Protonic Conductivity and Hydrogen Bonds in (Haloanilinium)(H2PO4

Article pubs.acs.org/JPCC

Protonic Conductivity and Hydrogen Bonds in (Haloanilinium)(H2PO4) Crystals Yuya Yoshii,† Norihisa Hoshino,†,‡ Takashi Takeda,†,‡ and Tomoyuki Akutagawa*,†,‡ †

Graduate School of Engineering, Tohoku University, 2 Chome-1-1 Katahira, Aoba Ward, Sendai 980-8579, Japan Institute of Multidisciplinary Research for Advanced Materials (IMRAM), Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai 980-8577, Japan

‡

Downloaded by NEW YORK MEDICAL COLLEGE on August 29, 2015 | http://pubs.acs.org Publication Date (Web): August 28, 2015 | doi: 10.1021/acs.jpcc.5b06665

S Supporting Information *

ABSTRACT: Brønsted acid−base reactions between phosphoric acid (H3PO4) and haloanilines in alcohols formed 1:1 proton-transferred ionic salts of (Xanilinium+)(H2PO4−) and 2:1 ones of (X-anilinium+)2(HPO42−) (X = F, Cl, Br, and I at o, m, and p positions of anilinium). Only the former 1:1 single crystals showed proton conductivity under the N2 condition, and the latter 2:1 crystals became protonic insulators. In crystals, diverse hydrogen-bonding structures from 1D to 2D networks were achieved by modification of the molecular structure of X-anilinium cations. The protonic conductivity was associated with the connectivity of H2PO4− anions in the hydrogen-bonding networks. The hydrogen-bonding ladder chains in (o-cloroanilinim)(H2PO4−) and (obromoanilinim)(H2PO4−) resulted in the highest protonic conductivity of ∼10−3 S cm−1. The protonic conductivity of the ladder-chain (H2PO4−) arrangements was higher than that of 2D sheets. The motional freedom of protons was analyzed by difference Fourier analysis of the single-crystal X-ray structure. The 2D layer, including (H2PO4−)2 dimers and (H2PO4−)4 tetramers, showed relatively low protonic conductivity, and the activation energy for proton conductivity was lowered by increasing the hydrogen-bonding connectivity and uniformity between H2PO4− anions. triazole)2(H2PO4)2, the increase in the value of μ in the 2D coordination environment resulted in a higher σH of ∼10−4 S cm−1 at relatively low temperatures (∼423 K),32 suggesting that an appropriate chemical design of the cationic part of the hydrogen-bonding H2PO4− anion network could optimize the proton hopping path and protonic mobility μ. On the contrary, the control in carrier density n of the protonic conductor has been reported to depend on the chemical modification of acidity in the 1,4-benzenedicarboxylate derivative of MIL-53, which is also known as a metal−organic framework (MOF). The introduction of a highly acidic substituent into 1,4benzenedicarboxylate ligand increased σH due to enhancement in the n value.33 A correlation between the hydrogen-bonding structures and proton conductivities was one of the essential points of view to design new superprotonic conductors. For simplicity, we employed quite significantly simplified proton-conducting passes of the hydrogen-bonding H2PO4− network.22,32 Herein, we focused on a simple, pure organic protonic conductor based on phosphate anions, created by the replacement of Cs+ cations in CDP or CHS with the organic ones. A simple haloanilinium derivative (XAni+) was utilized as the countercation of the

1. INTRODUCTION Various types of organic or inorganic materials (e.g., Nafion membrane,1 porous materials (MOFs and COFs),2−7 metal oxides,8−14 ionic salts,15−18 and polymer films bearing phosphoric acid19,20) have been applied as proton-conducting electrolytes for fuel-cell applications. Among them, crystalline materials of cesium dihydrogen phosphate (CsH2PO4, CDP) and cesium hydrogen phosphate (Cs2HPO4, CHS) have attracted much attention because of their relatively simple structures and conduction mechanisms.21−29 A high-temperature phase (Phase I at T > 503 K) and low-temperature phase (Phase II at T < 503 K) of CDP showed protonic conductivity (σH) of approximately 10−2 and 10−6 S cm−1, respectively,20 which were well characterized by solid NMR spectra and neutron quasielastic scattering.21−28 In Phase I, the superprotonic conducting state (σH ≈ 10−2 S cm−1) was achieved by thermally activated isotropic molecular rotation of tetrahedral H2PO4− anions. Relatively high proton conduction under low humidity was realized in the temperature range 100−300 °C of CDP crystals, in which the platinum metal showed high catalytic activity in a fuel-cell application.29,30 Protonic conductivity σH is proportional to the carrier density (n), proton charge (e), and mobility (μ) and is expressed as σH = neμ.31 Because the charge of a proton is constant (+1.602 × 10−19 C), the controls in n and μ increase the σH value. In the metal-coordination complex of Zn(1,2,4© XXXX American Chemical Society

Received: July 11, 2015 Revised: August 24, 2015

A

DOI: 10.1021/acs.jpcc.5b06665 J. Phys. Chem. C XXXX, XXX, XXX−XXX

Article

The Journal of Physical Chemistry C

Scheme 1. Brønsted Acid−Base Reaction between Haloaniline and Phosphoric Acid: Both the 1:1 and 2:1 Binary Ionic Organic Salts of (XAni+)(H2PO4) and (XAni+)2(HPO42−) Were Obtained by the Proton-Transfer Reaction

