Octanuclear Cubic Coordination Cages

Two new bis-bidentate bridging ligands have been prepared, Lnaph and Lanth, which contain two chelating pyrazolyl-pyridine units connected to an aromatic spacer (naphthalene-1,5-diyl and anthracene-9,10-diyl respectively) via methylene connectors. Each of these reacts with transition metal dications having a preference for octahedral coordination geometry to afford {M8L12}16+ cages (for Lanth, M = Cu, Zn; for Lnaph, M = Co, Ni, Cd) which have an approximately cubic arrangement of metal ions with a bridging ligand spanning each of the twelve edges, and a large central cavity containing a mixture of anions and/or solvent molecules. The cages based on Lanth have two cyclic helical {M4L4} faces, of opposite chirality, connected by four additional Lanth ligands as “pillars”; all metal centers have a meridional tris-chelate configuration. In contrast the cages based on Lnaph have (noncrystallographic) S6 symmetry, with a diagonally opposite pair of corners having a facial tris-chelate configuration with the other six being meridional. An additional significant difference between the two types of structure is that the cubes containing Lanth do not show significant interligand aromatic stacking interactions. However, in the cages based on Lnaph, there are six five-membered stacks of aromatic ligand fragments around the periphery, each based on an alternating array of electron-rich (naphthyl) and electron-deficient (pyrazolyl-pyridine, coordinated to M2+) aromatic units. A consequence of this is that the cages {M8(Lnaph)12}16+ retain their structural integrity in polar solvents, in contrast to the cages {M8(Lanth)12}16+ which dissociate in polar solvents. Consequently, the cages {M8(Lnaph)12}16+ give NMR spectra in agreement with the symmetry observed in the solid state, and their fluorescence spectra (for M = Cd) display (in addition to the normal naphthalene-based π−π* fluorescence) a lower-energy exciplex-like emission feature associated with a naphthyl → pyrazolyl-pyridine charge-transfer excited state arising from the π-stacking between ligands around the cage periphery