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Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

Unique Hydration/Dehydration-Induced Vapochromic Behavior of a Charge-Transfer Salt Comprising Viologen and Hexacyanidoferrate(II) Rikako Tanaka,†,‡ Atsushi Okazawa,§ Hisashi Konaka,⊥ Akito Sasaki,⊥ Norimichi Kojima,∥ and Nobuyuki Matsushita*,†,‡ †

Department of Chemistry, College of Science and ‡Research Center for Smart Molecules, Rikkyo University, Nishi-Ikebukuro, Toshima-ku, Tokyo 171-8501, Japan § Department of Basic Science, Graduate School of Arts and Sciences, The University of Tokyo, Komaba, Meguro-ku, Tokyo 153-8902, Japan ⊥ XRD Application and Software Development Department, X-ray Instrument Division, Rigaku Corporation, Matsubara-cho, Akishima-shi, Tokyo 196-8666, Japan ∥ Toyota Physical and Chemical Research Institute, Yokomichi, Nagakute-shi, Aichi 480-1192, Japan S Supporting Information *

ABSTRACT: 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.


induce changes in crystal packing and intermolecular CT interaction. The (EV)(H3O)2[Fe(CN)6] salt obtained in this manner (1-Wet), contains two oxonium ions (H3O+). Interestingly, 1-Wet displays vapochromic behavior upon exposure to methanol and water vapors. Vapochromic compounds that show reversible color change induced by solvent vapor are attracting growing interest for chemical sensing applications.9−13 A variety of vapochromic compounds have been reported to date. In most cases, the absorption/ desorption of solvent molecules leads to changes in π−π interactions,10 metal−metal interactions,11 coordination structures,12 and ligand conformations,13 among others, that result in color changes. In 1-Wet, physical measurements and crystal structure analyses reveal that the unique hydrations and dehydrations of oxonium ions cause reversible crystal-packing changes that induce a shift in the intermolecular CT

Charge-transfer (CT) salts are known to exhibit physical properties based on intermolecular CT interactions, such as electric conduction and magnetism, among others.1−3 The salt of methyl viologen (MV2+, 1,1′-dimethyl-4,4′-bipyridine-1,1′diium) and hexacyanidoferrate(II), (MV)2[Fe(CN)6]·8H2O, is one such CT salt.4−6 In Prussian blue, one of the most famous intervalence CT compounds, the [Fe(CN)6]4− species acts as the electron-donor, while MV2+ is a well-known electronacceptor that readily forms intermolecular CT compounds with electron-donating species.7 While both [Fe(CN)6]4− and MV2+ ions are colorless, the CT salt of MV2+ and [Fe(CN)6]4− exhibits deep colors based on CT interactions; deep blue in the solid phase and red-purple in aqueous solution. These observations stimulated our interest in the colors of CT salts composed of organic acceptor cations and [Fe(CN)6]4−.4,8 Herein, a new CT salt of ethyl viologen (EV2+, 1,1′-diethyl4,4′-bipyridine-1,1′-diium) and the [Fe(CN)6]4− anion was prepared, because the modification of viologen was expected to © XXXX American Chemical Society

Received: December 9, 2017


DOI: 10.1021/acs.inorgchem.7b03100 Inorg. Chem. XXXX, XXX, XXX−XXX


Inorganic Chemistry

Kα radiation (0.71075 Å) and a Rigaku low-temperature unit. Intensity data were corrected for Lorentz-polarization effects and absorption factors based on a multiscan method. Its structure was solved by direct methods. Refinement was performed on F2 by full-matrix least-squares method with anisotropic displacement parameters for non-H atoms. The H atoms in EV2+ were geometrically placed and refined using a riding model. C(methyl)H, C(methylene)H, and C(aromatic) H bond distances were constrained to 0.98, 0.92, and 0.95 Å, respectively. Isotropic displacement parameters of the H atoms, Uiso(H), were tied to their parent atoms and set to 1.5Ueq(C). The H atoms of the oxonium ions were located in a difference Fourier map and refined using a riding model with the constraint Uiso = 1.5Ueq(O). The following programs were used: SHELXT-201417 for solutions, SHELXL-201418 for refinements, and ABSCOR19 for absorption corrections. The crystal data and refinement details are listed in Table 1.

interactions responsible for color of the compound. To the best of our knowledge, this is the first report of a vapochromic compound based on intermolecular CT interactions.


