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Synthesis of Maleimide Dyes with Colourful Fluorescent Emissions

18 Apr 2018

A group of researchers based at universities spanning the UK, China and Spain have synthesised a diverse library of fluorescent maleimide dyes with the aim of developing a structure-function relationship, relating substituent effects to the optical properties of such molecules. This work is not only important to build upon fundamental understanding of the fluorescence mechanism, but to develop knowledge that may be used to guide the synthesis of organic fluorophores which demand particular optical properties.

Organic fluorescent molecules are used as tools in many areas such as forensics, genetic analysis, DNA sequencing and biotechnology. Maleimides are commonly used as fluorescent labels for proteins, as they can couple with the thiol groups of cysteine residues. They are suited to this purpose as they are stable, easily functionalised, give strong emissions and do not perturb the protein structure to a large extent.

Molecules fluoresce upon absorption of UV or visible light, elevating an electron from a ground state orbital to a higher-energy orbital and resulting in a singlet excited state. Relaxation to the ground state occurs rapidly (~ 10 ns) with concomitant emission of a photon – this is what we observe as ‘fluorescence’. The emitted photon almost always has a longer wavelength than the absorbed light, a phenomenon known as the ‘Stokes shift’.

Structures of selected amino-halo-maleimides and alkoxy-halo-maleimides synthesised for the study

With three dihalomaleimide precursers in hand (Cl, Br and I) the researchers assembled a library of amino-halo-maleimides, amino-alkoxy-maleimides, and amino-thio-maleimides. They varied the R groups bound to the N, O and S heteroatoms to include aliphatic, phenyl and benzyl examples.

The optical properties of the amino-halo-maleimides in diethyl ether were examined and the emission wavelengths were measured to be 461-487 nm, giving green-blue fluorescence. The fluorescence quantum yields, a measure of the quantity of emitted photons compared to absorbed photons and an indication of emission brightness, decreased with the electronegativity of the halide (Cl: 37%, Br: 30%, I: 8%). Like many fluorescent molecules in solution the compounds exhibited solvafluorochromism: when the polarity of the solvent alters the optical properties. In protic solvents (methanol and water) the fluorescence quantum yields decreased to below 1% and the emission wavelengths increased by 73-109 nm. On the other hand, in non-polar solvents (cyclohexane) the fluorescence quantum yield increased, up to 56% for the chloro analogue.

a) The UV and emission spectra of fluorescent maleimides bearing amino (2a-c) and alkoxy (3a, 3b) substituents. b) The quantum yields of selected amino and alkoxymaleimides. c) The solvafluorochromism effect for three maleimides (2a-c) in various solvents.

Compared to their amino-substituted counterparts, alkoxy-halo-maleimides have lower quantum yields (reduction of 20-25%), indicating the increased electron-donating capacity of the amine substituent is important for fluorescence intensity. Furthermore, the slight decrease in the emission wavelengths of alkoxy-halo-maleimides (458-465 nm) gives them blue fluorescent emissions. Amino-thio-maleimides, with greater electron-donating capacity than both the amino and alkoxy analogues, have increased emission wavelengths (526-564 nm), thus yellow fluorescent emissions.

This study is a worthwhile read for anyone who uses fluorescent molecules in their work, those wishing to understand a little more about the practical principles of fluorescence and all those curious minds who like to form their own hypotheses.

To find out more please read:

Rational design of substituted maleimide dyes with tunable fluorescence and solvafluorochromism

Yujie Xie, Jonathan T. Husband, Miquel Torrent-Sucarrat, Huan Yang, Weisheng Liu, Rachel K. O’Reilly.
Chem. Commun., 2018, 54, 3339 – 3342
DOI: 10.1039/C8CC00772A

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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Synthesis of Oxazolines with a Triple Activation Twist

04 Apr 2018

A group of researchers based in Valencia, Spain, have developed an enantio- and diastereoselective method to synthesize oxazolines, using a combination of silver and an organocatalyst that is only one degree of separation away from a classic British cocktail (to get that one, you might have to keep reading). This method complements the current literature by employing unactivated ketone substrates to prepare products with two adjacent chiral centres, one of which is quaternary.

Oxazolines are a versatile functional group, found in synthetic precursors and functional molecules and materials. They are a common structural feature in many natural products and in medicinal chemistry they are incorporated into synthetic drug candidates as bioisosteres of other functional groups, such as thiazoles and imidazoles. Oxazolines can also be used to prepare poly(2-alkyl/aryl-oxazoline) polymers, which have utility in biomedical fields to prepare hydrogels, nanoparticles for drug delivery and imaging, and polymer-protein conjugates. In an organic synthesis laboratory oxazolines are encountered as ligands in asymmetric catalysis, such as the bis(oxazoline) ‘BOX’ family of compounds.