Table 1. Crystal Stoichiometry and Qualities of Crystals 1−13


entry 1 2 3 4 5 6 7 8 9 10 11 12 13

cation +

o-FAni o-ClAni+ o-BrAni+ o-IAni+ m-FAni+ m-ClAni+ m-BrAni+ m-IAni+ m-BrAni+ p-FAni+ p-ClAni+ p-BrAni+ p-IAni+

ratio

pKa (X-Ani+)

ΔpKa1a

ΔpKa2a

δNH (cm−1)

m.p. (°C)b

1:1 1:1 1:1 1:1 2:1 2:1 2:1 2:1 1:1 2:1 1:1 1:1 1:1

3.20 2.66 2.53 2.54 3.59 3.52 3.53 3.58 3.53 4.65 3.98 3.89 3.81

−1.23 −0.69 −0.56 −0.57 −1.62 −1.55 −1.56 −1.61 −1.56 −2.68 −2.01 −1.92 −1.84

3.62 4.16 4.29 4.28 3.23 3.30 3.29 3.24 3.29 2.17 2.84 2.93 3.01

1541 1530 1545 1541 1597 1585 1578 1564 1583 1570 1560 1551 1549

133 142 148 147 126 132 103 125 142 152 140 172 156

ΔpKa1 and ΔpKa2 values were defined by pKa1 (H3PO4) − pKa (XAni+) and pKa2 (H3PO4) − pKa (XAni+), respectively. bMelting point of the crystals. a

calorimetry (DSC) analyses were carried out using a Rigaku Thermo plus DSC8120 thermal analysis station with an Al2O3 reference in the temperature ranging from 300 K to the temperature below the melting points or decomposition temperature, with a heating rate of 5 K min−1 under nitrogen. Infrared (FT-IR, 400−7600 cm−1) spectra were acquired with samples in the form of KBr pellets, using a Thermo Fisher Nicolet 6700 FT-IR spectrophotometer with a resolution of 4 cm−1. Temperature-dependent σH values were measured using the two-probe AC-impedance method over the frequency varying from 102 to 1000 × 103 Hz (HP4194A). The single crystal was placed on the heating stage of Linkam LTS350, and the measurement axis was determined by X-ray analysis of the single-crystal structure. The electrical contacts were prepared using gold paste to attach the 10 μm φ gold wires. Crystal Structure Determination. Temperature-dependent crystallographic data (Table S2) of 13 kinds of single crystals were collected using a Rigaku RAPID-II diffractometer equipped with a rotating anode fitted with a multilayer confocal optic and using Cu−Kα (λ = 1.54187 Å) radiation from a graphite monochromator. Structural refinements were carried out using the full-matrix least-squares method on F2. Calculations were performed using crystal-structure software packages. Parameters were refined using anisotropic temperature factors except in the case of hydrogen atoms CCDC 1410275−1410292.

phosphate anions (X = F, Cl, Br, and I at o, m, and p positions of XAni+) to construct a variety of hydrogen-bonding network structures. In these, the changes in the size, dipole moment, and basicity of XAni+ affected the hydrogen-bonding network structure of the phosphate anions. The 12 possible combinations between haloaniline and H3PO4 were investigated to prepare single crystals by modifying the acid dissociation constant (pKa) of XAni+ cation from pKa = 2.53 to pKa = 4.65.34 Changes in the structure of XAni+ cations affected the cation−anion packing structure and the hydrogen-bonding network in the crystal, which also influenced the μ value of protonic conductivity. Correlation between the crystal structure and protonic conductivity of the binary ionic salts of 1:1 stoichiometric (XAni+)(H2PO4−) and 2:1 stoichiometric (XAni+)2(HPO42−) was systematically examined by changing the structural parameter of the X substituent.

2. EXPERIMENTAL SECTION Preparation of Single Crystals. Commercially available aniline derivatives (o-fluoroaniline, o-chloroaniline, o-bromoaniline, o-iodoaniline, m-fluoroaniline, m-chloroaniline, m-bromoaniline, m-iodoaniline, p-fluoroaniline, p-chloroaniline, p-bromoaniline, and p-iodoaniline) and phosphoric acid were used for the single-crystal preparations without further purification. Single crystals were grown by the slow evaporation of 1:1 and 2:1 mixtures of haloaniline derivatives and phosphoric acid in CH3OH or C2H5OH, and the precipitate crystals were washed by acetonitrile with a yield of ∼44%. Elemental analyses and IR spectra were consistent with the formation of protontransferred 1:1 stoichiometric (XAni+)(H2PO4−) or 2:1 stoichiometric (XAni+)2(HPO42−) salts (Table S2). Physical Measurements. Thermogravimetry (TG) analyses of each crystal were conducted using a thermal analysis station of Rigaku Thermo plus TG8120, with Al2O3 as a reference in the temperature range 293−675 K, with a heating rate of 10 K min−1 in ambient air. Differential scanning

3. RESULTS AND DISCUSSION Crystal Preparation. Thirteen kinds of single crystals were successfully obtained by combining 12 kinds of X-aniline derivatives with X = F, Cl, Br, and I at o, m, and p positions, and H3PO4. The three-step acid dissociation constants of H3PO4 were pKa1 = 1.97, pKa2 = 6.82, and pKa3 = 12.5,35 whereas the pKa values of 12 kinds of conjugated acid of the XAni+ cation were observed to have pKa values from 2.53 to 4.65 (Table 1). The proton-transfer reaction from H3PO4 to B


