A recent email has caused quite a buzz in the astronomy community.  The email pertains to the expectations placed on graduate students.  We are reminded that similar letters have been written in the chemistry community – see this post on Chemistry Blog.  Our friends over at Astrobites are conducting a survey to look at what faculty, graduate students, undergraduates, and those not in science think about academic culture.  The survey takes at most 5 minutes and it will provide us with very interesting and important information.  You can take a look at the Astrobites post here or go straight to the survey here.

Title:  Stibonium Ions for the Fluorescence Turn-On Sensing of F^– in Drinking Water at Parts per Million Concentrations

DOI: 10.1021/ja308194w

Author:  Iou-Sheng Ke ^a, Mykhaylo Myahkostupov ^b, Felix N. Castellano *^b, and François P. Gabbaï *^a

Journal:  Journal of American Chemical Society

Affiliation:  a) Department of Chemistry, Texas A&M University, College Station, Texas, USA; b) Department of Chemistry and Center for Photochemical Sciences, Bowling Green State University, Bowling Green, Ohio, USA.

Take-home Importance According to the Authors: The 9-anthryltriphenylstibonium cation, [1]⁺, has been synthesized and used as a sensor for the toxic fluoride anion in water. This stibonium cation complexes fluoride ions to afford the corresponding fluorostiborane 1-F. This reaction, which occurs at fluoride concentrations in the parts per million range, is accompanied by a drastic fluorescence turn-on response. It is also highly selective and can be used in plain tap water or bottled water to test fluoridation levels.

Take-home Importance According to the Blogger:  Stibonium cation is not the cation of a new element; we call the element antimony  This simple complex fluoresces in the presence of fluoride and is very sensitive. Take some tweaking, but it works very well.

Tidbit from the Blogger: First, I would like to utter a word of remembrance for Neil Armstrong, who has inspired and will continue to inspire generations to bravely go forth and explore the unknown realms. 

On an unrelated note, there are rather alarming reports on journal article retraction and academic integrity in Nature (10.1038/489346a) and PNAS (10.1073/pnas.1212247109). Retractions due to honest mistakes only constitute about 20% of the total number investigated, which resonated with me in particular as I just completed the Responsible and Ethical Conduct of Research (RCR), a part of my NSF fellowship requirement. In all honesty, it was one of the most informative web courses I have taken and gave me great pleasure in discovering my previous misconceptions. Strongly recommended.

At last, I recently found an old editorial on science blogs (10.1021/ac102628p) and in summary, caveat emptor. I agree wholeheartedly that all words here should be read with a grain of salt as said in disclaimers and such, since I am not yet an expert. By the way, all the editorial posts in Analytical Chemistry by Professor Royce Murray are very good. Interesting and informative read, again strongly recommended. Well now, back to chemistry.

Summary: Fluoride is an interesting ion that it is regularly added to drinking water and preventing tooth decay, but at the same time, it could be rather toxic at higher concentrations. The article itself is rather straightforward. Fluoride ions (Lewis basic) can interact strongly with trialkyl boron molecules or tetraalkyl pnictogen molecules (Lewis acidic) in some solvents while weakly in others. The weak interaction could be due to stronger ligand strength from the solvent, as predicted by the spectrochemical series. In this case, water could displace ligated flouride ions, forming a switch mechanism.

Comparing the acidity of the tetraphenyl pnictogen cations, the antimony analog is greater than the phosphorus or the arsenic analogs, due to both steric and electronic reasons. The increase in ion size allowed greater electron density polarizability and greater orbital overlapping during electron donor-acceptor interactions, as in electropositivity. In the anthryl-substituted structure, binding of the fluoride ions caused a blue shift in the UV absorption spectrum from the triflate complex. At the same time, fluorescence quantum yield was increase more than ten-fold due to decrease of nonradiative decay and removal of intramolecular ligand-metal charge transfer that might otherwise quench the fluorescence.

