1997

Coordination Frameworks

Makoto Fujita (b. 1957), Omar Yaghi (b. 1965)

The chemistry of metal coordination compounds has provided innumerable catalysts for the laboratory, various dyes for industry, and even platinum anticancer drugs and gadolinium MRI contrast agents for the medical field. In 1997, Jordanian-American chemist Omar Yaghi produced a paper investigating a new sort of metal complex. This type starts by making a rigid and symmetrical organic molecule, with groups at its ends that can easily complex metals. If these groups are reacted in solution with the appropriate metal ions, a crystalline solid can grow as the organic molecules arrange themselves around the metal atoms in a repeating three-dimensional lattice. There are many possible coordinating groups, which can be built onto all sorts of molecular scaffolds. And the various metals (as first shown by Swiss chemist Alfred Werner late in the nineteenth century) can take all sorts of geometries around themselves. This sounds like a recipe for a bewildering variety of different structures, and it certainly is. At present, every month brings reports of still more new materials, sometimes called metal-organic frameworks or coordination polymers.

The names vary because the crystals that form can be very odd structures with correspondingly odd properties. If the rigid organic units form large enough spacers, the resulting solid can have very large channels and pores—in fact, the crystals can be almost entirely empty space! These ordered cavities can be filled, in turn, by all sorts of other molecules, and the hope is that these materials might be used to store hydrogen, sequester carbon dioxide, serve as a matrix for new battery technologies, and more. A 2013 paper from the lab of Japanese chemist Makoto Fujita even reported that other small molecules can soak into some of these frameworks in such an orderly way that X-ray crystallography can be done on them, perhaps opening a convenient way to get crystal structures of things that otherwise might never crystallize at all.

The structure of the coordination framework MIL-53. The backbone is made up of benzene rings (with carbons as black spheres) that have carboxylic acid groups on each side (oxygens as red spheres). These coordinate around metal atoms (inside the blue polyhedrons), and the yellow spheres illustrate the amount of empty space in the resulting pores.