Azide Chemistry – An Inorganic Perspective,Part II[‡] [3+2]‐Cycloaddition Reactions of Metal Azides and Related Systems

In whatever state of bonding – whether covalent to an organic residue or a heteroatom, or polar to ionic in contact with a metal – the azide moiety N₃ is characterized by its high potential of reactivity which essentially manifests itself in two basic processes: the elimination of dinitrogen and the entry into 1, 3‐dipolar cycloadditions with suitable dipolarophiles, the latter of which clearly predominates the chemistry of azide, also that of its metal compounds. In a preceding review entitled “Part I – Metal Azides: Overview, General Trends and Recent Developments” which was meant to lay the foundations for the present paper, these and other reactions have already been touched upon. The present review – Part II – now focusses in great detail on the formation of five‐membered heterocycles – tetrazol(at)es, triazol(at)es, triazolin(at)es, thiatriazol(at)es, etc. as well as various consecutive products – from azide and nitriles, isocyanides, alkynes, alkenes and heteroallenes (CS₂, RN=C=S) in the ligand sphere of the metal. Generally, these [3+2]‐cycloadditions are found to proceed under much milder conditions in comparison with the strictly organic case whose triumphant progress since the 1960s is intimately bound up with the name of Huisgen. Mechanistic considerations on the matter are presented. A secondary aspect still occupying quite a part of the discussion is concerned with the role of metals in [3+2]‐cycloadditions particularly of the highly topical “click”‐type, e.g. (CuAAC), (RuAAC). Likewise, a short chapter deals with the question of pentazol(at)e (N₅^–) which according to numerous theoretical studies could well be stabilized and isolated in combination with metals, e.g., in the form of azametallocenes. A last chapter is devoted to a cursory survey of related systems, in particular fulminato complexes, metallo nitrile ylides and metallo nitrile imines, in which the metal acts as a substituent on the 1, 3‐dipole (metallo‐1, 3‐dipole). Other systems with a metal substituent on the dipolarophile (metallo‐dipolarophile), or, with metal itself in the three‐ (two‐) atom arrangement constituting the dipole (dipolarophile) [metalla‐1, 3‐dipole, metalla‐dipolarophile] are only quoted by way of example.