Encapsulation of Polyhedral Borane Cluster trans-[B20H18]2- in Cyclodextrins: Stabilizing Effect in Aqueous Solution.
Abstract
The preparation of polyhedral boranes/cyclodextrin (CD) inclusion complexes was studied to develop novel supramolecular assemblies that could be useful for biological applications. The encapsulation of the trans-[B20H18]2- anion in cyclodextrins was investigated using nuclear magnetic resonance (NMR) spectroscopy, isothermal titration calorimetry (ITC), and molecular dynamics (MD) combined with density functional theory (DFT) calculations. The results revealed the formation of strong inclusion complexes with γ- and β-CD, while weaker interactions were observed with the smaller α-CD. Very large binding constant values of K1:1 = 137 ± 4 × 103 M-1 and K2:1 = 3.3 ± 0.8 × 1010 M-2 were determined for 1:1 and 2:1 [B20H18]2-:CD complexes with β- and γ-CD, respectively, consistent with the host-guest size-matching effect and the chaotropic effect of this large, low-charged boron cluster. This exceptionally strong affinity in host-guest complexation provides enhanced stability of the boron cluster in water against hydrolysis. It has been shown by NMR of [B20H18]2-/CD solutions in D2O that γ- and β-CD prevent the H/D exchange process, considered as the primary step of hydrolysis. The protective role of CD against anion degradation was found to be effective for more than four months, paving the way for the design of boron-based drugs with improved stability suitable for medical use.