The synthesis and characterisation of immobilised palladium carbene complexes and their application to heterogeneous catalysis

Silica supported palladium NHC complexes have been prepared by two different routes: one involving the reaction of silica-supported imidazolium salts with palladium acetate and a direct immobilisation of a pre-formed complex by reacting a (trimethoxysilylpropyl)-N-aryl-imidazolylidene palladium complex with surface hydroxyl groups. A small range of catalysts of varying steric bulk were prepared in order to evaluate the effect on catalytic conversion. The activity of the palladium catalysts in Suzuki cross-coupling reactions has been established. The catalysts prepared by immobilising pre-formed palladium complexes gave superior results for the conversion of aryl bromides and aryl chlorides. In addition, use of sterically bulky NHCs (such as the N-2,6-(diisopropyl)phenyl-substituted ligand) resulted in increased catalytic activity, which is analogous to the trends noted in homogeneous catalysis.

Graphical abstractThe following work documents the synthesis, characterisation and testing of a small range of silica-supported palladium NHC complexes. The complexes could be prepared by two different routes allowing for a comparison of synthetic methodologies. Also, as would generally be predicted from studies of the homogeneous analogues of these complexes, increasing the steric bulk around the metal centre has a positive effect on the activity of the catalyst in Suzuki cross-coupling reactions.Figure optionsDownload full-size imageDownload as PowerPoint slideHighlights► Silica supported bis-NHC complexes of palladium were synthesised by two different routes. ► One route allows for characterisation of the NHCs prior to immobilisation and greater control over the palladium loading. ► Moderate activity in Suzuki coupling reactions was observed. ► The presence of steric bulk on the NHC has a positive effect on catalytic activity.