TEMPO in metal complex catalysis

Nitroxyl radicals, illustrated by 2,2,6,6-tetramethylpiperidine-N-oxyl (TEMPO), constitute a versatile class of stable free radicals that are extensively used in metal complex catalyzed synthetic transformations of a great interest. The Cambridge Structural Database reveals more than 260 hits, concerning crystal structures of metal complexes of N-oxyl ligands and most of them (ca. 240) relate to complexes comprising TEMPO and its derivatives. In order to prepare multifunctional catalysts, supramolecular architectures, drugs, magnetic materials, etc., the para-position of TEMPO has been functionalized with OH, COOH, CN, NH2 or N3, leading to an increase of the coordination sites beyond the NO· synthon. Herein, we highlight the coordination mode and reactivity of TEMPO ligands in the design of coordination compounds, which can be applied in catalysis. The review mainly focuses on the auxiliary/crucial role of TEMPO in functionalization of alkanes, alkenes, alkynes, aromatics, heterocycles, aldehydes, ketones and alcohols, catalyzed by metal complexes.