Materials and Preparation. (EV)Br2 and H4[Fe(CN)6] were prepared following literature methods (see Supporting Infomation).14,15 (EV)(H3O)2[Fe(CN)6] (1-Wet). A solution of H4[Fe(CN)6] (100 mg, 0.46 mmol) in water (5 mL) was added to a solution of (EV)Br2 (170 mg, 0.45 mmol) in water (5 mL). Fine purple crystals immediately formed. The crystals were collected by filtration, washed with water and air-dried. Yield: 120 mg (55%). Elemental analysis: Found: C, 51.64; H, 5.03; N, 23.90%; calcd for C20H24FeN8O2: C, 51.73; H, 5.22; N, 24.14%. Thermogravimetry: Loss of weight, 7.35%; calcd for C20H24FeN8O2: 2H2O, 7.76%. Selected IR bands (KBr pellet, cm−1): 3125−2871 (sh, C−H, str), 2679 (br, O−H, str), 2069 (sh, CN, str). (EV)(H)2[Fe(CN)6] (1-Dry). The ground purple powder of (EV)(H3O)2[Fe(CN)6] (1-Wet) was exposed to methanol vapor. The 10 min exposure gave brown powder. Elemental analysis: Found: C, 55.96; H, 4.71; N, 25.77%; calcd for C20H20FeN8: C, 56.09; H, 4.72; N, 26.16%. Selected IR bands (KBr pellet, cm−1): 3131−2877 (sh, C− H, str), 2069 (sh, CN, str). A sample of 1-Dry powder was also obtained by drying 1-Wet over silica gel. Powder X-ray diffraction (PXRD) patterns of the 1-Dry powders obtained by each method are in good agreement, as shown in Figure S3. Vapochromic Studies. Color change experiments involving exposure to vapor or drying over silica gel were carried out at ambient temperature. Powder samples of each phase were placed on pieces of filter paper on Petri dishes filled with solvent (water or methanol) or silica gel. Each sample was placed in a closed glass container (Figure S4). Physical Measurements. Elemental analyses were performed on an Elementar vario Micro cube analyzer. A JEOL GSX-4000 spectrometer was used to obtain the 1H NMR spectrum (400 MHz, D2O). Infrared (IR) spectra were collected by the KBr-disc method on a JASCO FT/IR-4000 spectrometer at room temperature. Routine PXRD patterns experiments were collected on a Rigaku RINT2000 diffractometer equipped with graphite monochromated Cu Kα radiation (1.5418 Å), with θ/2θ scans. Diffuse reflectance spectra were recorded on a JASCO V-650 spectrophotometer equipped with an integrating-sphere attachment (ISV-722); the spectra were referenced to powdered MgO packed in the same cell used for the samples. The spectra were displayed as Kubelka−Munk functions (1 − Rd)2/2Rd vs wavenumber (cm−1), where Rd is the relative reflectance. Thermogravimetric (TG) analyses were performed on a Rigaku Thermo Plus EVO2 TG8121 TG-DTA apparatus at a heating rate 10 °C/min under a flow of N2 gas (100 mL/min). Gravimetric analysis through humidification was performed on the same apparatus equipped with a Rigaku HUM-1F humidity generator. The sample was humidified to 60% relative humidity at 25 °C. Magnetic susceptibilities of the powder samples (ca. 30 mg) were measured using a Quantum Design MPMSXL5 SQUID magnetometer in the temperature range of 2−300 K under an applied magnetic field of 0.5 T. The magnetic response was corrected with diamagnetic blank data of the sample holder obtained separately. The core diamagnetism of the sample itself was estimated from the Pascal’s constant. 1-Wet was measured during heating, while 1-Dry was measured during cooling, at a sweep rate of 0.5 K min−1. 57Co in Rh was used as the γ-ray source for the 57Fe Mössbauer spectroscopy. The sample of 1-Wet was sealed tightly with silicone grease in an acrylic holder to avoid unexpected dehydration. The spectra were calibrated using the six lines of a bodycentered cubic iron foil (α-Fe), and the isomer shifts are quoted relative to α-Fe at room temperature. 57Fe Mössbauer spectra were fitted using MossWinn4.0 program.16 Single Crystal X-ray Structure Determination. A single crystal of 1-Wet was mounted on a glass fiber. Intensity data were measured using a Rigaku R-AXIS RAPID imaging plate diffractometer at 173 K. The diffractometer was equipped with graphite-monochromated Mo

