Azide-Substituted-1,2,3-Triazolium Salts as Useful Synthetic Synthons: Access to Triazenyl Radicals and Staudinger type Reactivity.

Abstract

Mesoionic carbenes (MIC) are a popular class of compound that are heavily investigated at the moment. The access to cationic MICs, and the ability of MICs to stabilize radicals are two highly attractive fields that have hardly been explored until now. Here we report the synthesis and characterisation of three different cationic azide-substituted 1,2,3-triazolium salts, used as building blocks for the study of their reactivity towards triphenylphosphine, where the reactivity is dependent on the nature of the starting triazolium salt. Furthermore, we have used the cationic triazolium salts to develop a series of unsymmetrical MIC-triazene-NHC/MIC' compounds, which can be readily converted to the radical form either via electrochemical or chemical methods. These radicals, which display NIR electrochromism, were investigated using a battery of techniques such as electrochemistry, UV/Vis/NIR and, EPR spectroelectrochemistry and theoretical calculations. Interestingly, the MIC plays an important role in the stabilization of the triazenyl radical, particularly in a competitive role vis a vis their NHC counterparts. These results shed new light into the ability of MICs to stabilize radicals, and possibly also on their π-accepting ability.