Polydentate Lewis acids with oriented acceptor functions

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Tailor-made structuring of molecular matter allows realizing specific chemical reactivity. Nature demonstrates in manifold ways how concerted spatial placement of reactive functions leads to specific reactivity (usually donor functions or hydrogen bridges). This is also frequently used in synthesis and in the construction of catalysts and receptors. In contrast to the donor functions used in this context (ligand design) examples with defined placement and orientation of acceptor functions (Lewis acids) are still scarce. However, such a concept would enable us to recognize anions and Lewis bases and consequently allow modifying their properties and reactivities. The aim of this project is therefore to place multiple Lewis-acid functions at suitable molecular scaffolds in a defined and oriented way for an optimized binding of Lewis basic substrates. The challenges were so far the very limited accessibility of suitable molecular scaffolds for attaching such functions. They have to be free of Lewis basic units; otherwise they would compete with the substrates for the acidic functions. In the first project period we have developed efficient access to rigid backbone molecules on the basis of hydrocarbons or organosilane scaffold molecules. These include 1,8-functionalized anthracenes, photochemically accessible anthracene dimers with four concordantly oriented functions, photochemically ‘dimerizable’ bridged bis(anthracenes) with switchable selectivities as well as triptycene with three concordantly oriented functions. In addition we found a very elegant synthesis of a syn-anthracen dimer with four alkyne functions pointing into one direction, whose high selectivity is the result of a hydrogen-bridged pre-complex during dimerization. Consequently, it is now clear which of the numerous tested systems provide practicable pathways for the multi-step syntheses to multidentate Lewis-acids, in order to give access to substantial amounts for further experiments aiming at their applications. We have demonstrated for several examples how to attach acceptor functions to these scaffolds. These functions are organometallic units of the elements Al, Ga and In (also Zn and Hg) and electronegatively substituted non-metallic functions of boron and silicon as well as gold functions binding via d10-d10 interactions. In the second project period we intend to bring in the harvest of the work and investment of the first period. We will prepare a variety of new multidentate Lewis-acids and study their chemical properties. These include possibilities of molecular recognition and complexing properties, the ability to enhance the acidity of Lewis acids by complexation with multidentate acceptors (aim ‘super Lewis-acids’), the generation of unusual complexes by chelating dispersive-type interactions, and the use of multidentate Lewis-acids in frustrated Lewis-pair chemistry.