Supramolecular Chemistry of Boron Clusters with Cyclodextrins: From Synthesis to Applications

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The host-guest chemistry of boron cluster compounds has remained unexplored until 2015, when our team not only published the first study on their host-guest complexation behavior, but also reported record affinities with cyclodextrins (CDs), which render this field of research not only original but also conceptually interesting and practically promising. Anionic icosahedral boron clusters of the dodecaborate type ((B12X12)2− with X = H, CI, Br, I) were found to form inclusion complexes with large CDs (e.g., β-,γ-, and ε-CD) in aqueous solution with exceedingly strong binding affinities (Ka =10^3 to 10^6 M^(−1)). We have interpreted the high affinities of the dodecaborate anions based on their high polarizability and the superchaotropic nature, revealing the chaotropic effect as a general driving force for the intrinsic binding of water-structure breaking anions (chaotropic anions) to hydrophobic cavities. Such high binding affinities offer new opportunities for future applications. Through this project, we would like to strengthen our contribution to this emerging research field by exploring new boron cluster derivatives with the formulae (B10X10)^2–, (B12X12)^2–, (B21H18)^–, and (B12X11R)^2–. Through changing the substituent X (H, F, Cl, Br, and I), not only the size of the cluster, but also its polarizability, can be systematically varied. Moreover, based on the chemical formula, the charge and the molecular geometries can also be varied. The proposed study will cover the parent and different functionalized CDs in order to mimic different types of intermolecular interactions, which are involved in the stability of the formed host-guest complexes, as well as other potential host molecules (e.g., cucurbiturils and calixarenes of different sizes). Moreover, we propose novel synthetic approaches for the monofunctionalization of boron clusters (halogenated and non-halogenated), which will be essential to further modify the clusters and open the perspective of future applications. Applications which serve to demonstrate the power of the dodecaborate binding motif, and which will be explored within the context of this proposal, include non-covalent labeling, surface modifications, constructing stimuli-responsive systems, chiral discrimination and resolution, and sensing. In order to tackle this task, a group working on synthetic chemistry (DG) and a group working on supramolecular chemistry (WN) have joined their complementary expertise.