Paraquat Substituent Effect on Complexation with a Dibenzo-24-crown-8-Based Cryptand

Dibenzo-24-crown-8-based cryptand 4 forms 1:1 inclusion complexes with three paraquat derivatives, P1, P2, and P3, as demonstrated by proton NMR spectroscopy and X-ray analysis. However, it was found that methyl-substituted paraquat derivatives, P2 and P3, can bind cryptand 4 more strongly than non-methyl-substituted paraquat derivative P1. The association constants (Ka) were determined in acetone by using a UV−vis titration method to be 5.0 × 103 M-1 for 4·P1, 1.0 × 105 M-1 for 4·P2, and 1.2 × 105 M-1 for 4·P3, respectively. In the solid state, 4·P2 and 4·P3 have similar T-type inclusion complexation conformations, which are very different from the pseudorotaxane-type complexation conformation of 4·P1. Theoretical calculations were done to explain these experimental results.