Article



Scheme 2. Schematic Hydrogen-Bonding Structures of H2PO4− or HPO42− Anionic Networks from the 1D Chains (type-I), 1D Ladder Chain (type-II), and Three Kinds of 2d Sheets (types IIIa, IIIb, and IIIc)

Crystal Structures. The hydrogen-bonding network structures between the H2PO4− anion or the HPO42− dianion in salts 1−13 were affected by the cation structures. These were classified into three main types: 1D single chain (type-I), 1D ladder chain (type-II), and 2D sheet (type-III), as shown schematically in Scheme 2. The anion arrangement of HPO42− in the 2:1 stoichiometric salts 5, 6, 7, 8, and 10 was classified as type-I. The other 1:1 stoichiometric salts had the type-II and type-III hydrogen-bonding structures. The type-II salts were further classified into a zigzag ladder (type-IIa) and dimer ladder chains (type-IIb). The type-III salts were also divided into three subclasses: type-IIIa, IIIb, and IIIc, according to the connectivity of H2PO4− anions in the 2D sheet. The hydrogenbonding connectivity was lower in the order IIIa > IIIb > IIIc, which affected the proton-conducting behavior. Table 2 summarizes the space group, type of hydrogen-bond, and hydrogen-bonded oxygen−oxygen distance (dO−O) of salts 1− 13. One-Dimensional Single Chains (Type-I). The protonic conductivity was not observed in the 2:1 stoichiometric salts of 5, 6, 7, 8, and 10, which had a 1D hydrogen-bonding single chain (type-I). Although salt 8 had an acentric space group of Pn, the other salts had centric space groups of P-1 or C2/c.

haloaniline occurred in alcohols to yield the 1:1 stoichiometric (X-Ani+)(H2PO4−) salts when ΔpKa1 = pKa1 (H3PO4) − pKa (XAni+) < 0. The formation of 2:1 stoichiometric (XAni+)2(HPO42−) salts using the second-step proton-transfer process of H3PO4 was not expected from positive ΔpKa2 = pKa2 (H3PO4) − pKa (XAni+) > 0; however, five kinds of 2:1 stoichiometric (XAni+)2(HPO42−) salts were obtained by using four kinds of m-haloaniline and p-fluoroaniline. Table 1 summarizes the crystal stoichiometry, pKa values of XAni+ cations, ΔpKa1, ΔpKa2, melting points, and N−H bending modes (δNH, cm−1) of 13 kinds of crystals. Single crystals of 1:1 stoichiometric (XAni+)(H2PO4−) were obtained by using o-haloaniline, resulting in (o-FAni+)(H2PO4−) (1), (o-ClAni+)(H2PO4−) (2), (o-BrAni+)(H2PO4−) (3), and (o-IAni+)(H2PO4−) (4). The combinations of H3PO4 with m-haloaniline formed four kinds of 2:1 stoichiometric single crystals [(m-FAni + ) 2 (HPO 4 2 − ) (5), (m-ClAni+)2(HPO42−) (6), (m-BrAni+)2(HPO42−) (7), and (mIAni+)2(HPO42−)] (8). Besides the 2:1 stoichiometric salt 7, the 1:1 stoichiometric salt (m-BrAni+)(H2PO4−) (9) was also obtained. The combination of H3PO4 with p-haloaniline formed the 2:1 stoichiometric single crystals of (pFAni+)2(HPO42−) (10) and 1:1 stoichiometric single crystals of (p-ClAni+)(H2PO4−) (11), (p-BrAni+)(H2PO4−) (12), and (p-IAni+)(H2PO4−) (13). The stabilization of Madelung energy in the ionic crystals played an important role in forming 2:1 stoichiometric salts with closely packed structures. The formation of proton-transferred ionic salts of crystals 1− 13 was confirmed by the vibrational spectra on KBr pellets. The energy of the vibrational N−H bending mode (δNH) of haloanilines has been observed in the energy range 1612−1632 cm−1 (Figure S2), whereas that of the N−H+ bending mode (δNH+) of haloaniliniums has been observed in the lower energy range 1530−1585 cm−1 with an ∼80 cm−1 red shift from those of haloaniline. The vibrational energies of the N−H bending mode of crystals 1−13 were consistent with the latter δNH+ mode of the protonated X-Ani+ species. The electrostatic interaction between the proton-transferred cations and anions also increased the thermal stability and melting point of crystals 1−13. The melting points of crystals 1−13 were approximately 70−140 K higher than those of the corresponding haloanilines, which was consistent with the formation of ionic salts through electrostatic interaction.

