Chemical Science Blog

Cyclic paraphenylenes (CPPs), first made in 2008, have potential roles in carbon nanotube synthesis as well as interesting optoelectronic properties and nano-sized cavities. Despite their potential, scientists haven’t explored them much for materials and nano applications because they’ve been really difficult to make at a reasonable scale – 10-15mg is typical – and they are expensive.

Now, scientists in the US have come up with a procedure to make 20g of a common intermediate within a week, which can be used to make gram quantities of cyclic paraphenylenes. They developed a macrocyclisation step that uses a much cheaper palladium source than before (ligand-free), reducing the cost significantly. They also report the first solid-state structure of the supramolecular complex between C60 and [10]CPP, illustrating the perfectly matched convex/concave pi-pi interactions (they describe it as a nanopeapod structure).

Link to journal article
Gram-Scale Synthesis and Crystal Structures of [8]- and [10]CPP, and the Solid-State Structure of C60@[10]CPP
J Xia, J W Bacon and R Jasti
Chem. Sci., 2012, DOI: 10.1039/c2sc20719b

Scientists in China have made a molecular spring that mimics stretchable systems in living systems, for example titin (a protein found in cardiac and skeletal muscles). 

Although molecular springs based on rotaxane are known, they can only change their length stepwise. This new rotaxane-based spring changes its length continuously as solvent polarity varies and so is a better mimic of biological springs.

Link to journal article
A solvent-driven molecular spring
Z Zhang et al
Chem. Sci., 2012, DOI: 10.1039/c2sc20728a

A new metal-organic framework (MOF) with the highest hydrogen uptake at 298K of all the MOFs that have been examined to date has been made by US scientists. The MOF exhibits exceptionally high hydrogen (58mg/g^-1 and 39g/L^-1 at 52 bar and 77K) and methane (276mg/g^-1 and 189g/L^-1 at 80 bar and 298K) uptake capacities, they say.

The team attributes the exceptionally high gas uptake capacity to the highly branched, aromatic-rich nature of the bridging ligand, optimal pore size and the open metal sites in the trizinc secondary building units.

The work highlights the potential of designing MOFs with even higher gas uptake capacities by further optimising their structural, chemical and topological characteristics. 

Link to journal article
A High Connectivity Metal-Organic Framework with Exceptional Hydrogen and Methane Uptake Capacities
D Liu et al
Chem. Sci., 2012, DOI: 10.1039/c2sc20601c

The first example of using an isolable carbene to dissociate homonuclear bonds (e.g. S-S, Br-Br) has been reported by researchers in the US.

The conditions are mild and metal-free – a surprisingly straightforward way to activate a variety of substrates. The dissociation of homonuclear bonds is critical to chemical reactions that range from the rearrangement of disulfide linkages in proteins to the synthesis of small molecules.

Link to journal article
Homonuclear Bond Activation Using A Stable N,N’-Diamidocarbene
K M Wiggins et al
Chem. Sci., 2012, DOI: 10.1039/c2sc20639k

Metallation is an entry point to constructing compounds, and by converting inert C-H bonds to reactive C-metal bonds, it opens up bond forming opportunities. Mixed metal reagents are widely used; combining an alkali metal with a softer, less reactive metal charges the softer metal component with super-reactivity, which, combined with good selectivity and functional group tolerances, makes the reagents superior to organolithium reagents. But how do they work? 

To remove the mystery surrounding these mixed-metal reagents, scientists in the UK have studied one such reagent in detail. They discover (surprisingly) that LiCd(TMP)₃ is unlikely to be an ate as previously thought, instead consisting of two independent homometallic amides. Rather than a synergistic metallation of the substrate, the metallation is a two step process: ortholithiation followed by transmetallation to cadmium.

Link to journal article
Opening the black box of mixed-metal TMP metallating reagents: direct cadmation or lithium-cadmium transmetallation
D R Armstrong et al
Chem. Sci. , 2012, DOI: 10.1039/c2sc20392h

Hydrogen sulfide is a gas best known for its rotten egg smell. Although generally considered toxic, our bodies produce it in small amounts. It is thought to help keep our heart and other organs healthy and may be involved in signalling. Altered levels of H₂S have been implicated in a number of diseases, including Alzheimer’s and Down’s Syndrome. 

Scientists in China have reported a fluorescent probe for detecting H₂S in blood and brain tissue. Although other probes are known, this one has the advantage of being fast in addition to selective and sensitive. The team used it to test H₂S levels in mice blood and brain tissue. They say it is the first probe that could allow parallel measurements of H₂S concentrations in both blood and tissues.

Link to journal article
A Fluorescent Probe for Rapid Detection of Hydrogen Sulfide in Blood Plasma and Brain Tissues in Mice
Y Qian et al
Chem. Sci., 2012, DOI: 10.1039/c2sc20537h

Hybridising metal nodes in metal-organic frameworks (MOFs) could open up a new route for activating CO₂ so it can be converted into useful chemicals, claim Chinese scientists. 

Using theoretical methods, they calculated the properties of a copper-based MOF into which they hybridised tungsten ions. They found that the asymmetric W-Cu centres in the MOF have unique catalytic reactivity towards CO₂ conversion that W-W or Cu-Cu centres don’t possess.

 Link to journal article
Catalyzed Activation of CO₂ by a Lewis Base Site in W-Cu-BTC Hybrid Metal Organic Frameworks
Q Zhang et al
Chem. Sci., 2012, DOI: 10.1039/c2sc20521a

US scientists have made nanoparticles that undergo enzyme-induced changes in structure which are detectable in complex environments.

Enzymes have previously been used to manipulate nanoscale structures, with methods such as electron microscopy used to analyse the morphology changes. But, says Nathan Gianneschi at University of California, San Diego, changes in nanoscale architecture will only be detectable in vivo if the enzyme’s action results in an output signal unique to the nanoscale assembly, such as a spectrophotometric response.

So Gianneschi and colleagues developed enzyme-responsive fluorogenic micellar nanoparticles made from peptide–polymer amphiphiles labelled with fluorescent dyes. They showed they could use Förster Resonance Energy Transfer (FRET) signals to sensitively monitor the nanoparticles’ response to enzymes.

The results are a step towards the use of enzyme-programmed materials for molecular diagnostics and drugs.

Link to journal article
Fluorogenic Enzyme-Responsive Micellar Nanoparticles
M-P Chien et al
Chem. Sci., 2012, DOI: 10.1039/c2sc20165h

Having synthesised the first example of a rigid acetylene-linked perylene bismuth macrocycle, scientists in Germany and Korea have made a series of these compounds with varying ring sizes and looked at their light-harvesting capabilities with regards to ring size, to establish a structure-property relationship.

They found that the larger the ring size, the slower the energy hopping. They add that the compounds have fast energy transfer capabilities compared to other artificial light-harvesting complexes such as porphyrin-based ones.

Enormous efforts have been invested into mimicking natural light-harvesting bacteria over the past two decades; investigations have focused on efficient energy transfer in particular.

Link to journal article
Excitation energy migration in covalently linked perylene bisimide macrocycles
F Schlosser et al
Chem. Sci., 2012, DOI: 10.1039/c2sc20589k