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.

Inchworm movement of two rings switching onto a thread by biased Brownian diffusion represent a three-body problem

“…More likely is the independent, stepwise threading of rings to produce an observable intermediate with a slower second step (Fig. 1C) (13). Last, two rings can thread in an inchworm movement ( Fig.…”

“…Molecular machines and switches with three components are well represented by [3]pseudorotaxanes (9)(10)(11)(12)(13)(14), [3]catenanes (15)(16)(17)(18), and [3]rotaxanes (19)(20)(21)(22)(23)(24)(25). Examples include unidirectional motion of two small rings around a larger ring (15) and muscle-like contraction of two rings along a dumbbell in a [3] rotaxane (20).…”

“…Studies on the impact of two rings on kinetics and mechanism are considerably rarer (11, 13, 15-17, 24, 25). Kinetics can be studied when timescales of motion match techniques used for observing switching (11,13,24,(26)(27)(28)(29)(30)(31)(32). Here, we take advantage of cyclic voltammetry (CV) to study redox-driven kinetics and mechanism of switching on 1-ms to 10-s timescales.…”

“…We have previously described switches composed of two rings and a single tetrazine thread exemplifying the three-body problem. With copper(I)-phenanthroline rings (13), there are no inter-ring interactions and the rings can be considered independent of each other. Stepwise threading of a first ring onto a tetrazine anion produced an observable [2]pseudorotaxane intermediate with a…”

Heteroleptic copper phenanthroline complexes in motion: From stand-alone devices to multi-component machinery

“…This detailed mechanistic investigation reveals an alternative switching modality to run molecular machines. A conceptually analogous approach was utilized by the same group for triggering a double switching protocol in palindromic [3]pseudorotaxanes [56].…”