Double Switching of Two Rings in Palindromic [3]Pseudorotaxanes: Cooperativity and Mechanism of Motion

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 <i>single</i>-site [2]­pseudorotaxane between bipyridine and <i>single</i>-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 <i>double</i>-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 <i>anti</i>cooperative 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

Self-assembly snapshots of a 2 × 2 copper(I) grid

<div><p>Self-assembled 2 × 2 grids have been characterised as high-fidelity species produced when the correct stoichiometric ratios are met, but rarely are the individual steps leading to and from their formation characterised. Here, we present such a study using equilibrium-restricted factor analysis to model a set of UV–vis spectra starting from a bis-bidentate ligand to the assembly of a 2 × 2 grid complex upon titration with 1 equiv. of [Cu(MeCN)₄](PF₆) and to disassembly upon further titration. Intermediate species [Cu<b>L</b>₂]⁺, [Cu₂<b>L</b>₃]²⁺, [Cu₃<b>L</b>₂]³⁺ and [Cu₂<b>L</b>]²⁺ are evidenced along the assembly and disassembly pathways. Complementary ¹H NMR titrations are consistent with the rich set of complexes and equilibria involved. Given the nature of the assembly process, the assembly is entropy driven and likely enthalpy driven as well. The disassembly process is both enthalpy and entropy driven according to the standard free energy values derived from the modelling of the spectrophotometric titration data.</p></div