Posted by: Mark Foreman | March 24, 2012

The borohydrides are back !

Dear Reader,

Some time ago a student asked me which hydride reducing agent he/she should select for a reduction, I had become mighty sick of just saying ”just remember that lithium aluminium hydride is much stronger than sodium borohydride”. So I started to think of why one reagent is so much stronger than the other.

This lead to my most popular blog item ever on organic chemistry, it tends to get about 300 hits per week which suggests to me that people want to read about hydride reducing agents. While it might not be as popular as “pop culture” (things like Brittney Spear’s latest hair cut etc etc) it does seem to be popular.

This have made me choose to follow it up with a bit more on the hydride reducing agents, I have been looking in the crystallographic literature and I have been trying to get in touch with my artistic side. Do not worries I have not started to hack off my ears yet.

I started by looking at the crystal structures of sodium, potassium, rubidium and cesium borohydrides. Now these all seem to roughly the same, it is a cubic cell which gets bigger as the metal atom gets larger. The arrangement of the boron and the alkali metals looks the same as it is in a rock salt lattice. This is according to S.C. Abrahams and J. Kalnajs, Journal of Chemical Physics, 1954, 22, 434-436.

Cation

Length   of one side of the cell (Å)

Na

6.164

K

6.727

Rb

7.029

Cs

7.419

What is happening is that the borohydrides are staying the same in size but the cations are getting bigger, as a result the unit cell gets larger. Due to something called disorder it is not possible to clearly see where the hydrogens are on the borohydrides. Disorder is a age old effect in crystallography, when some of the unit cells have a different arrangement of the atoms or one atom replaced randomly with another (good examples are homogenous mixtures of uranium and plutonium dioxides and cobalt/nickel alloys) you also get a form of disorder.

Here is a picture of the unit cell of potassium borohydride with some nearby atoms, I have shown all the possible locations of the hydrogens. Here it is, note that the borons look like they are eight coordinate (me thinks impossible) but understand that they are in real life only four coordinate.

The unit cell of potassium borohydride plus some nearby atoms showing you the disordered hydrogens

Now if we remove half of the hydrogens we will get what one of the possible unit cells.

One of the two arrangements of the hydrogens in the unit cell.

Now here is the other form.

The other possible arrangement of the hydrogens in the unit cell.

Real life is part way between the two, it is an equal mixture of the two forms. This is what “disorder” means.

Always bear in mind that disorder is not something which some lack of skill in a crystallographer has caused, instead it is just a fact of life. So my advice is “if you do not like disorder then try and get over it !”.

I then looked at lithium borohydride; this is not a cubic solid. For details see J.P. Soulie, G. Renaudin, R.Y. Cerny and K. Yvon, Journal of Alloys and Compounds, 2002, 346, 200-205.

A unit cell of lithium borohydride, with some of the nearby atoms added to make it more clear what is going on.

A second view of the unit cell

I suspect that the sodium chloride structure could not tolerate the large change in the size of the cation. The general trend is that a 1:1 solid where the cation and anion are about the same size will have a sodium chloride like cell. But when the size ratio of the cation and the anion moves a long way away from 1:1 then the crystal structure will change to something else. Such as the cesium chloride structure.

Now I guess for many of you the alkali metal borohydrides were all well and good, but let’s move onto something else. Now one of the great things in inorganic chemistry are the diagonal relationships. For example vanadium, molybdenum and rhenium share some chemistry as do carbon and phosphorus. Another classic is lithium and magnesium. These two elements share a lot of chemistry.

I got the crystallographic data for magnesium borohydride (Y.Filinchuk, R.Cerny, H.Hagemann, Chemistry of Materials, 2009, 21, 925-933) and I found it was a much more complex solid than lithium borohydride. As the magnesium is a larger radius but more charged cation it is able to form a solid with more bonds between the metal and the borohydride anions.

If we look at the coordination environment around a single magnesium we will see that four borohydrides are attached.

The environment around a magnesium in magnesium borohydride

These are attached in a tetrahedral manner which gives us a very cross linked solid. The best way to think of the solid is as a 3D polymer which like a thermoset “plastic” extends in all directions.

Here is a picture of the solid in all its glory.

Magnesium borohydride

I then moved onto consider zinc borohydride (P. Choudhury, V.R. Bhethanabotla, E. Stefanakos,  Physical Review, Serie 3. B – Condensed Matter, (18,1978-) (2008), 77, 134302-1-134302-9), I was hoping that zinc borohydride would have the same structure as magnesium borohydride (it would have saved me doing a new set of drawings). But I found that the zinc compound is different. I suspect it is because the zinc cations are larger than the magnesium cations. The zinc solid is less cross linked, it is a layered solid rather than being a 3D network. The layers in the zinc solid are rather corrugated like an old fashioned shed roof.

Here is the zinc solid showing you the layers, I think that as the zinc is larger it has a smaller ability to form covalent bonds to the hydrogens of the borohydrides and as a result we get less cross linking.

Zinc borohydride

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Responses

  1. A nice review on the crystal chemistry of metal borohydrides! You can find more information in Z. Kristallogr. 2008. V. 223. No 10. P. 649-659. Also, there is quite an amazing story about a porous borohydride, which similar to MOFs is capable to store guest molecules, such as hydrogen, see Angew. Chem. Int. Ed. 2011. V. 50. No 47. P. 11162-11166.

    • Thanks for the comment, I am glad that you like what I wrote about the borohydrides.


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