Bilability is Defined when One Electron is Used to Switch between Concerted and Stepwise Pathways in Cu(I)-Based Bistable [2/3]Pseudorotaxanes

Abstract

Supramolecular switches operate as simple machines by using a stimulus to turn stations off and on, generating thermodynamic differences that define bistability and enable motion. What has not been previously investigated, yet is required to gain further control over molecular movements for complex operations, is an understanding of how the same stimulus can also switch pathways off and on, thus, defining the kinetic property of bilability. To address this challenge, the mechanisms of the forward and return reactions of redox-switchable Cu(I)-based [2/3]pseudorotaxanes have been quantitatively characterized utilizing mechanistic cyclic voltammetry and employing a series of isosteric bis-bidentate ligands. First, the bistability of the switch is retained across the series of ligands: Reduction of the ligand drives the reaction forward where a [2]pseudorotaxane switches into a reduced [3]pseudorotaxane and reoxidation drives the switching cycle back to the beginning. Second, the switch is bilabile with the forward reaction following an association-activated interchange pathway (concerted), whereas the reverse reaction follows a different dissociation-based dethreading pathway (stepwise). The forward reaction is more sensitive to denticity (bidentate tetrazinyl ligand, k(2) = 12,000 M(-1) s(-1), versus the monodentate pyrazinyl ligand, k(2) = 1500 M(-1) s(-1)) than to electronics (k(2) = 12,000 M(-1) s(-1) for methyl and trifluoromethyl substituents). The rate of return with the pyrazinyl ligand is k(1) = 50 s(-1). Consequently, both the mechanism and the thermodynamics of switching are stimuli dependent; they change with the oxidation state of the ligand. These findings have implications for the future design of molecular motors, which can be built from systems displaying allosterically coupled bistability and bilability.

Measurement of the ground-state distributions in bistable mechanically interlocked molecules using slow scan rate cyclic voltammetry

“…Understanding the mechanism by which intramolecular noncovalent bonding interactions occur-especially in those systems that can undergo reversible switching eventsholds the key to how artificial molecular machines (14) can be engineered to fit the demands of a given function. Gaining intimate knowledge of the mechanisms (15)(16)(17)(18) governing the relative molecular motions of their components is, therefore, a pursuit that yields crucial information for the design of artificial molecular machines. Bistable mechanically interlocked molecules (MIMs) [namely, bistable catenanes (19,20) and rotaxanes (21), whose syntheses are often templated by noncovalent bonding interactions that "live-on" in the MIMs] have experienced a frenzy of intense research activity in recent years.…”

Interconverting Two Classes of Architectures by Reduction of a Self‐Sorting Mixture

“…Most selfassemblies will disassemble upon stimulation (e.g., pseudorotaxanes [6,7] and tetrahedra [8] ). Herein, we demonstrate this capability with the facile and reversible interconversion between two different architectures (Scheme 1) using electrons instead of chemicals.We self-assembled different architectures around a dynamic Cu I ion [13] and used ligand reduction [14] to switch between them. While the ability to transform between two different self-assembled architectures (pincer and grid, [10] ring and cage, [11] from 2D to 3D [12] ) is growing in number, these demonstrations make use of chemical stimuli.…”

Design and Synthesis of Nonequilibrium Systems

“…49,88 A beginning to this endeavor has been provided by Flood 89,90 in his design of a bilabile switch where the pathway going between switch states depends on the external conditions. Recently, Leigh 91,92 has discussed mechanisms for turning catalytic functions off or on by moving a ring from one recognition site to another.…”