Table 2. Space Groups and Bonding Characteristics of Crystals 1−13

C

entry

space group

type

1 2 3 4 5 6 7 8 9 10 11

P21/n P21/n P21/n P21/n P-1 C2/c C2/c Pn P21/c P-1 Pbca

IIa IIb IIb IIb I I I I IIIc I IIIa

12 13

Pbca Pbca

IIIb IIIb

dO−O (Å) O1−O4 O1−O4 O1−O4 O1−O4 O1−O6 O1−O3 O1−O3 O1−O3 O1−O3 O1−O6 O1−O7 O3−O5 O1−O3 O1−O3

= = = = = = = = = = = = = =

dO−O (Å)

2.524(2) 2.542(2) 2.535(2) 2.575(2) 2.583(2) 2.604(6) 2.911(3) 2.46(3) 2.580(3) 2.555(8) 2.545(4) 2.542(5) 2.528(2) 2.531(5)

O2−O4 O2−O4 O2−O4 O2−O4 O2−O5

= = = = =

2.597(2) 2.593(2) 2.586(2) 2.565(4) 2.581(1)

O5−O7 O2−O4 O2−O5 O4−O6 O2−O8 O2−O4 O2−O4

= = = = = = =

2.47(3) 2.636(3) 2.545(8) 2.560(5) 2.644(5) 2.593(2) 2.603(6)


Article



Figure 1. Crystal structure of type-I salts: (a) Unit cell of salt 5 viewed along the a axis, (b) antiferroelectric hydrogen-bonding chain of (HPO42−)∞ connected by alternate O1−H···O6 and O2−H···O5 interactions along the a axis, and (c) ferroelectric hydrogen-bonding chains of (HPO42−)∞ connected by uniform O1−H···O3 and O5−H···O7 interactions along the a axis.

Figure 2. Crystal structures of type-II salts: (a) Unit cell of salt 1 viewed along the b axis (type-IIa), (b) ferroelectric dipole arrangement of H2PO4− anions along the ladder-leg direction in type-IIa salt 1, (c) zigzag−type anion arrangement of type-IIa salt 1 in the bc plane, (d) antiferroelectric dipole arrangement of H2PO4− dimer along the ladder-leg direction in type-IIb salts 2, 3, and 4, and (e) dimer-type anion arrangement of type-IIb salts 2, 3, and 4 in the bc plane.

D


Article



Figure 3. Crystal structure of type-IIIa salt 11 with the tetramer network structure: (a) unit cell viewed along the b axis, (b) hydrogen-bonding (H2PO4−)4 tetramer unit by dimeric O2−H···O8 and O6−H···O4 interactions, which was further connected by interdimer O3−H···O5 interaction, and (c) 2D tetramer arrangement through the intertetramer O1−H···O7 interactions in the ab plane.

ladder-rung dO2−O4 = 2.597(2) Å in the type-IIa salt. The hydrogen-bonding (H2PO4−)2 dimer in salts 2, 3, and 4 was connected by the two equivalent ladder-leg hydrogen-bonding interactions dO2−O4 (Figure 2d), and the dO2−O4 distances of salts 2, 3, and 4 were 2.593(2), 2.586(2), and 2.565 (4) Å, respectively. Increasing the cation size from o-ClAni+, o-BrAni+, to o-IAni+ expanded the cation volume and had a tendency to decrease the ladder-rung dO2−O4 distance along the a + c axis. In contrast, the ladder-leg dO1−O4 distances of salts 1, 2, 3, and 4 were 2.524(2), 2.542(2), 2.535(2), and 2.575(2) Å, respectively, which did not show clear cation size dependence. Although the macro dipole moment was not observed in typeII salts due to the centric space group of P21/n, the dipole arrangements of type-IIa and type-IIb salts were different. The two dipole moments of the neighboring chains in the type-IIa salt were arranged in the ferroelectric [↑]···[↑] orientation (Figure 2b), which was canceled due to the antiferroelectric ∼ [↑]···[↑] ∼ [↓]···[↓]∼ interladder arrangement (Figure 2c). In contrast, the antiferroelectric [↑]···[↓] dipole orientation was achieved in the neighboring zigzag ladder chain of type-IIb salts (Figure 2c,d). Two-Dimensional Tetramer Networks (Type-IIIa). Only one kind of salt (salt 11) had the 2D tetramer network structure of type-IIIa, which was crystallized in the centric space group of Pbca. Alternate cation and anion layers in the ab plane were elongated along the c axis (Figure 3a). The two kinds of hydrogen-bonding interactions of dO2−O8 = 2.644 (5) Å and dO4−O6 = 2.560(5) Å formed the (H2PO4−)2 dimer unit, which was further connected by the two interdimer dO5−O3 = 2.542(5) Å interactions, forming the (H2PO4−)4 tetramer unit (Figure 3b). The neighboring tetramer units were orthogonally arranged in the ab plane and were connected by the intertetramer hydrogen-bonding interaction of dO1−O7 = 2.545(4) Å, forming the 2D tetramer network structure (Figure 3c). Because the hydrogen-bonding network structure was governed by the O1−H···O7 intertetramer interaction, the hydrogen-bonding connectivity of type-IIIa salt was quite significantly lower than that of type-II salts. Two-Dimensional Dimer Networks (Type-IIIb). The 2D dimer network structure (type-IIIb) was observed in salts 12 and 13 with the centric space group of P21/c. Alternate cation and anion layers in the ab plane were elongated along the c axis

