| Femke Jannsen |  |
Crystal engineering is defined as the understanding of intermolecular interactions in the context of crystal packing and the utilization of such understanding in the design of new solids with desired physical and chemical properties.[1] These intermolecular interactions can roughly be divided into three categories; covalent, coordination and non-covalent.
Metal-Organic Frameworks (MOF's) are molecular assemblies constructed by coordination bonds, where metal atoms form the vertexes and organic polydentate molecules the linkers. As illustrated in Fig. 1 the topology of the network depends on the coordination environment of the metal atom and the organic linker, for instance a tetrahedral metal geometry and a tridentate ligand leads to a 3 dimensional structure (Fig. 1C) with a diamonoid topology.
Figure 1. The combination of metal geometry and linker lead to networks in different dimensions
Since the 1990's Metal-Organic Frameworks have gained a lot of interest, because of their promising application as e.g. gas storage devices, non-linear optics, and molecular sensors. Furthermore, they can be used in catalysis and size/shape selective separations. In these fields a comparison with zeolites can be made. Although zeolites as well as MOF's are networks, the topology of zeolites is rather limited, because they are mainly constructed of tetrahedral Al and Si units. The advantage of Metal-Organic Frameworks is that the variability in coordination geometry of metal atoms is much larger. Therefore, the pores in MOF's can have different geometries and sizes compared to zeolites.
Another promising advantage of MOF's is the opportunity of introducing chirality into the networks. This has been attempted with zeolites, but has not been successful yet. Currently, the number of stable porous metal-organic frameworks, which are catalytically active, is rather limited, but the few examples known do show great potential.Furthermore, the understanding of the intermolecular interactions that play a role in the formation and stability of a framework can be investigated by changing several parameters, like temperature, solvent, counterion, etc.
A nice example of a porous metal-organic framework was published by J.S. Seo et al.[2] The enantiopure framework (POST-1) consists of [Zn(&mu3-O)(1-H)6] 2H3O 12H2O units (Fig. 2). It was tested in a transesterification reactions and yielded esters in ~8% enantiomeric excess. Although a modest enantioselectivity, it does indicate the possibilities for the use of (chiral) MOF's as (enantioselective) catalyst.
Figure 2.
The objective of this PhD project is to design stable porous frameworks (preferably 3D) that show catalytic activity and to investigate the influence of e.g. temperature, stoichiometry, counterions on the network formation.
Student projects
If you are a student looking for a place to do your internship and you are interested in my project, you can always stop by for information about my projects. But to give you some idea of what you might be doing if you decide to do your internship with me, I will give you a short summary of two possible projects (see below). Of course, if you want to know more details you are always welcome to have a talk with me.
Student project #1
The building blocks I use for the construction of metal-organic frameworks are newly designed metalloligands. Studying the influence of for instance the effect of counterions associated with these linkers in metal-organic frameworks, requires a good understanding of the reactivity and catalytically activity of the isolated building blocks. This is the basic goal of this project. Your project will start with the synthesis of these building blocks to get a feeling of the synthetic route leading to these metalloligands. The next step is to study the activity of these compounds as catalysts under different conditions.The project combines nice organic and organometallic synthesis (to obtain the building blocks) with an application of these compounds in catalysis.
Student project #2
If you are interested in (organic) synthesis, this might be something for you.
Creating new organic linkers based on existing ligands can be achieved on paper easily. However, synthesizing them is not as easy as it seems. Since these linkers are new compounds, the synthetic pathway leading to them has to be designed. So, a nice result of your project can be a unique building block that can be used for the synthesis of new selective catalysts. That does not mean that you will be synthesizing only these compounds. With these new building blocks available your project could go into two directions. You can either start with exploring the catalytically activity of these compounds or you could start making Metal-Organic Frameworks of them. The choice is entirely yours!
Techniques
During internships you will come across different techniques to analyze your compounds. You might already know some of them, since these belong to standard characterization techniques (e.g. NMR, IR, elemental analysis). However, a good characterization of Metal-Organic Frameworks is very difficult with these techniques, because the insoluble nature of these compounds prevents using these techniques. That means that the information of these compounds must come from solid state techniques. So, analyzing compounds is done with the aid of for instance X-ray diffraction, DSC, and TGA.
References
[1] G.U.Desiraju, Crystal Engineering: The design of Organic Solids, Elsevier, 1989,
[2] J. S. Seo, D. Whang, H. Lee, S. I. Jun, J. Oh, Y. J. Jeon, K. Kim, Nature, 2000, 404, 982-986
Other nice references:
[3] S. Kitagawa, R. Kitaura, S.-I. Noro, Angew. Chem. Int. Ed., 2004, 43, 2334-2375
[4] C. Janiak,
Rowan group for molecular materials
