Electrically responsive structural transformations triggered by vapour and temperature in a series of pleochroic bis(oxalato)chromium(iii) complex salts

Abstract

Stimuli-responsive structural transformations of metal–organic materials are attracting considerable attention due to their potential use in functional switchable devices. In this study, we describe the successful demonstration of a reversible structural transformation triggered by humidity, methanol vapours, and temperature in a series of complex salts containing bis(oxalato)chromium(III) anions and bicyclic ammonium cations. Synthesis of complex salts with cationic DABCO and ABCO derivatives (DABCO = 1,4-diazabicyclo[2.2.2]octane; ABCO = 1-azabicyclo[2.2.2]octane), and bis(oxalato)chromium(III) [Cr(bpy)(C₂O₄)₂]⁻ and [Cr(phen)(C₂O₄)₂]⁻ anions (bpy = 2,2′-bipyridine, phen = 1,10-phenanthroline) was carried out using a green chemistry mechanochemical approach. The salts readily dissolve and can be prepared in the form of single-crystals that show pleochroism when viewed under polarised light, changing their colour from purplish red to orange, similar to the gemstone ruby, which also contains Cr(III) ions. The characterization of the new complex salts was performed by X-ray diffraction analysis, thermal analysis, infrared spectroscopy, UV-vis diffuse reflectance spectroscopy and impedance spectroscopy. Interestingly, this series of compounds exhibits reversible single-crystal-to-single-crystal transformations involving dehydration/desolvation and rehydration/resolvation processes. Moreover, these structural transformations are accompanied by significant changes in current response, which makes these compounds interesting for use as sensors and thermistors.