Thermodynamic and electrochemical study of tailor-made crown ethers for redox-switchable (pseudo)rotaxanes

Abstract

Crown ethers are common building blocks in supramolecular chemistry and are frequently applied as cation sensors or as subunits in synthetic molecular machines. Developing switchable and specifically designed crown ethers enables the implementation of function into molecular assemblies. Seven tailor-made redox-active crown ethers incorporating tetrathiafulvalene (TTF) or naphthalene diimide (NDI) as redox-switchable building blocks are described with regard to their potential to form redox-switchable rotaxanes. A combination of isothermal titration calorimetry and voltammetric techniques reveals correlations between the binding energies and redox-switching properties of the corresponding pseudorotaxanes with secondary ammonium ions. For two different weakly coordinating anions, a surprising relation between the enthalpic and entropic binding contributions of the pseudorotaxanes was discovered. These findings were applied to the synthesis of an NDI-[2]rotaxane, which retains similar spectroelectrochemical properties compared to the corresponding free macrocycle. The detailed understanding of the thermodynamic and electrochemical properties of the tailor-made crown ethers lays the foundation for the construction of new types of molecular redox switches with emergent properties.

Keywords: crown ether; isothermal titration calorimetry; redox chemistry; rotaxanes; supramolecular chemistry.

Figure 1

Structures of the compounds used in this study: a) crown-8 analogs; b) crown-7 analogs; c) secondary ammonium axles. BArF₂₄⁻ represents tetrakis(3,5-bis(trifluoromethyl)phenyl)borate.

Scheme 1

Schematic representation of synthetic routes towards TTFC7, exTTFC7, NDIC7, and NDIC8.

Figure 2

Solid-state structures of a) exTTFC7 (CH₃CN molecule omitted for clarity), b) NDIC7 (CH₃CN molecule omitted for clarity) and c) NDIC8.

Figure 3

a) Synthesis of the [2]rotaxane NDIRot. b) Stacked ¹H NMR spectra (700 MHz, CDCl₃, 298 K) of NDIC8 (top), NDIRot (middle), and A1·BArF₂₄ (bottom). The signal assignment was done by 2D NMR spectroscopy.

Figure 4

UV–vis–NIR spectra obtained by spectroelectrochemical measurements (0.1 M n-Bu₄PF₆, CH₂Cl₂/CH₃CN 1:1, 298 K) of a) NDIC7 (0.5 mM), b) NDIC8 (0.5 mM), and c) NDIRot (1 mM). Potentials are referenced against a silver wire pseudo-reference electrode.