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Chapter 2: Ligands used for aqueous organometallic catalysis

Solubility of the catalysts in water is determined by their overall hydrophilic nature which may arise either as a consequence of the charge of the complex ion as a whole, or may be due to the good solubility of the ligands. Although transition metal complexes with small ionic ligands, such as halides, pseudohalides or simple carboxylates can be useful for specific reactions the possibility of the variation of such ligands is very limited. As in organometallic catalysis in general, phosphines play a leading role in aqueous organometallic catalysis (AOC), too. There is a vast armoury of synthetic organic chemistry. | Chapter 2 Ligands used for aqueous organometallic catalysis Solubility of the catalysts in water is determined by their overall hydrophilic nature which may arise either as a consequence of the charge of the complex ion as a whole or may be due to the good solubility of the ligands. Although transition metal complexes with small ionic ligands such as halides pseudohalides or simple carboxylates can be useful for specific reactions the possibility of the variation of such ligands is very limited. As in organometallic catalysis in general phosphines play a leading role in aqueous organometallic catalysis AOC too. There is a vast armoury of synthetic organic chemistry available for preparation and modification of various phosphine derivatives of which almost exclusively the tertiary phosphines are used for catalysis. The main reason for the ubiquity of tertiary phosphines in catalysis is in that most transformations in AOC involve the catalysts in a lower valent state at one or more stages along the catalytic cycle and phosphines are capable of stabilizing such low oxidation state ions such way hindering metal precipitation. For the same reason ligands posessing only hard donor atoms e.g. N or O are not common in AOC and used mainly for synthesizing catalysts for oxidations or other reactions where the oxidation state of the metal ion remains constant throughout the catalytic cycle examples can be the heterolytic activation of dihydrogen or certain hydrogen transfer reactions . Some of the neutral that is non-ionic ligands are water-soluble due to their ability of forming several strong hydrogen bonds to the surrounding water molecules. These ligands usually contain several N or O atoms such as the l 3 5-triaza-7-phosphaadamantane PTA the phosphorus analog of urotropin tris hydroxymethyl phosphine P CH2OH 3 or several phosphines containing long polyether e.g. polyethyleneglycol- PEG-type chains. Most of the ligands in AOC however are derived from waterinsoluble .