. Single crystal X-ray structures and spectroscopic data for all compounds are discussed.