Transition metals and organoazides

Fachbereich Chemie, Technische Universitdt Kaiserslautern, Erwin-Schrodinger-Str. Get. 54, D-67663 Kaiserslautern, Germany 

In coordination chemistry, azido complexes wherein azide ions are coordinated to metal sites are known for long. The azide ion was found in terminal as well as in bridging (l,l-/r, l,l,l-/r, l,3-/r) coordination geometries. It is well established in the literature that the stability of such compounds decreases with an increase of the covalence of the M-N3 bond and with an increasing M/N3 ratio. Binary systems of the type MtNslx usually decompose under formation of dinitrogen and elemental M. 

Organic Azides Syntheses and Applications Edited by Stefan BrSse and Klaus Banert 2010 John Wiley Sons, Ltd. ISBN 978-0-470-51998-1 

If the metal site of a coordination compound is in a sterically and/or electronically saturated situation, coordination of an organo azide will be hindered and thus metal centred reactivity of the N3-fragment will not occur. If additionally the other ligands coordinated to the metal site are inert against an attack by the organoazide too, no reactivity will be observed at all. There must be a multitude of such intrinsically nonreactive combinations of organoazides and (transition) metal complexes which have not found their way into literature. 

The authors did not report the thermal stability of the copper(II) complex. However, they mention, that it is noteworthy that picryl azide reacts with a variety of olefins, norbomene, pinene, cyclopentene, cyclooctene, and others, with exceptional ease . The fact, that the copper(II) does not decompose this organoazide can clearly be attributed to the efficient shielding of the copper(ll) centre by the two 8-hydroxyquinolinato ligands. 

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However, transition metal sites not only possess Lewis acidic properties, they are often redox active too. Metal compounds in low and middle oxidation states can deliver electrons to substrates like organoazides. By a formal two electron reduction, [RN3] dianions are generated, which again can be described by at least two mesomeric forms (Scheme 12.2) and which should be stabilized due to the presence of electronegative nitrogen atoms. 

The ferrocenyl fragment is a key example for a sterically and electronically saturated transition metal site. Therefore it is not surprising, that organoazides will not undergo reaction with this system under normal conditions. However, there is a series of other structurally well defined organoazides being in connection to a transition metal centre via a ligand backbone in the literature. 

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