Template synthesis, characterization and crystal structure of heptaazadentate Schiff base lanthanide aminopodates

The Schiff base condensation reactions between 2,6-diacetylpyridine and diethylenetriamine in the presence of lanthanum(III) and praseodymium(III) ions as template agents yield, exclusively, heptaazadentate complexes of a podate type with two terminal amine groups, irrespective of the molar ratio of the starting materials used in the synthesis. These complexes, which might be regarded as possible intermediates in the template synthesis of macrocyclic compounds, appear to be the final products. The ring-closure of the partial Schiff base condensation products does not occur either by the transamination mechanism or by the 2,6-diacetylpyridine addition. The rare heptaazadentate behavior of the Schiff base aminopodand in lanthanide complexes is proved by single-crystal X-ray diffraction analysis correlated with spectroscopic characterization. All the seven potential nitrogen donor atoms are available for coordination to the metal ions. The high coordination number of eleven is achieved by the incorporation of two bidentate nitrate anions in the lanthanide coordination sphere.