Figure 1a shows the unit cell of salt 5 viewed along the a axis. The alternate cation and anion layers in the ab plane were elongated along the c axis, where the cation layer became a bilayer structure due to the 2:1 stoichiometry. The orientational disorder of F atom at the m position was observed at the 0.5:0.5 occupancy factor, and the HPO42− dianions formed a 1-D O− H···O hydrogen-bonding single chain of (HPO42−)∞ along the a axis. Relatively strong alternate hydrogen-bonding interactions were observed at dO1−O6 = 2.583(2) and dO2−O5 = 2.581(1) Å in salt 5. A uniform 1D chain was observed in salts 6 and 7 due to the average structure of the disordered HPO42− dianions in the hydrogen-bonding chains along the c axis, whereas an alternate structure was confirmed in salts 5, 8, and 10. The intermolecular O−H···O hydrogen-bonding interactions in salts 6, 7, 8, and 10 were observed at the dO−O distances of 2.47(3)−2.911(6) Å, where the dO−O distances have a tendency to become longer due to increase in the size of the m substituent from F, Cl, and Br (except for the I group). The expansion of the cationic volume increased the volume of the anionic hydrogen-bonding layer as well as the dO−O distances. Further expansion of the cationic layer to m-IAni+ in salt 8 resulted in an acentric space group and exceptionally short hydrogen-bonding distances (dO1−O3 = 2.46(3) and dO5−O7 = 2.47(3) Å). The neighboring 1D hydrogen-bonding single chain of (HPO42−)∞ in the centric space group (salts 5, 6, 7, and 10) had the antiferroelectric [↓]···[↑] dipole arrangement (Figure 1b), whereas those in the acentric one (salt 8) formed the ferroelectric [↑]···[↑] one (Figure 1c). One-Dimensional Ladder Chains (Type-II). The 1D hydrogen-bonding ladder chain was observed in type-II salts 1, 2, 3, and 4 with the use of o-XAni+ cations. The 1D (H2PO4−)∞ hydrogen-bonding chain was constructed by the ladder-leg O1−H···O4 interaction, which was further connected by the ladder-rung O2−H···O4 interaction using the second proton of the H2PO4− anion. In the type-II salts, there were two kinds of ladder-chain arrangements: zig-zag (type-IIa: salt 1) and dimer (type-IIb: salts 2, 3, and 4). Two kinds of effective O1−H···O4 and O2−H···O4 interactions were observed along the ladderleg and ladder-rung directions, respectively. Alternate cation and anion layers in the a + c plane were elongated along the b axis (Figure 2a). The ladder-leg hydrogen-bonding interaction of dO1−O4 = 2.524(2) Å was ∼0.07 Å shorter than that of E


Article



Figure 4. Crystal structure of type-IIIb salts with (H2PO4−)2 dimer network structure: (a) unit cell of salt 12 viewed along the b axis, (b) hydrogenbonding (H2PO4−)2 dimer through the two equivalent O2−H···O4 interactions, and (c) 2D orthogonal (H2PO4−)2 dimer arrangement in the ab plane through the interdimer O1−H···O3 interaction.

Figure 5. Crystal structure of type-IIIc salt 9: (a) unit cell viewed along the c axis, (b) four intermolecular hydrogen-bonding interactions around one H2PO4 anion by O1−H···O3 along the c axis and one O2−H···O4 along the b axis, and (c) 2D uniform anion arrangement and antiferroelectric dipole orientation in the bc plane.

(Figure 4a). The dimeric (H2PO4−)2 units of salts 12 and 13 were constructed by two equivalent hydrogen-bonding interactions of dO2−O4 = 2.593(2) and 2.603(6) Å, respectively. Each dimer was orthogonally arranged to form the 2D sheet structure in the ab plane by the interdimer hydrogen-bonding interaction of dO1−O3 2.528(2) and 2.531(5) Å, respectively, in salts 12 and 13 (Figure 3c). The replacement of the p-XAni+ cation from p-ClAni+ to p-BrAni+ or p-IAni+ drastically modified the hydrogen-bonding sheet structures from the tetramer to dimer networks by increasing the size of halogen atoms. The hydrogen-bonding dimensionality of type-IIIb salts was higher than that of type-IIIa salt but was lower than that of type-II salts. Two-Dimensional Uniform Networks (Type-IIIc). A 2D uniform hydrogen-bonding interaction (type-IIIc) was observed in salt 9 with the centric space group of P21/c. Alternate cation and anion layers in the bc plane were elongated along the a axis (Figure 5a). It should be noted that each cation and anion layer was constructed by the bilayer arrangement, which was completely different from those of the other 1:1 salts. One

H2PO4− anion was surrounded by four H2PO4− anions with intermolecular hydrogen-bonding distances of dO1−O3 = 2.580(3) Å along the c axis and of dO2−O4 = 2.636(3) Å along the b axis. Although the former distance was ∼0.06 Å shorter than the latter one (Figure 5b), formations of the tetramer or dimer units were not observed in the 2D hydrogenbonding layer. Strictly speaking, the two hydrogen-bonding distances should be equivalent in the perfectly uniform anion layer; however, two kinds of 1D hydrogen-bonding chains were orthogonally elongated along the b and c axes, resulting in an almost 2D hydrogen-bonding (H2PO4−)∞ × (H2PO4−)∞ network. Because the hydrogen-bonding connectivity of typeIIIc salt was greater than that of type-II, type-IIIa, and typeIIIb salts, the most uniform intermolecular hydrogen-bonding interaction was achieved in salt 9. Although the ferroelectric 1D hydrogen-bonding chain was observed along the c axis, the antiferroelectric (H2PO4−)∞ arrangement along the b axis canceled the total dipole moment of the crystal. Protonic Conductivity. The σH value of single crystals along the hydrogen-bonding chain was evaluated using an F


