Unique Hydration/Dehydration-Induced Vapochromic Behavior of a Charge-Transfer Salt Comprising Viologen and Hexacyanidoferrate(II)

We successfully prepared and crystallographically characterized the first intermolecular charge-transfer (CT)-based vapochromic compound, (EV)(H₃O)₂[Fe(CN)₆] (1-Wet, EV²⁺: 1,1′-diethyl-4,4′-bipyridine-1,1′-diium), an ethyl viologen-containing CT salt. 1-Wet, which is purple in color, is transformed into a brown powder (1-Dry) upon exposure to methanol vapor, drying over silica gel, or heating; 1-Dry returns to 1-Wet upon exposure to water vapor. These color changes are induced by hydration and dehydration, and gravimetric analyses suggest that 1-Dry is the dehydrated form of 1-Wet, namely, (EV)(H)₂[Fe(CN)₆]. Interestingly, desorption of water molecules from the oxonium ions in 1-Wet produces isolated protons (H⁺) that remain in 1-Dry as counter cations. Powder X-ray crystal structure analysis of 1-Dry reveals the presence of very short contacts between the nitrogen atoms of adjacent [Fe(CN)₆]^4- anions in the crystal. The isolated protons are trapped between the nitrogen atoms of cyanido ligands to form very short N···H···N hydrogen bonds. A detailed comparison of the crystal structures of 1-Wet and 1-Dry reveals that hydration and dehydration induce changes in crystal packing and intermolecular CT interactions, resulting in reversible color changes.