Archive for the ‘Hot Articles’ Category

Making Gyroid Polymer Films to Speed up Proton Conduction

Proton exchange membranes (PEMs) are essential to the functionality of fuel cells. They conduct protons in electrolytes and drive electricity generation by oxidizing fuels. Following the success of Nafion® –– a family of commercial proton-conductive fluoropolymers –– materials researchers around the globe are developing innovative PEMs with high proton conductivities and affordable prices.

A group of Japanese researchers has recently synthesized self-standing polymer films with a gyroid nanostructure. These films possess two unique characteristics that other PEMs rarely have: a high proton conductivity in the order of 10-1 S/cm and retention of the conductivity across a wide temperature range (20-120 °C). This finding has been published in Chem. Sci. (doi: 10.1039/C9SC00131J).

The authors used a tailor-made macromolecule, Diene-GZI (Figure 1a), as the building block. It had an amphipathic structure, with one end being a hydrophilic zwitterionic group and another end of a hydrophobic alkyl chain. When mixed with bis(trifluoromethanesulfonyl)imide and water, multiple Diene-GZI molecules could assemble together into a gyroid network –– an infinitely periodic minimal surface (Figure 1b). After the self-assembly, ultra-violet-irradiation-induced polymerization solidified the morphology of the gyroid nanostructure.

Figure 1. (a) The molecular structure of Diene-GZI. (b) Solidification of the self-assembled gyroid via polymerization.

The high proton conductivity of the polymer film originated from its three-dimensional gyroid structure. Since the gyroid surface was densely coated with the hydrophilic zwitterionic chains, the film could readily uptake as high as 15.6 wt.% of water at a relative humidity of 90%. The adsorbed water layers formed a three-dimensional continuous pathway along the gyroid surface, serving as proton-conduction expressways and resulting in a high conductivity in the order of 10-1 S/cm. Due to the strong binding force between water and the zwitterionic groups, heating the polymer film to 120 °C did not decrease the water content significantly, and thus, the proton conductivity remained high. Additionally, the control films with no gyroid structures were unable to compete with the gyroid film in terms of proton conductivities within the measured temperature range (Figure 2).

Figure 2. The dependence between proton conductivities and temperature. Legends: red solid circles – gyroid film; others – control samples without the gyroid nanostructure.

This work highlights the critical role of rational design of raw materials to augment the proton conductivities of PEMs. The advantage of the gyroid phase in speeding up ion diffusion could also inspire innovative materials in applications demanding ultrafast ion transport, e.g., supercapacitor electrodes.

 

To find out more, please read:

Gyroid Structured Aqua-Sheets with Sub-Nanometer Thickness Enabling 3D Fast Proton Relay Conduction

Tsubasa Kobayashi, Ya-xin Li, Ayaka Ono, Xiang-bing Zeng, and Takahiro Ichikawa

Chem. Sci., 2019, 10, 6245-6253

About the blogger:

Tianyu Liu obtained his Ph.D. (2017) in Chemistry from University of California, Santa Cruz in the 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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Tryptophan, Featuring BN/CC Isosterism

Ever wanted to find a way to replace a carbon-carbon (CC) double bond with another bond that will change the physical and chemical properties of a molecule without significantly altering its sterics? Look no further than a boron-nitrogen bond! BN/CC isosterism involves substituting a CC double bond with a BN bond, which can substantially change the electronic properties of a molecule while keeping it the same size. This isosterism could be a powerful tool in biomedical studies of biologically relevant arene-containing organic molecules, which are plentiful. However, few studies report on the differences in functions cause by substituting a BN bond into an arene. Initial results suggest that the BN compounds can have similar or increased activity and availability when compared to the natural, all carbon molecules.

Figure 1. Image of naturally occuring tryptophan and the BN-tryptophan analogue.

