Polyaminocarboxylic acid ligands

The ability of polyaminocarboxylic acids to form stable, water-soluble chelates over a wide pH range accounts for their diversity of uses. Ethylenediaminetetraacetic acid, H4EDTA, has played a central role in this development and stimulated interest in other complexones with a view to finding ligands with increased affinity and selectivity for metal ions. 

Another class of Mn( III) complexes involves the polyaminocarboxylic acid ligands. The earliest study appeared in 1962 ( 44) and was followed shortly by studies of the cyclohexane analog of EDTA as well as other derivatives of EDTA (45). A recent paper discusses the reactivity of the manganese (III )-diaminocyclohexanetetraacetate complex with hydrogen peroxide (46). A mechanism is proposed which involves complexation by the peroxide anion followed by subsequent electron transfer to produce the Mn(II) complex and the H02 radical. The results are interesting and indicate the potential for selective catalysis by the higher oxidation state manganese complexes. 

Mixed Donors. The 4-methylpyridine adduct of bis-[l-(2-thienyl)-4,4,4-trifluoro-butane-l,3-dionato]nickel(ii) is shown to have weaker co-ordination than the nonadduct by X-ray studies. The crystal structure of bis-(8-amino-2-methylquinoline) nitratonickel(ii) nitrate shows that the nitrato ligand is bidentate (66). A kinetic study has been made of nickel(ii) murexide complex formation in DMS0-MeN02- Nickel(ii) complexes of some optically active ethylenediamine-NN -diacetic acid-type polyaminocarboxylic acids have been prepared and solid-state spectra and t.g.a. recorded. N.m.r. temperature dependence for racemization of Ni(edta) ,... 

The inability of DTPA to completely coordinate the tetra-valent actinides is shown by the easy formation of ternary complexes between Th(DTPA) and many bidentate ligands (16, 17, 18). The hydrolysis of Th(IV) and U(IV) DTPA complexes at pH near 8 is explained by the dissociation of H+ from a coordinated water molecule (19, 20, 21, 22). In addition, the polyaminocarboxylic acids are toxic because they indiscriminately complex and remove biologically important metals, especially zinc (23, 24, 25, 26). Thus there is a need to develop new and powerful chelating agents highly specific for tetravalent actinides, particularly Pu(IV). 

Gd + complexes with macrocyclic and acyclic polyaminocarboxylic acid ligands 78-80, 8 are typical MRI contrast agents (Caravan et al., 1999). Although they effectively encapsulate the Gd + cation and form stable complexes in water (log K = 16-25), their metal centers have free coordination sites for water molecules and their unpaired electrons reduce the relaxation times Ti and T2 (fig. 29) (Rulofif et al., 1998 Aukmst et al., 2001 Dubost et al., 1991 Kumar et al., 1994). The exchange processes of the coordinated water molecules (inner-sphere water) and hydrogen-bonded water molecules (2nd-sphere water) with bulk water molecules have a significant influence on the reduction of the relaxation times (fig. 30). Therefore, the structural and electronic factors of the Gd + complex should be controlled to attain dynamic water exchange. 

While acychc polyaminocarboxylate (DTPA, ethylenedi-aminetetraacetic acid (EDTA)) complexes of copper(n) have high thermodynamic stabihty, these complexes are kinetically labile to ligand exchange. Cu(ll) has been found to have much greater kinetic stability (and consequently greater serum stability) with macrocyclic chelates, such as TETA and DOTA,... 

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