Anion recognition by dimetallic cryptates

Bis-tren cryptands (i.e. octamine cages consisting of two tripodal tetramine subunits covalently linked by given spacers) are able to incorporate first two metal ions, then an ambidentate anion, according to a cascade mechanism. In particular, dicopper(II) cryptates behave as effective receptors for anions, which fill the empty cavity of the cage and place their donor atoms in the two axial sites left available by each Cu(II) centre (which adopts a trigonal bipyramidal stereochemistry). Anion encapsulation by dicopper(II) cryptates often induces the development of a rather intense anion-to-metal charge transfer absorption band in the visible region, so that the recognition process is signalled by the appearance of a bright colour. Two examples are considered in detail: (i) that of a rigid bis-tren cryptate containing 1,3-xylyl spacers, which does not recognise the shape, but the bite of the polyatomic anion (i.e. the distance between two consecutive donor atoms); and (ii) that of the flexible cryptate containing 2,5-furanyl spacers, which is able to include also monoatomic anions, in particular halides, displaying peak selectivity in favour of Cl-. (C) 2001 Elsevier Science B.V. An rights reserved.