Archive for the ‘Uncategorized’ Category

An Organometallic Toolbox for the Study and Synthesis of Unique N-Heterocyclic Carbenes

N-heterocyclic carbenes (NHCs) are an interesting example of chemical curiosity turned commonplace. NHCs are stable singlet carbenes located within an N-heterocycle, in which the carbon centre bears an sp2 hybridised pair of electrons. As early as 1835 chemists were thinking about carbenes, with Dumas’ optimistic (if unsuccessful) attempt to synthesise the methylene carbene by dehydrating methanol. For many years the intentional study of carbenes was considered too demanding because of their instability, and so they remained in relative obscurity. A number of seminal papers changed this preconception; in particular, a report by Wanzlick in 1968 reporting the synthesis of the first NHC-metal complex using mercury and the first synthesis of a stable and isolable NHC by Arduengo in 1991.

Intensification in research and interest in NHCs over the past thirty years may have originated with these seminal reports, but it continues because of the success of NHCs in catalysis: both as strongly σ-donating metal ligands and nucleophilic organocatalysts. One of the most valuable features of NHCs is the ability to tailor their steric and electronic properties by altering the heterocyclic ring and N-bound substituents. Accordingly, the study of NHC reactivity and the development of methods to functionalise NHCs are essential for continued innovation in this field.

Drs Marina Uzelac and Eva Hevia at the University of Strathclyde, Scotland, have written a review article summarising organometallic methods to metallate N-heterocyclic carbenes. The work summarises metallation of all three components of the NHC: the carbenic carbon, the heterocyclic backbone and the N-bound substituents.

The lithiated complex (1), synthesised by treatment of the N-heterocyclic carbene NHC with nBuLi, can be transmetallated at the C4 position by a number of main group elements to give a variety of bimetallic complexes (2). These complexes can be selectively quenched to generate NHC complexes with unconventional regiochemistry (3).

The lithiated complex (1) can be transmetallated at the C4 position by a number of main group elements to give a variety of bimetallic complexes (2). These complexes can be selectively quenched to generate NHC complexes with unconventional regiochemistry (3).

To exemplify the breadth of research discussed; beginning with 2,6-diisopropylphenyl (dipp) substituted imidazole-2-ylidenes, the reactivity of the NHC can be unlocked by initial addition of an alkali metal such as lithium, sodium or potassium (see figure). Metallation at the C4 position occurs by deprotonation of the vinyl protons in the NHC backbone, while a second metal coordinates to the carbene electron pair at the C2 position. From this species (1) it is possible to transmetallate the C4 position with a less-polar metal such as zinc, aluminium, gallium, boron or iron to furnish a bi-metallic NHC (2). Interestingly, addition of an electrophilic methyl or proton source to this species exclusively quenches the C2 position, generating a suite of unconventional complexes (3) with the carbene electron pair positioned on the C4 carbon.

Lithiation of NHC complexes: a) deprotonation of the backbone of NHC-borane complex; b) co-complexation of NHC-zinc complex with alkyllithium affording lithium zincate; c) deprotonation of the abnormal carbene complex.

Reactivity of main-group NHC complexes towards lithiation.

Further studies investigate how different reagents influence the regioselectively and extent of metallation, how metallated NHCs can activate small-molecules such as carbon dioxide, conditions which can lead to the metallation of N-dipp substitutents, as well as products and speciation following treatment of NHCs with a variety of bimetallic reagents.

In addition to expanding the knowledge of NHC reactivity, the work summarised in this review provides a reference and inspiration to researchers seeking to tailor NHCs for unique applications in synthesis and catalysis.

To find out more please read:

Polar organometallic strategies for regioselective C-H metallation of N-heterocyclic carbenes

Marina Uzelac and Eva Hevia.
Chem. Commun., 2018, Advance Article
DOI: 10.1039/c8cc00049b

About the author:

Zoë Hearne is a PhD candidate in chemistry at McGill University in Montréal, Canada, under the supervision of Professor Chao-Jun Li. She hails from Canberra, Australia, where she completed her undergraduate degree. Her current research focuses on transition metal catalysis to effect novel transformations, and out of the lab she is an enthusiastic chemistry tutor and science communicator.

