Homogeneous catalysis with metal chelates

Of particular interest as catalysts are the incompletely coordinated metal chelate compounds, which are sufficiently stabilized by the ligand to be stable in solution at pH values much higher than that at which the aquo metal ion would precipitate as the hydroxide and thus to become unavailable for homogeneous catalysis. Such a metal chelate would be particularly effective as a catalyst for the activation of a substrate which can coordinate to the metal ion in the chelate compound. The interaction of the substrate with the metal ion would increase its reactivity toward nucleophilic reagents such as solvent molecules or hydroxyl ions, in accordance with the following scheme ... 

Abstract This review deals with the synthesis and the catalytic application of noncyclopentadienyl complexes of the rare-earth elements. The main topics of the review are amido metal complexes with chelating bidentate ligands, which show the most similarities to cyclopentadienyl ligands. Benzamidinates and guanidinates will be reviewed in a separate contribution within this book. Beside the synthesis of the complexes, the broad potential of these compounds in homogeneous catalysis is demonstrated. Most of the reviewed catalytic transformations are either C-C multiple bond transformation such as the hydroamination and hydrosilylation or polymerization reaction of polar and nonpolar monomers. In this area, butadiene and isoprene, ethylene, as well as lactides and lactones were mostly used as monomers. 

Another class of chelating phosphines has received considerable attention in recent times for potential application in homogeneous catalysis. These are called pincer ligands. As shown in structure 232, here two phosphorous and one carbon atom occupy three coordination sites of the metal atom. Pincer ligands are therefore tridentate ligands, where two phosphorous atoms form two dative bonds, but with the carbon atom there is a formal covalent bond. 

Scheme 41 outlines the essence of chiral catalysis. The chiral catalysts in general work homogeneously which means that they are small molecules, mostly monomeric and contain one (mononuclear) or sometimes two (binuclear) metal atoms in a chelate complex with chiral organic ligands. Typical metals are Pd(0), Pd(II), Rh(I), Rh(II), Cu(II) which are used for essentially non-polar reactions... 

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