Researchers in the United States synthesized a BN-analogue of tryptophan (Figure 1) for use as an unnatural amino acid (UAA) to study and intentionally alter the properties of proteins. Tryptophan, in addition to making Americans sleepy at Thanksgiving, is relatively rare, but participates in pi system interactions and is the primary source of native protein fluorescence. This makes it an important target for UAA research. The researchers synthesized the sodium salt of BN-tryptophan in a 6-step process, which can be modified to resolve the two enantiomers by chiral HPLC. The BN-tryptophan exhibits noticeably red-shifted absorbance and emission spectra, with the fluorescence maximum shifted by almost 40 nm in the BN compound.

In order to test whether the BN-tryptophan could be incorporated into proteins, researchers incorporated it into media without tryptophan and monitored whether E. coli cells that lacked the ability to produce tryptophan would grow. They found that the cells grew when in the presence of BN-tryptophan, but to a significantly lesser degree than with an equivalent quantity of natural tryptophan. However, cell growth increased when the media contained both BN-tryptophan and natural tryptophan. This suggests that cells will accept BN-tryptophan as a tryptophan analogue, but they don’t tolerate full replacement well.

Figure 2. Representation of the protein sequence, structures of other tryptophan analogues, and fluorescence plot for the studied substrates.

Further studies incorporated BN-tryptophan and three other previously utilized tryptophan analogues into a green fluorescent protein (GFP). For fluorescence to be detected, the analogue must be incorporated into the protein and then accurately read by cells. The BN-tryptophan performs as well or better than the established tryptophan analogues, proving its functionality (Figure 2). The proteins with BN-tryptophan also demonstrate several different properties than those containing natural tryptophan; their fluorescence is red shifted and they are more susceptible to oxidation by hydrogen peroxide. These alterations in activity could prove useful in future studies.

To find out more please read:

Synthesis and characterization of an unnatural boron and nitrogen-containing tryptophan analogue and its incorporation into proteins

Katherine Boknevitz, James S. Italia, Bo Li, Abhishek Chatterjee and Shih-Yuan Liu

Chem. Sci., 2019, 10, 4994–4998.

About the blogger:

Beth Mundy is a PhD candidate in chemistry in the Cossairt lab at the University of Washington in Seattle, Washington. Her research focuses on developing new and better ways to synthesize nanomaterials for energy applications. She is often spotted knitting in seminars or with her nose in a good book. You can find her on Twitter at @BethMundySci.

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HOT Chemical Science articles for April

We are happy to present a selection of our HOT articles for April. To see all of our HOT referee-recommended articles from 2019, please find the collection here.

As always, Chemical Science articles are free to access.

The full dynamics of energy relaxation in large organic molecules: from photo-excitation to solvent heating

Vytautas Balevičius Jr, Tiejun Wei, Devis Di Tommaso, Darius Abramavicius, Jürgen Hauer, Tomas Polívka and Christopher D. P. Duffy*

Chem. Sci., 2019, 10, 4792-4804

DOI
: 10.1039/C9SC00410F, Edge Article

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Chiral diversification through the assembly of achiral phenylacetylene macrocycles with a two-fold bridge

Ryo Katoono,* Keiichi Kusaka, Yuki Saito, Kazuki Sakamoto and Takanori Suzuki

Chem. Sci., 2019, 10, 4782-4791

DOI
: 10.1039/C9SC00972H, Edge Article

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NosL is a dedicated copper chaperone for assembly of the CuZ center of nitrous oxide reductase

Sophie P. Bennett, Manuel J. Soriano-Laguna, Justin M. Bradley, Dimitri A. Svistunenko, David J. Richardson, Andrew J. Gates* and Nick E. Le Brun

Chem. Sci., 2019, 10, 4985-4993

DOI
: 10.1039/C9SC01053J, Edge Article

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Brønsted acid catalysis – the effect of 3,3′-substituents on the structural space and the stabilization of imine/phosphoric acid complexes

Maxime Melikian, Johannes Gramüller, Johnny Hioe, Julian Greindl and Ruth M. Gschwind*

Chem. Sci., 2019, 10, 5226-5234

DOI
: 10.1039/C9SC01044K, Edge Article

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Theoretical design of a technetium-like alloy and its catalytic properties

Wei Xie* and Michihisa Koyama*

Chem. Sci., 2019, 10, 5461-5469

DOI
: 10.1039/C9SC00912D, Edge Article

 

