Download The catalytic hydrogenolysis of esters to the two corresponding

The catalytic hydrogenolysis of esters to the two corresponding alcohols is a difficult transformation,
since esters are very stable compounds. Therefore, the development of suitable homogeneous
catalysts is very challenging. One of the current focuses in our group is the preparation of ruthenium
complexes bearing tridentate ligands which consist of N-Heterocyclic Carbenes (NHCs) functionalized
with phosphines. Upon coordination to transition metals, these ligands give the resulting metal
complex stability and electron density that is needed for the formation of hydride species. Hydride
species are important intermediates in the catalytic cycle of hydrogenolysis and hydrogenation
reactions. The aim of this project is to develop ruthenium complexes bearing bidentate NHC ligands
functionalized with phosphines or amines (see Figure 1) as an extension to the tridentate ligands.
The amine functionalities are chosen as donor group because this group has shown to enable
bifunctional mechanisms in hydrogenation reactions. The NH-group allows ‘outer sphere’
mechanisms through metal-ligand bifunctional catalysis, where the substrate does not have to bind
to the metal centre.
Figure 1: ligands used in this project
Ligand CNHC-PPh2 and ligand CNHC-NH2 are complexated to ruthenium successfully and characterized
by NMR and mass spectroscopy. The preparation of ruthenium complexes bearing ligand CNHC-NHtBu
was troublesome and not accomplished. The two complexes are applied in the hydrogenation of
acetophenone of which complex CNHC-NH2 showed good results. Complex CNHC-PPh2 gave a negligible
conversion to 1-phenylethanol. Moreover, catalysis with in situ prepared catalysts was performed.
Some of the in situ generated catalysts gave good conversions to the alcohol but the catalysts were
less active compared to similar catalysts from literature.