Chemical Science Blog

Photochemical HX splitting reactions (X = halogen) provide a route for solar-to-fuel energy conversion and halogen photoelimination is a critical step in the process.

Scientists in the US have investigated the photoreduction mechanism of a pair of valence-isomeric dirhodium phosphazane complexes and suggest that a common intermediate is accessed in the photochemistry of both mixed-valent and valence-symmetric complexes.

They conclude that halogen photoelimination proceeds by two sequential photochemical reactions: ligand dissociation followed by subsequent halogen elimination, and they hypothesise that complexes that can directly assume a halide-bridged structure will have the highest quantum efficiencies for energy conversion.

Read the ‘HOT’ article for free today:

Halogen Photoelimination from Dirhodium Phosphazane Complexes via Chloride-Bridged Intermediates
David C Powers, Matthew B Chambers, Thomas S Teets, Noémie Elgrishi, Bryce L Anderson and Daniel G Nocera
Chem. Sci., 2013, DOI: 10.1039/C3SC50462J

Spongistatin 1 is a highly cytotoxic natural product derived from a marine sponge.  In addition to its exquisite biological activity against human cancer cell lines, the structural complexity of spongistatin 1 has captured the imagination of several organic chemistry groups.

A number of total syntheses of the spongistatins exist, but more recently, simplified analogs of the natural products have been tested for anti-cancer activity.

In order to further probe the biological efficacy of simplified spongistatin analogs– or indeed, fragments– researchers at Columbia University have developed new methodology for the synthesis of the A-B fragment 3 and its merger to the C-D spiroketal fragment 3.

In their most recent Chemical Science Edge article, the Leighton research group have described a synthetic blueprint that could enable the synthesis and testing of a library of ‘C-D spiroketal-modified’ analogs of spongistatin 1.  Towards this overarching goal, Samuel Reznik was able to synthesise 34.5 g of fragment 3 in around 60 days; an achievement that demonstrates both the ‘scalability and step-economy’ of this synthetic route.

The Leighton group hope to apply this route to the synthesis of a series of C-D analogs and also hope to develop a complementary synthesis of the E-F fragment to further explore the anti-cancer therapeutic potential of spongistatin 1 derivatives.

Marilyn Monroe once sang that ‘diamonds are a girl’s best friend.’  If that is the case then it could be said that graphene, another form of carbon, has become the scientist’s best friend.  The relatively recent explosion of research conducted on graphene has been remarkable, and in 2010 resulted in a Nobel Prize for Andre Geim and Konstantin Novoselov at the University of Manchester.

Energy minimized structure from DFT calculations for a large PAH with the hydrogens removed.

Graphene can be thought of as the largest member of the polycyclic aromatic hydrocarbon (PAH) family, molecules made up of fused aromatic rings.  The interest in these carbon-based materials arises from their amazing electronic and optical properties.  The smaller members of the PAH family provide chemists an opportunity to create molecules of fixed and uniform dimensions, something which is challenging when making graphene.  Furthermore, by synthesising PAHs in a logical, stepwise fashion, we can adapt their properties to suit our own needs.

In a recent Chemical Science Edge article, the groups of Hexing Li (Shanghai Normal University) and Colin Nuckolls (Columbia University) have shown that they can construct large polycyclic aromatic hydrocarbons (PAHs) in a relatively straightforward and high yielding process. As might have been expected, such a large aromatic molecule is not particularly soluble but, emphasising the utility of their method, they are able to overcome this by adding groups to the molecule which make it more soluble.  The versatility of their approach will surely lead to the creation of even larger PAHs which will eventually serve to bridge the gap between these small molecules to the extended structures of graphene.

Interested in more?  Read this HOT Chem Sci Edge article now!

Supersized contorted aromatics
Shengxiong Xiao, Seok Ju Kang, Ying Wu, Seokhoon Ahn, Jong Bok Kim, Yueh-Lin Loo, Theo Siegrist, Michael L. Steigerwald, Hexing Li and Colin Nuckolls
Chem. Sci., 2013, 4, 2018-2023
DOI: 10.1039/C3SC50374G

Ruaraidh McIntosh is a guest web-writer for Chemical Science.  His research interests include supramolecular chemistry and catalysis.  When not working as a Research Fellow at Heriot-Watt University, Ruaraidh can usually be found in the kitchen where he has found a secondary application for his redoubtable skills in burning and profanity.

Scientists in the UK are attempting to understand the host–guest chemistry of molecular cages in solution, by separately quantifying the free energy contributions of (at least) three host–guest interaction mechanisms that usually work in unison: hydrogen-bonding, non-polar interactions and solvophobic effects.

