Host-Guest Complexation Between Cyclodextrins and Hybrid Hexavanadates: What are the Driving Forces?

Host-guest complexes between native cyclodextrins (alpha-, beta- and gamma-CD) and hybrid Lindqvist-type polyoxovanadates (POVs) [V6O13((OCH2)(3)C-R)(2)](2-) with R = CH2CH3, NO2, CH2OH and NH(BOC) (BOC = N-tert-butoxycarbonyl) were studied in aqueous solution. Six crystal structures determined by single-crystal X-ray diffraction analysis revealed the nature of the functional R group strongly influences the host-guest conformation and also the crystal packing. In all systems isolated in the solid-state, the organic groups R are embedded within the cyclodextrin cavities, involving only a few weak supramolecular contacts. The interaction between hybrid POVs and the macrocyclic organic hosts have been deeply studied in solution using ITC, cyclic voltammetry and NMR methods (1D H-1 NMR, and 2D DOSY, and ROESY). This set of complementary techniques provides clear insights about the strength of interactions and the binding host-guest modes occurring in aqueous solution, highlighting a dramatic influence of the functional group R on the supramolecular properties of the hexavanadate polyoxoanions (association constant K-1:1 vary from 0 to 2 000 M-1) while isolated functional organic groups exhibit only very weak intrinsic affinity with CDs. Electrochemical and calorimetric investigations suggest that the driving force of the host-guest association involving larger CDs (beta- and gamma-CD) is mainly related to the chaotropic effect. In contrast, the hydrophobic effect supported by weak attractive forces appears as the main contributor for the formation of alpha-CD-containing host-guest complexes. In any cases, the origin of driving forces is clearly related to the ability of the macrocyclic host to desolvate the exposed moieties of the hybrid POVs.

Automated summary

The study investigated host-guest complexes formed between cyclodextrins and hybrid Lindqvist-type polyoxovanadates in aqueous solution. The results showed that the nature of the functional R group strongly influences the conformation and crystal packing of the complexes. Various techniques demonstrated the strength and modes of interactions between the hybrid polyoxoanions and macrocyclic organic hosts, indicating a significant impact of the functional group R on the supramolecular properties of the complexes.