Organometallic 3d transition metal NHC complexes in oxidation catalysis

The development of processes for the selective oxidation of hydrocarbons is a major focus in catalysis research. Making this process simultaneously environmentally friendly is still challenging. 3d transition metals are highly promising candidates to replace expensive and/or toxic heavier metals in their applications as oxidation catalysts. Optimizing such catalytic systems is closely connected to the improvement of stabilizing ligands. In this context, N-heterocyclic carbenes (NHCs) have attracted particular attention. Their strong σ-donor properties, compared to nitrogen or oxygen-donor ligands, allow the stabilisation of transition metals in high oxidation states, known to be key intermediates in oxidation catalysis. This review provides an overview of recent applications of 3d transition metal NHC complexes in oxidation catalysis, by summarising the advances towards the catalytic oxidation of the most important substrates namely alkenes, alkanes, aromatics, alcohols and amines until the end of 2021. Additionally the activation of molecular oxygen, representing a green and abundantly available oxidant, is described with first row transition metal NHC complexes.