Archive for the ‘Subject Areas’ Category

Novel concept for switchable olefin metathesis catalysts

Chemists have discovered a novel triggering mechanism for olefin metathesis catalysts while unravelling the mechanism of a crucial ligand rearrangement step.

Olefin metathesis is a widely used reaction in organic and polymer synthesis. cis Dichloro ruthenium benzylidene complexes bearing an N-heterocyclic carbene ligand have recently gained considerable attention as stable metathesis catalysts. They are known as latent catalysts as they initiate slowly, which is thought to be because the cis dichloro precursors have to rearrange to their trans dichloro counterparts to become active.

While studying the reaction mechanism, Christian Slugovc and colleagues at Graz University of Technology, Austria, discovered that a cationic complex is an intermediate in the rearrangement step. They also observed that pyridine, which is a donor ligand, facilitates the displacement of one of the chloride ligands, the first step in the rearrangement.

Graphical abstract: Pyridine as trigger for chloride isomerisation in chelated ruthenium benzylidene complexes: implications for olefin metathesis

But most striking, says Slugovc, is that the chloride counterion (or, more generally, a counterion that can coordinate to ruthenium) is indispensible for the catalytic activity. Changing the counterion for the non-coordinating hexafluorophosphate ion produced an almost inactive catalyst, but the activity was triggered again by adding chloride. Slugovc says this constitutes a novel concept for switchable olefin metathesis catalysts.

Download Slugovc’s ChemComm communication to find out more.

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New polymer hydrogels offer step forward in desalination

Chemists working in Australia have used polymer hydrogels as a ‘draw’ agent in a desalination process.

Forward osmosis (FO) desalination is an emerging area of interest for chemists as it provides a low energy method of obtaining salt free water from the sea. Typically in FO, saline water is separated by a membrane from a ‘draw’ solute. Water passes through the membrane from the saline side to the ‘draw’ solute via osmosis. The water is then recovered from the ‘draw’ agent using distillation.

Now Huanting Wang and colleagues at Monash University have investigated the use of polymer hydrogels as the draw agent for FO desalination. Polymer hydrogels can reversibly change their volume when exposed to certain stimuli such as temperature and pressure. This gives them an advantage over traditional draw agents as they can potentially be recycled and release the water at lower energy and therefore cost.

Graphical abstract: Stimuli-responsive polymer hydrogels as a new class of draw agent for forward osmosis desalination

The team found that it is indeed possible to release significant amounts of water from the polymer hydrogels tested and are investigating other stimuli, such as light, to further increase the efficiency of this process.

Read more about this exciting advance by downloading  the full ChemComm communication today and let us know what you think below.

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Lack of luminescence quenching does not prove DNA intercalation by Ru(II) complexes

The binding of luminescent complexes to DNA is a popular area of research, with applications ranging from molecular switches to photodynamic therapy. It has commonly been reported in the literature that a lack of luminescence quenching by ferrocyanide, [Fe(CN)6]4−, can be used as evidence of the intercalation of a complex with DNA. 

Claudia Turro and colleagues at Ohio State University have now shown that a Ru(II) complex that binds strongly to DNA electrostatically rather than by intercalation is equally resistant to 3MLCT (metal-to-ligand charge transfer) emission quenching as one known to be a DNA intercalator.

These findings indicate that the absence of emission quenching by [Fe(CN)6]4− cannot be used alone as proof of DNA intercalation by a complex. Hence, researchers using this method will need to take extra care when interpreting their results.

To find out more and start a discussion download the communication (for free until 18th Feb 2011) and leave your comments below.

 

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