Octadentate ligand

Octadentate ligands with four catechol units like 140 exhibiting eight coordination sites are capable of effective complexation of not only iron ions, but also much larger Pu4+ [49]. 

Neutral N-derivatized octadentate ligands based on cyclen (1,4,7,10-tetraazacy-clododecane) form tripositive cationic complexes with the trivalent lanthanides [116-124]. The N-substituted tetraamide derivatives have proven useful in understanding the relationship between the solution structure of the Ln3+ complex and its water exchange rate, a critical issue in attaining optimal relaxation efficiency of CA s [125-131]. 

All three complexes have a decreased lifetime of the coordinated water molecule as compared to octadentate ligands such as GdDOTA. In the case of GdD03A and GdPCTA[12] complexes, coordinated water exchange is occurring by a dissociative mechanism... 

Mixed N—O donor atom sexa- and octadentate ligands... 

Incorporation of (111) into a kinetically stable complex suitable for attachment to an antibody calls for an octadentate ligand, and so pendant-arm cyclen derivatives are appropriate. In particular, two or three antigen-binding fragments of an antibody (Fabs) have been attached to the di- or trimaleimide ligands 80-82 (Scheme 8) (90, 91)... 

The Gd + complex of a diamide (22) has an octadentate ligand in a nine-coordinate tricapped trigonal prismatic molecule with one coordinated water molecule. ... 

Relaxivity and other studies on other Gd DTPAbis(amide) complexes have been reported Dissociation kinetics of Ce and Gd complexes of bis(amide) ligands derived from DTPA have been studied. Complexes of a new octadentate ligand H5BOPTA (23) similar to DTPA have been made. 

The potentially octadentate ligand DOTA (l,4,7,10-tetra(earboxymethyl)-l,4,7,10-tetraazacyclo-dodeeane, has been widely studied. It forms very stable lanthanide complexes with one water molecule coordinated, [Ln(D0TA)(H20)] (26). Log K for [Gd(D0TA)(H20)] is 24.7, compared with the value of 22.46 for [Gd(DTPA)(H20)]. ... 

The other two complexes in Fig. 2 are based on the 2,4,6,8-tetrakispyridine bispidine backbone, with a hexadentate [Fig. 2(a)] and an octadentate ligand... 

Fig. 2(/ )]. These are expected to be stable organic molecules. However, based on the problems of stereocontrol (see Section II.B) it is unlikely that this chemistry will soon start to develop. The computed stmctures indicate that the hexadentate ligand will enforce a trigonal-prismatic coordination geometry [Fig. 2(a)], and the octadentate ligand has a preference for square-prismatic structures [Fig. 2 b)]. 

Fig. 6. Structure of the octadentate ligands used for the synthesis of biomimetic trinuclear copper complexes.

As some trivalent lanthanides (Ln " ) specifically displace Ca " in biological systems and in consequence are used a paramagnetic probes for Ca sites in NMR studies, it is of interest to examine the relative labilities of Ca " and Ln " in similar coordination sites. The potentially octadentate ligand 2-[(2-bis[carboxy-methyl]amino-5-methylphenoxy)methyl]-6-methoxy-8-bis[carboxymethyl]amino-... 

The synthetic octadentate ligand in Figure 13-3 is being evaluated as an anticancer agent. This chelate binds a metal through four N atoms and four O atoms. The chelate is covalently attached to a monoclonal antibody, which is a protein produced by one specific type of cell in response to one specific foreign substance called an antigen. In this case, the antibody binds to a specific feature of a tumor cell. The chelate carries a short-lived radioisotope such as or which delivers... 

The interpretation of the observed proton and C PCS led to the conclusion that the two structural isomers are enantiomeric pairs of diastereoisomers differing in the octadentate ligand conformation, namely, capped square antiprismatic (SA or M) and twisted capped square antipris-matic (TSA or m) geometries, respectively (Figure 5). " The Ln + coordination polyhedral of SA and TSA have two opposite parallel faces occupied by the ligand ring nitrogen... 

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