Shrinking in the heat – lanthanoid hexacyanidocobaltates, a.k.a. LnCo(CN)6

While most materials expand when heated, a few show the opposite behaviour, known as negative thermal expansion (NTE). The record holder, which has the greatest rate of volume contraction upon warming over a wide temperature range, is single-network cadmium cyanide (we previously blogged about the double-network version). This unusual phenomenon is useful – if you combine an NTE material with a normal material in just the right ratio, you can make a mixture with zero thermal expansion. Such a composite is immune to the undesirable effects of thermal expansion, from the buckling of railway tracks in the heat, to quartz-crystal clocks gaining or losing time when it’s too hot or cold.

A related group of frameworks which also show NTE are the lanthanoid hexacyanidocobaltates, with chemical formula LnCo(CN)6, where Ln can be any element from the lanthanoid row of the periodic table from lanthanum (La) to lutetium (Lu), Co is cobalt, and CN is cyanide. The cyanide is tightly bound to the cobalt, so these materials aren’t particularly toxic, unlike potassium cyanide.  They can be easily crystallised from a solution as shown in this time-lapse video, recorded over the course of a few hours.

LaCo(CN)6·5H2O is easy to grow, and forms nice hexagonal crystals. By the end of the video, they are about 2 mm in size and have started to merge together. Once the water is removed from the crystal structure by heating, the NTE properties are activated.

The key to the NTE is the networked structure of the materials. The lanthanoid and cobalt atoms are ‘bridged’ together by the cyanide (CN) ions. As the temperature rises, these linkers start to vibrate more and more in skipping-rope-like transverse (sideways) vibrations. Just like the ends of a skipping rope get closer together when you vibrate it, these transverse motions cause the average metal-metal distances to decrease, thereby causing contraction of the crystal.

The LnCo(CN)6 frameworks’ lanthanoid and cobalt metal atoms are linked together by cyanide bridges. ‘Skipping rope’ transverse thermal vibrations of these bridges bring the metal atoms closer together and cause the contraction of the whole material as it heats up.

The LnCo(CN)6 frameworks’ lanthanoid and cobalt metal atoms are linked together by cyanide bridges. ‘Skipping rope’ transverse thermal vibrations of these bridges bring the metal atoms closer together and cause the contraction of the whole material as it heats up.

Crystallography (powder diffraction in this case) allows the dimensions of the unit cell to be precisely measured, which lets us monitor the thermal expansion of materials by taking diffraction measurements at a number of different temperatures. The results for a series of LnCo(CN)6 compounds are shown in the plot below, and demonstrate that we can tune the NTE properties just by making the material with different lanthanoid metals. Swapping Lu for La doubles the steepness of the plot. This is because La bonds more loosely to the cyanide linkers than Lu does, making a more flexible framework with bigger transverse vibrations, and therefore bigger NTE.

NTE_2

The crystal structure of LaCo(CN)6 is made up of trigonal prismatic lanthanum (green) and octahedral cobalt (blue) metal atoms linked together by cyanide bridges (grey/blue). The plot shows the negative thermal expansion (NTE) behaviour of several different LnCo(CN)6 frameworks (with Ln = La, Sm, Y, Ho and Lu) measured using X-ray powder diffraction at the Advanced Photon Source. Larger lanthanoid metals, such as La, give a more flexible framework and a stronger NTE effect. The neutron diffraction measurements (black) allow us to go to lower temperatures, revealing how the NTE behaviour dies out, deviating from the nice straight line as the transverse vibrational modes “freeze out”. These results were published in S.G. Duyker et al. (2013), Angew. Chem. Int. Ed., 52: 5266–5270.

Merry Christmas – Ho Ho Ho

Dave T tells us of his crystallographic Christmas inspiration!

Dave T tells us of his crystallographic Christmas inspiration!

What does it look like?

HoHoHo What is it?

Sitting at my desk on the last working Friday of the year, just minutes away from a barbeque, Dr Maynard-Casely lays down the challenge for a “cheesy-corny Christmas-themed post”. Distracted by Steve Smith’s century (on captaincy debut), thinking was not easy, until my mind turned to a project in which I am currently involved.