Synthesis of oxazolines from ketones and isocyanaoacetate esters via a formal [3+2] cycloaddition reaction.

The researchers optimised the reaction with acetophenone and tert-butyl isocyanoacetate, and found that Ag₂O (2.5 mol%) and a dihydroquinine squaramide organocatalyst (5 mol%) promoted the reaction to obtain a quantitative yield of the cis-oxazoline (80:20 cis/trans) in 24 hours with excellent enantiomeric excess (99/93% e.e.). The researchers tested the reaction scope and found that acetophenone derivatives substituted with electron-withdrawing and donating groups (NO₂, Cl, Br, Me) gave the cis-oxazoline products in reliable yields (60 – 99%), fair to good diastereoselectivity (56:44 – 95:5 cis/trans) and excellent enantioselectivities (91 – 99% e.e.). Other examples tested included cyclohexanone, 2-acetylthiophene, deoxybenzoin and acetone as well as aliphatic ketones with a combination of methyl, isopropyl and cyclopropyl groups (63-99% yield, 45:55 – 98:2 cis/trans, 56 – 98% e.e.).

Diagram showing the triple activation of the substrates achieved using a dihydroquinone squaramide organocatalyst combined with a silver lewis acid.

Diagram showing the triple activation of substrates using a dihydroquinine squaramide organocatalyst combined with a silver lewis acid.

The dihydroquinine squaramide organocatalyst used is a composite structure comprising a rigid cyclobutendione ‘squaramide’ and a reduced quinine analogue. The former makes strong hydrogen bonds with carbonyl derivatives while the latter confers chirality on the catalyst and has a Brønsted basic quinuclidine moiety. Quinine is an alkaloid isolated from the cinchona plant, and is the same stuff in anti-malarial tonic that is better washed down with a measure of gin. The reaction proceeds via silver-coordination to the terminal carbon of the isocyanate, accelerating deprotonation of the alpha protons by the quinuclidine base and forming a nucleophilic enoate. The electrophilicity of the ketone is enhanced by the formation of two hydrogen bonds with the N-H bonds of the squaramide, shaping a doubly activated system which rapidly undergoes a formal [3+2] cycloaddition to give the products.

At first glance this seems to be a simple and straightforward transformation, but in reality the authors have succeeded in optimising a reaction for unactivated substrates which has stereocontrol over two chiral centres, one of which is quaternary. Furthermore, the reaction uses a combination of two catalysts reacting via three modes of activation.

To find out more please read:

Enantioselective synthesis of chiral oxazolines from unactivated ketones and isocyanoacetate esters by synergistic silver/organocatalysis

Pablo Martínez-Pardo, Gonzalo Blay, M. Carmen Muñoz, José R. Pedro, Amparo Sanz-Marco and Carlos Vila.
Chem. Commun., 2018, 54, 2862 – 2865
DOI: 10.1039/c8cc00856f

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An Industrial Revolution on the Nanoscale

27 Mar 2018

“What are the possibilities of small but movable machines? They may or may not be useful, but they surely would be fun to make”. In December 1959 Richard Feynman addressed the annual meeting of the American Physical Society at Caltech with a talk entitled ‘There’s Plenty of Room at the Bottom’, imploring the scientific community to start thinking small, like ‘entire 24 volumes of the Encyclopaedia Britannica on the head of a pin’ kind of small. Many quote this lecture as when the notion of nanomachines first entered the scientific sphere – with talk of miniature cars, injectable molecular ‘surgeons’ and machines that place atoms side by side to synthesize any molecule imaginable. The lecture reads like the description of the futuristic setting in ‘Back to the Future’, an exploration of possibilities at a time when we fundamentally lacked the tools to make them a reality.

As in ‘Back to the Future’, which predated yet predicted the emergence of mobile-banking technology, video calling and personal drones, Richard Feynman’s plea for scientists to prepare molecular-scale machines has also become a reality, and for their successes in this field Jean-Pierre Sauvage, Sir Fraser Stoddart and Ben Feringa were jointly awarded the Nobel Prize in Chemistry in 2016.

