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

Abstract

We successfully prepared and crystallographically characterized the first intermolecular charge-transfer (CT)-based vapochromic compound, (EV)(HO)[Fe(CN)] (1-Wet, EV: 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)[Fe(CN)]. 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)] 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.

The Exploration and Analysis of the Magnetic Relaxation Behavior in Three Isostructural Cyano-Bridged 3d–4f Linear Heterotrinuclear Compounds

“…Due to the addition of the HCl solution, the lattice water molecule, and the terminal cyano group of the [Fe(CN) 6 ] 3− entity along the c axis were both protonated by a half-occupied hydrogen atom. Similar hydronium ions as counter cations has recently been found in a charge-transfer (CT) salt of hexacyanidoferrate(II) [44]. And the hydrogen isocyanide (HNC) system has been discovered in the crystal lattice of similar "cyanometallic acids" [45].…”

MnIII–FeIII Heterometallic Compounds within Hydrogen-Bonded Supramolecular Networks Promoted by an [Fe(CN)5(CNH)]2– Building Block: Structural and Magnetic Properties

“…To our best knowledge, 1 and 2 represent the very first example of [M–CN–M′] coordination complexes featuring [Fe­(CN) 6– x (CNH) x ] n − as a building block, although the protonation of hexacyanometallates have been studied spectroscopically and crystallographically in few cases . Note that this type of molecular architecture was obtained only with m -halogenated anilinium derivatives as cations.…”

Mono- and Di-Quaternized 4,4′-Bipyridine Derivatives as Key Building Blocks for Medium- and Environment-Responsive Compounds and Materials

“…The solvatochromism of such a CTC has not been given much attention but constitutes an interesting chromic effect of the response of the complex to its environment. Acknowledging the chromic behavior of this class of CTCs, Tanaka et al recently reported on the reversible vapochromic and environment-sensitive character of crystalline CTCs between ethyl-viologen and hexacyanoferrate(II) anions [91]. The color of a dry sample of CTC 53b (see Table 2) was found to be significantly different from that of a wet sample (see Figure 24a).…”