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Highly regioselective complexation of tungsten with Eu@C82/Eu@C84: interplay between endohedral and exohedral metallic units induced by electron transfer

Lipiao Bao, Pengyuan Yu, Ying Li, Changwang Pan, Wangqiang Shen, Peng Jin,* Shuquan Liang* and Xing Lu*

Chem. Sci., 2019, 10, 4945-4950

DOI
: 10.1039/C9SC01479A, Edge Article

 

 

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One-Pot MOFs

Metal-organic frameworks, commonly known as MOFs, are one to three-dimensional structures composed of metal ions coordinated to organic linkers. They’ve drawn substantial research interest given their highly porous nature, the extreme tunability of their properties, and, in the early days, relative ease of synthesis. As the field has matured, the syntheses of the organic linkers have increased in complexity. New linkers require substantial expertise in synthetic organic chemistry and can be time and cost intensive to produce. One strategy to avoid the linker-induced bottleneck in MOF development is to create one-pot procedures, generating both the linker and the MOF in a single vessel. While the idea is straightforward, in practice it involves carefully balancing conditions to crystalize the MOF without producing unwanted side reactions.

Figure 1. Reaction motifs utilized for in-situ ligand generation. a) nitro-compound reduction, b) diazo coupling of nitro compounds, c) condensation of boronic acids, and d) imidization of an anhydride and an amine.

Researchers in China recently examined several classes of organic reactions to test the viability of in-situ ligand and MOF synthesis. The basic procedure involves complex ligand generation, formation of small metal clusters, and finally crystallization of the final MOF structure. They chose reduction and diazo coupling of nitro compounds, condensation of boronic acids, and imidization between anhydrides and amines (Figure 1). A rigid, nitro-containing dicarboxylic acid proved the most robust for the reduction studies.  When combined with a hydrated metal salt (copper, zinc, and indium nitrates and manganese chloride), exposed to a protic solvent, and heated, a MOF formed in a single vessel without the addition of a purposeful reductant. This specific ligand didn’t have the proper geometry to produce MOFs via diazo couplings, but a similar motif was used to create a new ligand with greater distance separating the carboxylic acid groups. The researchers dissolved the ligand and various metal salts in DMF, then added proton source, and heated the mixture. The reactions with zirconium, zinc, cadmium, and indium all produced MOFs. The reaction conditions varied from metal to metal, producing different forms of the ligand in-situ that resulted in MOFs of a range of morphologies (Figure 2).

Figure 2. Structures of zirconium, zinc, cadmium, and indium-based MOFs synthesized by ligands generated via diazo coupling.

While these MOFs formed via strong, irreversible reactions, the covalent organic framework literature utilizes the plethora of reversible reactions to expand the scope of possible MOFs. This inspired the researchers to use boronic acid derivatives as a proof of concept. When a ligand with both boronic and carboxylic acid motifs reacted with zirconium or hafnium and formic acid, the researchers isolated a MOF with tetrahedral cages. This approach also proved successful when combining two ligands in a Schiff base synthesis to form a zirconium-based MOF. The reversible reactions required meticulous tuning of the acid source to effectively crystalize and assemble the MOFs. However, the ease of reaction set up allows more rapid screening of conditions than full-scale ligand synthesis.

This relatively simple and efficient strategy for producing new MOFs likely has broader applications than the few reactions currently explored. This will hopefully increase the speed of new MOF discovery with increasingly complex ligands.

To find out more please read:

Constructing new metal-organic frameworks with complicated ligands from “One-Pot” in situ reactions

Xiang-Jing Kong, Tao He, Yong-Zheng Zhang, Xue-Quian Wu, Si-Nan Wang, Ming-Ming Xu, Guang-Rui Si, and Jian-Rong Li

Chem. Sci., 2019, 10, 3949 – 3955.

About the blogger:

Beth Mundy is a PhD candidate in chemistry in the Cossairt lab at the University of Washington in Seattle, Washington. Her research focuses on developing new and better ways to synthesize nanomaterials for energy applications. She is often spotted knitting in seminars or with her nose in a good book. You can find her on Twitter at @BethMundySci.

