Molecular meccano, 49 - Pseudorotaxanes and catenanes containing a redox-active unit derived from tetrathiafulvalene

Two bis(2-oxy-1,3-propylenedithio)tetrathiafulvalene-containing acyclic pol yethers and two macrocyclic polyethers, each incorporating one bis(2-oxy-1, 3-propylenedithio)tetrathiafulvalene unit and one p-phenylene ring, have be en synthesized. The two acyclic polyethers are bound by cyclobis (paraquat- p-phenylene) with pseudorotaxane geometries in solution. The two macrocycli c polyethers have been mechanically interlocked with this tetracationic cyc lophane to form [2]catenanes in a kinetically controlled self-assembly proc ess. The X-ray crystallographic analysis of one of the two [2]catenanes and H-1-NMR-spectroscopic studies of both compounds showed that the p-phenylen e ring of the macrocyclic polyether is located inside the cavity of the tet racationic cyclophane, while the bis(2-oxy-1,3-propylenedithio)tetrathiaful valene unit resides alongside. The [2]pseudorotaxanes and [2]catenanes show broad bands around 780 nm, arising from the charge-transfer (CT) interacti on between the electron-donor tetrathiafulvalene(TTF-)type unit and the ele ctron-acceptor units of the tetracationic cyclophane. H-1-NMR-spectroscopic studies have shown that the [2]pseudorotaxanes dissociate into their separ ate components upon oxidation of the TTF-type unit, as a result of disrupti on of the CT interaction and electrostatic repulsion between the tetracatio nic host and the newly formed monocationic guest. The subsequent reduction of the guest to its neutral state affords back the pseudorotaxane-type comp lex restoring the original equilibrium. The results obtained from electroch emical experiments are consistent with the reversible, redox-driven dethrea ding/rethreading process observed by H-1-NMR spectroscopy. Variable-tempera ture H-1-NMR-spectroscopic investigations have revealed two dynamic process es, both involving the relative movements of the mechanically interlocked c omponents in the [2]catenanes. The two consecutive oxidation processes invo lving the TTF-type unit, observed electrochemically, are displaced toward m ore positive potentials compared with the free cyclic polyethers. The two r eversible two-electron reduction processes, characteristic of free cyclobis (paraquat-p-phenylene), separate into four reversible one-electron processe s because of the topological difference between the "inside" and "alongside " electron-acceptor units in the [2]catenane.