Symmetry in copper and silver cryptates
The cryptand ligands imBT (l,4,7,10,13,16,21,24-0ctaaza-bicyclo[8.8.8]hexacosa-
4,6,13,15,21,23-hexaene) and amBT (1,4,7,10,13,16,21 ,24-0ctaazabicyc10[
8.8.8]hexacosane) form interesting disilver(I) and trisilver(I) cryptates, as well as a
dicopper(I) and a well studied average valence dicopper(1.5) cryptate. Detailed structural
and spectroscopic studies of the silver cryptates show that complex equilibria exist in
solution, and the trisilver form appears to be thermodynamically favoured, the additional
stabilisation apparently being due to argentophilic interactions.
An optically pure form of the dicopper(I) imBT cryptate was successfully obtained, and is
undergoing X-ray diffraction studies aimed at determining whether a copper-copper
interaction exists, by direct examination of the electron density.
Synthetic studies aimed at introducing substitution to the imBT and amBT ligands were
complicated by competing reactions. A strategy to modify cryptand cavity size by
incorporating asymmetric tetraamine caps succeeded, yielding dicopper(I) and disilver(I)
cryptates with properties intermediate between cryptates incorporating the related
Manganese(II) cryptates of imBT and amBT were investigated as potential MRl contrast
agents, the iminocryptate showing surprisingly high relaxivity, despite the fact that no
water molecules were located in the crystal structure.
The observation of high mass peaks in the FAB mass spectra ofimBT and amBT cryptates
suggests the presence of 6+4 Schiff base condensation products, as well as the more
abundant 3+2 products. It has not proved possible to isolate these molecules as yet,
however initial studies aimed at a rational synthesis of the 6+4 condensation products were
made, as these ligands could be of great interest for modelling the recently reported CUz