Table 1. Crystallographic Data for 1-Wet and 1-Dry compound



formula formula weight crystal system space group temperature (K) radiation type crystal/powder color a (Å) b (Å) c (Å) β (deg) V (Å3) Z Dcalcd (g cm−3) μ (mm−1) 2θ range (deg) no. of measured reflns no. of independent reflns no. of observed reflns no. of parameters Ra/wRa Rpb/Rwpb S

C20H24FeN8O2 464.32 monoclinic P21/n 173 Mo Kα purple 8.4891(5) 13.8151(7) 9.9530(7) 95.726(2) 1161.44(12) 2 1.328 0.681 6.02−63.96 25076 3995 3047 143 0.0605/0.1647 − 1.100a

C20H20FeN8 428.29 monoclinic P21/n 298 Cu Kα brown 8.71314(16) 11.38979(19) 10.63734(17) 99.4532(4) 1041.32(3) 2 1.37 2.74 6.0−90.0 838 − − 140 − 0.0090/0.0123 1.276b

a R = (Σ||F0| − |Fc||)/Σ|F0| for observed reflections, wR = [{Σ(w(F02 − Fc2)2)}/Σ(w(F02)2)]1/2 for independent reflections. S = [{Σ(w(F02 − Fc2)2)}/(n − p)]1/2, where n and p are the numbers of the reflections and the parameters in the refinement, respectively. bRwp = [Σ{w(I0 − Ic)2}/Σ{w(I0)2}]1/2 is a weighted fitness metric, while Rp = Σ|I0 − Ic|/ ΣI0 is an unweighted fitness metric. S = {Rp/(N − M)}1/2, where N and M are the numbers of measured points and the parameters in the refinement, respectively.

Powder X-ray Structure Determination. PXRD data for 1-Dry were collected on a Rigaku SmartLab diffractometer operating with monochromated Cu Kα radiation (1.54187 Å) and equipped with a D/teX Ultra detector, in the 6−90° range with θ/2θ scans at ambient temperature. The powder was encapsulated in a 0.5 mm-diameter borosilicate glass capillary, which was rotated for isotropic data collection. All calculations were performed using the Rigaku PDXL2 software package. Diffraction data were indexed using DICVOL0620 program. The number of formula units in the cell (Z) was estimated to be two from the unit cell volume and density, and the space group was determined to be P21/n. Profile fitting was carried out using Pawley method. The structure was solved using the simulated annealing approach. The initial geometries and molecular models of EV2+ and [Fe(CN)6]4− were derived from the crystal structure of 1-Wet. The initial solution was used as the starting point for the Rietveld B

