Alternative energy input for transfer hydrogenation using iridium NHC based catalysts in glycerol as hydrogen donor and solvent

Abstract

Four Ir(I) and Ir(III) N-heterocyclic carbene (NHC) based complexes with 3,4,5-trimethoxybenzyl N-substituents have been obtained and fully characterized. The new complexes have been used as catalysts in the reduction of aldehydes and ketones with glycerol, and their activities have been compared to those shown by other Ir(III) NHC-complexes previously reported. The reactions were carried out under oil bath heating, and a detailed comparative study has been carried out using microwave (MW) and ultrasound (US) activation. The new Ir(III) complexes proved to be most efficient in the reduction of ketones, while the Ir(I) complexes are more active in the reduction of aldehydes. The use of ultrasound has a tremendous impact in shortening reaction times, and good results have been obtained in the reduction of aldehydes. In some experiments transmision electron microscopy (TEM) and UV−vis analysis showed the presence of iridium-containing nanoparticles after MW or US activation.