Host–guest interactions in an inverted cucurbit[7]uril with α,ω-alkyldiammonium guests

Abstract

Host–guest interactions in an aqueous solution of an inverted cucurbit[7]uril (iQ[7]) with α,ω-alkyldiammonium guests (H₃N(CH₂)_xNH₃Cl₂, x = 2, 4, 6, 8, 10, and 12 named 1–6) were investigated by nuclear magnetic resonance (NMR) and isothermal titration calorimetry (ITC). ¹H NMR spectroscopy revealed that all alkyldiammonium cations are involved in interactions with iQ[7] that fall into three groups. Averaged signals of free and bound guests with shorter alkyl chains including 1–3 indicate a higher binding and unbinding exchange ratio, within the time scale of the NMR experiments. Averaged signals for the guest 1 compound exhibited a downfield shift indicative of a dominant portal interaction between 1 and iQ[7], in contrast to guest 2 and 3 compounds that exhibited an upfield shift. Averaged signals of guests with longer alkyl chains such as 4–6 displayed a lower binding and unbinding exchange ratio on the NMR time scale. The occurrence of upfield shifts for all guests except 1 indicate a dominant inclusion interaction between the guests and iQ[7]. ITC experiments showed that the thermodynamics of complexation of the alkyldiammonium guests with iQ[7] were both enthalpically and entropically driven. Interestingly, complexation was predominantly entropically driven for the first guest group, with the exception of guest 3, but was enthalpically driven for the third guest group. Host–guest interactions of the more conventional cucurbit[7]uril (Q[7]) with these α,ω-alkyldiammonium guests were also investigated under the same conditions for comparison.