Chelation lanthanide-coordination chemistry

The importance of the chelate effect combined with the construction of multidentate ligands is well known in lanthanide chemistry. This is expressed in the rich coordination chemistry of / -diketonates [88] or complexes with Schiff bases [89] and macrocyclic polyethers [90] where lanthanide cations achieve steric saturation by high coordination numbers. Entrapment of the cation in a macrocyclic cavity results in greater complex stability. However, simply functionalized ligands such as ethanolamines can also supply a suitable ligand sphere [91-93],... 

Figure 1.8 Three possible intra-molecular energy transition mechanisms [9]. (Reprinted from Coordination Chemistry Reviews, 99, G.E. Buono-core, H. Li, and B. Marciniak, Quenching of excited states by lanthanide ions and chelates in solution, 55-87, 1990, with permission from Elsevier.)...

Because of their basic resemblance to porphyrins, it was initially expected that the sapphyrins would mimic, at least on some level, the rich coordination chemistry displayed by the porphyrins. However, the larger core size ca. 5.5 A inner N-N diameter vs. ca. 4.0 A for porphyrins), the greater number of potentially chelating heteroatom centers, and the fact that pentaazasapphyrins when fully deprotonated are potentially trianionic ligands made sapphyrin a likely candidate for large metal chelation, particularly as a potential ligand for the trivalent lanthanides and actinides. Unfortunately, in spite of extensive effort, this hope remains largely unrealized. Nonetheless, some metal complexes of sapphyrins and heterosapphyrins have been successfully prepared and characterized. Their preparation and properties are reviewed in this section. 

While the coordination chemistry of lanthanide ions in water is extensive, it is of course not limited to this solvent. In the 1970s, considerable interest developed in the synthesis of lanthanide ion complexes soluble in apolar solvents for their use as lanthanide shift reagents in NMR spectroscopy. Typically, such complexes were neutral and based on chelating 1,3-diketonate ligands, one of their attractive features being that not only did they cause normally overlapping resonances to be spread out and resolved but that they could readily be prepared from optically active ligands and thus used to... 

Of the known cyclic oxocarbon acids, the systems based on squaric (68) and croconic (69) acids have been most widely studied. The loss of two protons from these acids gives rise to aromatic dianions as shown in equations (18) and (19), and these can coordinate to metal anions in a variety of ways. Unidentate coordination (70,77) is known for both systems but is not common. Simple bidentate chelate coordination (78) is also relatively uncommon but is observed in a number of croconate complexes. The squarate anion adopts this mode only with larger cations, such as the group 2 and lanthanide metals, and then only in association with additional bridging interactions. Bridging coordination modes dominate the chemistry of these anions, some of which are shown here (71-76), (79-81). The various modes of coordination can usually be distinguished by IR spectroscopy, and the use of NMR spectroscopy has also been investigated. 

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