Biomimetic catalysis with immobilised organometallic ruthenium complexes: substrate- and regioselective transfer hydrogenation of ketones

Chloro-(eta6-arene) complexes of ruthenium(II) with N-sulfonyl-1,2-ethylenediamine ligands that have one or two styrene side chains have been synthesised and characterised. The chloro ligand was substituted with a diphenylphosphinato ligand and the resulting organometallic complexes are transition state analogues for the ruthenium-catalysed transfer hydrogenation of benzophenone. Following the protocol of molecular imprinting, these complexes were copolymerised with ethylene glycol dimethacrylate (EGDMA) in the presence of a porogen. The polymers were ground and sieved, and the phosphinato ligand was substituted with a chloro ligand, thus generating a shape-selective cavity in close proximity to the catalytically active metal centre. When tested for their ability to catalyse the reduction of benzophenone, the imprinted polymers showed a significantly higher activity (up to a factor of seven) than control polymers without cavities. Out of a mixture of seven different aromatic and aliphatic ketones, benzophenone was preferentially reduced when the imprinted polymer was used. Furthermore, the specificity of the catalyst for diaryl ketones has been confirmed in a reaction with a bifunctional substrate, 4-acetyl-benzophenone; the diaryl ketone was reduced faster with the imprinted catalyst than the acetyl group. The opposite regioselectivity was observed with the control polymer. Both the activity and the selectivity of the imprinted catalysts are dependent on how the ruthenium complexes are attached to the polymer backbone. A double connection proved to give superior results.