Article


the anhydrous proton conductors of imidazole@MOFs and CDP derivatives showed a relatively high Ea values of 0.6 to 1.0 eV,4,32,37 where the rotation of H2PO4− or imidazole molecules under the steric hindrance mediated the proton transfer rate through the intermolecular hopping. Although relatively low Ea value of 0.22 eV was observed in crystal 9, the proton hopping between the oxygen or nitrogen sites resulted in the Ea value of ∼0.7 eV. Therefore, the proton conducting in crystals 1−4, 9, and 11−13 occurred in the absence of H2O molecules. σH of Type-II Salts. Figure 6 summarized the Arrhenius plots of log (σH) − T−1 of type-IIa (purple), type-IIb (red),

alternate current (AC) impedance method in N2 gas at temperatures near the melting point (Table S3). The measurement axis and temperature range were determined by single-crystal X-ray diffraction and TG analyses (Figure S2). Proton-conducting behavior was not confirmed in the 2:1 stoichiometric type-I salts of 5, 6, 7, 8, and 10. The 1:1 stoichiometric salts showed proton-conducting behavior according to the hydrogen-bonding nature, and the absolute σH values in type-II salts were higher than those in type-III salts. Among them, type-IIb salts of 2 and 3 indicated the highest σH values of ∼10−3 S cm−1 at 388 K. Table 3 summarizes the crystal type, measurement axis, measurement temperature range, maximum σH value, and activation energy (Ea) of salts 1−13.


Table 3. Types, Orientations, and Other Characteristics of Crystals 1−13 crystal

type

axis

T range (K)

1 2 3 4 5 6 7 8 9 10 11 12 13

IIa IIb IIb IIb I I I I IIIc I IIIa IIIb IIIb

[010] [010] [010] [010] [100] [001] [001] [100] [001] [100] [100] [100] [100]

r.t.−376 r.t.−386 r.t.−388 r.t.−387 r.t.−357 r.t.−357 r.t.−353 r.t.−379 r.t.−388 r.t.−374 r.t.−385 r.t.−405 r.t.−415

σHmax (S cm−1) 3.2 2.2 2.2 1.7

× × × ×

10−6 10−3 10−3 10−4

−6

Ea (eV) 0.40 0.72 0.74 0.70

6.0 × 10

0.22

8.8 × 10−5 1.6 × 10−5 2.9 × 10−6

0.60 1.10 1.18

Figure 6. Arrhenius plots of log (σH) − T−1 of type-IIa (purple), typeIIb (red), type-IIIa (green), type-IIIb (black), and type-IIIc (gray) salts.

type-IIIa (green), type-IIIb (black), and type-IIIc (gray) salts. The type-IIa and type-IIb salts had hydrogen-bonding interactions originating from the zigzag and dimer ladder chains, respectively, whose σH values were measured along the hydrogen-bonding a axis. Figure 7a showed the Nyquist plot of salt 3, indicating the temperature-dependent semicircular trace characteristic for the protonic conductor. The σHmax value of ∼3.2 × 10−6 S cm−1 at 376 K in salt 1 (type-IIa) was 3 orders of magnitude lower than that of type-IIb salts 2 and 3. The σHmax values of type-IIb salts 2, 3, and 4 were 2.2 × 10−3, 2.2 × 10−3, and 1.7 × 10−4 S cm−1, respectively. The lower σHmax value of salt 4 (than of salts 2 and 3) was consistent with the longer dO1−O4 distance along the ladder-leg direction. In contrast, the activation energy Ea = 0.40 eV in type-IIa salt became almost half the magnitude of those of type-IIb salts with Ea ≈0.74 eV. This is due to the difference in the hydrogenbonding structures. The dimeric protons in the ladder-rung direction had a tendency to be localized in the hydrogen bonds, and this increased the magnitude of the Ea value in type-IIb salts. The single-crystal X-ray structural analyses of salts 1 (typeIIa) and 3 (type-IIb) were successfully obtained in the hightemperature proton conducting state at 350 K. The electron density (ρe) of hydrogen-bonding protons at 100 and 350 K was evaluated using differential Fourier synthesis to discuss the localization-delocalization property of the protons. Figure 7b,c indicated the ρe profile of protons on the ρe = 0.4e−/Å3 surface at 100 (left figure) and 350 K (right figure) of salts 1 and 3. The ρe profile for the two protons at O1−H···O4 and O2−H··· O4 in salt 1 at 100 K clearly indicated the proton localization at the O1 and O2 sites, respectively, whereas those at 350 K showed the disappearance of the ρe profile with the delocalization property of two protons. This was due to

Anhydrous proton conductors based on phosphate anions have been reported in Zn(1,2,4-triazole)2(H2PO4−)2 and CDP under low humidity.32,37 The hydrogen-bonding network structure of phosphate anions can form a continuous protonhopping path adequate for long-range proton transport. Our system also has diverse hydrogen-bonding network structures of H2PO4− anions suitable for proton conduction. Among them, the 1D dianion chain of (HPO42−)∞ did not act as a proton conducting path, although the type-II and type-III salts bearing H2PO4− anion networks could form excellent protonconducting paths. The magnitude of σH and Ea values of 1:1 salts depended on the connectivity of the hydrogen-bonding interactions. The cleavage of O−H bonds and intermolecular proton transfer to the neighboring anionic -O− site (namely OH···O− → O−···HO) in the hydrogen-bonding network should occur simultaneously to enable long-range proton conduction. The proton cleavage in the O−H bond was affected by the magnitude of pKa value, where a highly acidic condition at low pKa could easily cleave O−H bonds and generate H+ carriers. Because the second acidic proton was already released form the HPO42− dianion of the type-I salts, only the third proton with low acidity pKa3 = 12.5 could contribute to the proton conducting behavior; however, σH was not observed in type-I salts due to the low acidity of the third proton. The magnitude of low Ea values for the past hydrous proton conductors was dominated by Grottuss mechanism of motional H2O molecules. For instance, hydrous proton conductors such as MOFs,33 COFs,3 and nafion36 typically showed Ea values of 0.21 to 0.47, 0.11, and 0.22 eV, respectively. On the contrary, G


