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The switching of two macrocycles to form a pseudorotaxane following input of a single electron was investigated. The action relies on a two-site tetrazine and the identification of an ideal complementary ligand; 5,5′-dimethyl-2,2′-bipyridine. The switching relies on cooperative thermodynamics and was found to display anticooperative kinetics, where the first stepwise movement is 4 times faster than the second.
The existence of two rings in pseudorotaxanes presents opportunities for those rings to undergo double switching and cooperative mechanical coupling. To investigate this capability, we identified a new strategy for bringing two rings into contact with each other and conducted mechanistic studies to reveal their kinetic cooperativity. A redox-active tetrazine ligand bearing two binding sites was selected to allow for two mobile copper(I) macrocycle ring moieties to come together. To realize this switching modality, ligands were screened against their ability to serve as stations on which the rings are initially parked, ultimately identifying 5,5′-dimethyl-2,2′-bipyridine. The kinetics of switching a macrocycle in a single-site pseudorotaxane between bipyridine and single-site tetrazine stations were examined using electrochemistry. The forward movement was rate-limited by the bimolecular reaction between reduced tetrazine and bipyridine pseudorotaxane. Two bipyridines were then used with a double-site tetrazine to verify double switching of two rings. Our results indicated stepwise movements, with the first ring moving 4 times more frequently (faster) than the second. While this behavior is indicative of anticooperative kinetics, positive thermodynamic cooperativity sets the two rings in motion even though just one tetrazine is reduced with one electron. Double switching in this pseudorotaxane uniquely demonstrates how a series of independent thermodynamic states and kinetic paths govern an apparently simple mechanical motion.
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