DOI: 10.1021/acs.inorgchem.7b03100 Inorg. Chem. XXXX, XXX, XXX−XXX


Inorganic Chemistry

approximately coplanar. The two ethyl groups of the EV2+ cation adopt a trans configuration. The C−C bond length between the two pyridinium rings of the EV2+ cation in 1-Wet (1.482(6) Å) is similar to the values reported for EV2+ salts, for example, 1.487(6) Å in (EV)I2,23 1.477 Å in (EV)(I3)2,24 and 1.484 Å in (EV)(tcnoet)2 (tcnoet = 1,1,3,3-tetracyano-2ethoxypropenide).25 This bond length is quite different to that of the methyl viologen radical cation (1.40 Å) in MV•+PF6−,26 indicating that the EV2+ cation has not been reduced to the EV•+ radical cation. The oxonium ion is surrounded by three [Fe(CN)6]4− anions and forms O−H···N hydrogen bonds with the cyanido ligands (Figure 1b, Table 2).

refinement. Atomic positions were refined subject to restraints that maintained atomic distances and angles in the ranges of those found for similar compounds in the Cambridge Structural Database (CSD).21 A global isotropic displacement parameter was used for all atoms. A profile fit and associated crystallographic data are shown in Figure S5 and Table 1. Crystallographic data in this paper have been deposited with the Cambridge Crystallographic Data Centre. The CCDC deposition numbers for 1-Wet and 1-Dry are 1585768 and 1585769. These data can be obtained free of charge from the Cambridge Crystallographic Data Centre via

RESULTS AND DISCUSSION Crystal Structure of (EV)(H3O)2[Fe(CN)6] (1-Wet). A solution of (EV)Br2 and H4[Fe(CN)6] gave deep purple crystals of (EV)(H3O)2[Fe(CN)6] (1-Wet), in which the ratio of [Fe(CN)6]4− to EV2+ was 1:1. Figure 1a reveals the

Table 2. Hydrogen Bonds Data for 1-Wet D−H···A

D−H (Å)

H···A (Å)

D···A (Å)

D−H···A (°)

O−H1A···N2 O−H1B···N1iiia O−H1C···N3iva

0.92 0.84 0.87

1.70 1.72 1.70

2.572(3) 2.531(3) 2.563(3)

157.2 163.0 170.0

Symmetry codes: (iii) −1/2 + x, 1/2 − y, 1/2 + z; (iv) 1/2 + x, 1/2 − y, 1/2 + z. a

As a result, a three-dimensional hydrogen-bonding network that stabilizes the crystal packing is constructed. As shown in Figure 2, the [Fe(CN)6]4− anions and the EV2+ cations are stacked alternately in columns that are parallel to the c axis.

Figure 1. (a) A view of the molecular structure of 1-Wet with the atomic numbering scheme. (b) A view of the hydrogen-bonding network around the oxionium ion in the crystal of 1-Wet. Displacement ellipsoids are drawn at the 50% probability level for non-H atoms. The magenta dashed lines in (b) represent hydrogen bonds [symmetry codes: (i) −x, −y, −z, (ii) 1 − x, 1 − y, −z, (iii) −1/ 2 + x, 1/2 − y, 1/2 + z, (iv) 1/2 + x, 1/2 − y, 1/2 + z].

Figure 2. A view of crystal packing of 1-Wet. Displacement ellipsoids are drawn at the 50% probability level for non-H atoms. The green square and the magenta dashed lines represent the unit cell and hydrogen bonds, respectively.

structures of the molecular components of 1-Wet. The asymmetric unit of 1-Wet contains one-half of an EV2+ cation, one-half of a [Fe(CN)6]4− anion, and one oxonium ion. All hydrogen atoms of the oxonium ion were located by difference Fourier map. The IR spectrum of 1-Wet also reveals the presence of the oxonium ion; a broad band at 2679 cm−1 assigned to its νOH stretch is evident (Figure S6).8,22 The iron atom is located at inversion centers with atomic coordinates of (0, 0, 0) and (1/2, 1/2, 1/2) and is coordinated by six cyanido ligands in a slightly distorted octahedral configuration. The EV2+ cation lies on the (0, 0, 1/2) and (1/2, 1/2, 0) inversion centers. The two pyridinium rings of the EV2+ cation are

