Homogeneous catalysis at extreme concentrations
Nanospheres contain up to 1M gold(I) catalyst
Using self-assembled molecular spheres, chemists of the Van 't Hoff Institute for Molecular Sciences (HIMS) have been able to study homogeneous transition-metal catalysis at very high catalyst concentrations. In the internationally leading journal Angewandte Chemie, the University of Amsterdam researchers report unusual activity and selectivity of a gold(I) catalyst. They expect that further exploration of high concentration catalysis will lead to new chemical conversions.
In industrial chemistry a wide range of valuable chemicals is produced by means of homogeneous catalysis. In this process the catalyst (quite often a transition-metal) is in the same phase as the reactants - most commonly everything is dissolved in a solvent. The performance of the catalyst can be adjusted through various parameters such as the molecular environment of the active metal, the temperature and pressure during the catalytic conversion and the concentration of the reactants.
The concentration of the catalyst, however, is hardly exploited. It is usually kept very low because of solubility issues and to optimize the catalyst/reactant ratio, both implying low catalyst concentrations. Now, HIMS researchers of the Homogeneous, Supramolecular and Bio-Inspired Catalysis research group led by professor Joost Reek publish a method to increase the local catalyst concentration a thousand- to a millionfold, while keeping the average concentration low.
The Amsterdam researchers managed to place up to 24 gold(I) catalysts in permeable nanosized molecular spheres. They used a self-assembly strategy based on 12 palladium metals and 24 so-called ditopic nitrogen ligands, to which the catalyst (AuCl) was attached. During sphere preparation the concentration of the catalyst in the sphere could be controlled by using gold containing ligands as well as catalyst free ligands and varying their ratio.
When all 24 ligands in the sphere contained the gold, the local catalyst concentration in the sphere was at the molar level - more than a million times the catalyst concentration commonly used for industrial reactions. However, because of the containing effect of the spheres, the solubility of the catalyst was not compromised and the average concentration was at the micromolar level.
Uncovering new reactivity
The HIMS researchers performed numerous experiments with varying catalyst concentration, thus performing, for the first time, a thorough investigation of the reactivity at high concentrations.
As it turned out, the high local catalyst concentration converts an inactive AuCl complex into an active system. Next to this unusual reactivity, the gold containing nanospheres also display high selectivities in intramolecular cycloisomerization reactions.
As the self-assembly strategy is simple and based on accessible building blocks, the HIMS researchers expect their high concentration approach to be widely applicable and likely leading to the uncovering of new reactivity.
Rafael Gramage-Doria, Joeri Hessels, Stefan H. A. M. Leenders, Oliver Tröppner,Maximilian Dürr, Ivana Ivana Ivanović-Burmazović, and Joost N. H. Reek: Gold(I) catalysis at extreme concentrations inside self-assembled nanospheres. Angew.Chem.Int.Ed 2014, 53 First published online: 12 sep 2014 DOI: 10.1002/anie.201406415