Wednesday, June 11, 2014

Interpnictogen Cations DOI: 10.1002/anie.201307658

Interpnictogen Cations: Exploring New Vistas in
Coordination Chemistry
Alasdair P. M. Robertson, Paul A. Gray, and Neil Burford*

Okay, pnictogens! Now, this is something I'll be able to understand actually. Still, I can remember a time, long long ago when I didn't really know what a pnictogen was. Yes, now to take us back to that time, when I got sent by my supervisor during my undergrad days, to the library, to look up some compound that I can't remember right now in chemical abstracts, and to figure out a way of making it. Now I may be dating myself here, but chemical abstracts still existed at the time. And after I did go to the library, I seriously thought about quitting chemistry. When you enter the rows with the abstracts there is an undescribable smell of old paper and boredom, and after half an hour of leafing through them your brain starts turning off and you begin to fall asleep, prevented from passing out by the rigid metal shelves and the stone floor of the library. Still, you find a chair and spend an hour sleeping. It was after that one hour of sleeping that I opened a random chemistry journal on another shelf, anything to get away from the sleep demon that is the chemical abstracts.

The article that I opened talked about pnictogens. It looked really crazy with phosphorous making some crazy structures when chlorophospines or alkylchlorophosphines were reacted together with something and water was present in small amounts. There was no way I could make any sense of the reactions, and now in retrospect, I'm guessing the authors couldn't either, but the products looked pretty cool. I couldn't really figure out what a pnictogen was, but I noticed that they talked about it with regards to nitrogens and phosphorous, so I guessed it was a group V element. It's not like we had Wikipedia back then. Later I asked somebody, and they said, 'yes, it is a group V element'. Then I probably forgot and learned that factoid again. Well that's it. Probably not that interesting of a story actually.

Now for something more interesting! A review about pnictogen-pnictogen bonds, where inevitably you end up with a cation when you go to the more heavy ones. The process of making them involves taking a halopnictogen RPnCl and reacting it with another pnictogen that has a free lone pair and can substitute the chloride. Because the chloride (or Br or I) is actually a pretty good Lewis Base itself, sometimes you need to activate it a bit with AlCl3, so you end up in the end with an AlCl4 counterion that leaves your resulting molecule stable. You can also do anion substitution in situ to get a PF6 in there and let the NaCl salt fall out of solution. That's it. Pretty simple, but it can get complicated as your products can dimerize to give you a cyclic product, a linear Pn-Pn-Pn-Pn compound (Figure 8) or something crazy like a sandwich in Figure 23.
The chemistry, after you look at a few mechanisms at the beginning, is easy to follow for the more simple examples, but complicated reactions that create a tetraphosphine cation that coordinates to As for example, are not so easy to predict.

I did get lost in the writing quite a bit, but I think it's because of my bad pnictogen experience back in undergrad. My eyes glaze over whenever I see that word, and all the proper name endings associated with various anions or cations of arsenic or antimony. But chemistry is a visual science and it has become much more visual with chemdraw. The authors do an excellent job with figures so that you can read almost the entire review by just looking at the pictures, which happen to be on the same page as the text about them, which is also very helpful. Of course since I was reading on a smartphone small-screen, this advantage proved to be moot. I did enjoy reading this review because I understood most of it right away and I think the figures played a very big part in that, but also when the product was predictable, the simple rules for making the products at the beginning helped me to figure out the mechanism after looking at a figure for a few seconds. If you're looking for a quick and fun review to read to make yourself smarter (but it probably won't actually, as I doubt more than 1% remains in my head a week after I slog through them), read this one.

Obviously a lot of it is basic science research. There is a big part of it where it's really unpredictable with bismuth (which has a much higher coordination number so you get 'surprised' a lot) and if you introduce water, oxygen, or even a bit too much AlCl3. It makes sense since as the authors say at the beginning the energy of a Pn-Pn bond is much less than that of many other bonds. And if one of them serves as a Lewis base for another, well water can serve as a pretty good Lewis base. Or oxygen...

All (okay, let's say 'most' since I don't know for sure) these compounds are very sensitive to moisture and air. Also halogen substituted amine starting materials are very hard to work with, plus a lot of the heavy pnictogens are toxic and especially their compounds, with a notable exception being bismuth for some reason. I did a few experiments with making air sensitive phosphines and inevitably, my lab would always have some few specialized pieces of equipment missing. You're supposed to have a Schlenk line, but also some specialty glassware to filter under nitrogen, distill under nitrogen, etc...  it's always a few more extra steps than common organic chemistry. But this is still some basic science that makes interesting stuff and needs to be done. Some of it can't be predicted so well. The review focuses on saying whether a bond is ionic or covalent by looking at bond lengths and comparing them to the sum of the covalent radii and making comments to the effect that the bond is short because such and such substitution makes one of the pnictogens a better Lewis base. I always found that as filler since you just get an interesting product, but you now need to describe it in detail. I don't really care much for the bond lengths and I won't remember them tomorrow, but I will remember the crazy bismuth compound with an antimony donor.

The authors end the review with: "We anticipate that the fundamental development of coordination chemistry between the pnictogens, and the p-block elements in general, will lead to the discovery of new materials and catalysts to rival those developed from the chemistry of carbon and transition metals."

This is not true. There is not even a single planet in the universe that doesn't have lots of oxygen in some sort of pnictogen devouring form probably. It's fun basic chemistry that maybe someone will use in a practical process in a reactor some years from now. But rival the chemistry of carbon and transition metals? No...

One last not to someone who wants a quick JACS Angewandte article, is to take a look at Figure 1 in the beginning and more importantly (since many of those in Figure 1 will remain imaginary forever) at Table 2 at the end. If you can make any of those that have not been made yet, make sure to submit them to the aforementioned journal first. Don't worry... I won't be your competition, but the Burford lab at UVic might be. Good luck.

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