Kotov et al. reported similar CT salts formed from EV2+ and [Fe(CN)6]4−, (EV)1.5K[Fe(CN)6]·12.5H2O, and (EV)1.5Li[Fe(CN)6]·14H2O.27,28 However, these crystal packings as well as the ratio of the EV2+ to the [Fe(CN)6]4− are completely different from that of 1-Wet. In our case, oxonium ions are incorporated into the crystal as counter cations, which seem to depend on the acidity of mother liquor. In fact, 1-Wet can be prepared from a dilute solution of sulfuric acid with K4[Fe(CN)6]·3H2O and (EV)Br2 (see Supporting Information C

DOI: 10.1021/acs.inorgchem.7b03100 Inorg. Chem. XXXX, XXX, XXX−XXX


Inorganic Chemistry for details). On the other hand, (EV)1.5K[Fe(CN)6]·12.5H2O and (EV)1.5Li[Fe(CN)6]·14H2O were obtained from solutions of A4[Fe(CN)6] (A = K or Li) and (EV)I2 in the absence of any mineral acid. These results support the hypothesis that the oxonium ion in 1-Wet forms due to the acidic conditions. Vapochromic Behavior. The 1-Wet powder changed color from purple to brown on exposure to methanol vapor at room temperature in 10 min (Figure 3). The PXRD pattern of the

Figure 5. UV−vis solid-state diffuse-reflectance spectra of 1-Wet (purple) and 1-Dry (brown).

Figure 3. Different appearances of 1-Wet and 1-Dry.

Figure 4. PXRD patterns of 1-Wet (purple) and 1-Dry (brown).

brown powder is different to that of 1-Wet (Figure 4). An ethanol-vapor exposure gave same brown powder as the methanol-vapor exposure (Figure S3). The brown powder was also obtained by drying 1-Wet over silica gel; the PXRD pattern of the silica-dried powder is in good agreement with that of the brown powder obtained by exposure to methanol vapor. This result suggests that the brown powder is a dehydrated form of 1-Wet. The brown powder, referred to as “1-Dry”, regained its purple color upon exposure to water vapor, and the PXRD pattern of the regained purple powder is in good agreement with that of the as-prepared purple 1-Wet powder. This result reveals that the change in color and crystal structure observed for 1-Wet and 1-Dry are reversible. The diffuse-reflectance spectra of 1-Wet and 1-Dry are shown in Figure 5. It is known that CT salts composed of viologen and [Fe(CN)6]4− display absorption bands in the visible region that correspond to intermolecular CT transition;4,5,27 1-Wet and 1-Dry exhibit absorption bands at 560 and 507 nm, respectively, which we assign to their intermolecular CT transitions. The band shift observed as 1-Wet transformed into 1-Dry suggests that each crystal phase exhibits different intermolecular CT interactions. Composition of 1-Dry. To further explore the vapochromic behavior, we characterized 1-Dry. At first, TG analyses were carried out. As shown in Figure 6a, 1-Wet lost 7.35% of its weight between 23 and 70 °C, which corresponds to the loss of two water molecules per chemical formula unit (7.76%). As mentioned above, the crystal structure of 1-Wet, (EV)(H3O)2[Fe(CN)6], contains oxonium ions rather than water. This TG analysis result, therefore, suggests that water molecules are released from the oxonium ions, leaving protons

Figure 6. (a) Thermogravimetric analysis trace for 1-Wet. (b) Gravimetric analysis trace for 1-Dry with humidification. The green and the light-blue lines represent weight gain and relative humidity, respectively.