Article



Figure 7. Proton conductivities and conducting mechanism of type-II salts: (a) Nyquist plot of type-IIb salt 3 in the temperature range 338−358 K (upper) to 368−388 K (lower). The ρe = 0.4e−/Å3 surface profiles of (b) type-IIa salt 1 and (c) type-IIb salt 3 at 100 K (blue) and 350 K (red).

type-IIIa (tetramer) > type-II (ladder). The lower σHmax value and higher Ea value of salt 13 (than that of salt 12) were consistent with the longer hydrogen-bonding dO−O distances in salt 13. A 2D uniform hydrogen-bonding network was observed in type-IIIc salt 9, which showed the σHmax value of 6.0 × 10−6 S cm−1 and Ea = 0.22 eV. This Ea value was the minimum value obtained in this study. Although the σH value of type-IIIa salt was not higher than those of type-II salts, the Ea value became lower than that of type-IIa salt. There were two kinds of independent O1−H···O3 and O2−H···O4 hydrogen-bonding interactions in salt 9, and achievement of the ideal 2D (H2PO4−)∞ hydrogen-bonding network would result in Ea values much lower than 0.22 eV.

intermolecular proton transfer. In contrast, the two kinds of ladder-leg and ladder-rung protons in type-IIb salts were clearly observed in the ρe profile at 100 K; however, the former O1− H···O4 protons in salt 3 disappeared in the ρe profile at 350 K due to delocalization via proton conduction, whereas the latter O2−H···O4 protons were still observed at 350 K due to proton localization. The protons to connect the two H2PO4− anions along the ladder-rung direction had a tendency to localize in the proton conducting path. On the contrary, the two kinds of protons in the zigzag ladder chain could contribute to the total σH value, whereas only one kind of proton in the dimer along the ladder-leg direction determined the total σH value. These differences were associated with the magnitude of the Ea values. σH of Two-Dimensional Networks. A 2D tetramer network was observed in type-IIIa salt 11, which showed a σHmax value of ∼8.8 × 10−5 S cm−1 in the temperature range 347−385 K with Ea = 0.60 eV. The σHmax value of salt 11 was 3 orders of magnitude less than those of type-IIb salts. Highquality single-crystal X-ray diffraction data were not obtained for salt 11, so that the proton localization−delocalization property could be discussed. The fundamental hydrogenbonding structural unit in salt 11 was the dimeric (H2PO4−)2 unit, and the Ea value was almost the same as that of type-IIb salts. From these results, we conclude that the intratetramer protons of O2−H···O8 and O6−H···O4 hydrogen-bonds in the dimer unit should be localized at O2 and O6 oxygen sites, and the intertetramer proton of O3−H···O5 plays an important role in showing the proton-conducting property. The 2D dimer network of (H2PO4−)2 was observed in typeIIIb salts 12 and 13, whose σHmax values were 1.6 × 10−5 and 2.9 × 10−6 S cm−1, respectively and which were lower than those of type-II and type-IIIa salts. The relatively high Ea values of 1.10 and 1.18 eV were observed in salts 12 and 13, respectively. The magnitude of the σH and Ea values was associated with the hydrogen-bonding connectivity between the H2PO4− anions. The σHmax increased in the order type-II (ladder) > type-IIIa (tetramer) > type-IIIb (dimer), and the lowest Ea values decreased in the order type-IIIb (dimer) >

4. CONCLUSIONS The combination of haloaniline and H3PO4 resulted in five kinds of 2:1 stoichiometric salts of (XAni+)2(HPO42−) and eight kinds of 1:1 stoichiometric salts of (XAni+)(H2PO4−). Among these, three kinds of hydrogen-bonding network structures from the 1D and 1D ladder chain to the 2D sheet were observed after modifications of the cation structure. The 2:1 salts were protonic insulators, whereas the 1:1 salts showed relatively excellent protonic conductivity in the single-crystal state along the hydrogen-bonding direction of the H2PO4− anions in networks. The proton-conducting behavior depended on the type of hydrogen-bonding network, and the highest σHmax value (∼10−3 S cm−1) occurred in salts 2 and 3 (typeIIb), which had the hydrogen-bonding ladder chain. In contrast, the 2D hydrogen-bonding network had a tendency to show lower σHmax value than those of the ladder chains. The (H2PO4−)2 dimer units decreased the hydrogen-bonding connectivity and increased the magnitude of Ea, whereas the uniform O−H···O network structures in type-IIa and type-IIIc salts resulted in relatively low Ea values of 0.22 and 0.40 eV, respectively. The protons in (H2PO4−)2 dimer units had a tendency to be localized due to the absence of proton transfer. The ideal uniform 2-D H2PO4− network is expected to show a H


