The reactions of group 13 metal trichlorides with aromatic azides were examined by CW EPR and pulsed ENDOR spectroscopies. Complex EPR spectra were obtained from reactions of aluminium, gallium and indium trichlorides with phenyl azides containing a variety of substituents. Analysis of the spectra showed that 4-methoxy-, 3-methoxy- and 2-methoxyphenyl azides all gave 'dimer' radical cations [ArNHC₆H₄NH₂](+•) and trimers [ArNHC₆H₄NHC₆H₄NH₂](+•) followed by polymers. 4-Azidobenzonitrile, with its electron-withdrawing substituent, did not react. In general the aromatic azides appeared to react most rapidly with AlCl₃ but this reagent tended to generate much polymer. InCl₃ was the least reactive group 13 halide. DFT computations of the radical cations provided corroborating evidence and suggested that the unpaired electrons were accommodated in extensive π-delocalised orbitals. A mechanism to account for the reductive conversion of aromatic azides to the corresponding anilines and thence to the dimers and trimers is proposed.
Keywords: ENDOR; EPR; aluminium; aromatic azides; gallium; indium.