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 –(OCH₂CH₂)_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., K_ass = 1.0 × 10³ M⁻¹ for n = 0 and K_ass = 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 (k_t) and dethreading (k_d) rate constants are larger in MeCN than in H₂O and decrease with increasing chain length (e.g., k_t > 10⁸ and k_t = 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.