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  • Title: Cu12 and Cd16 coordination cages and their Cu3 and Cd3 subcomponents, and the role of inter-ligand π-stacking in stabilising cage complexes.
    Author: Stephenson A, Sykes D, Ward MD.
    Journal: Dalton Trans; 2013 May 21; 42(19):6756-67. PubMed ID: 23508360.
    Abstract:
    The bridging ligand L(14Nap), which contains two chelating pyrazolyl-pyridine units separated by a naphthalene-1,4-diyl spacer, has been used in self-assembly of polyhedral coordination cages. The largest such cage is [Cd16(L(14Nap))24](BF4)32 which has a tetra-capped truncated tetrahedral Cd16 core with a bridging ligand spanning every edge. The complex is indefinitely stable in dilute solution, which makes it quite different from the previously-reported isostructural cage [Cd16(L(14Ph))24](BF4)32 (based on a 1,4-phenyl bridge) that forms on crystallisation but slowly rearranges to smaller cages in solution. The additional inter-ligand π-stacking between ligand fragments associated with replacement of a phenyl group by a naphthyl group allows the complex to be stable in solution, providing conclusive proof of the importance of inter-ligand π-stacking in the assembly of these cages. With Cu(II) in place of Cd(II) a smaller cage [Cu12(L(14Nap))15](ClO4)24 was formed which contains a mixture of tris-chelated (six-coordinate) and bis-chelated (four-coordinate, or five-coordinate if an additional monodentate ligand is present) Cu(II) ions; the difference between the two structures arises in part from the different stereoelectronic preferences of the two metal ions. Despite this difference both the Cd16 and Cu12 cages contain {M3(L(14Nap))3}(6+) triangular helical units as subcomponents which form the triangular faces of the polyhedra. By using a 1 : 1 ligand : metal ratio in the synthesis examples of these can be isolated and characterised; the structures of the trinuclear cyclic helicates [Cd3(L(14Nap))3(BF4)4(EtOAc)2](BF4)2 and [Cu3(L(14Nap))3(BF4)(MeCN)2](BF4)5 have also been determined.
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