Carefully designed molecular building blocks that recognize one another through specific noncovalent interactions can be used to construct large supramolecular structures in a spontaneous self-assembly process. Coordination bonds that are formed between metal ions and organic ligands are particularly effective for self-assembly because they are not only associated with well-defined geometries, but are also relatively strong and so lead to robust structures.

Now, a team of researchers in India1 have used coordination-bond-driven self-assembly to make a hexagonal molecular box—that is open at the top and bottom—in which each square face is a porphyrin that has a pyridyl ligand at each of its four corners. The six porphyrin molecules are held together by a total of twelve platinum metal complexes that act as bridges between pairs of pyridyl ligands along the upper and lower rims of the box.

The team, led by Partha Sarathi Mukherjee (IPC) from the Indian Institute of Science, Bangalore, mixed together the porphyrin molecules and metal complexes in solution and an immediate color change indicated that a reaction had occurred between them. A red-brown solid precipitated from the reaction mixture by adding diethyl ether and, this product was subsequently analyzed.

Nuclear magnetic resonance (NMR) spectroscopy indicated that just a single symmetrical product had been formed in the reaction and mass spectrometry clearly showed that the product contained six porphyrin molecules and twelve of the metal complexes. This particular combination of building blocks could result in either a closed cube type structure or an open hexagonal box — and a more detailed analysis of the NMR data suggested the hexagonal box as the most likely structure.

Fig. 1: A hexagonal molecular box — open at the top and bottom — is comprised of six porphyrins connected together by twelve metal complexes that contain iron and platinum.

Single-crystal X-ray analysis confirmed the hexagonal box structure (Fig. 1) and showed that there is an internal cavity estimated to have volume of greater than 40,000 Å3 — a value that is quite large for such discrete coordination cages.

Mukherjee and co-workers are now beginning to explore the properties and potential applications of these boxes. “We are planning to expand the size and shape of such boxes by changing the position of pyridyl nitrogen,” comments Mukherjee, “and complementary approaches may lead to the formation of fluorescent sensors.”