The team have systematically looked at a series of guests (with/without π-stacking ability and different hydrogen-bond acceptor capabilities), within a water-soluble cubic coordination cage and an isostructural organic-soluble cage.

Being able to quantify solvent effects associated with specific molecular substituents or functional groups will help in the design of hydrophobic cavities within coordination cages. Hydrophobic cavities can stabilise otherwise unstable molecular guests and catalyse reactions where the transition state matches the cavity size and shape.

Read this ‘HOT’ article, hot off the press:

Quantification of solvent effects on molecular recognition in polyhedral coordination cage hosts
Martina Whitehead , Simon Turega , Andrew Stephenson , Chris Hunter and Mike D Ward
Chem. Sci., 2013, Accepted Manuscript
DOI: 10.1039/C3SC50546D

Chloride ions are central to many biological processes and measuring the concentration of chloride ions in biological fluids can help diagnose a number of diseases, including cystic fibrosis.

Scientists from the US have synthesised a squaraine rotaxane which can detect chloride ions via colorimetric naked eye detection and fluorescence. This is the first example of a ratiometric chloride sensor that emits in the deep red region.

Read the ‘HOT’ Chemical Science article for free:

Squaraine rotaxane shuttle as a ratiometric deep-red optical chloride sensor
Carleton G. Collins, Evan M. Peck, Patrick J. Kramer and Bradley D. Smith
Chem. Sci., 2013, DOI: 10.1039/C3SC50535A

In a recent Edge article, Chemical Science Associate Editor Professor Haw Yang and his group at Princeton University use Brownian motion to transport a two-sided micro-particle to a target, with the help of ‘photon-nudging’ to keep it on track.

The micrometer precision of this simple method of locomotion is notable.  It could potentially be used to deliver drugs and molecules in cells, precisely and in real-time.

Normally, Brownian motion causes micro-particles to move randomly in fluids.  This presents a challenge for the creation of controllable ‘micro-swimmers’.  Yet, with some external direction, this motion can deliver particles to their targets.

The adaptive photon-nudging idea.

Yang and co-workers ‘steer’ a two-sided microsphere in a fluid, as it undergoes Brownian motion.  One side has a gold coating, whilst the other is polystyrene.  When the polystyrene side is facing the target, a laser beam is activated.  The light then moves this ‘Janus’ particle in one of two ways, either by ‘radiation-pressure’ (pushing by photons) or by heat-induced locomotion via the photophoretic effect.  The latter effect forces this particle, when it has a large temperature difference across its two faces, to move in the direction of the cooler side (polystyrene side) (see also Crooke’s radiometer).

The authors do not use ‘photon-nudging’ constantly.  It is used occasionally, just to steer the particle in the right direction. Brownian motion does most of the work transporting the particle to the target– making this a good example of a man-made Brownian motor.

It will be interesting to see how soon researchers can refine this ‘photon-nudging’ method to achieve nanometer or molecular scale accuracy.  Many would welcome that level of control for particles and molecules in cells and chemical solutions.

Read this HOT Chem Sci cover article in full!

Harnessing thermal fluctuations for purposeful activities: the manipulation of single micro-swimmers by adaptive photon nudging
Bian Qian, Daniel Montiel, Andreas Bregulla, Frank Cichos and Haw Yang
Chem. Sci., 2013, 4, 1420-1429
DOI: 10.1039/C2SC21263C

Geoff Nelson is a guest web-writer for Chemical Science.  He currently works as a post-doctoral research associate in Dr David Payne’s research group in the Department of Materials at Imperial College, London.  Geoff’s current research concerns the synthesis and characterization of post-transition metal oxides for use in the energy sector.  His other research interests include carbon-based materials, biophysical chemistry, and surface science.

Scientists in the US have prepared three UiO-68 network topology metal-organic frameworks (MOFs) using phosphorylurea substituted organic ligands and used them as sorbents to extract actinide elements from water and artificial sea water. 

The MOFs were shown to be highly efficient at sorbing uranyl ions, with saturation sorption capacities as high as 217 mg U g^-1 which is equivalent to binding one uranyl ion for every two sorbent groups.

In the future, the technology could be used to extract uranium from seawater for nuclear fuel production and for the removal of radionuclides from waste streams and drains at nuclear power plants.

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Highly porous and stable metal-organic frameworks for uranium extraction
Wenbin Lin , Michael Carboni , Carter Abney and Shubin Liu
Chem. Sci., 2013, DOI: 10.1039/C3SC50230A