Metal Organic Frameworks (MOFs) as catalysts for organic reactions


Metal – Organic Frameworks (MOFs) are crystalline, porous, three-dimensional coordination polymers in which inorganic building units are joined by organic links.[1] Through judicious choices of bridging ligands and metal centres their porosities and functionalities can by fine tuned for multiple applications: gas storage, in particular hydrogen and methane, separation processes, drug delivery, ion exchange, sensor technology, non-linear optics, magnetism, luminescence and many others.

MOFs are also dream compounds for catalysis,[2] mainly because:

1) they can be rationally designed and their structure studied with atomic resolution using X-ray diffraction,

2) catalytic site isolation allows for assessment of structure – activity relationships,

3) they are heterogeneous catalysts and as such can be easily separated and recycled (‘green catalysts’)

4) their tunable pore structure offer the possibility of shape-selective catalysis,

5) they can be easily made homohiral, what offers great opportunities to design heterogeneous asymmetric catalysts.[3]

What makes MOFs truly distinct is their easily tunable, regular, porous architecture, which provides unique nanoenvironment to the immobilised catalysts; environment that might be compared to that of active sites of enzymes.[4]

Microsoft Word - Graphical Abstract.doc

Objectives. Within this Project (“IDEAS PLUS” grant from the Ministry of Polish Science and Higher Education) we would like to demonstrate that the incorporation of homogeneous catalysts into metal-organic frameworks greatly facilitates the development of new tandem catalytic systems by preventing the catalysts form mutual deactivation. To reach this strategic goal we plan to synthesize a range of catalytic MOFs, both new and previously described, and then study their catalytic properties with particular focus on tandem reactions.

[1] Ch. Janiak, J. K. Vieth, ‘MOFs, MILs and more: concepts, properties and applications for porous coordination networks (PCNs)’, New J. Chem. 2010, 76-79.

[2] a) A. Corma, H. García, F. X. Llabrés i Xamena, ‘Engineering metal organic frameworks for heterogeneous catalysis’, Chem. Rev. 2010, 110, 4606-4655; b) D. Farrusseng, S. Aguado, C. Pinel, ‘Metal-organic frameworks: opportunities for catalysis’, Angew. Chem. Int. Ed. Engl. 2009, 48, 7502-7513; c) W. Lin, ‘Asymmetric Catalysis with Chiral Porous Metal–Organic Frameworks’, Topics in Catalysis 2010, 53, 869-875.

[3] M. Yoon, R. Srirambalaji, K. Kim,‘Homochiral Metal–Organic Frameworks for Asymmetric Heterogeneous Catalysis’, Chem. Rev. 2012, 112, 1196–1231.

[4] K. P. Lillerud, U. Olsbye, M. Tilset, ‘Designing Heterogeneous Catalysts by Incorporating Enzyme-Like Functionalities into MOFs’, Top. Cat. 2010, 53, 859-868.

Supramolecular chemistry of anions

Negatively charged species play fundamental roles in a range of biological, chemical, medical, and environmental processes.[1] Accordingly, the development of receptors highly proficient in anion recognition, sensing or transport has become one of the major areas of supramolecular chemistry. Recent progress in the construction of anion receptors is, to a large extent, the result of the development of simple, organic, hydrogen bond donating units that exhibit strong affinities towards anions. Fundamental knowledge about their precise structure, conformational preferences, anion affinities etc., allow chemists to successfully construct more and more potent receptors with improved affinities, selectivities and sensitivities for anions.

Moreover, building blocks with high affinity enabled the use of anions in controlling self-organisation processes, like for instance anion templated synthesis of rotaxanes and catenanes.

[1] J. L. Sessler, P. A. Gale and W. S. Cho “Anion Receptor Chemistry”, RSC Publishing, 2006.

Receptors, sensors and transporters of anions based on 1,8-diaminocarbazole skeletonFig 2_Anion induced conformational switch

Within the project founded by National Science Centre we study uncharged receptors derived from a new building block develoed in our group – 1,8-diaminocarbazole. It combines several attractive features such as:

  • ease of synthesis and derivatisation,
  • strong hydrogen bond donor – carbazole NH,
  • rigid skeleton facilitating preorganisation of hydrogen bond donors,
  • chromophore and fluorophore directly coupled with anion binding sites, what makes possible anion detection through binding induced changes in UV-Vis and fluorescence spectra.

The project concerns the synthesis of a range of model derivatives of 1,8-diaminocarbazoles, such as amides, thioamides, ureas, thioureas, both acyclic and macro(bi)cyclic, systematic investigations of their structure, conformational preferences (X-ray analysis, ROESY and DOSY NMR spectroscopy; VT NMR, molecular modelling) and then investigations of their anion binding and sensing properties (binding constants measurment by NMR and UV-Vis titrations, changes in colour and fluorescence upon anion binding, deprotonation by basic aninos, etc.).

In collaboration with dr Roberto Quesada from Universidad de Burgos we study also the ability of model receptors to transport anions through lipid bilayer.

Rotaxanes and catenanes – anion templated synthesis and application in anion sensing

Fig 3_Pseudorotaxane

The use of anions as templates in supramolecular synthesis is a rapidly developing field of research in supramolecular chemistry. Rotaxanes and catenanes obtained with the use of anionic templates have built-in 3D cavities, which allow them to strongly and selectively bind anions, in particular the anions that templated their formation. Moreover, when equipped with fluorophores, chromophores or electrochemically active groups, these interlocked structures may become selective sensors for anions (see scheme).

RESEARCH_Catenanes, rotaksanes_scheme_sensory rotaxaneEN

Currently we are working on the construction of fluorescent rotaxanes and catenanes based on 1,8-diaminocarbazole building block developed by us.

Photoswitchable receptors and catalysts

Photocontrol over molecular recognition is an important step towards the construction of molecular machines and opens interesting perspectives in supramolecular chemistry and nanotechnology. Potential applications include an active, light-driven transport of molecules through lipophilic membranes or controlled binding and release of active substances.

Within the “Homing” project from the Foundation for Polish Science we try to construct novel, photoswitchable hydrogen bonding donors and study their potential in anion binding and catalysis.

Conducting Polymers

The structure of unsubstituted 1,8-diaminocarbazole is a hybrid of aniline and pyrrole – precursors of two important conducting polymers. In collaboration with prof. Skompska from our Department we discovered that this new precursor has the unique ability to effectively electropolimerise from both aqueous and organic solutions, and the resulting polymer is an excellent support for the immobilisation of enzymes, synthesis of metal nanoparticles and other applications.

Therefore we developed a practical, multigram synthesis of this interesting monomer.

Self-organisation of bioactive multivalent structures

Fig 4_ConA + Grid
Multiplication of functional units by self-assembly is a powerful way to new properties and functions. Self-organisation of components decorated with recognition groups leads to multivalent entities, amenable to strong and selective binding with multivalent targets, such as protein receptors. In collaboration with prof. Jean-Marie Lehn we have illustrated this concept with an efficient, supramolecular, one-pot valency multiplication process proceeding through self-organisation of monovalent components into well-defined, grid-shaped [2×2] tetranuclear complexes bearing eight sugar residues for multivalent interaction with tetrameric lectin, concanavalin A. Read more in our recent paper in Chemistry – A European Journal.