Self-assembly process of a quadruply interlocked palladium cage

A supramolecular approach is effective to construct topologically complicated molecules with the aid of reversible bond formation. Although topologically complicated molecules have been synthesized for the past three decades, their formation mechanisms have rarely been discussed. Here we report the formation process of a tetranuclear interlocked palladium cage composed of two binuclear cages, which are quadruply interlocked with each other. In the main pathway, the binuclear cages are produced with binuclear partial cages. The ditopic ligand that does not bridge the two palladium(II) ions in the binuclear partial cage then threads into the binuclear cage to afford a tetranuclear partially interlocked cage, with partial conversion of the binuclear cage into the binuclear partial cage. The tetranuclear partially interlocked cage interlocks intramolecularly through repetitive cleavage and formation of Pd(II)–N coordination bonds mediated by a free pyridyl group, finally leading to the tetranuclear interlocked cage.