title: Double Switching of Two Rings in Palindromic [3]Pseudorotaxanes: Cooperativity and Mechanism of Motion.
abstract: The existence of two rings in [3]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 [2]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 [2]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 [3]pseudorotaxane uniquely demonstrates how a series of independent thermodynamic states and kinetic paths govern an apparently simple mechanical motion.
authors.affiliation: Department of Chemistry, Indiana University 800 East Kirkwood Avenue, Bloomington, Indiana 47405, United States.
authors.forename: Christopher R
authors.initials: CR
authors.lastname: Benson
authors.affiliation: Department of Chemistry, Indiana University 800 East Kirkwood Avenue, Bloomington, Indiana 47405, United States.
authors.forename: Andrew I
authors.initials: AI
authors.lastname: Share
authors.affiliation: Department of Chemistry, Indiana University 800 East Kirkwood Avenue, Bloomington, Indiana 47405, United States.
authors.forename: Matthew G
authors.initials: MG
authors.lastname: Marzo
authors.affiliation: Department of Chemistry, Indiana University 800 East Kirkwood Avenue, Bloomington, Indiana 47405, United States.
authors.forename: Amar H
authors.initials: AH
authors.lastname: Flood