Complexation thermodynamics of cucurbit[6]uril with aliphatic alcohols, amines, and diamines

Using the isothermal calorimetric titration technique, we determined the stability constants (K), reaction enthalpy (Delta H degrees), and entropy (Delta H degrees) for complexation of cucurbit[ 6] uril (CB[ 6]) with a series of aliphatic alcohols, monoand diamines, as well as spermidine and spermine, in aqueous solutions of alkali metal chlorides. The K for spermine reached 5.4 x 10(10) M-1 in 0.2M LiCl, which is the largest amongst the values reported for CB[ 6]. Propylamine forms the strongest 1: 1 complex with CB[6] (K = 21000 M-1) in 0.1 M Na acetate buffer, which is driven exclusively by entropy. A comparison of K for 1,3-propanediamine versus 1,4-butanediamine reveals an extraordinary > 60000-fold enhancement in affinity, which is the largest increment/CH2 ever observed in supramolecular chemistry. The present results in combination with our ESI-MS data reported recently unambiguously demonstrate that CB[6] exists in aqueous solution of alkali metal salts exclusively as a dicationic species, e. g. [CB[ 6]center dot 2Na](2+).