Toward Understanding the Origin of Structural Phase Transition in

Article pubs.acs.org/crystal

Toward Understanding the Origin of Structural Phase Transition in Guanidinium Pyridinium 1,5-Naphthalenedisulfonate Chao Shi,† Bin Wei,† and Wen Zhang* Ordered Matter Science Research Center, Southeast University, Nanjing 211189, P. R. China S Supporting Information *

ABSTRACT: A phase transition compound 9PY was screened from a series of organic salts (Cat)·G·NDS, where the Cat is protonated amine or N-containing heterocycle (Cat = methylammonium, 1MA; ethylammonium, 2EA; propylammonium, 3PA; dimethylammonium, 4DMA; isopropylammonium, 5iPA; tert-butylammonium, 6tBA; imidazolium, 7IM; pyrazolium, 8PZ; pyridinium, 9PY; 2-methylimidazolium, 10MIM; 2-ethylimidazolium, 11EIM; (R,S)-3-methylpiperidium, 12MP), the G is guanidinium, and the NDS is 1,5naphthalenedisulfonate. Detailed crystal structural analysis of 9PY shows the competing hydrogen-bonding interactions among the pyridinium and sulfonates are the driving force for the structural phase transition at 211 K.

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INTRODUCTION Crystal engineering is expected to be a powerful tool to manipulate the structures and properties of crystalline materials through a deep understanding and full control of intermolecular interactions among the molecular building blocks.1,2 Concepts, such as supramolecular synthons, have proved practicable in some situations by exploiting relatively strong hydrogenbonding interactions.3 However, the goal to predict crystal structures with desired properties and functions is still far beyond reach currently because of the limited knowledge of complicated interplays among various intermolecular interactions which are believed to be the key factor to determine crystal packings. This is particularly true in molecule-based phase transition compounds (e.g., ferroelectric compounds), whose properties are primarily dominated by cooperative effects of coupling among molecules and/or ions through both strong and weak intermolecular interactions.4−7 In experiments, a phase transition compound is usually found serendipitously. In order to gain deep insight into the rational design of phase transition compounds, systematic synthesis and structural analysis of various families of compounds are prerequisite and fundamental. A good candidate for screening phase transition compounds is host−guest compounds (e.g., inclusion compounds) in which the host backbones and guests can fit each other well. In some specific situations, the delicate match of the components, upon external stimuli such as temperature and pressure, becomes instable to result in a structural reorganization, i.e., a phase transition. Guanidinium organo(di)sulfonate inclusion compounds would be such a candidate. The guanidinium (G) cation is a unique hydrogen-bonding component with a powerful structure-directing capability in constructing predictable organic architectures. The six hydrogen (H) atoms of G cation can be involved in formation of three pairs of H bonds. The sulfonate (S) group is a good acceptor in H bonds by means of three O atoms. Ward and co-workers performed a © 2014 American Chemical Society

systematic investigation of the family of guanidinium organo(di)sulfonate compounds.8−27 The attracting features of these compounds include (i) two-dimensional H-bonding networks, (ii) adjustable porosity, and (iii) ordered arrays of guest molecules. Modifications of the G and S components significantly alter the crystal packings of the corresponding guanidinium organo(di)sulfonate compounds.28−36 Herein we report the synthesis and structural characterization of a series of organic salts, (Cat)·G·NDS, based on protonated amines or N-containing heterocycles (Cat), G, and 1,5-naphthalenedisulfonate (NDS): Cat = methylammonium (1MA), ethylammonium (2EA), propylammonium (3PA), dimethylammonium (4DMA), isopropylammonium (5iPA), tert-butylammonium (6tBA), imidazolium (7IM), pyrazolium (8PZ), pyridinium (9PY), 2-methylimidazolium (10MIM), 2ethylimidazolium (11EIM), and (R,S)-3-methylpiperidium (12MP) (Scheme 1). Compounds 1MA−12MP can be seen as a subgroup of the family of guanidinium organo(di)sulfonate inclusion compounds. The introduction of the second Cat other than the G cation modulates the whole anionic supramolecular H bond network formed by the G and NDS. Compound 9PY is screened from the serial compounds as a phase transition compound, verified by thermal analysis and dielectric constant measurement. Detailed structural analysis discloses striking changes in the crystal structure around 211 K as shown in cell parameters, dihedral angles, atom displacements, and H bonds. It is found that competing H-bonding interactions among the pyridinium and sulfonates are the driving force for the structural phase transition. Received: October 6, 2014 Revised: November 9, 2014 Published: November 13, 2014 6570

