Organocatalysis – very trendy!

 

In the heart of the periodic table sits a group of elements which most people would instantly recognise, but more than likely take for granted. This is not surprising – many of the transition metals, including iron, nickel, copper, silver and gold, are so familiar, so ordinary, that it is hard to imagine that they could be at the centre of a cutting-edge branch of chemistry. In fact, this group contains some of the most interesting and useful elements in existence – and there is still much to be learned about them.

Transition metals are fascinating, both structurally and functionally. Not only do they form very regular, predictable structures, they can also carry out a variety of interesting chemical activities, and have the potential to be put to a range of exciting new uses. Dr Shane Telfer, a lecturer in chemistry at Massey University and a Principal Investigator with the MacDiarmid Institute, is researching how transition metals can be put to new uses in the fields of supramolecular chemistry and nanochemistry.

Although Dr Telfer joined Massey University just 18 months ago, he already has several projects in the pipeline. The first of these involves fabricating metalorganic frameworks, which consist of organic ligands ‘glued together’ with transition metals into nanostructured crystalline arrays. This is part of the burgeoning field of supramolecular chemistry, which involves creating extended arrays of molecules arranged into precisely ordered configurations. 

The unique structural and functional characteristics of transition metals mean they can be used to not only drive the assembly of metal-organic frameworks, but they also have great potential to play interesting active roles within them. Actually creating metal-organic frameworks is not new, explained Dr Telfer, “but what is really going to be interesting is to have functional frameworks that actually serve a specific purpose.”

Dr Telfer plans to create metal-organic frameworks that contain functional organic groups that mimic known organocatalysts – small organic molecules that accelerate chemical reactions. “Organocatalysis is an active sub-field of organic chemistry – it is currently very trendy. Our idea is to take some of the concepts from that fi eld and merge them with the field of metal organic frameworks.”

The long term plan is to create a heterogeneous catalyst – a catalyst that is in a different phase from the reagent. “A solid crystalline metal organic framework could float around in a solution containing the components of the reactions that you want to catalyse,” said Dr Telfer. “The neat thing about heterogeneous catalysts is that you can filter them off once the reaction is completed, and ideally re-use them on a different reaction.” In the future, such catalysts could have a variety of practical uses. “Catalysis is hugely important in industrial processes, such as making pharmaceuticals. If you can improve their efficiency, then it is better for the environment, better for the economy, and you can access novel materials and chemicals. That is very distant, but these long term implications are the kind of things we use to motivate ourselves.

“There is also is a lot of interest in the ability of metal-organic frameworks to absorb and store gases. The hydrogen economy is potentially around the corner, but you can’t put a tank of hydrogen in the back of a hydrogen car – it is too dangerous and impractical. If you take hydrogen gas and adsorb it to a surface, however, it becomes much easier to store and safer to transport. Metal-organic frameworks are good at adsorbing gases because they are nanoporous, and have very large surface areas, and so there is a particular interest in applications in the hydrogen economy. They are looking quite promising.”

Another of Dr Telfer’s projects, in collaboration with Massey colleague Dr Mark Waterland, involves using transition metals to assemble gold nanoparticles – small clusters of up to a few thousand gold atoms. “You can make individual gold nanoparticles pretty easily, but getting them to form assemblies in a controlled manner is difficult. We are going to start simple and make just a dimer – two nanoparticles linked together in a specific arrangement, using transition metals as the glue, the link. Gold nanoparticles are always surrounded by capping agents, made of various types of organic compounds – if they are not there, the particles aggregate randomly into a big glob of gold. One end of these capping agents is fixed to the gold, while the other end is floating out free in space. We want to attach a specific functional group to the free end of the molecule, which will enable it to be attached to a transition metal. On the surface of each nanoparticle there are typically thousands of capping agents, but, crucially, only one of these is going to have this special functionality that attracts it to the transition material. This means that there is only one point of attraction and we can achieve a controlled assembly.”

As well as being interesting in their own right, gold nanoparticle dimers could be used to investigate fundamental questions in chemistry and physics. “When we tie the nanoparticles together this way, there is a kind of ‘wire’ between the two, and we can use it to conduct electrons between the particles,” said Dr Telfer. “Using spectroscopy, we can look at the motion of the electrons and study it very explicitly knowing that there is one and only one wire. We hope to be able to follow the electron transfer process.”

Dr Telfer is enthusiastic about the opportunities that Principal Investigator status within the MacDiarmid Institute has afforded him. “I have a MacDiarmid Institute-funded PhD student starting next month, which will provide a huge boost to the metal-organic framework project.” he notes. “As a beginning researcher it’s a great bonus being associated with New Zealand’s leading physical scientists. It’s really inspirational.”