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

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 19F 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.