behind. The color of the sample was brown following the TG experiment, and its PXRD pattern was in good agreement with that of 1-Dry (Figure S3). The TG analysis of 1-Dry revealed no weight loss until its decomposition at 180 °C (Figure S10). 1-Wet is transformed into 1-Dry not only by drying over silica gel, but also by exposure to methanol vapor. The TG analysis indicates that methanol is not incorporated into 1-Dry, rather it removes water from 1-Wet. Gravimetric analysis was performed on 1-Dry with humidification. The observed weight gain (7.88%) corresponds to the incorporation of two water molecules per chemical formula unit (7.76%), as shown in Figure 6b. These gravimetric results indicate that the changes in color and crystal packing between 1-Wet and 1-Dry are accompanied by the reversible adsorption and desorption of two water molecules per chemical formula unit. IR spectroscopy provides further evidence in support of these conclusions (Figure S6). As mentioned above, the oxonium-ion stretching band (νOH) was observed at 2679 cm−1 in 1-Wet; this band is absent in the IR spectrum of 1-Dry. D

DOI: 10.1021/acs.inorgchem.7b03100 Inorg. Chem. XXXX, XXX, XXX−XXX


Inorganic Chemistry

values that remain at ∼0 cm3 K mol−1 over the entire temperature range, which reveals that the iron atoms in both phases are in divalent low-spin states (S = 0). Hence, the color change is not originated in any redox reaction and spin-state transition of the iron atoms. On the basis of the results described above, the composition of 1-Dry is strongly suggested to be (EV)(H)2[FeII(CN)6], which is the dehydrated form of 1-Wet, (EV)(H3O)2[FeII(CN)6]. Presumably, desorption of water molecules from oxonium ions produces isolated protons, H+, and these protons remain in 1-Dry as counter cations. The elemental analysis data also support the formula of 1-Dry presented above (see Experimental Section). Crystal Structure Determination of (EV)(H)2[Fe(CN)6] (1-Dry). Although the composition of 1-Dry has been determined, its crystal structure is necessary in order to provide important information that clarifies the different CT interactions operating within 1-Wet and 1-Dry responsible for the observed color difference. Unfortunately, single crystals of 1-Wet disintegrate into crystalline 1-Dry powder during dehydration through exposure to methanol vapor, drying over silica gel, or heating. Consequently, the crystal structure of 1Dry was determined by PXRD analysis. As described above, the suggested formula of 1-Dry is (EV)(H)2[FeII(CN)6]. On the basis of this formula, the structure model for initial structure solution was modeled by 1:1 mixture of EV2+ and [Fe(CN)6]4−. In this structure model, the isolated protons, H+, as counter cations, were omitted because protons contribute poorly to XRD data. Following final refinement, a void calculation using Mercury30 was performed; the calculational results reveal that no void exists for solvent molecules. Figure 8a displays the structures of the molecular components of 1-Dry. The asymmetric unit of 1-Dry contains one-half of an EV2+ cation and one-half of a [Fe(CN)6]4− anion. The FeII atom of [Fe(CN)6]4− lies at (0, 0, 0) and (1/2, 1/2, 1/2) inversion centers. The EV2+ cation is also located at (0, 1/2, 0) and (1/2, 0, 1/2) inversion centers. Figure 8b shows the crystal-packing structure of 1-Dry. The [Fe(CN)6]4− anions and the EV2+ cations are arranged in the same manner as in 1Wet, although the stacking direction, which runs parallel to the b axis, is different from that of 1-Wet (c axis). It is notable that two [Fe(CN)6]4− anions are located close to each other, and two nitrogen atoms (N1 and N3) of their cyanido ligands are extremely close (Figure 9). The N1···N3 distance of 2.481(4) Å is much shorter than the sum of the van der Waals radii of two nitrogen atoms (3.10 Å). The isolated proton, H+, as a counter cation is likely to be trapped within this short contact to form a very short N···H···N hydrogen bond. This type of hydrogen bonding has been reported only in a few cyanido metal compounds, for example, in H4[Fe(CN)6] (N···N = 2.679 and 2.882 Å),31 H4[Mo(CN)8]·2O(C2H5)2·CH3OH·2H2O (N···N = 2.567, 2.583 Å),32 (AsPh4)[(OC)5Cr{CNHNC}Cr(CO)5] (N···N = 2.569 Å),33 and (MQ)2[Fe(CN)4(CNH)2] (MQ: 1methylquinoxalinium) (N···N = 2.572 Å).34 As a result of these N···H···N hydrogen bonds, a two-dimensional hydrogenbonding network is formed in the (−101) plane, as shown in Figure S13. Structural Comparison of 1-Wet and 1-Dry. A comparison of the crystal data of 1-Wet and 1-Dry reveals a shrinkage in the cell volume of ∼120 Å3 in transforming 1-Wet into 1-Dry. This shrinkage in volume is equal to the volume of four water molecules, which corresponds to the number of released water molecules per unit cell. As mentioned above, the

