Barium Templating Schiff-Base Lateral Macrobicycles

Schiff-base lateral macrobicycles containing two different binding units, a rigid and unsaturated N2X set (X: N,
O) and a flexible and cyclic N2On set, linked by two aromatic bridges, have been prepared by reaction of the
appropriate bibracchial diamines N,N‘-bis(aminobenzyl)-diaza-crown and diformyl precursors in the presence of
Ba(II) as templating agent. The expected cryptands do not form in the absence of the cation; the presence of this
metal ion is necessary to orient the diamine precursor in a syn conformation. Comparison of the X-ray crystal
structures of the barium complex of the bibracchial diamine N,N‘-bis(2-aminobenzyl)-1,10-diaza-15-crown-5 and
the barium complex of the cryptand derived from it incorporating a pyridine unit indicates that the encapsulation
of the metal ion is clearly more effective in the case of the cryptand. The coordination of Ba(II) ion to the
pyridine nitrogen atom promotes the displacement of the metal into the cavity of the cryptand as well as important
changes in the conformation of the crown unit, although the fold of the pendant arms remains practically unchanged.
The cryptands reported constitute the first example of a novel family of macrobicycles.