Anion recognition by dimetallic cryptates

Bis-tren cryptands (i.e. octamine cages consisting of two tripodal tetramin e subunits covalently linked by given spacers) are able to incorporate firs t two metal ions, then an ambidentate anion, according to a cascade mechani sm. In particular, dicopper(II) cryptates behave as effective receptors for anions, which fill the empty cavity of the cage and place their donor atom s in the two axial sites left available by each Cu(II) centre (which adopts a trigonal bipyramidal stereochemistry). Anion encapsulation by dicopper(I I) cryptates often induces the development of a rather intense anion-to-met al charge transfer absorption band in the visible region, so that the recog nition process is signalled by the appearance of a bright colour. Two examp les are considered in detail: (i) that of a rigid bis-tren cryptate contain ing 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, w hich is able to include also monoatomic anions, in particular halides, disp laying peak selectivity in favour of Cl-. (C) 2001 Elsevier Science B.V. An rights reserved.