 

 

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Gold Rocks the Casbah

Researchers at the University of Texas have developed an inventive method to deliver molecules into the cell’s nucleus. Advances in gene therapy and the development of drugs that target DNA, the transcription machinery and other regulatory systems all rely on effective transport of molecules into the nucleus. Furthermore, achieving selective delivery of drugs reduces toxicity to non-target organs while maintaining the therapeutic effect.

Towards this aim, the authors delivered liposomes coated with clusters of gold nanoparticles into the cytoplasm. Laser irradiation of the cells heats the nanoparticles to high temperatures resulting in vapourisation of the water-based cytosol, and the transient formation of nanobubbles. The effect of this is an increase in the porosity of the nuclear envelope, enabling the transfer of various macromolecules from the cytoplasm into the nucleus. The authors describe this technique as ‘nanomechanical transduction’ because it is hypothesised that the mechanical effects brought on by the rapid growth and collapse (20 – 50 ns lifetimes) of the bubbles is responsible for the observed increase in porosity.

Local heating of gold nanoparticles and the subsequent formation of nanobubbles occurs due to ‘plasmon resonance’, whereby an electromagnetic field interacts with gold on the surface of the liposome and drives free-electron oscillation in resonance with the incident laser.

A diagram showing nanomechanical transduction. A gold-coated nanoparticle liposome enters the cell and, upon activation by a laser pulse, creates nanobubbles which causes mechanical disruptions in the cell and increased permeability of the nuclear membrane so molecules such as plasmids can enter.

A diagram showing nanomechanical transduction. A gold-coated liposome enters the cell and, upon activation by a laser pulse, creates nanobubbles and mechanical disruption within the cell, resulting in increased permeability of the nuclear membrane.

As a proof-of-concept the authors investigated whether nanomechanical transduction can improve the nuclear localisation of two different types of macromolecule: a dextran polymer labelled with a fluorescent dye, and a plasmid encoding the green fluorescent protein. In the first experiment, cells containing the dextran polymer were incubated with plasmonic liposomes and subjected to a near-infrared laser pulse. Up to 70% fluorescence intensity was observed in the nucleus compared to the cytoplasm, far exceeding the result from control experiments using electroporation to increase cell membrane permeability. In a similar way, nanomechanical transduction resulted in a 2.7 fold increase in the expression of the green-fluorescent protein compared to using electroporation, demonstrating efficient delivery of the plasmid into the nucleus.

The authors entitle their paper ‘rock the nucleus’ and, unintentional reference or not, I think a Casbah (one meaning is the central keep, or citadel, of a walled city) is a rather fitting analogy for the nucleus: the command post of the cell, and safeguard of genetic information. The authors of this work offer a sophisticated yet general method for molecules to breach the walls.

To find out more please read:

Rock the nucleus: significantly enhanced nuclear membrane permeability and gene transfection by plasmonic nanobubble induced nanomechanical transduction

Xiuying Li, Peiyuan Kang, Zhuo Chen, Sneha Lal, Li Zhang, Jeremiah J. Gassensmith and Zhenpeng Qin.
Chem. Commun., 2008, Advance Article
DOI: 10.1039/c7cc09613e

About the author:

Zoë Hearne is a PhD candidate in chemistry at McGill University in Montréal, Canada, under the supervision of Professor Chao-Jun Li. She hails from Canberra, Australia, where she completed her undergraduate degree. Her current research focuses on transition metal catalysis to effect novel transformations, and out of the lab she is an enthusiastic chemistry tutor and science communicator.

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HOT ChemComm articles for January

All of the referee-recommended articles below are free to access until 5th March 2018.

Kinetic Stabilisation of a Molecular Strontium Hydride Complex using an Extremely Bulky Amidinate Ligand
Caspar N. de Bruin-Dickason, Thomas Sutcliffe, Carlos Alvarez Lamsfus, Glen B. Deacon, Laurent Maron and Cameron Jones
Chem. Commun., 2018,54, 786-789
DOI: 10.1039/C7CC09362D, Communication

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The antibacterial activity of polyoxometalates: Structures, antibiotic effects and future perspectives
Aleksandar Bijelic, Manuel Aureliano and Annette Rompel
Chem. Commun., 2018,54, 1153-1169
DOI: 10.1039/C7CC07549A, Feature Article