A group of researchers based in London and Singapore have written a feature article introducing both the foundational work in this field and state-of-the-art examples. Nanomachines are single molecules or molecular assemblies on the nanoscale (this review defines a 1 – 100 nm scope) that have the ability to perform ‘useful work’ upon application of an external energy source. To extract work (often in the form of controlled mechanical movement) molecular machines are designed to operate at a thermodynamically far-from-equilibrium state, maintained by an energy input, with movement occurring as the system relaxes towards equilibrium. At the synthetic level, molecules are designed with components which have restricted translational and rotational movements with respect to each other, and the ability to control these movements is key to obtaining the desired function.

A catalytically active rotaxane synthesised by Nolte and co-workers acts as a tiny epoxidising machine , moving along a polybutadiene polymer

The catalytically active rotaxane synthesised by Nolte and co-workers acts as a tiny epoxidising machine, moving along a polybutadiene polymer

One of the first advances towards the synthesis of nanomachines was by the research group of Jean-Pierre Sauvage, who achieved the templated synthesis of catenanes; structures with two circular molecules that are interlocked like two links in a chain. It was subsequently shown that a catenane motor could be prepared, with one ring rotating with respect to the other in a controlled manner. Fraser Stoddart further contributed to the field with ‘rotaxanes’, composite molecules comprising a ring threaded onto an axle. Nanomachines based on rotaxanes have been developed and include switches, shuttles and ‘molecular elevators’. A state-of-the-art example of a catalytically active rotaxane synthesised by Nolte and co-workers in 2003 demonstrates the potential of nanomachines to revolutionise organic synthesis. The rotaxane is constructed with a magnesium-bound porphyrin, which threads onto a polybutadiene polymer (300 kDa, 98% cis) and catalyses the epoxidation of the double bonds (turnover number: 140, cis/trans ratio of the polyepoxide: 1:4).

Ben Feringa’s electric nano-car, a single molecule with four fluorene ‘wheels’ capable of driving across a copper surface

In 2011 Ben Feringa and co-workers synthesized the worlds tiniest electric car using the same design principles they had used to create a spinning motor in 1999. The car is a single molecule with the ability to propel itself across a crystalline copper surface upon activation by a voltage pulse, with 10 pulses moving the car 6 nm across the surface. The car itself is comprised of a central diyne strut bonded at each end to carbazole ‘axles’. Each axle is bound through alkenes to two fluorene ‘wheels’. The key design elements are the alkenes and two chiral methyl substituents on each axle which forces each wheel to twist out of the plane. For one wheel rotation: an electronic excitation induces trans–cis isomerisation of the alkene causing a quarter turn of the wheel such that it sits adjacent to the methyl group. Next, a vibrational excitation induces helical inversion, allowing the wheel to push past the methyl group another quarter turn. Another isomerisation and helical inversion completes a full rotation. Research achievements like these demonstrate mechanical work on the nanoscale, with the vision of achieving movement on the macroscale via synchronised motion.

These examples represent a small subset of those discussed in the feature article review, which not only spans the current scope of molecular-scale machines, but reviews the design principles guiding their development and the possibilities nanomachines represent in the future of scientific research.

To find out more please read:

Artificial molecular and nanostructures for advanced nanomachinery

Elizabeth Ellis, Suresh Moorthy, Weng-I Katherine Chio and Tung-Chun Lee.
Chem. Commun., 2018, Advance Article
DOI: 10.1039/c7cc09133h

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Buckyball’s Hydrogen Spillover Effect at Ambient Temperature Observed Experimentally for the First Time

26 Mar 2018

A group of scientists from Tohoku University, Japan experimentally demonstrated the hydrogen spillover effect of buckyball (a.k.a. fullerene or C₆₀). They achieved this breakthrough using mass spectroscopy, and their findings were published recently in Chem. Commun.

Certain transition metal nanoparticles (e.g. Ru, Pt and Ni) can capture hydrogen molecules. The capture process generally involves three sequential steps. Firstly, hydrogen molecules split into hydrogen atoms on the metal surface. Secondly, the yielded hydrogen atoms migrate on the surface towards substrates under the metal nanoparticles and, finally, these atoms fix onto the substrates. The second step is termed the “spillover effect” (Figure 1a). Previous studies predicted that curved graphene sheets could enhance the hydrogen spillover effect at ambient temperatures, but solid experimental evidence has remained inadequate.

To gather evidence for this prediction, Nishihara et al. studied the material buckyball, a spherical carbon nanosphere that represents an extremely curved graphene sheet. The researchers selected ketjenblack (KB), a type of porous carbon sheet, as the substrate, and deposited Pt nanoparticles (1-3 nm in diameter) and buckyball molecules onto it (Figure 1b). They found that the Pt and buckyball-decorated KB stored a higher amount of hydrogen compared to the Pt-loaded KB. This observation indirectly confirmed the previous prediction, as hydrogen storage capacity may be improved by enhancing the spillover effect.

