BCA CCG Autumn Meeting: "Increasing Complexity in Crystal Engineering of Materials"

This year the British Crystallographic Association (BCA), Chemical Crystallography Group (CCG) autumn meeting was co-hosted with the Directed Assembly Grand Challenge or the Directed Assembly Network, DAN for short and held in the beautiful city of Leeds (no bias being a Yorkshireman) at the rather aptly named “Rose bowl”, located within the Leeds Metropolitan University.

Expertly and efficiently organised by Louise Male (CCG secretary) and sponsored by Bruker AXS.

The running theme and title for the meeting, “Increasing Complexity in Crystal Engineering of Materials“. Drawing across a wide range of scientific areas from chemistry, chemical engineering, pharmaceuticals to polymers and of course crystallography. A title developed to try and put flesh on the bones of three questions hopefully addressed through the talks.

How to increase the complexity of organic molecular materials?
How can we engineer complex materials with more than 2 or 3 building blocks? and
Can we begin to predict this degree of complexity?

An interesting set of challenges to address in just one day of talks. But with an ‘A’-list of speakers this meeting look easily set to meet those ‘grand challenges’.

Introductions

The meeting started out with general introductions, safety, and the days line up from the chair of the morning session Dr Simon Coles (University of Southampton).

Dr Jenny Woods (Directed Assembly Network coordinator) continued the introductions with a brief background of the Directed Assembly Grand Challenge and Network.

Rather importantly telling us all about the potential funding opportunities offered through DAN - a “one women research council”. The next closing date for applications** 30/11/2013** applicable to early career and established researchers alike.This was followed by the first of the days speakers.

Prof Harry Anderson FRS - University of Oxford

Harry’s talk entitled: “Directed Assembly of Porphyrin Nanorings: Engineering Complex Functional Materials from Multiple Building Blocks” gave great insights in to template mediated molecular synthesis. Punctuated with amazing images and animations.

Drawing from inspiration and parallels in natural systems such as photosynthesis (LH1/LH2) as well as nano-wires through his talk Harry developed the underlying synthesis of complex macro cyclic porphyrin assemblies using a range of techniques. Turning porphyrin “wires” into more and more complex and beautiful molecules.

Starting out with a “simple” 6-porphyrin “ring” (cyclic oligomers), bridging Zn porphyrins by butadiyne-linkers, produced materials challenging to characterise. Confirmation that the system was not just a linear oligomer was achieved using techniques such as SAXS (I22, DLS) and through some beautiful single crystal data determined by Dr Amber Thompson and collected at station I19, DLS.

Three template strategies were described. Starting out with the use of large rigid templates molecules. Moving from rigid templates to flexible cyclodextrin based templates allowed progression to larger oligomer rings rapidly approaching the state of nano-rings. Then finally as the large template molecules becoming more and more complicated to synthesise a process of limiting returns started to develop. A change of tack and the use of “vernier complexes” - using small templates where the number of binding sites on the template does not match, either directly or via a multiple, the number of binding sites of the linear porphyrin oligomer - producing “figure of eight” - “∞” shaped intermediate molecules which on template removal unfold to produce nano-rings. Studied through amazing STM images from Prof Peter Beton, Nottingham University.

This approach was then progressed through the talk showing how the cyclic oligomers themselves could be used as templates producing systems similar to B850 (photosynthesis) but also bringing together larger and larger nano-rings, gasp from the audience as a 10-‘nmer’ was displayed, all the way to 50 porphyrin units.

Prof Howard Colquhoun - University of Reading

Even by his own admission, Howard “had a hard act to follow - with Harry”. However this was rapidly proven wrong as he presented his talk entitled: “Complementary π-π stacking interactions in crystalline materials”.

Starting out with an astute observation that even today π-π stacking interactions are in a state of flux - with definitions changing almost every year - his research utilised these very properties to explore novel materials.

Howard explored the background of the research from his ICI days looking into interleaved complexes with paraquat and diquat - retelling how he “almost got sacked” due to a poor choice in sub-storey by an editor. He also showed how sometimes the most interesting part of the** crystal structure** is in the packing diagram.

A review followed into how the presence of a guest can cause a “breathing” effect with these early systems moving on to more complicated multicomponent systems utilising two pyrene ends in the search for shape complementarity and molecular “tweezers”. Molecules designed to bind to macrocycles utilising π-π interactions between electron poor moieties.

The first examples showing infinite π-π electron-rich-poor-rich-poor stacks and opening up the prospect of reverse engineering to the system through ring opening chemistry to make high molecular weight polymers. Ironically a side product often made during the synthesis of the macrocycle target molecule. However when the linear species was the target the macrocyle became a very frustrating impurity.

The presence of the tweezer molecule interaction with the host could readily be determined from NMR. Where a significant magnetic shielding effect causes shifts in the observed proton position, for example in the imide protons and can be effective as far as 10 Å away. These results were confirmed with elegant crystallographic studies from Prof Christine Cardin and once again I19, DLS data. Ironically though in this case a similar motif was determined in Reading on their home laboratory X-ray system, Gemini S Ultra, after months of trial at DLS as well.