Well, sounds great, but the complex binds with water strongly enough to precipitate. How can this fluoride sensor function in water? The solution is simple. Just like how soap removing grease by creating bipolar micelles, a similar approach was utilized with cetyltrimethylammonium bromide as an additive. This way, the stibonium complex was kept away from water binding, but still remain soluble in aqueous environments. Remarkably, the sensor was only selective for fluoride ions and did not respond to any of the following:  Cl^-, Br^-, I^-, NO₃^-, N₃^-, HCO₃^-, or SO₄^2-, avoiding a major problem of many sensors–false positive response by species with similar chemical properties. Impressively, at a concentration of at least 1 ppm, the color change is visible in about one minute.

All based on a simple organometallic complex.

Title:  Ambient-Temperature Isolation of a Compound with a Boron-Boron Triple Bond

DOI:  10.1126/science.1221138

Author:  Holger Braunschweig, Rian D. Dewhurst, Kai Hammond, Jan Mies, Krzysztof Radacki, Alfredo Vargas

Journal:  Science

Affiliation:  Institut für Anorganische Chemie, Julius-Maximilians-Universität Würzburg, Am Hubland, 97074 Würzburg, Germany

Take-home Importance According to the Authors:  Homoatomic triple bonds between main-group elements have been restricted to alkynes, dinitrogen, and a handful of reactive compounds featuring trans-bent heavier elements of groups 13 and 14. Previous attempts to prepare a compound with a boron-boron triple bond that is stable at ambient temperature have been unsuccessful, despite numerous computational studies predicting their viability. We found that reduction of a bis(N-heterocyclic carbene)-stabilized tetrabromodiborane with either two or four equivalents of sodium naphthalenide, a one-electron reducing agent, yields isolable diborene and diboryne compounds. Crystallographic and spectroscopic characterization confirm that the latter is a halide-free linear system containing a boron-boron triple bond.

Take-home Importance According to the Blogger:  I am again very impressed by the illustration of a very simple synthetic principle–“You can’t make it yet? Give it more energy.” Although the structure and the approach are both very straightforward, synthesis and subsequent characterization can take quite a bit of work. This work is at the very forefront of what we call fundamental research.

Tidbit from the Blogger:  First, there is a pretty interesting (or somewhat controversial) story about a rather significant application of research in real life in a previous issue of science.  I will let you decide if the rest is worth your time or not. Personally, it was rather inspiring. Second, I am very impressed by the recent development in the Chinese manned space program. Sometimes I wonder if science can be a force uniting people around the world for a common goal, but so far I only see furious fighters trying to stay ahead of competitions.

For some chemistry related thoughts, if you have not had your recent dose of JACS, things are going quite…funny? First, theorists are claiming the existence of Zn’s formal +3 oxidation states (DOI: 10.1021/ja3052409). I would respond by saying, “Great, I will believe you when you make it.” Second, experimental data is suggesting that fluoride ion can be a better reducing agent than iodide ion in non-aqueous solvent by forming fluorine radicals (DOI: 10.1021/ja303173n). Similarly, if the fluorine radical cannot be isolated (which the authors are claiming reaction with the glassware), I will remain skeptical about their conclusions. Although I would like to remind all chemists that from your freshmen chemistry book, a Lewis base is a molecule that can donate LONE PAIRS of electrons. Electrochemical reduction potential can be correlated to Lewis basicty when considering a donation of LONE PAIRS. Good reducing agent but a poor base? Authors here are using one.

Now, results I can actually confidently comprehend.

Summary:

If I have not mentioned theorists calculating carbon-carbon quadruple bond (10.1038/nchem.1263) earlier this year, I will mention again. Sometimes for a synthetic chemist, it is gut-instinct to ask, “Can I make it?” The case of boryne, a triple bond between two boron atoms, holds true.