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Controlled Preparation of Amphiphilic Triblock-Copolyether in a Metal- and Solvent-Free Approach for Tailored Structure-Directing Agents
Alexander Balint, Marius Papendick, Manuel Clauss, Carsten Müller, Frank Giesselmann and Stefan Naumann
Chem. Commun., 2018, Advance Article
DOI: 10.1039/C7CC09031E, Communication

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Alkene functionalization for the stereospecific synthesis of substituted aziridines by visible-light photoredox catalysis
Wan-Lei Yu, Jian-Qiang Chen, Yun-Long Wei, Zhu-Yin Wang and Peng-Fei Xu
Chem. Commun., 2018, Advance Article
DOI: 10.1039/C7CC09151F, Communication

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Microwave-induced covalent functionalization of few-layer graphene with arynes under solvent-free conditions
M. V. Sulleiro, S. Quiroga, D. Peña, D. Pérez, E. Guitián, A. Criado and M. Prato
Chem. Commun., 2018, Advance Article
DOI: 10.1039/C7CC08676H, Communication

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Barbiturate end-capped non-fullerene acceptors for organic solar cells: tuning acceptor energetics to suppress geminate recombination losses
Ching-Hong Tan, Jeffrey Gorman, Andrew Wadsworth, Sarah Holliday, Selvam Subramaniyan, Samson A. Jenekhe, Derya Baran, Iain McCulloch and James R. Durrant
Chem. Commun., 2018, Advance Article
DOI: 10.1039/C7CC09123K, Communication

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Hiding Carbon Dioxide in Oxazolidinones

Sometimes it feels as though the pinnacle of synthetic achievement is represented by 20 step total syntheses (with 10 contiguous stereocentres and 5 fused rings…). The level of chemical complexity that can be fashioned from simple building blocks is undoubtedly impressive, but amid such feats it is important not to lose sight of the elegance and worth of simple chemistry, especially when it aims to play a part in resolving profound challenges. One such challenge, which will increasingly confront future generations, is how to reduce the load of carbon dioxide in the atmosphere. One solution is to ‘fix’ carbon dioxide by integrating it into chemical building blocks of added complexity in a sustainable way.

The porosity and high surface area of metal organic frameworks (MOFs), a class of three-dimensional coordination networks, proffers them as ideal materials for capture and storage of carbon dioxide. A team of researchers have designed a MOF which consumes carbon dioxide in a different way: by transformation into value-added chemicals. The group have developed a catalytic MOF embedded with lewis-acidic copper centres capable of converting aziridines to oxazolidinones by the addition of carbon dioxide. Oxazolidinones are used as auxiliaries in chiral synthesis, and are structural components of some antibiotics.

The MOF, termed MMPF-10, is a metal-metalloporphyrin framework constructed from a copper-bound porphyrin ring chemically modified to incorporate 8 benzoic acid moieties, generating an octatopic ligand. These carboxylic acids groups form a second complex with copper in situ, termed a ‘paddlewheel’ for its appearance, with the formula [Cu2(CO2)4]. The resulting network contains hexagonal channels measuring 25.6 x 15.6 Å flanked by four of each of the two copper complexes. With 0.625 % of the catalyst at room temperature, 1 bar CO2 pressure, and in a solvent free environment, MMPF-10 catalyses the transformation of 1-methyl-2-phenylaziridine to yield 63% of the product.

metal-metalloporphyrin MOF catalyses catalyzes carbon dioxide fixation aziridines to oxazolidinones

Topology of MMPF-10 showing hexagonal channels in a) and c), and pentagonal cavities in b). Turquoise: copper, red: oxygen, grey: carbon, blue: nitrogen.

This work, a simple reaction to prepare oxazolidinones, shows that carbon dioxide can be fixed in specialised synthetic building blocks in a sustainable way. This is the way the first paragraph ended, ‘in a sustainable way’, because the challenge of developing such a reaction is two-fold: it must use carbon dioxide, and the reaction conditions must be sustainable. There will be no beneficial offset if the reaction uses a lot of energy, requires many resources, or generates larges quantities of waste. In this reaction the researchers have remained mindful of developing a mild, solvent-free reaction with low catalyst loading employing an earth abundant metal, reflecting an earnest aim to develop practical and sustainable chemistry.

