Extended and Contorted Conformations of Alkanediammonium Ions in Symmetrical α,α′,δ,δ′-Tetramethylcucurbit[6]uril Cavity

Binding
interactions between symmetrical α,α′,δ,δ′-tetramethylcucurbit[6]­uril
(TMeQ[6]) and a series of alkyldiammonium ions in aqueous solution
and in the solid state were investigated by 1H NMR spectroscopy,
MALDI-TOF mass spectrometry, X-ray crystallography, and isothermal
titration calorimetry (ITC). Their 1H NMR spectra reveal
that the actual binding behaviors vary depending upon the alkyl chain
length. Their single-crystal X-ray diffraction analyses indicate the
guest 1,2-ethanediammonium is located outside of the TMeQ[6] portal,
while the other four alkyldiammonium guests can be accommodated in
the TMeQ[6] cavity, forming 1:1 inclusion complexes. Most importantly,
the long-chain alkyldiammoniums (1,8-octanediammonium and 1,10-decanediammonium)
take a contorted conformation when bound within the TMeQ[6] cavity.
Additionally, ITC experiments show that the complexation of the alkyldiammonium
guests with TMeQ[6] is mainly enthalpy driven, which benefits from
ion–dipole interactions.