Chelates with Acyclic Ligands

Gd-DTPA (gadopentetate, Magnevist) is a good example of a chelate , because the metal ion is covered by the polydentate ligand like a claw (Pig. 3). 

The central metal ion has nine coordination sites. It is attached to the three nitrogen atoms and to five carboxylate moieties (oxygen atoms). A single water molecule is able to coordinate at the vacant ninth site resulting in a strong enhancement of the water proton relaxation rate. The chelate can be described as a distorted capped square antiprism according to X-ray analysis [6]. 

Generic name LD50 (mmol/kg) body weight References  

Special formulations have been developed to visualize the gastrointestinal tract (see below). Magnevist has also been applied succesfully as an X-ray contrast agent in patients who were allergic to iodine because of the attenuation of X-rays by the gadolinium ion [14]. 

---

The interaction of oxygen-containing acyclic ligands with alkali and alkaline earth metal cations has provided a burgeoning area of interest. In historic terms, this was preceded by the advent of crown ethers and the accompanying almost retrospective look at their acyclic precedents. This section is sub-divided into five parts simple chelates, metal complexes as ligands, podands, polypodands and sugars. 

A common drawback with macrocyclic ligands is their slow complexation rate (Jang et al. 1999). A way to overcome this problem is to increase the ring size. Following this idea, two larger chelators have been developed, one based on a pentaaza macrocycle (PEPA) and the other on a hexaaza macrocycle (HEHA) O Fig. 45.3). Both these chelators show much faster labeling kinetics with some radiometals (by a factor of about 100 times faster). Still, they have slower labeling kinetics than the acyclic chelator DTPA (by a factor of about 10) (O Table 45.4). 

It has been found that a number of bidentate ligands greatly expand the scope of copper catalysis. Copper(I) iodide used in conjunction with a chelating diamine is a good catalyst for amidation of aryl bromides. Of several diamines that were examined, rra s-yV,yV -dimethylcyclohexane-l,2-diamine was among the best. These conditions are applicable to aryl bromides and iodides with either ERG or EWG substituents, as well as to relatively hindered halides. The nucleophiles that are reactive under these conditions include acyclic and cyclic amides.149... 

---