Cryptands catechol

As with the crowns, the situation becomes more complicated when there are other heteroatoms or substituents in one of the cryptand bridges. The symbol B is used to designate a benzo- or catechol unit in the bridge and subscripts are used to designate heteroatoms when non-oxygen heteroatoms are present. Examples of this are shown in structures 12 and 13 above. 

Amidocryptands were until recently unknown in anion circles [39-41,54-56], However, Raymond and coworkers previously synthesized catechol-based amidocryptands as models for siderophores [57], The appeal of polyamide cryptands over polyamines is related to their solution properties, namely the former class of ligands is less susceptible... 

Iron transporters known as siderophores occur in various bacteria. They coordinate iron in a complex that involves three catechol residues. A natural host molecule called entero-bactin is shown along with a cryptand-like molecule (9) that is one of several that were devised to mimic this complexation behavior (reviewed in Roosen-berg, 2000 Raymond, 2003). The catechols deprotonate to the catecholate anions, which provide six oxygen donors for ferric ion The host thus completely envelops the cation permitting transport as the complex. 

The mechanism of 1 1 complex formation between palladium(II) and catechol and 4-methylcatechol has been studied in acidic media, and the rate of 1 1 (and 1 2) complex formation between silver(II) and several diols is an order of magnitude higher in basic solution than in acidic. The kinetics of formation and dissociation of the complex between cop-per(II) and cryptand (2,2,1) in aqueous DMSO have been measured and the dissociation rate constant, in particular, found to be strongly dependent upon water concentration. The kinetics of the formation of the zinc(II) and mercury(II) complexes of 2-methyl-2-(2-pyridyl)thiazolidine have been measured, as they have for the metal exchange reaction between Cu " and the nitrilotriacetate complexes of cobalt(II) and lead(II). Two pathways are observed for ligand transfer between Ni(II), Cu(II), Zn(II), Cd(II), Pb(II) and Hg(II) and their dithiocarbamate complexes in DMSO the first involves dissociation of the ligand from the complex followed by substitution at the metal ion, while the second involves direct electrophilic attack by the metal ion on the dithiocarbamate complex. As expected, the relative importance of the pathways depends on the stability of the complex and the lability and electrophilic character of the metal ion. 

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