Supramolecular Self-Assembly of Macrocycles, Catenanes, and Cages through Coordination of Pyridine-Based Ligands to Transition Metals

Recent developments in the study of the transition-metal mediated supramolecular self-assembly are reviewed. Focus is on the self-assembly of macrocycles, catenanes, and cages from (en)Pd(NO3)2 (1) and pyridine-based bridging ligands. Coordination of linear 4,4′-bipyridine on the cis coordination site of palladium complex gives a macrocyclic square supramolecule, whereas macrocyclic dinuclear Pd(II) complexes self-assemble from 1 and flexible bridging ligands. Unprecedented formation of catenanes through rapid slippage of two preformed molecular rings can be achieved by exploiting the labile character of a Pd(II)-linked macrocycle: i.e., a macrocycle assembling from 1 and PyCH2C6H4CH2Py exists in rapid equilibrium with its catenated dimer and the equilibrium is strongly pushed toward the catenane (> 99 : 1) in a polar media. The combination of 1 with tridentate ligands gives three-dimensional cagelike hosts. A spherical M3L2 complex organizes from 1 and a flexible tridentate ligand only in the presence of specific guests providing a model for induced fit. On the other hand, a nanometer-sized hollow supramolecule self-assembles from four rigid tridentate ligands held together by six protected Pd complex 1.