Cu12 and Cd16 coordination cages and their Cu3 and Cd3 subcomponents, and the role of inter-ligand π-stacking in stabilising cage complexes

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.