To find out more please read:

A metal-metalloporphyrin framework based on an octatopic porphyrin ligand for chemical fixation of CO2 with aziridines

Xun Wang, Wen-Yang Gao, Zheng Niu, Lukasz Wojtas, Jason A. Perman, Yu-Sheng Chen, Zhong Li, Briana Aguila and Shengqian Ma
Chem. Commun., 2018, Advance Article
DOI: 10.1039/c7cc08844b

About the Author

Zoë Hearne is a PhD candidate in chemistry at McGill University in Montréal, Canada, under the supervision of Professor Chao-Jun Li. She hails from Canberra, Australia, where she completed her undergraduate degree. Her current research focuses on transition metal catalysis to effect novel transformations, and out of the lab she is an enthusiastic chemistry tutor and science communicator.

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Tech red unmasked

Tech red, an enigmatic technetium compound that has resisted characterisation for half a century, has been identified using chemical detective-work and computer modelling. The molecule’s unusual chemistry may explain why it has proven so difficult to unmask.1


Source: © Bradley Childs
Tech red forms a red, oily liquid upon condensation

Nuclear chemists have been running into a volatile red oxide of technetium – Tc, a radioactive metal – since at least the 1960s.2 ‘Everybody seems to have accidentally made this a couple of times,’ notes Keith Lawler, a postdoctoral researcher at the University of Nevada Las Vegas (UNLV), US. Although the telltale hue makes tech red easy to spot, it has gone unidentified over the intervening decades. Tech red refuses to form crystals, so can’t studied by crystallographic methods, while technetium’s radioactivity is an inherent barrier to researching its compounds. ‘There are only a handful of laboratories who can work with large amounts of technetium, and even fewer who have access to anything other than simple characterisation techniques,’ explains John McCloy, who investigates radioactive materials at Washington State University, US.

Read the full story by Alexander Whiteside on Chemistry World.

 

1 K V Lawler et al, Chem Commun., 2018, DOI: 10.1039/c7cc09191e (This article is free to access until 7 March 2018.)

2 C Rulfs, R Pacer and R Hirsch, J. Inorg. Nucl. Chem., 1967, 29, 681 (DOI: 10.1016/0022-1902(67)80323-3 )

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Shoot the Messenger: Circular DNA-Graphene Oxide Material Targets mRNA in Living Cells

Schematic of the circular DNA cDNA/GO graphene oxide platform fabrication for intracellular mRNA messenger RNA imaging and gene therapy.

Scheme showing how cDNA/GO enters the cell and interacts with mRNA

Did you know that the combined length of DNA in your body’s cells is a number so large that the only references I could find use cosmic distances as a reference? Try twice the diameter of the solar system, or the distance to the moon and back 1500 times. Despite the complexity and infinite detail encountered when studying science, it is often something so simple as size that gives us pause. How can DNA be both uncomprehendingly huge and tiny at the same time?

The major function of DNA is to encode proteins, a process which begins with the transcription of genes into single-stranded messenger RNA (mRNA) molecules. It is mRNA that is directly translated into the strands of amino acids which fold to form proteins.

A team of researchers at Fuzhou University in China have developed a graphene oxide and circularised single-stranded DNA (cDNA/GO) hybrid material capable of penetrating living cells and binding mRNA. The material’s utility is shown in two practical applications: mRNA imaging and nucleotide therapeutics. The authors chose the mRNA of survivin and c-raf kinase as targets, because the enzymes are involved in carcinogenesis, and the mRNA are overexpressed in cancer cells and can be used as biomarkers.

cDNA was chosen for its increased stability over linear single-stranded DNA, which is rapidly degraded in vivo by exonucleases. For mRNA imaging the material is designed with a fluorescent dye coupled to the cDNA. GO was chosen as a hydrophilic delivery scaffold capable of adsorbing cDNA and quenching the dye. When cDNA/GO was incubated with HeLa cells (a cancer cell strain) a time-dependent increase in fluorescence was observed in the cytoplasm. Fluorescence is restored when cDNA encounters the target and desorbs from the GO to form a duplex with the mRNA.