Modification of the tweezer at the backbone between the two aromatic rings allowed a tweezer design which switched target away from the pyromellitimide moiety. Unfortunately where NMR was elucidative the single crystal structure proved not to be so much.

Howard concluded by talking abut more recent applications of “tweezer” moieties to act as terminal groups in polymeric/oligomeric chains. Allowing designed ends to join in a “Roman handshake” manner and through which larger molecular weight polymers could readily be synthesised. This research greatly helped by results from the station 11.3.1 ALS, Berkeley laboratory.

Prof Euan Brechin - University of Edinburgh

Euan’s talk entitled: “Molecular Magnets: Understanding and Exploiting the Structure-Property Relationship” started out with a pre-warning that “Molecular Nano Magnets” was now a meaningless term. However the application of molecular magnetism covered a whole host of research themes: Information storage, Quantum computing, Magnetic refrigeration, Spintronics, Biocompatible sensors, and Molecular recognition just being a few of them.

The talk today however focused on magneto-structure correlations and single molecule magnets (SMM). Can we understand and importantly can we exploit?

__Euan also showed the view from his office perhaps a motivation for a “Friday afternoon reaction” using “dirty solvents” and this approach can lead to some serendipitous self-assembly. Turning today’s talks on their heads somewhat. “As all approaches work in synthesis”. He went on to explain what made a “Friday afternoon reaction” and most importantly defined a “dirty solvent”. Which was in short pretty much every miscible solvent in the laboratory you can lay your hands on and although they could be frustrating for the crystallographers when it came to refinement. In general the “molecule picks its own complementary solvent” in most cases. The crystal structure of which then elucidating which solvent to use when synthesising more material in future reactions.

The rest of the talk focussed around a deceptively simple [MnIII3] triangle complex and the efforts required to turn this antiferromagnetic complex - ferromagnetic through manipulation of the triangular plane via axial and basal plane ligand modification. As well as the application of high pressure in diamond anvil cells. All in the effort to “pucker” the ring and by doing so break the communication between the Mn-centres. Through analysis of hundreds of structures only a single parameter in the Mn-coordination environment could be correlated with the observed magnetic property changes. Euan emphasised the dangers of fitting a single technique results from analysis. Showing that you need to backup your analysis with other techniques which can provide complementary and useful information. Such as EPR, INS, FDMRS, MCD and SQUID.

The remainder of the talk moved into other more complex materials from triangles to paddle-wheels. But still one ligand and one metal combinations giving a variety of products. What interesting things could be created with shapes based on triangles?

Euan ended his talk with the rather provocative statement - which he sometimes uses with his students “Everything crystallises - if not you’re doing it wrong”. Perhaps a lesson for us all?

Dr Jonathan Nitschke - University of Cambridge

“Complex Functions from Self-assembling Metal-organic Systems with Simple Precursors” **was the title of the talked presented by Jonathan Nitschke. Which completely undersells an interesting and inspiring introduction into **deterministic chemical pathways and product formation in complex cage coordination systems.

As with Harry Anderson’s talk in the morning elegant sciences was punctuated with amazing images and crystallography.

Jonathan considers himself a “synthetic chemist which looks at systems” rather than single molecules. Using stoichiometry to** control** the final product through deterministic processes. Simply put the elegant molecules you want to form, form because the “leftover” reactants do not want to form “forbidden products”. With this simple theory complex reaction mixtures with many reagents can be used to repeatable control the final product formed. Plus by adding additional reagents such as different metals product distributions can be altered and used as “signal events” as they will always seek the “most accommodating host”. Air unstable and transient guest **could be **captured within the coordination cages such as SF6 **and P4. In the case of P4 producing a material that was - when **contained with the cage even stable in air and water - probably due to the reaction transition state being too large to fit in the cage and therefore prohibitively energetically expensive to form.

The talk continued, showing how a simple quadrant model based on rate of exchange of guests could be generated and with this knowledge guests could be actively shuttled and swapped within cages. Again potential signalling uses. Then from the tetrahedral cage motif to other geometries such as cubic cages. These large cages being able to accommodate molecules like, C60, C70 and coronene. The mixtures of fullerenes produced from electric-arc soot being difficult to separate could the** cubic cages** be used as a separating medium? Jonathan also showed how binding affinity was related to the existing guest presence. Which could allow for a simple method for tuning guest affinity.

Concluding his talk by showing how the geometry of the building blocks can control the cube formation as well as how counter ions could also control morphology and be used as a signal to bind a second anion.

Throughout crystallography was emphasised as a vital front-line technique for characterisation.