Article


(11) Katahira, K.; Kohchi, Y.; Shimura, T.; Iwahara, H. Protonic Conduction in Zr-Substituted BaCeO3. Solid State Ionics 2000, 138, 91−98. (12) Yamazaki, Y.; Hernandez-Sanchez, R.; Haile, S. High Total Proton Conductivity in Large-Grained Yttrium-Doped Barium Zirconate. Chem. Mater. 2009, 21, 2755−2762. (13) Pergolesi, D.; Fabbri, E.; D’Epifanio, A.; Di Bartolomeo, E.; Tebano, A.; Sanna, S.; Licoccia, S.; Balestrino, G.; Traversa, E. High Proton Conduction in Grain-Boundary-Free Yttrium-Doped Barium Zirconate Films Grown by Pulsed Laser Deposition. Nat. Mater. 2010, 9, 846−852. (14) Pogorzelec-Glaser, K.; Pawlaczyk, C.; Pietraszko, A.; Markiewicz, E. Crystal Structure and Electrical Conductivity of Imidazolium Succinate. J. Power Sources 2007, 173, 800−805. (15) Ponomareva, V.; Shutova, E. High-Temperature Behavior of CsH2PO4 and CsH2PO4−SiO2 Composites. Solid State Ionics 2007, 178, 729−735. (16) Norby, T.; Friesel, M.; Mellander, B. E. Proton and Deuteron Conductivity in CsHSO4 and CsDSO4 by In Situ Isotopic Exchange. Solid State Ionics 1995, 77, 105−110. (17) Xu, H. R.; Zhang, Q. C.; Zhao, H. X.; Long, L. S.; Huang, R. B.; Zheng, L. S. Anisotropy of Proton Transport in an Organic−Inorganic Compound [(C6H10N2)2(SO4)2·3H2O]n(C6H10N2= Phenylenediammonium Dication). Chem. Commun. 2012, 48, 4875−4877. (18) Stevens, J. R.; Wieczorek, W.; Raducha, D.; Jeffrey, K. R. Proton Conducting Gel/H3PO4 Electrolytes. Solid State Ionics 1997, 97, 347− 358. (19) Petty-Weeks, S.; Zupancic, J. J.; Swedo, R. Proton Conducting Interpenetrating Polymer Networks. Solid State Ionics 1988, 31, 117− 125. (20) Otomo, J.; Minagawa, N.; Wen, C.; Eguchi, K.; Takahashi, H. Protonic Conduction of CsH2PO4 and its Composite with Silica in Dry and Humid Atmospheres. Solid State Ionics 2003, 156, 357−369. (21) Yamada, K.; Toshiko Sagara, T.; Yamane, Y.; Ohki, H.; Okuda, T. Superprotonic Conductor CsH2 PO4 Studied by 1H, 31P NMR and X-ray Diffraction. Solid State Ionics 2004, 175, 557−562. (22) Kim, G.; Blanc, F.; Hu, Y.; Grey, C. Understanding the Conduction Mechanism of the Protonic Conductor CsH2PO4 by Solid-State NMR Spectroscopy. J. Phys. Chem. C 2013, 117, 6504− 6515. (23) Ishikawa, A.; Maekawa, H.; Yamamura, T.; Kawakita, Y.; Shibata, K.; Kawai, M. Proton Dynamics of CsH2PO4 Studied by Quasi-Elastic Neutron Sscattering and PFG-NMR. Solid State Ionics 2008, 179, 2345−2349. (24) Hayashi, S.; Mizuno, M. Proton Diffusion in the Superprotonic Phase of CsHSO4 Studied by 1H NMR Relaxation. Solid State Ionics 2004, 171, 289−293. (25) Mizuno, M.; Hayashi, S. New Highly Ion-Conductive Crystals Precipitated from Li2S-P2S5 Glasses. Solid State Ionics 2004, 171, 317− 323. (26) Ke, X.; Tanaka, I. Atomistic Mechanism of Proton Conduction in Solid CsHSO4 by a First-Principles Study. Phys. Rev. B: Condens. Matter Mater. Phys. 2004, 69 (165114), 1−13. (27) Wood, B.; Marzari, N. Proton Dynamics in Superprotonic CsHSO4. Phys. Rev. B: Condens. Matter Mater. Phys. 2007, 76 (134301), 1−13. (28) Belushkin, A. V.; Adams, M. A.; Hullb, S.; Shuvalov, L. A. PT Phase Diagram of CsHSO4. Neutron Scattering Study of Structure and Dynamics. Solid State Ionics 1995, 77, 91−96. (29) Boysen, D. A.; Uda, T.; Chisholm, C. R. I.; Haile, S. M. HighPerformance Solid Acid Fuel Cells through Humidity Stabilization. Science 2004, 35, 68−70. (30) Haile, S. M.; Boysen, D. A.; Chisholm, C. R. I.; Merle, R. B. Solid Acids as Fuel Cell Electrolytes. Nature 2001, 410, 910−913. (31) West, A. R. Basic Solid State Chemistry; Wiley: New York, 1999. (32) Umeyama, D.; Horike, S.; Inukai, M.; Itakura, T.; Kitagawa, S. Inherent Proton Conduction in a 2D Coordination Framework. J. Am. Chem. Soc. 2012, 134, 12780−12785.

low Ea value (

Protonic Conductivity and Hydrogen Bonds in (Haloanilinium)(H2PO4

Recommend Documents