If you look at the Lewis dot structure of boron, three unpaired electrons, and of nitrogen, three unpaired electrons. Boron and nitrogen can sit together with a triple bond and form a nice octet for the nitrogen side, but the boron side will not be too happy as it is clearly electron poor. Well, it is a stable nitrogen molecule analog, but you are making it super unhappy by ripping a lone pair from one side. A likely results would be reacting with surrounding molecules and losing the high electron density in the triple bond. In the compound boron nitride, instead of a linear geometry, electron density was delocalized in a graphite-analog of fused six-membered rings, which helps the compound to de-stress by going to sp2 hybridization. When borons form three single bonds, BH₃ or BF₃ for example, the species is a good Lewis acid, where it can accept a lone pair of electrons to become more stable. Borons with four bonds can be fairly stable, such as BF₄^- non-coordinating counter ions that usually like to be alone in aqueous solution. (However, if you start to boil an aqueous solution containing BF₄^-, take heed that the fluoride ion will dissociate and you get HF by proton extraction from water. Then you might lose your glassware, if not something else, not a fluorine radical hopefully. You have been warned.)

Reaction Scheme for forming a dibromodiborene and a diboryne from a tetrabromoborane

But, if we can supply a large electron density to the boron side without forming a bond, the boron-nitrogen could actually be quite happy. We may even make boron-boron triple bond by supplying both side with a lot of “nonbonding” electron density, and the corresponding product should be stable, at least we think so as chemists. Previous work has demonstrated both alkyl and NHC (N-heterocyclic carbene, a member of stable carbenes) substituents could successfully stabilize a boron-boron double bond (diborene), which implied that with the right modification, it was possible to stabilize a boron-boron triple bond (diboryne). In this case, starting from tetrabromodiborane, a boron-boron single bonded species, reduction in the presence of stabilizing NHC (large side arms of 2,6-diisopropylphenyl) allows removal of bromide from the borane center (as forming charge neutral Na-Br), consequently generating dibromodiborene after two equivalences of reduction and finally diboryne after four equivalences of reduction. The diboryne was then characterized and its existence verified via ¹¹B NMR, IR, and single-crystal structure. Reaction of the boryne with tetrabromodiborane resulted in full conversion to dibromodiborene via comportionation (opposite of disporportionation). The diboryne is also thermo-stable till 234 degrees Celsius.

The authors also claimed that this result would contribute to the knowledge of boron-based functional material, which people always do to justify their use of taxpayer money for research (and it is a good thing).

But in this case, no one has ever made a boron-boron triple bond and it is just so COOL, BEAUTIFUL, and AWESOME.

Would you really need a reason to do that chemistry?

Title:  Electrocatalytically Active Graphene–Porphyrin MOF Composite for Oxygen Reduction Reaction

DOI:  10.1021/ja211433h

Author:  Maryam Jahan, Qiaoliang Bao, and Kian Ping Loh

Journal:  Journal of the American Chemical Society

Affiliation:  Graphene Research Centre, Department of Chemistry, National University of Singapore, Singapore

Take-home Importance According to the Authors: By reacting the pyridine-functionalized graphene with iron–porphyrin, a graphene–metalloporphyrin MOF with enhanced catalytic activity for oxygen reduction reactions (ORR) is synthesized. The structure and electrochemical property of the hybrid MOF are investigated. The results show that the addition of pyridine-functionalized graphene changes the crystallization process of iron–porphyrin in the MOF, increases its porosity, and enhances the electrochemical charge transfer rate of iron–porphyrin, showing a facile 4-electron ORR and can be used as a promising Pt-free cathode in alkaline Direct Methanol Fuel Cell.

Take-home Importance According to the Blogger: Composite material that takes advantages of each component, metalloporyphrin’s high reactivity with metal-organic framework’s high porosity. As someone works in adjacent fields, this concept is fairly inevitable. I am glad someone finally made it work and I applaud their effort (and I will be using something else for my third year original research proposal).

