Inorganic click reactions: recent studies on the reactivity of metal azides of groups 6–9 towards alkynes and nitriles

Click reactions have been comprehensively explored and employed in bio-orthogonal coupling owing to their remarkable specificity. In these bio-orthogonal reactions, biomolecules are modified by covalently associating them with fluorophores and radioisotopes, specifically tagging, for detecting biomolecules in complex systems. The most notable example of a click reaction is the [3 + 2] cycloaddition coupling of azides and electrophiles to afford five-membered heterocyclic rings. To date, almost all metals have produced an azide complex, whether it is a coordination or an organometallic compound. Unlike the catalysed well-known azide-alkyne reactions, the inorganic click referred to as "iClick" reactions occur directly in the inner metal coordination sphere without the need for a catalyst. We herein provide an overview of the reactions of metal azides of groups 6-9 with alkynes and nitriles, focusing on the nature of iClick product(s) and how the reaction conditions, electrophile type, and steric hindrance affect the isomerization and preference of one isomer over the other(s).