A personal look at the history of aqueous organometallic catalysis

1 A personal look at the history of aqueous organometallic catalysis 

Nowadays we look with other eyes at organometallic compounds the family of which has expanded enormously. Some members of this family are soluble in water due to their ionic nature the legions of anionic carbonylmetallates (e.g. [Ni(CN)(CO)3]-) and cationic bisphosphine Rh-chelate complexes (e.g. [Rh(BDPP)(COD)]+) just come to mind. Others obtain their solubility in water from the well soluble ligands they contain these can be ionic (sulfonate, carboxylate, phosphonate, ammonium, phosphonium etc. derivatives) or neutral, such as the ligands with polyoxyethylene chains or with a modified urotropin structure. 

One of the most important metal complex catalyzed processes is the hydroformylation of light alkenes. In the early years the catalyst was based on cobalt and this brought about an intense research into the chemistry of cobalt carbonyls. A key intermediate, [CoH(CO)4] is well soluble and stable in water and behaves like a strong acid [1] in aqueous solution  

For a decade or so [CoH(CN)5]3 was another acclaimed catalyst for the selective hydrogenation of dienes to monoenes [2] and due to the exclusive solubility of this cobalt complex in water the studies were made either in biphasic systems or in homogeneous aqueous solutions using water soluble substrates, such as salts of sorbic acid (2,4-hexadienoic acid). In the late nineteen-sixties olefin-metal and alkyl-metal complexes were observed in hydrogenation and hydration reactions of olefins and acetylenes with simple Rh(III)- and Ru(II)-chloride salts in aqueous hydrochloric acid [3,4], No significance, however, was attributed to the water-solubility of these catalysts, and a new impetus had to come to trigger research specifically into water soluble organometallic catalysts. 

In addition to the outstanding achievements in connection with the RCH-RP process other breakthroughs of aqueous organometallic catalysis deserve mentioning, too. The first attempts of enantioselective hydrogenation in water with soluble catalysts were described already in 1978 and today there are several examples of almost complete 

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