The complex shown above is a trinuclear complex containing three holmium ions – HoHoHo. This was reported in 2005 by Junk and Deacon and contains quinolinolate anions as the ligating groups. This anion is typically used in gravimetric analysis as it coordinates to metal ions to form highly insoluble complexes. A derivative of this ligand has been used in anti-diarrhoea medication and is also being investigated in anti-Alzheimer’s medication.

The structure has three holmium atoms in close proximity, bridged by the oxygen atoms of the quinolinolate ligands. A recent analogue of this compound containing dysprosium in place of holmium has been shown to act as a single molecule magnet at low temperatures (Chilton et al., Inorg. Chem., 2014, 53, 2528). This means that each molecule acts like a minute magnet, albeit at temperatures below that of liquid nitrogen. Solid state packing of these complexes is, unsurprisingly, dominated by pi-pi interactions.

Where did the structure come from?

The structure was published in Z. Anorg. Alleg. Chem., 2005, 631, 2647.

Seasons greetings. The crystal structure of Cocoa Butter.

What does it look like?

Image generated by the Mercury Crystal structure visualisation software from the Cambridge Crystallographic Database Centre http://www.ccdc.cam.ac.uk/Solutions/CSDSystem/Pages/Mercury.aspx

Image generated by the Mercury crystal structure visualisation software http://www.ccdc.cam.ac.uk/Solutions/CSDSystem/Pages/Mercury.aspx

What is it?

Who’d have thought that a bundle of hydrogen, carbon and oxygen could be so tasty!  Here is the structure of cocoa butter fat, one of the main tasty ingredients in chocolate.  Chocolate is a very sophisticated material, and requires the right blend of sugar, milk and cocoa butter fats for it to taste right.  An added complication is that you can get different crystal structures of cocoa butter depending on how you solidify it.  These have different melting temperatures, so can affect the whole experience of the chocolate tasting.  The type you want to have is called ‘Type V’ or ‘Beta 2’, and is why chocolate has to be tempered – to make sure you get the right type.

Where did the structure come from?

The image was generated using the structure determined by van Mechelen et al. in 2006. They used synchrotron x-rays to determine this structure and were trying to understand how fat bloom (which is, in fact, a different structure of cocoa butter ‘type VI’) forms in chocolate.

Molecule of deceit – raspberry ketone

What does it look like?

The crystal structure of the raspberry ketone molecule, drawn with VESTA.

The crystal structure of the raspberry ketone molecule, drawn with VESTA.

What is it?

This small molecule packs a lot of flavour.  Raspberry ketone is one of the most expensive food additives currently about, used to add that raspberry fruity-ness to all sorts of things.  Part of the reason that it’s so expensive is because there’s actually very little of it in nature (it doesn’t take much raspberry ketone to make your raspberry taste good!).  But it can be manufactured synthetically much cheaper – and probably the reason that there’s so many raspberry flavoured things about.  This means that knowing the crystal structure is very important, as it can be used to check that you have synthesised the right molecule to add to your ice-cream!

The reason it’s a deceitful molecule is that recently it’s been marketed as a weight-loss supplement, but there is no clinical evidence that it does help humans loose weight.  The claims come from a study that showed some effects in rats. In the small doses used for flavour this molecule has little effect on our bodies, but the concerning thing is the companies suggesting people take large does for weight loss – we just don’t understand the effect these would have on our bodies. 

Where did the structure come from?

The crystal structure of the raspberry ketone molecule was reported by Wang in 2011, in Acta Crystallographia E. The structure is #2230481 in the Crystallography Open Database.

The MOFIA & the boss of the crown family: the UiO-66

What does the boss UiO-66 look like?

This picture was drawn using Diamond structure visualisation software and 'extra' images from http://eslkidsgames.com/2012/08/esl-mafia-game-for-teens.html.

This picture was drawn using Diamond structure visualisation software and ‘extra’ images from http://eslkidsgames.com/2012/08/esl-mafia-game-for-teens.html.

 “MOFIA”, what is that?

It is just a nickname for the scientific community working on Metal Organic Frameworks, acronymed MOFs, thus the community is the MOFIA.

