Biomimetic Catalysis




Polyoxometalates (Fig.1) constitute a structurally diverse class of discrete, negatively charged transition metal oxide clusters with many technologically interesting properties (electrochromism, photochromism, support of multi-electron transfer). Therefore, methods gain in importance which target on tailoring the surface properties of these compounds. Utilizing tailored surfactants or polyelectrolytes, respectively, the brittle polyoxometalate salts are transformed into soft hybrid materials which can be processed into materials of arbitrary shape. The realisation of structurally well-defined ultra-thin films and coatings incorporating polyoxometalates as a functional component opens prospectives for new fields of applications and technological developments. Thus, first prototypes of electrochromic displays, gas sensors and intelligent information storage devices were recently developed.



Fig.1: Representative examples of polyoxoxmetalates which are used in our group


Our current interest focusses on the use of polyoxo-metalates in artificial enzymes. We have been searching for strategies of augmenting the catalytic activity typical of many polyoxometalates (especially in oxydation catalysis and oxo transfer reactions) with an improvement in substrate selectivity. A possible solution to this challenging problem is derived from nature (Fig.2)): The protein shells of many metallo enzymes contain tailored channels and pockets, which direct the appropriately sized substrate to the active site. We think that we might be able to achieve a similar function through the construction of “dendrizymes” which are composed of a catalytically active polyoxometalate that is nested in a shell of dendritically branching amphiphiles.



Fig.2: Dendrizymes (schematically), (A): construction principle of a natural enzyme; (B) construction elements of a dendrizyme; (C) Representative example of a dendritically branched amphiphile


Previous structure analytical investigations have demonstrated that the complexes of cationic surfactants and anionic polyoxometalates do form discrete core-shell particles in non-polar solvents. Computer simulations (Fig.3) lead us to assume that in the case of shells which consist of dendritically branched amphiphiles, cavities and channels build up spontaneously which could provide a means of substrate size selection.



Fig.3: Computer-simulation of a dendrizyme. Cavities inside of the dendrimer shell are high-lighted in different colours


Dendritic surfactants of different generation numbers have been prepared very recently and the structures of the corresonding polyoxometalate complexes have been studied in depth. Investigations including extensive catalytic test reactions are currently being performed which shall help to confirm our concepts.



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