Tidbit from the Blogger: Sorry for the delay as this article is intended a month ago for April, but there is the Ph.D. qualification oral exam, a matter which I have to attend to. Either way, I intend a double-issue with a following post, so hopefully your hunger for science (or just the commentary) can be satisfied.

If you have not heard already, there is a recent (for me at least) movement for open access of journals, which is probably prompted by the mathematics community against the publishing giant Elsevier (one example here by Time Growers). Although the lack of subscription to the Journal of Electroanalytical Chemistry is to my dismay, I got over it after checking the library fact sheet. As many research is funded by the public by taxpayers, I am all for open access of those work or even a small fee to fund the management so the general public can clearly see where their support went to. However, for the chemistry community in particular, I believe open peer-review of research articles may be a separate but more immediate goal. When you became a graduate student and started to check literature for references (at least in my field in particular), you began to realize that not all published procedures are reproducible if followed word-by-word (such as adding 1 L of solvent to a 10 mL reaction vessel). Many review the peer-reviewing process as an irritating duty and mistakes were probably made on both sides. But having a larger body of scientists reviewing a pre-published work and commenting on the tricks in utilizing a post-published work can be extremely beneficial. Maybe this idea can be adapted eventually. Anyways, as always, thanks for listening.

Summary:  There are several “hot” topics in material and inorganic chemistry nowadays: 1) catalytic small-molecule transformation on a metal center (complexes such as metalloporphyrins), 2) porous materials and their modifications as high-surface-area scaffolds (polymers such as metal-organic frameworks, or MOFs), 3) electron and redox process management bridged by highly conductive structures (conjugated carbon derivatives such as graphene). Combine everything together, an efficient electrocatalyst is born. Earlier works by Hupp and coworkers have established that metal-organic frameworks (or MOFs) with metalloporphyrins as building units can sustain energy-transfer interactions with other ligands in the same molecule. Equally as important, they have constructed a system where the active metal centers do not bind to the building units and are free to interact with small-molecule targets. Thus, it is expected for the community to explore more interesting structures that can enhance the catalytic process.

Graphene−Porphyrin MOF: (I) G-dye is synthesized from r-GO sheets via diazotization with 4-(4-aminostyryl) pyridine, (II) Porphyrin MOF is synthesized from TCPPs and Fe ions, (III) (G-dye-FeP)n MOF formed via reaction between Porphyrin MOF and G-dye

A very typical coordination scheme in MOFs is a di-metal-tetracarboxylate-paddlewheel unit where the axial position on the cluster nodes are coordinationally unsaturated if the metal species present are divalent. For example, Zn₂(COOR)₄ construction motif is typical, while higher oxidation states may involve metal-metal bonding or axial ligand binding to provide charge balance. In this case, by appending benzenecarboxylate groups (a common strategy in porphyrin-based MOF construction), individual porphyrins can be linked and fixed into a square grid via coordination chemistry. However, in this case, the so-called MOF is an aggregation of two-dimensional sheets of linked porphyrin, not truly a three-dimensional framework. However, as earlier works have shown, these two-dimensional sheets can be easily linked together by 4,4′-bipyridine neutral basic linkers that will coordinate to the unsaturated axial sites and self-assemble to form a three-dimensional structure. A pretty neat trick that is, and the authors here take advantage of that linkage motif by diazotizing the reduced-graphene oxide (rGO) sheets with 4-(4-aminostyryl) pyridine to provide the lock and key necessary for a three-dimensional structure.

All said and done, it is a quite interesting structure. The internal space of the rGO is now expanded by the introduction of porphyrin sheets (which I disagree with author’s claim that the graphene existed as a structural impurity, as additional powder x-ray diffraction peaks were observed. Observed x-ray diffraction peaks are definite indications of structural changes that may result from highly organized local structures, which then should not be considered as an impurity. Seriously, those formations are the heart of this study).

