Alternative Energy Input for Transfer Hydrogenation using Iridium NHC Based Catalysts in Glycerol as Hydrogen Donor and Solvent

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.