Supramolecular chemistry and molecular design: self-assembly of molecular squares

Modem supramolecular chemistry has been described as organized polymolecular systems held together by noncovalent interactions and represents one of the major frontiers in the chemical sciences.'-' At present the field is dominated by the hydrogen bonding motif that mimics biological systems and the classical covalent macrocyclics such as crown ethers, cyclophanes, cyclodextrins, calixarenes, etc., that represent the roots of the field!?' Much less is known about the use of coordination and transition metals as a motif for the assembly of supramolecular species.