Catalysis In Crystals

A single crystal of an iridium complex can exchange small-molecule ligands, such as CO and NH₃, for N₂ and selectively catalyze the conversion of ethylene to ethane, report researchers from the University of North Carolina, Chapel Hill (Nature 2010, 465, 598).

“Reactions of molecular crystals with gases involving covalent bond breaking and formation are as yet quite a rare observation,” says Lee Brammer, a chemistry professor at the University of Sheffield, in England. That the nonporous molecular crystals studied by the UNC group can hydrogenate ethylene is groundbreaking, he adds.

In the new work, Zheng Huang, Peter S. White, and Maurice Brookhart used a bulky, electron-deficient ligand, C₆H₃[OP(C₆H₂(CF₃)₃)₂]₂, and N₂ to create the square planar iridium complex [Ir]–N₂ (shown). Crystallization of the complex from toluene yielded light red single crystals. Structures of the crystals showed that the iridium complex’s four CF₃-substituted aryl rings form a pocket around the N₂ site. Multiple iridium complexes stack in such a way that disordered toluene molecules line a channel that runs along the small-molecule site.

When they exposed the [Ir]–N₂ crystals to gases of small molecules, Huang and colleagues found that the N₂ exchanged to form crystals of different colors: dark red [Ir]–NH₃, orange [Ir]–CO (end-on with an Ir–C bond), deep red [Ir]–C₂H₄ (side-on), green [Ir]–O₂ (side-on), and light red [Ir]–(H)₂(H₂) (two Ir–H bonds and one side-on H₂). The crystals’ ability to exchange ligands depends on the size of the incoming molecule. For example, although the crystals took up ethylene to form [Ir]–C₂H₄, they would not take up propylene.

Huang and coworkers believe that the gases enter the crystal through the channel occupied by the disordered toluenes. Vacuum experiments demonstrated that the small-molecule ligands are not simply lost, leaving an open coordination site for the new ligand to fill. Instead, a ligand-exchange mechanism seems to be at work.

The group also found that when crystals of [Ir]–N₂ were exposed to a mix of ethylene, propylene, and hydrogen, ethylene was selectively hydrogenated. That selectivity is not seen in common heterogeneous palladium-on-carbon hydrogenation catalysts, Sheffield’s Brammer notes.

Although growing single crystals would be impractical for large-scale catalysis, details gained from studying this system could provide insight into the catalytic mechanism and lead to better catalyst design, says Paul R. Raithby, a chemistry professor at the University of Bath, in England. Raithby points to measurements of solid-gas reaction kinetics and the possible development of new gas sensors based on single crystals as possible applications of the work.