Homo- and hetero-dinuclear anion complexes

The tripodal system 4, in which urea fragments are appended to the three terminal amine nitrogen atoms of a tris(2-aminoethyl)amine (tren) subunit, includes a Cu-II ion and two anions X-, according to a cascade mechanism through three well defined stepwise equilibria in a DMSO solution. The first anion X- (halide, N-3(-), NCS-, NO2-, H2PO4-) seeks the Cu-II centre coordinated by the tren moiety; the second anion X- interacts with the trisurea cavity, but this occurs only if the stronger H-bond acceptors, such as N-3(-) and H2PO4-, are used. Binding of the second X- ion is favoured by the preorganising effect exerted by the metal and disfavoured by the steric and electrostatic repulsions between the anions. Under the appropriate conditions, heterodinuclear complexes of formula [Cu-II(4)(Cl)(H2PO4)] can be obtained in solution, in which Cl- is bound to the metal centre and H2PO4- interacts with the trisurea compartment.