Inclusion of 1,8-octanediphosphonic acid in cucurbit[7]uril: a combined solid state and solution study

The inclusion of 1,8-octanediphosphonic acid (OA) in cucurbit[7]uril (Q[7]) has been investigated via a combined solid state and solution study. The reaction of OA and Q[7] in water led to the formation of a 1:1 inclusion compound 1 as a white crystalline material. Synchrotron single crystal X-ray diffraction revealed its detailed structure validated by Raman spectroscopy with featured vibrational modes from both organic molecules. Subsequent ¹H solution NMR data supported the formation of a 1:1 inclusion complex in aqueous solution. The changes in NMR chemical shifts are consistent with the solid-state orientation in which the phosphonic acid groups of OA are oriented externally to the carbonyl portals and the intervening alkyl chain lies within the cavity of Q[7]. However, the solution study also inferred possible formations of other complexes in solution. This work highlighted the advantage of combining solid state and solution studies to reveal the detailed molecular interactions involving complex inclusion phenomena.