The expanded sheets should allow easier access to catalytically active prophyrins. Besides in-depth characterization with infrared spectroscopy (functional group mapping) and UV-vis absorption spectroscopy (electronic structure mapping, as well as doping concentration correlation), the more interesting properties–the electrochemical properites–are characterized in depth with cyclic voltammetry, with ferrocyanide control and then with oxygen reduction in 0.1 M KOH. There is indeed an increase in current density when greater amount of iron-porphyrin was doped in the system.

a) Surface area measurements via nitrogen absorption isotherm indicate that with increasing doping, greater surface area is obtained per gram basis; b) similarly, with increasing doping, the current density increases in cyclic voltammetry with 10 mM ferrocyanide standard.

Increase in iron-porphyrin doping shifts the reduction potential anodically and the composite generates more current density than graphene and graphene oxide.

Great! Interesting structures and interesting properties.

Well, maybe.

Although the concept is great, there are a lot of experimental details that may sound more amazing than what they actually are (IN MY OPINION). Let’s start with the Support Information.

1) As I have mentioned before, considering the paddlewheel structure formed, divalent metal ions will charge balance with the carboxylate and allowing binding with neutral pyridine. If the metal is not divalent, then the basic pyridine groups will be less likely to bind comparing to a charged ion like chloride. As I sort through the information that was swpet under the rug, the starting material is ferric chloride [Fe(III) Cl3], which means the axial positions on the paddlewheel are likely occupied by chloride ions. If not, there there must be some counter ions hanging out, somewhere.  Now if you check the synthetic scheme above, hmmm… of course porphyrin will be a metal-scavenger and take up excess iron in solution. However, if the paddlewheel positions are occupied, then the most Lewis acid sites would be directly on the porphyrin iron, which deactivate the catalytic activity of the porphyrins.

And then, your catalytic cycle became a little bit messy.

Also, the structure analysis by XPS alone was insufficient (considering the raw data was not very smooth to start, further analysis is stretchy to say at the least). Of course two iron environments are expected within the structure with difference in electron density, but that is about all XPS can show. There is no information on how iron coordinate in this composite material. To convince a chemist that the pyridine actually coordinates to iron, the shift in binding energy is not enough (considering strong pi interaction between the porphyrin sheets, which presumably is nonexistent in the composite). The oxidation state problem needs to be addressed if not with Mossbaur spectroscopy or EPR. Then, if you really want something more definite, write a proposal for EXAFS.

2) Potentiostat never reads current density directly, which means a scientist has to manually input the surface area of the electrode. See the absorption isomer plateau that correlates with surface area, then see the current density. As much as I trust the authors corrected for the increase in surface area due to expanded graphene surface from MOF intercalation, it seems a bit too close. I certainly hope the surface area used in the calculation is not just the GC electrode surface area, because greater electrode surface leads to greater current, just as expected.

And of course, how would the current density differ in the case of free porphyrin? In the control as well, care to explain the irreversible wave seen in the nitrogen control?

3) The Koutecky-Levich analysis on the mechanism is a nice touch. In fact, the section on the analysis is very nice, although it doesn’t hurt to spend a little more effort analysis the rather strange number of electron transferred or justifying the reaction is only oxygen reduction.

In the end, this is one of the first step towards MOF-graphene based composite material. Though it has flaws, at least someone somewhere is trying to make the idea work. So, just to throw some project aims out there:

1) Can a framework be constructed where structure binding groups do not interfere with active catalytic centers? (examples in Hupp’s porphyrin systems)

2) Can electron transfer interactions between the backbone graphene and the reaction centers be closer? (reducing electron transfer cost from graphene, or electrode material, to the metal center, when the overpotential is already amazingly low)

3) Can the transformation be catalytic? (since no turnover rate was reported at least for ORR)

4) Can the mechanism involve more than just electron transfer? (i.e., atom transfer chemistry by applying a potential to a pre-treated substrate; not quite catalytic, but could reduce the activation barrier)