Figure 1. (a) A schematic illustration showing how a hydrogen molecule is split on Pt surface [process (1)] followed by the spillover effect [processes (2) and (2′)]. (b) A schematic illustration of the structure of Pt and buckyball-decorated KB. The inset panel displays two forms of hydrogen bound to the composite: the physically adsorbed di-hydrogen molecules, and the spillover hydrogen atoms anchored on the KB substrate and buckyballs.

The authors then sought time-of-flight mass spectroscopy to obtain more evidence. This spectroscopic technique is capable of identifying molecules with different mass to charge ratios (m/z). As shown in Figure 2, after treating the buckyball and Pt-loaded KB with deuterium molecules (D₂), the spectrum (red) exhibited two additional peaks with m/z of ~723.5 and ~724.5 (highlighted by arrows in the figure) compared to those of the buckyball reference (black) and the buckyball and Pt-loaded KB prior to D₂ dosage (blue). The authors ascribed these two new peaks to single D atom-adsorbed buckyballs with different amounts of carbon isotopes (¹²C and ¹³C). The presence of the two new peaks clearly showed that buckyballs could host hydrogen atoms to enhance the spillover effect. In addition, upon exposing the D-containing buckballs to air, both of the newly-merged peaks disappeared, suggesting that D atom adsorption was reversible.

Figure 2. The time-of-flight mass spectroscopy spectra of buckyball (black), Pt and buckyball-decorated KB before (blue) and after (red) exposure to D2, and after exposure to air (green). Pictures on the right show the molecular structure of a buckyball molecule and two deuterium-incorporated buckyball molecules (with different number of 13C isotope). Deuterium is used to avoid the interference from the 13C isotope.

This work could serve as a reference for future studies of the spillover effect induced by buckyballs interacting with other metal nanoparticles. The increasing availability of in-depth fundamental insight could refine our understanding of ambient-temperature hydrogen storage.

To find out more please read:

Enhanced Hydrogen Spillover to Fullerene at Ambient Temperature

Hirotomo Nishihara, Tomoya Simura and Takashi Kyotani

Chem. Commun. 2018, DOI: 10.1039/c8cc00265g

About the blogger:

Tianyu Liu obtained his Ph.D. (2017) in Physical Chemistry from University of California, Santa Cruz in United States. He is passionate about scientific communication to introduce cutting-edge research to both the general public and scientists with diverse research expertise. He is a blog writer for Chem. Commun. and Chem. Sci. More information about him can be found at http://liutianyuresearch.weebly.com/.

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

05 Mar 2018

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

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Gold Rocks the Casbah

20 Feb 2018

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

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Anchoring Arynes on Graphene with Microwave but No Solvents

15 Feb 2018

Recently in ChemComm, an international team from Italy and Spain reported a non-conventional way to anchor arynes onto graphene surface using microwave. Their developed method is fast, efficient, mild and solvent-free.

Attaching functional groups onto graphene surface, i.e. functionalization, allows the physical and chemical properties of graphene to be fine-tuned, such as electrical conductivity and solubility. Conventional solvent-based functionalization strategies usually involve time-consuming reactions and tedious purification steps. The poor suspension stability of graphene in solvents, particularly in polar organic solvents, greatly hinders the overall functionalization efficiency. Therefore, establishing easy and solvent-free functionalization protocols for graphene is highly needed.

M. Prato, A. Criado and coworkers made a breakthrough in addressing this challenge by developing a microwave-assisted functionalization method. Their method to functionalize graphene consists of cycloaddition reactions between few-layer graphene (FLG) and arynes (Figure 1). These reactions proceed by mixing the dry powder of FLG and arylene anhydrides, the precursors of arynes, followed by rapid heating under microwave irradiation. The whole process is solvent-free and occurs within half a minute. It is also applicable to a variety of arynes (Figure 2).

Figure 1. The schematic illustration of the microwave-assisted functionalization of graphene with arynes. This process can be carried out within half a minute and is solvent-free.

Figure 2. A variety of arynes capable of being anchored on graphene surface. 1~6 represent the arylene anhydrides and f-G(7)~f-G(12) are corresponding arynes attached onto graphene.

The most unique feature of the demonstrated method is the dual role of FLG. In addition to being one of the reactants, FLG is capable of absorbing microwave energy, and enables its surface to rapidly reach high temperatures that significantly accelerate the cycloaddition reactions.

