Metal complexes, water-soluble catalysts

Examples of non-platinum metal hydrogenation catalysts include (arene)-chromiumtricarbonyls which will hydrogenate dienes, alkynes, and so on, while ReH7(PCy3)2 will selectively hydrogenate acenaphthalene. Lanthanides and early transition metals are discussed later. Those catalysts operate via non-radical processes, but a few systems are known to involve radical reactions. The complex [CoH(CN)5]3 is a water-soluble catalyst that is selective for the hydrogenation of ajS-unsaturated compounds. 

The hydrogenation of unsaturated fatty acids as part of membrane stmctures of a wide range of lipid classes offers important opportunities [16, 17, 169, 170, 244 f]. Water-soluble catalysts are much more suitable than their lipophilic derivatives, as no solvent vector such as THF or DMSO is needed for the transport of the metal complexes into the membranes. Furthermore, water-soluble catalysts are easier or remove from the still-intact membrane at the end of the reaction, and even the polarity of the catalyst may be controlled by suitable choice of ligand, thus offering a bundle of advantages over monophasic hydrogenations [173]. 

The aim of the most studies on the water-soluble phosphines is to find separable, active, and selective catalysts. The water-soluble catalysts are additionally useful for the reactions of hydrophobic substrates with inorganic salts. So far, amphiphilic solvents or mixed catalyst systems of normal PTCs and normal transition metal complexes have been used for some reactions. Though the mixed system enables the easy separation of inorganic salts from the reaction mixture, the separa-... 

Recent research in the application of supercritical (sc) fluids and ionic liquids (IL) as solvents in homogeneous catalysis (see Sections 7.3 and 7.4), opened the way to the development of biphasic water/scCOz [171, 172] and water/IL [173] systems for the hydrogenation of various substrates, e.g., alkenes, aldehydes, etc. with water-soluble catalysts. The catalytically highly active, versatile and robust transition metal - N-heterocyclic carbene complexes [174] have also been applied for hydrogenation reactions [175], Given that water-soluble complexes with N-heterocy-clic carbene ligands are known [176], catalytic applications in aqueous systems are also foreseen. 

Synthesis of the first water-soluble catalyst complexes was reported in 1973 [3] and was based on the use of sulfonated triphenylphosphine to replace TPP. The sulfonated derivative was found to stabilize the lower oxidation states of a number of transition metals such as Rh, Ru, Ir, Pt, Ni, and Cu in aqueous systems and these water-soluble catalysts facilitated hydrogenation of soluble substrates like pyruvic acid. 

Hydroformylation of olefins using water-soluble catalysts in two-phase systems has been extensively studied [22, 23], the role of different types of water-soluble ligands and metal complexes in the activity, selectivity, and stability has been discussed for a variety of olefinic substrates. A few case studies in which kinetics and rate behavior have been addressed are reviewed here. 

Herrmann, W.A. Kohlpaintner, C.W. Manetsberger, R.B. Bahrmann, H. Kottmann, H. (1995) Water-soluble metal-complexes and catalysts. 7. New efficient water-soluble catalysts for 2-phase olefin hydroformylation BINAS-Na, a superlative in propene hydroformylation, J. Mol. Catal A - Chem., 97,65-72. 

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