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A Single Nickel Site Reduces Nitrate Ions to Nitrogen Gas

Surface runoff of agricultural or landscape areas with excessive nitrate fertilizers has resulted in increasing nitrate ion concentrations in freshwater. This issue leads to eutrophication and algae blooms that significantly threaten aquatic lives and ecosystems. Eliminating nitrate ions is thus necessary to address the nitrate-borne water pollution. Nature presents a delicate yet complicated process for turning NO3 into N2 by four metalloenzymes. Is it possible to develop an artificial method with just one catalyst to drive the same process?

Lee, Baik, and coworkers at the Institute for Basic Science (IBS) and Korea Advanced Institute of Science and Technology (KAIST) answered yes. The researchers synthesized a square-planar nickel(II)-based complex, (PNP)Ni(ONO2) (PNP = N[2-P’Pr-4-methyl-C6H3]2), which converted NO3 to N2 in the presence of CO and NO. Ni2+ is the only active site. This breakthrough has been published in Chemical Science (DOI: 10.1039/C9SC00717B).

The reaction mechanism involves four successive redox reactions among (PNP)Ni(ONO2), CO, and NO (Figure 1). The first two steps consecutively transfer two oxygen atoms from (PNP)Ni(ONO2) to CO at room temperature, yielding two molecules of CO2 and a Ni-nitrosyl complex, (PNP)Ni(NO). Afterwards, the (PNP)Ni(NO) undergoes a disproportionation reaction with NO, generating (PNP)Ni(NO2) and N2O. The as-formed N2O interacts with the remaining (PNP)Ni(NO) and is eventually reduced to N2. The yield of N2 is 46% (based on the amount of N2O).

Figure 1. The schematic shows the artificial nitrate reduction with a Ni(II)-based complex, (PNP)Ni(ONO2), as the catalyst. Pr: propyl group.

Besides nitrate reduction, the (PNP)Ni(ONO2) could act as a potential alternative to the platinum-containing catalysts used in the catalytic converters of gasoline cars, because it transforms hazardous CO and NO into less toxic CO2 and N2.

To find out more please read:

One Metal is Enough: A Nickel Complex Reduces Nitrate Anions to Nitrogen Gas

Jinseong Gwak, Seihwan Ahn, Mu-Hyun Baik and Yunho Lee

Chem. Sci., 2019, 10, 4767-4774

About the blogger:

Tianyu Liu obtained his Ph.D. (2017) in Chemistry from University of California, Santa Cruz in the 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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Molecular Switching: from Blue Biradicals to Colorless Housanes

As chemists, we typically first encounter molecular switches, the general term for any molecule that exists in at least two stable or meta-stable states, as pH indicators in introductory chemistry classes. The external factor that causes the conversion can vary from a redox event to UV light. This makes molecular switches attractive sensors for a range of chemically relevant applications. The reversible and controllable bond-making and bond-breaking also provides a system within which to interrogate the nature of chemical bonding.

Figure 1. The biradical (left) and housane (right) in solution along with their simplified chemical structures.

Chemists at the University of Rostock in Germany explored light-driven molecular switching behavior in a class of heterocyclic molecules. These can exist as either biradicals or housanes (see Figure 1 for why the name makes sense) with a bond between the two phosphorus atoms. Understanding the mechanism by which the photo-isomerization occurs is important for future applications of the biradical system in small molecule activation. The in-depth studies used the 2,6-dimethylphenyl (2Dmp) derivative, since it was the most stable of the synthesized derivatives. The researchers monitored the conversion between species by examining their dramatically different 31P NMR spectra, with the housane resonances between -50 and -200 ppm and the biradical resonances between 200 and 300 ppm. An additional distinguishing feature between is color: the starting biradical is blue while the housane is colorless. The researchers found that biradical converts to the housane, which has a half-life of about 7 minutes at room temperature in solution, with an almost 25% quantum yield.

Figure 2. A single crystal of the biradical. Upon irradiation, the transition to the colorless housane can be observed, along with extensive cracking of the crystal.

