Size-dependent compression of threaded alkyldiphosphate in head to head cyclodextrin [3]pseudorotaxanes

The encapsulation of guests in a confined space enables unusual conformations and reactivities. In particular, the compression of akyl chains has been obtained by self-assembled molecular capsules but such an effect has not been reported in solution for pseudorotaxane architectures. By exploiting the tendency of cyclodextrin (CD) to form head to head [3]pseudorotaxanes and the hydrogen bonding abilities of phosphate groups, we have studied the effect of the CD dimer cavity on the conformation of threaded α,ω-alkyl-diphosphate axles. The formation of [2]pseudorotaxanes and [3]pseudorotaxanes was investigated by a combination of NMR, ITC and X-ray diffraction techniques. In the solid state, the [3]pseudorotaxane with a C₈ axle presents a fully extended conformation with both terminal phosphate groups interacting with hydroxyl groups of the primary rim of CDs. Such hydrogen bonding interactions are also present with the C₉ and C₁₀ axles resulting in a compression of the alkyl chain with gauche conformations in the solid state. NMR studies have shown that this effect is maintained in solution resulting in a size-dependent progressive compression of the alkyl chain by the CD [3]pseudorotaxane architecture for C₉, C₁₀ and C₁₁ axles.