Loptr - 30-9-2019 at 06:47
I found this interesting paper that described the reaction between boron trioxide with carbon at a temperature over 1000C, and the subsequent
formation of diborane.
If so, this might be a good route to prepare diborane adducts with solvents like THF, pyridine, dimethylamine, etc.
Obviously, there are safety precautions that must be taken due to the explosive nature of diborane, and the high temperatures. Is there any easier
route to boranes that do not require borohydride?
Thoughts?
https://www.sciencedirect.com/science/article/abs/pii/002219...
I can't seem to be able to post this as an attachment right now. I get an error page.
[Edited on 30-9-2019 by Loptr]
Pumukli - 30-9-2019 at 07:17
Hydrogenating something in a vessel heated to 1000 C is beyond most of us. 
DavidJR - 30-9-2019 at 07:28
It'd probably be easier to make NaBH4 from scratch then use that to make diborane...
Loptr - 30-9-2019 at 08:38
I really dont think it is beyond us. They talk about the passage of a stream of hydrogen over it at high temp.
This is what pops into my mind when I think about it. The apparatus wouldn't have to be too complicated. Just a stream of hydrogen, generated however
you wish, whether that be electrolysis with a membrane, metal + acid or base, etc. This is lead into a quartz tube containing the mixture that is
heated from below with a flame, and the outlet led into the solvent. The hydrogen would have to be vented somewhere, and you might have to cool the
gas as it comes out so that it doesnt boil the solvent.
https://youtu.be/u50mLkExs6o
EDIT: I forgot to mention that it can be done at atmospheric pressure, so it's not really a hydrogenation in terms as one usually hears of
hydrogenations whereby high pressure is used.
[Edited on 30-9-2019 by Loptr]
Praxichys - 1-10-2019 at 09:46
What did they use in that paper? A platinum boat, perhaps? If I've done my research correctly, boric oxide is a liquid at that temperature and will
likely be highly corrosive to quartz glass, forming borosilicate which will melt. I thought about 304 stainless or even nickel tube (available on
eBay) but the formation of dangerous metal carbonyls might be a problem for both.
I did some math on the required cooling. Hydrogen from room temp to 1200K has a specific heat of between 14.5 and 15.3 kJ/KgK. Pretend you need to
catch 2 mol (4g) of extra H2. Putting 900K on 4g of hydrogen with a nominal specific heat of 15 kJ/KgK will require around 54 kJ.
Pick a solvent - toluene is around 1.8 kJ/KgK, so cooling that hydrogen is enough to raise almost 400mL of toluene from room temp to boiling.
Precooling will definitely be necessary, especially since the solvent will have to be kept cold to capture and hold the diborane.
I have looked into producing sodium borohydride before. Really the only holdup for the "traditional way" is the sodium metal since producing NaH from
sodium is fairly straightforward and well-documented, and using that to reduce trimethyl borate (from boric acid and methanol, another easy prep) to
NaBH4 is also facile, albeit a bit of a fire hazard.
Loptr - 1-10-2019 at 10:17
The idea really isnt to prepare sodium borohydride, but a borane adduct. These seem to also be extremely handy, albeit quite a bit more risky.
And the part about the quartz glass and liquid boric oxide is something I hadn't thought of, and to be honest, dont really think it's an issue if you
just dont use glass or quartz. The idea was that you could use a tube capable of being flushed with an intert gas, plugged at two ends with nipples
that allow gas in and out, and can be opened to be able to place a bunch of powder inside. Also, that you be able to heat it with a flame. Quartz
apparently would have issues, which is fine, since a metal tube or pipe would probably work better. I would have to look into the metal carbonyl
issue.
EDIT: On second thought, are you sure carbonyl formation would be an issue? Doesnt that require finely divided metals and carbon monoxide to have any
sort of reasonable rate of reaction? Also, I just read in the carbonyl process that they decompose to the metals at 170C. If it formed, it sounds like
it would be transient. We are also talking about a reducing atmosphere, too. Good points, though.
[Edited on 1-10-2019 by Loptr]
[Edited on 1-10-2019 by Loptr]