Chemical Science Blog

As we transition to an energy future composed primarily of intermittent, renewable technologies, finding ways to store the excess generated charges will be critical. While the general public typically thinks of batteries, another area of massive interest is storing the electrons in a chemical bond. This can be accomplished by creating electrocatalysts that can couple an electric current with chemicals like water and oxygen to generate stable new bonds in molecules for later use as fuels. These renewably generated fuels would then be available whenever needed, like at night. However, finding efficient electrocatalysts composed of earth-abundant materials has proven challenging. Many researchers have turned to nature for inspiration by designing molecules that mimic the active sites of enzymes.

Researchers in the United States used this approach, focusing on creating high-valent iron-oxo species, which others previously identified as the key catalytic intermediates in multiple enzymatic reactions. These types of species have traditionally been synthesized by reacting a reduced iron complex with an oxygen transfer reagent, but the researchers developed a system to generate highly reactive species using electricity as the reaction driving force and water as the oxygen source. The studied catalyst is a commercially available iron(III)-aquo complex with a tetraamido macrocyclic ligand (TAML) as the ancillary ligand (Figure 1B).

Figure 1. A. Cyclic voltammagram of (TAML)Fe in acetonitrile. B. Structure of (TAML)Fe and cyclic voltammagram showing increased current upon the addition of ethylbenzene.

When analyzed by cyclic voltammetry, an electrochemical technique where you cycle the voltage between set points and measure the current output, the (TAML)Fe shows two redox events at around 650 and 1250 mV that the researchers attributed to generating the Fe^IV-OH and Fe^V(O) species (Figure 1). Addition of ethylbenzene, which should react with the Fe^V species, increased the current at voltages of 1250 mV and higher, indicating (TAML)Fe turnover. However, isolating the Fe^v(O) species proved challenging as it reacts rapidly with the (TAML)Fe to form an Fe^IV dimer. This also limits the efficiency of the overall system by decreasing the amount of the most reactive species in solution.

Figure 2. A, B. Products generated by oxidation of various substrates screened with (TAML)Fe for electrocatalysis with isolated yield and calculated conversion in parenthesis. C. Substrates that did not react with the (TAML)Fe complex.

Both the Fe^IV dimer and Fe^V(O) species proved capable of oxidizing C-H bonds in ethylbenzene, but the Fe^V(O) is much more reactive and increases the oxidation rate at high electrochemical potentials. The researchers tested the scope of (TAML)Fe reactivity using a series of compounds with benzylic C-H bonds (Figure 2). They found that the (TAML)Fe performed well with electron-rich and electron-neutral derivatives, with an electron-deficient nitro-substituted derivative showing lower reactivity. Several substrates with non-benzylic C-H bonds showed high selectivity for oxidation at the benzylic C-H bond. (TAML)Fe also showed high electrocatalytic activity for oxidizing alcohols and converted substrates as simple as cyclohexanol and as complex as a steroid to ketones in high yields (up to 97%).

This study of an earth-abundant, stable, and commercially available electrocatalyst acts as a baseline for further studies with other similar metal complexes. Despite the efficiency limits attributed to dimerization, the high stability and selectivity of the (TAML)Fe could lead to its use with a broader range of substrates with varied functional groups.

To find out more please read:

Electrochemical C–H oxygenation and alcohol dehydrogenation involving Fe-oxo species using water as the oxygen source

Amit Das, Jordan E. Nutting and Shannon S. Stahl

Chem. Sci., 2019, 10, 7542-7548

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.

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/.

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.