Boranes

[Hephaestus]

Does anyone have a prep for or has any experience with any of the various boranes? What is the best way of storing these?

Lastly - a list of borane reactions would be nice to cover in this thread.

[RyanJ]

As far as I remember the light boranes are quite unstable and burn easily in air but become more stable as they get heavier. It should also be noted that boranes are toxic so must be handled with care

 

Cheers,

 

Ryan Jones

[Tartaglia]

Boranes can be quite treacherous, particularly the lower ones. The higher caged type boranes/carboranes I have used in the past and are much more stable.

[woelen]

What do you count as boranes. Do you only call compounds BxHy borane, or do you also include ions like BH4(-) as borane.

 

There are some ionic B/H compounds, which are quite stable. NaBH4 is decomposed by water, but that reaction is quite slow. In practice, one can have solution of this salt in water for quite some time.

I have tried to isolate BH3 (B2H6, IIRC, H3BBH3 is a better representation) by adding acid to a solution of NaBH4, but that does only yield hydrogen. Apparently, borane molecules are quite unstable in the presence of water.

 

Boranes are quite strong reductors. The sodium borohydride is probably the strongest water-soluble reductor I have, but it is not capable of reducing chromium ions to the +2 oxidation state, nor is it capable of reducing vanadyl ions to vanadium in the +2 or +3 oxidation state. For those reductions in aqueous media, I need powdered zinc.

[Hephaestus]

Boron as an element is often neglected in forums. It is readily available as boric acid or borax and surely a whole spectrum of BxHy and ions are accessible. I've explored quite a few of the ugly but interesting P, As, Sb compounds and thought it nice to play with B.

 

It would be nice to synthesise our own NaBH4 and caged boranes. Caged boranes are currently being investigated as novel cancer targeters and NaBH4 has an endless number of uses. It's also difficult to purchase these days.

 

Lastly, a general procedure for boronic acid esters would open up a whole of carbon chem.

[woelen]

I have 25 grams of NaBH4 (purchased it from a photography chemical supplier), and it is used as a reductor for those purposes. It is also used in organic chemistry as a more selective/less destructive reductor than LiAlH4. It is really striking to me, however, how stable NaBH4 is in water. In DMSO it gives rise to extremely smelly sulphurous compounds (I tried it ), probably due to reduction of the DMSO to (CH3)2S, but I'm not sure about that. I only noticed the smell, I did not find any details about that reaction though.

 

Furthermore, I use NaBH4 mainly for keeping very air-sensitive precipitates, such as Mn(OH)2, and Fe(OH)2 good for a somewhat longer time. With just a very small pinch of NaBH4, dissolved in water, I can do experiments with air-sensitive materials which I otherwise could not do. The NaBH4 protects these compounds, byt taking away the oxygen.

[budullewraagh]

don't forget hydroboration!

[Hephaestus]

NaBH4 + HCl -> B2H6

B2H6 + NH3 ->(300C) borazine (inorganic benzene)

 

Dont want to waste the sodium borohydride, but I tried yesterday it and it worked. Product decomposes in water though. Keep you all posted on other B expts.

[woelen]

If you have solid NaBH4, then you can make B2H6 by adding the solid to almost pure H3PO4. With conc. HCl the B2H6 is hydrolysed by the water in the acid.

 

If you have 85% H3PO4, then boil it a little to get rid of most of the water, and then add NaBH4.

 

Yield is not 100%, but sufficiently good to obtain reasonable amounts of B2H6.

 

Btw, the bonding mechanism of B2H6 is very interesting. It has structure

 

H2B-(μ-H)2-BH2

 

Here, the H-atoms are bridging between the two boron atoms. These H-atoms use a special bond-type, in which 3 atoms are involved, which together share 2 electrons. The B-H-B bonds are 3-centered 2-electron bonds, one electron donated by the H atom, and one electron donated by one of the boron atoms. In this way, three atoms share 2 electrons and by doing so, both boron atoms have 8 electrons and all hydrogen atoms have 2 electrons. The H2B and BH2 units are builtup by ordinary B-H bonds, each donating one electron.