Synthesis of Novel Polyazacryptands for Recognition of Tetrahedral Oxoanions and Their X-ray Structures

Abstract

The Mannich reaction of tris(pyrrolyl-α-methyl)amine (H3tpa) with a mixture of formaldehyde and primary amine hydrochloride (methyl or ethyl) gave two novel macrobicyclic molecules: a hexapyrrolic pentaazacryptand and an unusual pentapyrrolic tetraazacryptand, which were separated by column chromatography and their structures were determined by X-ray diffraction method. The analogous reaction using benzylamine hydrochloride yielded the encapsulated chloride anion complex in 16% yield even after neutralization with aqueous K2CO3, which was also characterized by X-ray diffraction. In addition, the anion binding properties of these macrobicycles were investigated by NMR titration methods, and the binding constants for halides and oxoanions were determined with the EQNMR program. The cavity of the hexapyrrolic pentaazacryptand is flexible and large enough enabling it to form inclusion complexes with the smaller size fluoride ion as well as with the bulkier oxoanions. This was demonstrated by (19)F NMR spectroscopy and by the X-ray structures of the encapsulated sulfate, phosphate, and arsenate ion complexes. Upon complexation the distance between the bridgehead nitrogen atoms changes. Further, anion induced conformational changes were observed in the structures of the oxoanion complexes, particularly in the arsenate structure which represents the first azacryptand encapsulated structure of an arsenate ion. Furthermore, the competition crystallization experiment showed that the phosphate ion complex of the hexapyrrolic pentaazacryptand is the sole crystallization product from an aqueous-organic medium, as confirmed by IR and powder X-ray diffraction studies.