Credit: © 2010 NPG

Void spaces inside single-crystalline materials have — by definition — very specific geometries. If a reaction can be conducted inside this space it should occur in a highly selective fashion. Single-crystal to single-crystal transformations are rare, however, because small changes in compound structure can lead to large changes in the crystal packing and result in collapse of the structure. Now, a series of crystalline iridium complexes have been discovered1 in which only small structural changes occur when the metal centre is coordinated to a variety of small gaseous ligands. The crystalline complex could selectively catalyse the hydrogenation of ethylene in the presence of propylene.

Maurice Brookhart and co-workers from the University of North Carolina at Chapel Hill found that square-planar iridium complexes with a bulky tridentate 'pincer' ligand and any nitrogen, carbon monoxide, ethylene or hydrogen ligands had the same space group and almost the same lattice parameters. Exchange of the gaseous ligand was possible in the solid state, probably through a channel in the crystal structure usually occupied by solvent molecules.

Ethylene is small enough to fit into the channels in the crystal but propylene is not. Exposure of the crystalline complex to a mixture of ethylene, propylene and hydrogen resulted in hydrogenation of both alkenes with a small selectivity for ethylene. Catalytically active sites at and near the surface of the crystal were passivated by exposing them to carbon monoxide — which binds more strongly than ethylene. Using this modified catalyst — albeit at a higher temperature — led to a significantly higher selectivity for hydrogenation of ethylene.