This switching occurs not only in solution, but also in the solid state (Figure 2). The cracking evident in the images of a single crystal is attributed to the stress caused by changes to the crystal lattice by conversion of the biradical to the housane. Despite various efforts by the team, including crystallization attempts under constant irradiation, they were unable to obtain high enough quality crystals to acquire a crystal structure of the housane. The specific isomerization mechanism was computationally modeled and showed that the photoexcitation of the biradical led to a bonding interaction and distortion, allowing for housane formation. The reverse, thermally activated process, occurs due to the intersection of the ground and excited state energies near a transition state.

Given this enhanced understanding of the isomerization mechanism, the researchers manipulated the reactivity of the biradical by adding tert-butyl isocyanide (tBuNC). tBuNC catalyzed the thermal conversion of the housane back to the diradical. While the mechanism of the catalysis is unknown, it’s an exciting proof-of-concept for easily tuning molecular switching behavior.

To find out more please read:

A chemical reaction controlled by light-activatedmolecular switches based on hetero-cyclopentanediyls

Jonas Bresien, Thomas Kröger-Badge, Stefan Lochbrunner, Dirk Michalik, Henrik Müller, Axel Schultz, and Edgar Zander

Chem. Sci., 2019, 10, 3486-3493

About the blogger:

 

Beth Mundy is a PhD candidate in chemistry in the Cossairt lab at the University of Washington in Seattle, Washington. Her research focuses on developing new and better ways to synthesize nanomaterials for energy applications. She is often spotted knitting in seminars or with her nose in a good book. You can find her on Twitter at @BethMundySci.

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HOT Chemical Science articles for March

We are happy to present a selection of our HOT articles for March. To see all of our HOT referee-recommended articles from 2019, please find the collection here.

As always, Chemical Science articles are free to access.

The antioxidant activity of polysulfides: it’s radical!

Jean-Philippe R. Chauvin, Markus Griesser and Derek A. Pratt*

Chem. Sci., 2019, 10, 4999-5010

DOI
: 10.1039/C9SC00276F, Edge Article

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Anisotropic strain release in a thermosalient crystal: correlation between the microscopic orientation of molecular rearrangements and the macroscopic mechanical motion

Tomohiro Seki,* Takaki Mashimo and Hajime Ito*

Chem. Sci., 2019, 10, 4185-4191

DOI
: 10.1039/C8SC05563G, Edge Article

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Alkali metal complexes of an enantiopure iminophosphonamide ligand with bright delayed fluorescence

Thomas J. Feuerstein, Bhupendra Goswami, Pascal Rauthe, Ralf Köppe, Sergei Lebedkin, Manfred M. Kappes and Peter W. Roesky*

Chem. Sci., 2019, 10, 4742-4749

DOI
: 10.1039/C9SC00629J, Edge Article

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Direct observation of prion protein oligomer formation reveals an aggregation mechanism with multiple conformationally distinct species

Jason C. Sang, Ji-Eun Lee, Alexander J. Dear, Suman De, Georg Meisl, Alana M. Thackray, Raymond Bujdoso, Tuomas P. J. Knowles and David Klenerman*

Chem. Sci., 2019, 10, 4588-4597

DOI
: 10.1039/C8SC05627G, Edge Article

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Using coligands to gain mechanistic insight into iridium complexes hyperpolarized with para-hydrogen

Ben. J. Tickner, Richard O. John, Soumya S. Roy, Sam J. Hart, Adrian C. Whitwood and Simon B. Duckett*

Chem. Sci., 2019, 10, Advance Article

DOI
: 10.1039/C9SC00444K, Edge Article

 

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Direct conversion of phenols into primary anilines with hydrazine catalyzed by palladium

Zihang Qiu, Leiyang Lv, Jianbin Li, Chen-Chen Li and Chao-Jun Li*

Chem. Sci., 2019, 10, 4775-4781

DOI
: 10.1039/C9SC00595A, Edge Article

 

 

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HOT Chemical Science articles for February

We are happy to present a selection of our HOT articles for February. To see all of our HOT referee-recommended articles from 2019, please find the collection here.