Mr James McKenzie - University of Sheffield

Jame’s** “H-bond interactions in Solution and Solid State” talk emphasised how **difficult it can be using intermolecular interactions to predict properties in an effort to manipulate those properties in novel materials. With the goal of looking at co-crystals and polymorphism.** **

His research, which was detailed and quite well developed for a first year PhD student, tried to rank the functional groups found in hydrogen bonding using two parameters: α defined as the donor and β defined as the acceptor. He then used the propensity of formation tool part of the CSD Solid Form Suite to look for correlations between αβ and the calculated propensity value from the CSD. Sadly the initial results showed no correlation. Further investigation showed that the α-values appeared to break the correlation.

So an alternate method of determining αβ was undertaken using values** calculated** from the computed electrostatic surface min/max values. These also proved to be problematic as the calculated αβ varied greatly with molecular conformation. As would the propensity values within the CSD.

This led James to undertake a “human” search of the CSD using conquest rather than using the propensity tool. By using additional filters such as the formula to constrain the returned results and interpreting the number of hits retrieved as a relative** value** for stability for that particular interaction. He continues to look for patterns and it may simply be a case of determining the correct parameters to plot to find them.

Prof Andy Cooper - University of Liverpool

Andy the final speaker started out his talk:** “F**unctional **organic porous crystals: Design or discovery?”** with a short review of existing porous material approaches. Showing plots of porosity categorised by material. He highlighted how strategies used in the isoreticular or IRMOF break down when your basic building blocks change from rigid linkers, Lego like, units to discrete molecular assemblies. Are molecular synthons - predictable? Is the statement “design is not possible” true or false? Andy speculated that perhaps it is just that the free energy landscape of the systems are just** difficult to understand**.

So is it possible to use in silico techniques to i) take starting reagents, build a molecular product “a cage”; ii) take that product compute its most stable geometry and iii) then compute how that molecule will pack and iv) from there what will be the properties such as surface area be?

His work has shown that subtle changes in ligands can have a large difference on the final cage structure. So can we at least predict the stable product? Andy asked. Dr Kim Jelfs, formerly University Liverpool, researcher at Imperial has developed a method, ”Cage Creator Software”, that works reliably for the molecular cage systems from the Cooper group. It allows for non-intuitive results which match real world reactions. It also can do this within time scales many times** faster** than in the** laboratory.**

So steps i and ii can be done and through collaboration with Dr Graham Day they are working of steps iii. However it would appear that although Graham can readily calculate the most stable packing the relative speed is prohibitively slow. Or in other words it is not so fast - at the moment. The** results** so far though have been very good. With blind trial predictions matching previously determined single crystal structures with a high degree of matching.

With a range of possible cages already synthesised there are the possibilities to** mix cages** to form co-crystals. Can the crystal structure prediction handle that? Even quasiracemate forms can be found - giving good confidence. Some predictions do fail but even so the** method is sound**.

So can this be extended from 1+1 to 1+1+1 or ‘ter’ crystals made from three different cages? In the lab it is possible and interestingly the products formed obey Vegard’s law - which is more akin to property changes observed in alloys.

Andy closed by asking what about iv - physical properties? Can we calculate them. So far it is possible to do for guest selectivity. Since discrete molecular systems with intrinsic porous space, as opposed to MOF whose pore space is a product of the framework, readily suit computational methods. They can also be analysed using solution based techniques such as NMR. So in this respect yes.

Conclusion And Wrap Up

Prof Paul Raithby, chair of the final session and Directed Assembly PI, brought proceedings to a close with a final round-up of the day. He highlighted the inspirational, elegant chemistry that that the speakers had shared with us. Suggested that perhaps there could be a set of rules for thermodynamic or kinetic control and that it is vital to build the link between in silico - computational studies and experiment. But we must always remember that every experiment is valuable no matter if it is successful or not. He also mused that perhaps there is a place for databases and information to be** shared** within the community to help inform and guide the design. To seek dynamics between the solution and solid state and that it was most likely up to us over the next 20, 30 or 40 years to look for patterns within the** data**.

His final words of the meeting emphasising that the “future is exciting” - and that directed assembly is a LONG term vision.

BCA CCG Autumn Meeting: “Increasing Complexity in Crystal Engineering of Materials” by John Warren is licensed under a Creative Commons Attribution 3.0 Unported License CC BY 3.0.

BCA CCG Autumn Meeting: "Increasing Complexity in Crystal Engineering of Materials"

Introductions

Prof Harry Anderson FRS - University of Oxford

Further Reading

Prof Howard Colquhoun - University of Reading

Further Reading

Prof Euan Brechin - University of Edinburgh

Further Reading

Dr Jonathan Nitschke - University of Cambridge

Further Reading

Mr James McKenzie - University of Sheffield

Further Reading

Prof Andy Cooper - University of Liverpool

Further Reading

Conclusion And Wrap Up

Introductions

Prof Harry Anderson FRS - University of Oxford

Further Reading

Prof Howard Colquhoun - University of Reading

Further Reading

Prof Euan Brechin - University of Edinburgh

Further Reading

Dr Jonathan Nitschke - University of Cambridge

Further Reading

Mr James McKenzie - University of Sheffield

Further Reading

Prof Andy Cooper - University of Liverpool

**Further Reading**

Conclusion And Wrap Up

Further Reading