Threading/dethreading processes in pseudorotaxanes. A thermodynamic and kinetic study

Abstract

Pseudorotaxanes are host-guest complexes made of a thread-like component penetrating a bead-like component. We have investigated, by a stopped-flow technique, the threading/dethreading kinetics of pseudorotaxanes formed between a tetracationic cyclophane, cyclobis(paraquat-p-phenylene), and thread-like compounds consisting of a naphthalene ring system with appended -(OCH 2CH2)nOH groups (n from 0 to 3) on the 1 and 5 positions. The association and rate constants have been measured in MeCN (from 276 to 293 K) and H2O (from 276 to 313 K). In MeCN solution, the association constant increases with increasing length of the appended chain (e.g., Kass = 1.0 × 103 M-1 for n = 0 and Kass = 4.0 × 104 M-1 for n = 3, at 293 K); in H2O, the association constants are larger than in MeCN, but almost independent of the chain length. Both the threading (kt) and dethreading (kd) rate constants are larger in MeCN than in H 2O and decrease with increasing chain length (e.g., kt > 108 and kt = 4 × 106 M-1 s-1 for n = 0 and n = 3, respectively, in H2O at 276 K). Thermodynamic and kinetic parameters have been obtained and the role played by the length of the chains appended to the naphthalene ring system is discussed.