In CT salts, CT interactions depend on the electrondonating and -accepting species. The iron atom of the hexacyanido complex is generally stable in both divalent and trivalent forms, namely [FeII(CN)6]4− and [FeIII(CN)6]3−. The former behaves as electron-donor, but the latter does not. In order to elucidate the factors that affect the observed color change, the valences of the iron atoms need to be confirmed. To determine these valences, 57Fe Mössbauer spectroscopy and magnetic susceptibility measurements were performed for both 1-Wet and 1-Dry. 57Fe Mössbauer spectra were measured at 300, 170, and 77 K; the spectra acquired at 300 K are shown in Figure 7, while those at 170 and 77 K are shown in Figure S11.

Figure 7. 57Fe Mössbauer spectra of (a) 1-Wet and (b) 1-Dry at 300 K.

Each spectrum was deconvoluted to give the 57Fe Mössbauer parameters listed in Table 3 (300 K) and Table S1 (170 and 77 Table 3. 57Fe Mössbauer Spectral Parameters at 300 K 1-Wet 1-Dry a

I.S.a (mm s−1)

Q.S.b (mm s−1)

fwhmc (mm s−1)

−0.1109(13) −0.1200(11)

0.204(2) 0.2565(18)

0.281(4) 0.241(3)

Isomer shift. bQuadrupole splitting. cFull width at half-maximum.

K). The spectra for 1-Wet and 1-Dry resemble each other with an isomer shift (I.S.) of about −0.1 mm s−1 and a quite small quadrupole splitting (Q.S.) of ∼0.2 mm s−1. Such 57Fe Mössbauer parameters are typical values of low-spin iron(II) ions in hexacyanidoferrates(II), while almost all the Q.S. values are smaller than the natural line width of 0.195 mm s−1.29 The subtle Q.S. values seem to be attributable to slightly distorted octahedral environments around iron(II) ions in 1-Wet and 1Dry. At all temperatures, doublet were observed with small isomer shift (I.S.) and small quadrupole splitting (Q.S.), attributable to slightly distorted octahedral low-spin Fe(II) species, in both 1-Wet and 1-Dry. The magnetic susceptibilities of 1-Wet and 1-Dry, measured in an applied field of 0.5 T over the 2−300 K temperature range, are displayed in forms of χmolT vs T plot (Figure S12). Both 1-Wet and 1-Dry exhibit χmolT E

DOI: 10.1021/acs.inorgchem.7b03100 Inorg. Chem. XXXX, XXX, XXX−XXX


Inorganic Chemistry

Figure 8. (a) A view of the molecular structure of 1-Dry with the atomic numbering scheme. (b) A view of the crystal packing of 1-Dry. The green square and the magenta dashed lines in (b) represent the unit cell and short contacts, respectively [symmetry codes: (v) 1 − x, 1 − y, 1 − z, (vi) −x, 1 − y, −z].

angle are defined as shown in Figure 10. In 1-Wet, the slippage angles are 9.38°and 22.5° for the long and short axes,