CLSM images acquired for HeLa cells treated with both survivin and c-raf targeted cDNA/GO for duplexed intracellular mRNA imaging

The mRNA of both survivin and c-raf kinase can be imaged in living cells with cDNA/GO.

The researchers also probed whether the material might serve as a therapeutic agent: if formation of the cDNA-mRNA duplex blocks translation it may reduce the load of c-raf kinase and survivin in the cell and influence cancer cell growth. Accordingly, the researchers found that when the HeLa cells were incubated with cDNA/GO, cell proliferation was inhibited in a dose-dependent manner.

This research contributes a robust design which can be applied to diverse mRNA targets because optimisable properties such as stability, bioavailability and selectivity are largely independent of the sequence of nucleotides.

To find out more please read:

Circular DNA: a stable probe for highly efficient mRNA imaging and gene therapy in living cells

Jingying Li, Jie Zhou, Tong Liu, Shan Chen, Juan Li and Huanghao Yang
Chem. Commun., 2018, Advance Article
DOI: 10.1039/C7CC08906F

About the author

Zoë Hearne is a PhD candidate in chemistry at McGill University in Montréal, Canada, under the supervision of Professor Chao-Jun Li. She hails from Canberra, Australia, where she completed her undergraduate degree. Her current research focuses on transition metal catalysis to effect novel transformations, and out of the lab she is an enthusiastic chemistry tutor and science communicator.

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ChemComm Prizewinners at the 44th Symposium on Main Group Element Chemistry

The 44th Symposium on Main Group Element Chemistry took place on the 7th to 9th December at the Tokyo Institute of Technology. Over 250 delegates attended the symposium which was chaired by Prof Kei Goto (TITech).

ChemComm was proud to sponsor two prizes, which were chosen out of 60 oral presentations and 108 poster presentations.

The winner of the ChemComm oral presentation was Mr Shogo Morisako (Graduate School of Science, Hiroshima University) who presented on ‘Syntheses and Reactivities of New Multiple Bond Boron Compounds’.

Mr Shogo Morisako

Mr Shogo Morisako (left) being presented his award by Dr Hiromitsu Urakami (right) on behalf of Chemical Communications

The winner of the ChemComm poster presentation was Mr. Tomoyuki Kosai (Graduate School of Science, Tohoku University) who presented on ‘Activation of Dihydrogen Using Disilenes Bearing Amino and Boryl Groups’.

Tomoyuki Kosai

Mr Tomoyuki Kosai (left) being presented his award by Dr Hiromitsu Urakami (right) on behalf of Chemical Communications

Congratulations to both Mr Morisako and Mr Kosai, we wish you both the best for the future!

 

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ChemComm poster prize winners at the Pan Africa Chemistry Network Congress

The Pan Africa Chemistry Network (PACN) Congress was held on 7 – 9 November in Accra, Ghana and was attended by approximately 250 researchers and policy-makers.

The PACN Congress is the flagship annual event for the PACN, which seeks to create a self-sustaining science based in Africa, helping to build capacity, solve local challenges and contribute to global knowledge. The topic for this year’s Congress was ‘Sustainable Agriculture: how the chemical sciences can contribute to food security for a growing population’.

ChemComm is proud to have supported the poster prizes for this wonderful event and the 1st prize was awarded to Francis Asiam, from KNUST in Ghana, for his poster entitled ‘Collection, distribution, extraction and characterisation of vegetable oils from 40 varieties of high yielding Allanblackia parviflora in Ghana’. A full list of the poster prize recipients and poster titles can be found below.

Well done to all of the well-deserved winners!

 

PACN Prize 1st place: Francis Asiam (KNUST, Ghana) for ‘Collection, distribution, extraction and characterisation of vegetable oils from 40 varieties of high yielding Allanblackia parviflora in Ghana

PACN Prize 2nd place: EA Asamoah (KNUST, Ghana) for ‘Development of Rabbit Meat Sausages’

PACN Prize, 3rd place: Ray Voegborlo (KNUST, Ghana) for ‘Human Exposure Assessment of Ochratoxin A through consumption of cocoa beans’

Agilent supported prize: Nkechinyere Isienyi (Forestry Research Institute of Nigeria) for ‘Impact of heavy metal on soil near Lapite dumpsite in Ibadan, Nigeria’

Syngenta supported prize: Flaure Essoung (ICIPE, Kenya) for ‘Welwitschianol A and B: Two cyclohexene derivatives and other insecticidal constituents of Caesalpinia Welwitschiana’

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Cram Lehn Pedersen Prize 2018 – call for nominations

The International Committee of the International Symposium on Macrocyclic and Supramolecular Chemistry is pleased to invite nominations for the Cram Lehn Pedersen Prize for young supramolecular chemists.