This microwave-assisted functionalization method shows great promise as a stepping stone for the fast and efficient modulation of graphene surface and subsequently, the performance of graphene-based electronics.

To find out more please read:

Microwave-Induced Covalent Functionalization of Few-Layer Graphene with Arynes Under Solvent-Free Conditions

V. Sulleiro, S. Quiroga, D. Peña, D. Pérez, E. Guitián, A. Criado and M. Prato

Chem. Commun. 2018, DOI: 10.1039/C7CC08676H

About the blogger:

Tianyu Liu obtained his Ph.D. (2017) in Physical Chemistry from University of California, Santa Cruz in United States. He is passionate about scientific communication to introduce cutting-edge research to both the general public and scientists with diverse research expertise. He is an online blog writer for Chem. Commun. and Chem. Sci. More information about him can be found at http://liutianyuresearch.weebly.com/.

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HOT ChemComm articles for January

05 Feb 2018

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

01 Feb 2018

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 CO₂pressure, 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

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Single-Crystalline NiFe-Hydroxide Nanosheets for Catalyzing Oxygen Evolution

29 Jan 2018

A group of scientists led by Prof. Shizhang Qiao has synthesized an oxygen evolution reaction (OER) catalyst combining the merit of low cost, excellent catalytic activity and long lifetime. This OER catalyst is composed of single-crystalline NiFe-hydroxide nanoflakes directly grown on nickel foams. The work has been published recently in ChemComm.

OER, the reaction of producing oxygen gas from water, is an indispensable component of electricity-generation devices using sustainable energy (e.g. fuel cells and photoelectrochemical water splitting cells). OER is usually the bottleneck limiting the overall energy conversion efficiency due to its sluggish kinetics and complex reaction pathways. As such, OER catalysts are needed to accelerate the OER reaction rate. Among the various OER catalysts, noble metal oxides stand out owing to their ultrahigh catalytic activity. However, the “shining” performance is dimmed by their high cost and short lifetime. Thus, obtaining alternatives with comparable OER catalytic activity as well as long-term stability is required to advance the utilisation of sustainable energy.

To address this challenge, the authors turned their attention to a low-cost transition metal, nickel. They developed a hydrothermal method using nickel foams to grow highly crystalline and near-vertically aligned NiFe-hydroxide nanosheets as OER catalysts (Figure 1a). The seamless integration between the hydroxide nanosheets and the nickel substrates reduces the contact resistance and facilitates interfacial electron transfer. The near-vertical orientation (Figure 1b) allows water molecules to fully contact the catalysts. Both of the characteristics render excellent OER catalytic activity. Additionally, the high crystallinity (Figure 1c) ensures the catalysts are robust enough to withstand extensive use without degradation in performance.

Figure 1. (a) The schematic illustration of the synthetic procedures of the NiFe-hydroxide [Fe-Ni(OH)2] nanosheets supported on nickel foams (NF). (b) The scanning electron microscopy image shows the near-vertically aligned nanosheets on a piece of nickel foam. (c) The transmission electron microscopy image reveals the crystallinity of the synthesized catalyst.

The NiFe-hydroxide nanosheets outperform most of the state-of-the-art OER catalysts, including those containing noble metal elements. Specifically, the nanosheets exhibit an onset potential of 1.497 V (Figure 2). The onset potential is a measure of the catalytic activity that equals the magnitude of potential required to yield a current density of 10 mA/cm² (when appreciable amount of oxygen gas is evolved). Outstandingly, the onset potential of the NiFe-hydroxide is the smallest among the catalysts selected for comparison.

Figure 2. The polarisation curves of different OER catalysts. The onset potential is marked by the dotted line in the inset.

The catalytic activity is also highly stable, with no loss in performance after at least 100 h of measurement. Interestingly, the onset potential further shifts to a lower value of 1.465 V after 100 h. The authors attributed this observation to a “self-activation” process that involves the formation and accumulation of nickel oxyhydroxide (NiOOH) on the surface of the nanosheets.

The hydrothermal method demonstrated here could be used to synthesize other cost-effective crystalline catalysts to develop catalysts for reactions beyond OER, such as hydrogen evolution and carbon dioxide reduction.

To find out more please read:

Free-Standing Single-Crystalline NiFe-Hydroxide Nanoflake Arrays: A Self-Activated and Robust Electrocatalyst for Oxygen Evolution

Jinlong Liu, Yao Zheng, Zhenyu Wang, Zhouguang Lu, Anthony Vasileff and Shi-Zhang Qiao

Chem. Commun. 2017, DOI: 10.1039/c7cc08843d

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