A spoonful of sugar makes the catalytic activity go down! [Dalton Trans.]

I’m delighted to finally publish this work, the first of my research carried out during my Marie Curie Fellowship in University of Bern to come out. A lot of hard work by Erasmus student Pauline went into gathering data behind this manuscript where we asked the question – what impact would incorporating carbohydrates into the structure of a Ruthenium(II)-triazolylidene complex have on its ability to convert a ketone to an alcohol via transfer hydrogenation catalysis.

There were challenges in isolating the desired compound, so it had to be generated in situ, but we were able to assess the activity, and the results were interesting, and can be found in detail here in Dalton Transactions.

To summarise the conclusions: The carbohydrate functionality does impact catalytic activity (transfer hydrogenation of ketones). In complexes with the glucose directly triazolylidene-bound,  turnover rates were substantially higher when compared to more remote carbohydrate functionalisation (i.e. with an ethylene spacer). Both new complexes, however, have reduced activity compared to  unfunctionalised carbene complexes. Insight was also gained into the nature of the catalytic cycle through a substrate scope analysis.

“Captured and put in chains” – new article in Angewandte Chemie

“Captured and put in chains” – new article in Angewandte Chemie

I recently finished working in Trinity College Dublin after five and a half productive years. My final project was completed in the weeks before I left and submitted to the prestigious journal Angewandte Chemie on the 1st April, the day I started in my new position at Universität Bern.

This article has just come out, and I’m rather proud of it, and happy that it is a fitting ending to my time in Dublin. My original goal in Thorri Gunnlaugsson’s research groupwas to form interlocked molecules, such as “catenanes”, but this challenging goal kept moving further down the queue as we discovered new and interesting ways to exploit the “btp” motif (which has been the topic of all my research to date). But finally, and somewhat unexpectedly, we achieved this goal.

Creating these new structures built upon interesting behaviour we reported in our Chemistry – A European Journal article early this year – where we saw that btp molecules could interact with each other, forming pairs through weak hydrogen bonding. We wondered if this could be used to pre-organise molecules together in such a way that they could be ‘clipped’ together into interlocked rings (by “RCM”, as outlined in the Scheme above). This approach has occasionally been reported before for amides,but not for molecules like the ones we describe.

A representation of the formation of catenanes from btp ligands

In fact, this reaction was more successful than expected – and in the first case I tried, we were surprised to find the majority product (50% yield) was the interlocked “catenane”, with independent non-interlocked rings also observed. We were able to fully identify and characterise these molecules using X-ray crystallographic analysis, giving the clear pictures below (thanks to Dr Salvador Blasco).

These were nice structures, but I wanted them to do something more than look pretty! Discussions with my friend Anna Aletti opened up the idea that the cavity in the middle of the structures might be a perfect fit for some negatively charged ion guest, such as chloride, nitrate or sulfate – none of these ions did very much, but phosphate (the tetrahedral H2PO4- ion), on the other hand, caused changes in the “catenane” host, indicating specific interactions between these two molecules. This was exciting. It makes these the first catenanes in the literature to have such interactions with tetrahedral anions.

The catenane acts as a selective host for phosphate, as can be seen from changes in the NMR spectrum

Detailed analysis of the formation of these interesting compounds, as well as their adducts with phosphate was important to strengthening these results and making sure we understood what we were seeing. Working closely with Dr Gary Hessman, Technical Officer at TCD, allowed further insight into the systems and their composition.

If any of this sounds interesting, then you should read the article (http://dx.doi.org/10.1002/anie.201603213) or at least look at the pictures! One fun advantage of publishing in Angewandte Chemie is that they translate your abstract into German (which I am currently learning), which means I now know the useful everyday term “Triazolylwasserstoffbrüken“, which – of course – means “triazolyl hydrogen-bonding interactions”. I now use that in the pubs of Bern almost daily!

Chiroptical probing of self-assembly with ligands formed in one-pot ‘click’ reaction from chiral amines (Chem. Eur. J.)

Chiroptical probing of self-assembly with ligands formed in one-pot 'click' reaction from chiral amines (Chem. Eur. J.)

After a lot of work, we finally got these results off my bench and into the literature. In our new article in “Chemistry – A European Journal” (Wiley), we present a convenient one-pot approach to synthesising chiral bis(triazolyl)pyridine ligands from enantiopure amines with the stereochemistry retained. This approach is broadly applicable.

Molecular structures of ligands 1a and 1b derived from X-ray crystallography
Molecular structures of ligands 1a and 1b derived from X-ray crystallography showing noteworthy dimer formation between the bis(triazolyl)pyridine cores

The beautiful mirror-image crystal structures were obtained by Dr Miguel Martínez-Calvo (now in Santiago) and show interesting supramolecular hydrogen bonding interactions between the ligands which we will exploit in the future.

Dr Bob Peacock in Glasgow performed circularly polarised luminescence spectroscopic measurements on the coloured lanthanide(III) complexes of these ligands, illustrating their optically active nature.

CD titrations for both enantiomers of 1 (a,b) and recalculated spectra arising from fitting of the titration of 1b with Eu(III) in acetonitrile

In this article, we were able to show interesting behaviour which has only really been studied so far by researchers in Prof Thorri Gunnlaugsson’s lab (with two other examples published recently in projects led by Dr Oxana Kotova and Sam Bradberry, respectively). This behaviour was the notable changes in the circular dichroism (CD) spectra of these chiral molecules upon addition of lanthanide(III) ions. These spectral changes could be fit to determine binding constants of this self-assembly. These clear chiroptical spectra are in contrast to a dissapointing aspect of results we published earlier this year in Inorganic Chemistry (with remote amino acid substituents giving rise to weak CPL and CD) and shows that placing the chiral centre nearer to the metal ion binding location enhanced the effect on the chiroptical properties of such systems.

URL: http://onlinelibrary.wiley.com/doi/10.1002/chem.201504257/full