The Cram Lehn Pedersen Prize, named in honour of the winners of the 1987 Nobel Prize in Chemistry, recognises significant original and independent work in supramolecular chemistry.

Previous winners include Tom F. A. de Greef, Ivan Aprahamian, Feihe Huang, Oren Schermann, Tomoki Ogoshi, Jonathan Nitschke, and Amar Flood.

Those who are within 10 years of receiving their PhD on 31st December 2017 are eligible for the 2018 award. The winner will receive a prize of £2000 and free registration for the ISMSC meeting in Québec, Canada. In addition to giving a lecture at ISMSC, a short lecture tour will be organised after the meeting in consultation with the Editor of Chemical Communications, the sponsor of the award.

Nomination Details:

You may nominate yourself or someone else. Please send your CV, list of publications (divided into publications from your PhD and post-doc, and those from your independent work), and if desired, a letter of support, or these materials for someone you wish to nominate, to Prof. Roger Harrison (ISMSC Secretary) at rgharris@chem.byu.edu by 31st December 2017.

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Negative press is not always a bad thing: a novel anode material for sodium-ion batteries

At a product launch in California last week Elon Musk introduced Tesla’s new electric semi-trailer truck. Musk sells a tantalising future: one where an electric fleet replaces vehicles which currently rely on fossil fuels. Central to this fleet are powerful rechargeable batteries. Lithium-ion batteries are favoured for many current applications, such as portable electronic devices and the current offerings of full and hybrid vehicles. In coming years they are projected to be the technology of choice for the large-scale applications mentioned above and for storing power generated from intermittent renewable energy sources.

A limiting factor in the widespread roll-out of lithium batteries is that lithium is an expensive resource with low natural abundance. Sodium offers a possible alternative and has the obvious benefits of being both very cheap, and one of the most abundant elements in the earth’s crust. The electrode materials used in lithium batteries cannot be used to make the sodium variant because the sodium ion is larger (1.02 Å compared to 0.76 Å for lithium) and damages the crystalline materials optimised for lithium.

Researchers Gu, Gu and Yang at Beihang University in Beijing have reported the synthesis and performance of a novel anode material optimised for sodium. The material is a graphene-tetrahydroxybenzoquinone (Na4C6O6) hybrid, and is comprised of a porous graphene-oxide scaffold decorated with nanocrystals of Na4C6O6. Furthermore, X-ray photoelectron spectroscopy (XPS) reveals the homogenous distribution of sodium throughout this conducting material.

The electrochemical performance contrasts with previously reported materials of this type by exhibiting high cyclic stability. The reversible capacity of graphene-Na4C6O6 at a current density of 74.4 mA g-1 is 268 mA h g-1, a value which is steady over 60 cycles. This is competitive with the graphite anode materials found in lithium batteries, which have specific capacities between 200 and 400 mA h g-1. Furthermore the material performs well over a range of current densities, with reversible capacities of 95 – 211 mA h g-1 measured over a range of 3720 – 186 mA g-1.

With this work the authors contribute, at most, a viable candidate for the next rechargeable sodium battery and, at the very least, continued research into sustainable technologies. This ensures that in addressing our current energy challenges we are solving the problem, not delaying it.

To find out more please read:

3D organic Na4C6O6/graphene architecture for fast sodium storage with ultralong cycle life
Jianan Gu, Yue Gua and Shubin Yang
Chem. Commun., 2017, Advance Article
DOI: 10.1039/C7CC08045J, Communication

About the author

Zoë Hearne is a PhD candidate in chemistry at McGill University in Montréal, Canada, under the supervision of Professor Chao-Jun Li. She hails from Canberra, Australia, where she completed her undergraduate degree. Her current research focuses on transition metal catalysis to effect novel transformations, and out of the lab she is an enthusiastic chemistry tutor and science communicator.

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