Water‐Soluble Ligands, Metal Complexes, and Catalysts: Synergism of Homogeneous and Heterogeneous Catalysis

Rapid developments in the field of catalysis are leading to an increased demand for tailor-made catalysts. Water-soluble complex catalysts, which are being intensively investigated at the present time, combine the advantages of homogeneous and heterogeneous catalysis: simple and complete separation of the product from the catalyst, high activity, and high selectivity. From the large number of available water-soluble ligands, the appropriate catalysts can be developed for many reactions. The industrial applications in the fields of hydrogenation and hydroformylation have already indicated the wide scope of this type of catalyst. In addition, the annual production of 300 000 tons of butyraldehyde through application of water-soluble rhodium complexes at Hoechst AG in Oberhausen, Germany, has demonstrated the industrial importance of the concept of complex-catalyzed reactions in aqueous two-phase systems. The efficient operation of catalytic processes increasingly requires the loss-free recycling of the noble metal catalyst, and this can be simply and economically realized in two-phase systems. Special applications in biochemical problems open up developments in the field of water-soluble transition metal complexes that far transcend the familiar kinds of homogeneous catalysis. In the near future, the investigation and application of metal complex catalysts that are compatible with the physiological, cheap, and environmentally friendly solvent, water, is likely to become a high priority in catalysis research.