As always, Chemical Science articles are free to access.

Cooperativity basis for small-molecule stabilization of protein-protein interactions

Pim J. de Vink, Sebastian A. Andrei, Yusuke Higuchi, Christian Ottmann, Lech-Gustav Milroy and Luc Brunsveld*

Chem. Sci., 2019, 10, 2869-2874

DOI
: 10.1039/C8SC05242E, Edge Article

 

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Acyclic 1,2-Dimagnesioethanes/-ethene Derived from Magnesium(I) Compounds: Multipurpose Reagents for Organometallic Synthesis

Deepak Dange, Andrew R. Gair, Dafydd D. L. Jones, Martin Juckel, Simon Aldridge and Cameron Jones*

Chem. Sci., 2019, 10, 3208-3216

DOI
: 10.1039/C9SC00200F, Edge Article

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Catalytic Radical Difluoromethoxylation of Arenes and Heteroarenes

Johnny W. Lee, Weijia Zheng, Cristian A. Morales-Rivera, Peng Liu* and Ming-Yu Ngai*

Chem. Sci., 2019, 10, 3217-3222

DOI
: 10.1039/C8SC05390A, Edge Article

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Revising measurement process in the variational quantum eigensolver: Is it possible to reduce the number of separately measured operators?

Artur F. Izmaylov,* Tzu-Ching Yen and Ilya G. Ryabinkin

Chem. Sci., 2019, 10, 3746-3755

DOI
: 10.1039/C8SC05592K, Edge Article

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A dinuclear ruthenium(II) phototherapeutic that targets duplex and quadruplex DNA

Stuart A. Archer, Ahtasham Raza, Fabian Dröge, Craig Robertson, Alexander J. Auty, Dimitri Chekulaev, Julia A. Weinstein, Theo Keane, Anthony J. H. M. Meijer, John W. Haycock,* Sheila MacNeil* and James A. Thomas*

Chem. Sci., 2019, 10, 3502-3513

DOI
: 10.1039/C8SC05084H, Edge Article

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Facile synthesis of AIEgens with wide color tunability for cellular imaging and therapy

Wenhan Xu, Michelle M. S. Lee, Zhihan Zhang, Herman H. Y. Sung, Ian D. Williams, Ryan T. K. Kwok, Jacky W. Y. Lam, Dong Wang* and Ben Zhong Tang*

Chem. Sci., 2019, 10, 3494-3501

DOI
: 10.1039/C8SC05805A, Edge Article

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HOT Chemical Science articles for January

We are happy to present a selection of our HOT articles for January. To see all of our HOT referee-recommended articles from 2019, please find the collection here.

As always, Chemical Science articles are free to access.

Enantioselective [1,3] O-to-C rearrangement: dearomatization of alkyl 2-allyloxy/benzyloxy-1/3-naphthoates catalyzed by a chiral π–Cu(II) complex

Lu Yao, Kazuaki Ishihara*

Chem. Sci., 2019, 10, 2259-2263

DOI
: 10.1039/C8SC05601C, Edge Article

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Through-space charge transfer hexaarylbenzene dendrimers with thermally activated delayed fluorescence and aggregation-induced emission for efficient solution-processed OLEDs

Xingdong Wang, Shumeng Wang, Jianhong Lv, Shiyang Shao,* Lixiang Wang,* Xiabin Jing and Fosong Wang

Chem. Sci., 2019, Advance Article

DOI
: 10.1039/C8SC04991B, Edge Article

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Heterolytic bond activation at gold: evidence for gold(III) H–B, H–Si complexes, H–H and H–C cleavage

Luca Rocchigiani,* Peter H. M. Budzelaar* and Manfred Bochmann*

Chem. Sci., 2019, 10, 2633-2642

DOI
: 10.1039/C8SC05229H, Edge Article

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Insights into mechanochemical reactions at the molecular level: simulated indentations of aspirin and meloxicam crystals

Michael Ferguson, M. Silvina Moyano, Gareth A. Tribello, Deborah E. Crawford, Eduardo M. Bringa, Stuart L. James,* Jorge Kohanoff* and Mario G. Del Pópolo*

