Octanuclear Cubic Coordination Cages

Two new bis-bidentate bridging ligands have been prepared, L-naph and L-anth, 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 L-anth, M = Cu, Zn; for L-naph, 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 L-anth have two cyclic helical {M4L4} faces, of opposite chirality, connected by four additional L-anth ligands as pillars; all metal centers have a meridional tris-chelate configuration. In contrast the cages based on L-naph have (noncrystallographic) S-6 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 L-anth do not show significant interligand aromatic stacking interactions. However, in the cages based on L-naph, 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 (M-8(L-naph)(12))(16+) retain their structural integrity in polar solvents, in contrast to the cages {M-8(L-anth)(12)}(16+) which dissociate in polar solvents. Consequently, the cages (M-8(L-naph)(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 pi-pi* fluorescence) a lower-energy exciplex-like emission feature associated with a naphthyl -> pyrazolyl-pyridine charge-transfer excited state arising from the pi-stacking between ligands around the cage periphery.