Macrocyclic ligands, lanthanide complexes efficiency

AC and AEC complexation is also effected efficiently by other macrocyclic ligands such as the spherands 13, cryptospherands 14 [2.9, 2.10], calixarenes [2.38, A.6, A.23], torands [2.39], etc., some of them, for instance the spherands displaying particularly high stabilities. A special case is represented by the endohedral complexes of fullerenes in which the cation (Sr2, Ba2+, lanthanides) is locked inside the closed carbon framework [2.40],... 

The inorganic lanthanide triflate complexes Ln(OTf)3 (made in aqueous solution) have been shown by Kobayashi to be efficient Lewis-acid catalysts for hydroxy-methylation (using commercial aqueous formaldehyde solutions) of silicon enolates in aqueous medium (water -i- THF) or even in water alone in the presence of a surfactant. In these reactions, activation proceeds by coordination of the aldehyde oxygen atom by the Ln center that is a strong Lewis acid due to its hard character. Among the lanthanide triflates, ytterbium triflate was found to be the most active catalyst, but scandium triflate can sometimes also be efficiently used. Enantio-selective versions are also known in the presence of chiral macrocyclic ligands. The water-soluble catalyst is recovered in water after extraction of the organic products. 

Porphyrin is one of the most widely studied macrocyclic systems suitable for complexation with lanthanide(III) ions. Porphyrins can absorb strongly in the UV-vis region so as to serve as efficient photo-sensitizers, making lanthanide(III)porphyrinate complexes ideal candidates for luminescence imaging agents. Indirect excitation of porphyrin antenna chromopheres in close proximity to lanthanide ions can make the energy in the triplet state of the porphyrin ligand transfer efficiently to the excited state of the lanthanide ion so as to sensitize the lanthanide luminescence, particularly NIR emission. 

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