In recent years chemists have been trying to construct zeolite analogues in which the frameworks of pores and cavities are made of more sophisticated materials (the zeolites are made only of silicates). Attempts to do this have been thwarted because the elaborate structures created simply collapsed when their resident molecules departed. Paper #8 in the current Hot Ten tells of the first successful structure to be sufficiently robust to survive this loss; moreover, it even withstands being heated to 400^oC without its cavities collapsing.

The work is the result of collaboration between Omar Yaghi and Mohamed Eddaoudi of the University of Michigan, and Michael O’Keeffe and graduate student Hailian Li at Arizona State University, where the structural aspects of the new materials were investigated. Together the two groups have produced a remarkable molecular framework that promises to outshine even the zeolites.

The new material is coded MOF-5, which is short for "metal organic framework number 5" (O’Keeffe says they number their new compounds sequentially and are now up to MOF-60). MOF-5 is the most striking example to date of a new class of highly porous hybrid metal-organic materials that promise all kinds of applications. MOF-5 has a porosity of almost 3,000 square meters per gram, five times that of the most porous zeolites. What is also attractive about the new frameworks is that they can be chemically modified to provide active sites built into the structure itself.

The cube-like cavities of MOF-5 have an oxygen atom at each of their eight corners. Every oxygen is bonded to four zinc atoms as OZn₄ (a rare situation in itself) and every zinc is then bonded to the two oxygens of a carboxylate group attached to a benzene ring. The benzene ring has two such carboxylate groups at opposite ends of the molecule and these link to different Ozn₄ units at adjacent corners. The compound is formed when a solution of zinc nitrate and 1,4-benzenedicarboxylic acid, dissolved in a mixture of dimethylformamide and chlorobenzene solvents, is treated with triethylamine (which removes the acid’s hydrogens) and hydrogen peroxide (which provides the oxygens for the cube’s corners). The result is colorless crystals of MOF-5 filled with solvent molecules; these can then be replaced with other molecules or simply removed. The density of the empty MOF-5 gives some indication of its unique porosity, being only 0.59 g/cc, the lowest recorded density for any crystalline material.

Yaghi’s MOF-5 offers several attractive possibilities, and it is these which he believes are the reason for paper #8 attracting a high number of citations. Currently the group is exploring the commercial opportunities that MOFs offer: as potential catalysts; as absorbent materials; and as a means of separating chemicals. The crystals might also be used for gas storage, and the paper reveals just how much gas they can contain. For example, a cavity of MOF-5 can accommodate around 30 atoms of argon. As an absorbate it could also accommodate seven molecules of carbon tetrachloride.

"MOFs are a new generation of porous materials that are amenable to design and tailored properties, and are expect to rival zeolites. Indeed they go beyond the abilities of zeolites in commercial importance," says Yaghi. "The design principles uncovered by our research on them will impact the way we view the synthesis of materials, and they represent an evolution in how solids are prepared and in bridging molecular and solid-state chemistry."

Other, more recent work from the group includes a paper on compounds with extra-large pores (Science291:1021, 2001), and a review of the molecular building units employed in the design of highly porous metal-organic frameworks (Accounts of Chemical Research, 34: in press).