Figure 9. A view of the unique short nitrogen-atom contact in 1-Dry. The magenta dashed line represents the short contact [symmetry code: (vii) 3/2 − x, −1/2 + y, 3/2 − z].

crystal packings of both 1-Wet and 1-Dry adopt structures in which [Fe(CN)6]4− and EV2+ ions stack alternately. The crystal packing of 1-Wet is stabilized by a three-dimensional hydrogenbonding network involving oxonium ions and the nitrogen atoms of [Fe(CN)6]4−. In 1-Dry, the sites formerly occupied by the oxonium ions in 1-Wet are replaced by isolated protons, that facilitate the construction of a two-dimensional hydrogenbonding network involving the nitrogen atoms of [Fe(CN)6]4−. Reversible hydration/dehydration behavior of oxonium ions is rare. Haushalter et al. reported the dehydration of the oxonium ion in the solid state;35 although the crystal structure of an oxonium-ion-containing compound was reported, that of the dehydrated form was not. To the best of our knowledge, the current study is the first that reports the crystal structures of both hydrated and dehydrated forms of an oxonium-ioncontaining compound, namely 1-Wet and 1-Dry. To elucidate the cause of the observed difference in CT interactions in 1-Wet and 1-Dry, we compared their crystal structures in detail. The electron-donor and -acceptor distances, defined as the distances between the centroids of the [Fe(CN)6]4− and the EV2+ ions, are 4.98 and 5.70 Å for 1Wet and 1-Dry, respectively. According to the literatures,27,36 CN···N (viologen) and Fe···N (viologen) distances are often evaluated as the electron-donor and -acceptor distances. Those distances are also listed in Table S2. To estimate the overlap between the [Fe(CN)6]4− and the EV2+ ions in the stacking direction, the long and short axes of EV2+ and the slippage

Figure 10. (a) The long and short axes definition of EV2+. (b) The definition of the slippage angle.

respectively, while the analogous slippage angles of 1-Dry are 27.8° and 28.1°, respectively, which are larger than those observed for 1-Wet (Figure S14). In summary, the electrondonor/-acceptor distance in 1-Dry is longer than that in 1-Wet; consequently the electron-donor/-acceptor overlap in 1-Dry is lower than that in 1-Wet. Hence, the intermolecular CT interaction in 1-Wet is stronger than that in 1-Dry, because the large overlap generally induces strong interactions. As mentioned above, the CT band of 1-Wet appears on lower energy side of 1-Dry. From these results, we conclude that the F

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Inorganic Chemistry

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stronger the CT interaction is, the lower the transition energy of the CT band is in present salts. A similar relationship is reported in the intervalence CT compound of [Pt(en)2][PtX2(en)2](ClO4)4 (X = Cl, Br, I).37

CONCLUSIONS In conclusion, we prepared the first vapochromic CT salt composed of EV2+ and [Fe(CN)6]4− and investigated the mechanism associated with its color change. We determined that changes in crystal packing induced by hydration and dehydration of the oxonium ion alter the intermolecular CT interactions, resulting in the observed color changes. This work provides a new approach for the production of novel vapochromic materials.


S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.7b03100. Method for the preparation of 1-Wet using dilute sulfuric acid, PXRD patterns, a photographic image of the experimental set up for vapor exposure, PXRD profile for the structure analysis of 1-Dry, a thermogravimetric analysis trace, IR spectra, 57Fe Mössbauer spectra, a table of 57Fe Mössbauer parameters, and crystal structures (PDF) Accession Codes

CCDC 1585768 and 1585769 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: + 44 1223 336033.


Corresponding Author

*E-mail: [email protected] ORCID

Nobuyuki Matsushita: 0000-0003-1022-430X Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENTS A part of this work was conducted at Advanced Characterization Nanotechnology Platform of The University of Tokyo, supported by “Nanotechnology Platform” of the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan. This study was partly supported by MEXT-Supported Program for the Strategic Research Foundation at Private Universities (project no. S1311027) from the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan.


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