Determination of the Purity of Cucurbit[n]uril (n = 7, 8) Host Samples

The formation of highly stable inclusion complexes in aqueous solution between the organometallic cobaltocenium cation (Cob+) and the hosts cucurbit[7]uril (CB7) and cucurbit[8]uril (CB8) was used to develop a simple method, based on UV–vis titrations, to assay the purity of samples of these two hosts. The equilibrium association constant (K) of the Cob+@CB7 complex had been previously reported by our group as 5.7 × 109 M–1 at 25 °C in 50 mM sodium acetate medium. In this work, we determine a K value of 1.9 × 108 M–1 at 25 °C in the same medium for the Cob+@CB8 complex. The high stability of these complexes and their decreased molar absorptivity coefficients (at 261 nm), compared to that for free Cob+, lead to straightforward titration plots when graphing absorbance versus concentration of added CB7 (or CB8) host, at constant Cob+ concentration.