The boss of the crown family, namely the UiO-66, is “probably” the most famous MOF of these past 6 years, now taking care of the family over the HKUST-11 and MOF-52. This solid has been reported by Cavka et al. in 20083 and has generated quite a lot of enthusiasm from the ‘Mofia’. There is probably UiO-66 in each lab around the world working on MOFs.

This MOF was basically the first built up from a tetravalent metal, the zirconium Zr4+ and thus an novel inorganic cluster (see below), which has given rise to a robust solid, i.e. much more stable than most of known MOFs. Indeed, it has been shown that, in general, by increasing the valence of the metal, the chemical and thermal stabilities of MOF increases.4 The linker is the terephthalate.

Untitled_2Now, the question is “how long will it remain the big boss?”, the challenge being that you cannot just kill it, you need to find a material that is better! The ‘Mofia’ war is opened!!!

Where did the structure come from?

The structure comes from a paper of Cavka et al. published in the Journal of American Chemistry in 2008.3

 

References

  1. Chui, S. S.-Y.; Lo, S. M.-F.; Charmant, J. P. H.; Orpen, A. G.; Williams, I. D., A Chemically Functionalizable Nanoporous Material [Cu3(Tma)2(H2o)3]N. Science 1999, 283, 1148-1150.
  2. Rosi, N. L.; Eckert, J.; Eddaoudi, M.; Vodak, D. T.; Kim, J.; O’Keeffe, M.; Yaghi, O. M., Hydrogen Storage in Microporous Metal-Organic Frameworks. Science 2003, 300, 1127-1129.
  3. Cavka, J. H.; Jakobsen, S.; Olsbye, U.; Guillou, N.; Lamberti, C.; Bordiga, S.; Lillerud, K. P., A New Zirconium Inorganic Building Brick Forming Metal Organic Frameworks with Exceptional Stability. Journal of the American Chemical Society 2008, 130, 13850-13851.
  4. Devic, T.; Serre, C., High Valence 3p and Transition Metal Based Mofs. Chemical Society Reviews 2014, 43, 6097-6115.

 

An impossible molecule – Hexaferrocenylbenzene

What is it?

With the discovery of ferrocene in the 1950’s, chemists realised that this was only the starting point for many other materials with properties that would be of use to us. From this point they started to build other molecules from a combination of metal atoms and organic molecules.

One molecule that was sought for a long time was, hexaferrocenylbenzene – which would be made up of six ferrocene sandwiches attached to a benzene ring. The community was split as to weather such a molecule was possible, but were encourages by it’s potential properties as a ‘molecular gear’.

What does it look like?

In 2006 it was finally synthesised an its crystal structure worked out – here’s the unit cell. b604844g_1

Breaking that down a bit, here is an image of the hexaferrocenylbenzene itself within the structure.

b604844g_2Where did the structure come from?

Hexaferrocenylbenzene was synthesised by a group at the University of California, Berkley in 2006. They subsequently worked with a group of crystallographers to determine its structure which was published in Chemical Communications. You can get the crystallographic information file for this structure directly from the supplementary information of the original paper.

An accidental molecule – ferrocene.

What does it look like?

ferrocene
What is it?

Ferrocene was (like many things) discovered by accident. Two groups, working independently, were both looking for something else where they in fact made an orange powder that they didn’t expect. Both knew that it was made from an iron atom and two cyclopentadiene (five carbon and atoms joined together with hydrogen’s attached), but thought that they would sit aligneded in a solid together.

Ferrocene kealy.svg
Ferrocene kealy” by Roland Mattern – Own work. Licensed under Public domain via Wikimedia Commons.

But x-ray crystallography showed that that was wrong, and the ferrocene molecule takes up a ‘sandwich’ structure with the two cyclopentadiene rings either side of the iron atom. This remarkable discoverey also revleaved the strange propoerties of this material, and paved the way for the subsequent rapid growth of organometallic chemistry .

Where did the structure come from?

You can read more about the history of the structure of ferrocene from an excellent C&EN article here. The structure we’ve plotted today is actually a high-temperature form of ferrocene, and is #210932 in the Crystallography Open Database.