Chem. Sci., 2019, Advance Article

DOI
: 10.1039/C8SC04971H, Edge Article

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Structure revision of cryptosporioptides and determination of the genetic basis for dimeric xanthone biosynthesis in fungi

Claudio Greco, Kate de Mattos-Shipley, Andrew M. Bailey, Nicholas P. Mulholland, Jason L. Vincent, Christine L. Willis, Russell J. Cox* and Thomas J. Simpson*

Chem. Sci., 2019, Advance Article

DOI
: 10.1039/C8SC05126G, Edge Article

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From nano-balls to nano-bowls

Helena Brake, Eugenia Peresypkina, Claudia Heindl, Alexander V. Virovets, Werner Kremer and Manfred Scheer*

Chem. Sci., 2019, Advance Article

DOI
: 10.1039/C8SC05471A, Edge Article

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Learning from Nature: A Cu(II)-Porphyrin Complex Produces Oxygen Gas from Water at Ultra-Small Overpotential

Sunlight-assisted water splitting represents a sustainable way to convert solar energy into chemical energy in hydrogen and oxygen gases. Due to its high activation energy, the oxygen evolution reaction (OER) requires large overpotential for initiation. Developing suitable OER catalysts to reduce the overpotential thus becomes instrumental for the feasibility of solar energy harvesting.

Recently, a group of scientists led by Rui Cao from Renmin University of China, and Shaanxi Normal University, China, has developed a water-soluble Cu(II)-porphyrin complex as a high-performance OER catalyst. This breakthrough has been published in Chemical Science (DOI: 10.1039/C8SC04529A).

Inspired by the molecular structure of a natural OER catalyst in the photosynthesis system – photosystem II (PSII), the researchers designed a Cu2+-coordination compound with a porphyrin ligand, tetrakis(4-N-methylpyridyl)porphyrin (Figure 1a), which mimics the structure of PSII. This biomimetic Cu2+-complex exhibits outstanding catalytic OER activity in a phosphate buffer solution at pH=7.0. The current of the cyclic voltammogram of the Cu2+-complex increases sharply (due to O2 evolution) at an onset potential of 1.13 V vs. normal hydrogen electrode (Figure 1b), corresponding to an OER overpotential of 310 mV. For comparison, the cyclic voltammograms of a blank buffer solution and a CuSO4-containing buffer solution show no pronounced current enhancement (Figure 1b), indicating the electrolyte itself and the un-coordinated Cu2+ cannot generate O2 within the tested potential range. The 310 mV overpotential is approximately two times smaller than the typical values exhibited by previously reported Cu complexes.

Figure 1. (a) The molecular structure of Cu2+-tetrakis(4-N-methylpyridyl)porphyrin complex. (b) Cyclic voltammograms of 1 mM Cu2+-tetrakis(4-N-methylpyridyl)porphyrin (red), bare buffer solution (black) and buffer solution containing 1 mM CuSO4 (green). The electrode is a piece of fluorine-doped tin oxide glass slide.

The authors ascribed the ultra-small OER overpotential to the formation of an oxidized form of the Cu2+-porphyrin complex. This oxidized species is generated after the complex loses one electron, and is active for O-O bond formation and subsequent O2 evolution. The energy barrier of this one-electron-oxidation pathway is expected to be much lower than those of conventional processes involving higher-valent Cu species (e.g., Cu4+-oxo), which facilitates OER at small overpotential.

With the complete catalytic cycle of water oxidation by the Cu2+-porphyrin complex being fully revealed, OER will become more efficient and energy-saving.

To find out more please read:

Low Overpotential Water Oxidation at Neutral pH Catalyzed by A Copper(II) Porphyrin

Yanju Liu, Yongzhen Han, Zongyao Zhang, Wei Zhang, Wenzhen Lai, Yong Wang and Rui Cao

Chem. Sci., 2019, DOI: 10.1039/C8SC04529A

About the blogger:

Tianyu Liu obtained his Ph.D. (2017) in Chemistry from University of California, Santa Cruz in the 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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