dx.doi.org/10.1021/cg5014895 | Cryst. Growth Des. 2014, 14, 6570−6580

Crystal Growth & Design

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analysis of the 12 compounds shows they belong to five types of crystal packings: (I) 1MA−4DMA; (II) 5iPA−6tBA; (III) 7IM−9PY; (IV) 10MIM−11EIM; and (V) 12MP (Tables S1− S2). In type I compounds, the Cat ions are protonated lower aliphatic amines, i.e., methylammonium in 1MA, ethylammonium in 2EA, propylammonium in 3PA, and dimethylammonium in 4DMA. Compounds 1MA−3PA are newly synthesized, while 4DMA was reported elsewhere.39 The common structural features of type I compounds are illustrated in detail by 3PA. As shown in Figure 1a, the PA, G, and NDS ions arrange in a supramolecular bilayer stacking along the c axis, similar to typical (Guest)·G2·NDS compounds.12,27 However, because of the introduction of the Cat ion instead of the second G ion, the H bond motifs in 3PA show a striking difference to (Guest)·G2·NDS (Figure 1b). Besides the most representative H bond motif R22(8), three additional new motifs, notated as R21(4), R24(10), and R68(20), emerge due to the existence of the PA cations (Table 1). As a consequence, the high-symmetric R36(12) motif that usually appears in (Guest)·G2·NDS is interrupted. Notably, the −NH3 head of the PA cation protrudes a little out of the plane defined by the G and S groups on either side of the bilayer structure and forms an additional H bond with the S group of the neighboring bilayer, resulting in a new motif R44(12). The H bonds involved in the bilayer motif are strong with the N···O distances between 2.835 and 3.093 Å. The arrangement of the naphthalene groups as pillars in the bilayer structure of the type I compounds is a herringbone motif (Figure 1c). It can be described by a dihedral angle ϕ between the mean planes of the naphthalene groups. In 3PA, the value of ϕ is 75.86°, close to a right angle.16 It is found that such an arrangement of the naphthalene groups constitutes two types of cavities, i.e., large A and small B. There are two PA cations in A with a tail-to-tail arrangement, of which the −NH3 heads project out from the cavity and join the H-bonding networks of the bilayer. Because the naphthalene group has a large aromatic plane, expected attractive aryl motifs or embraces develop in the molecular packings in the serial compounds (Cat)·G·NDS. The combination of short contact interactions and π−π stackings among various aryl rings can be described as edge-to-face (EF) and offset face-to-face (OFF) motifs.40 In 3PA, two EF motifs exist with contacts of 2.652 and 3.240 Å, respectively, between

Scheme 1. Cations Used in the Series of Compounds (Cat)· G·NDS

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RESULTS AND DISCUSSION Crystal Structures of 1MA−12MP. Compounds 1MA− 12MP, having a common formula of (Cat)·G·NDS, are different from the (Guest)·G2·NDS serial compounds by the exploitation of cationic organic bases instead of neutral guest molecules, disclosing a new aspect of the well-studied guanidinium organo(di)sulfonate inclusion compounds. The major role of the polar cations in (Cat)·G·NDS compounds is to act as counterions to compensate the charge of the anionic backbones and build H-bonding networks in the crystal lattices. It is well-known that the (Guest)·G2·NDS compounds usually have relatively stable architectures because of the well organized and self-consistent H-bonding networks. The G and S components can easily form quasihexagonal and shifted ribbon moieties (Scheme 2).12 The basic H bond motif in the networks is notated as R22(8) where the R is ring, the superscript 2 is number of proton acceptors, the subscript 2 is number of proton donors, and the 8 is number of atoms in the ring.37,38 However, the introduction of Cat more or less undermines the stable H-bonding network between G and S. As a compensation for such an instability, the resulting compounds 1MA−12MP show various H-bonding networks depending on the sizes of the Cat ions. A crystal structural

Scheme 2. Typical H-Bonding Networks and Motifs in (Guest)·G2·NDS Compounds: Quasihexagonal (Left) and Shifted Ribbon (Right) Moietiesa

a

Only the −SO3 groups of the organodisulfonates are depicted for clarity. 6571



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Figure 1. Crystal structure of 3PA (type I): (a) Packing diagram viewed along the b axis, showing a bilayer structure; (b) H bond motifs in the bilayer plane. H bonds are depicted as dotted lines; (c) Arrangement of the PA, G, and NDS ions in one layer. Cavity A and B are highlighted as larger and smaller disks. Hydrogen and oxygen atoms are omitted for clarity; (d) EFF motifs between adjacent naphthalene groups. The balls represent the centroids of the C6 rings of the aryl groups.

Table 1. Structural Features for (Cat)·G·NDS Compounds compound 1MA 2EA 3PA 4DMA 5iPA 6tBA 7IM 8PZ 9PY 10MIM 11EIM 12MP

type I I I I II II III III III IV IV V

H-bond architecture bilayer bilayer bilayer bilayer layer layer chain chain chain chain chain discrete

θa 11.69°, 12.87°, 21.67°, 13.17°, 19.96°, 18.30°, 62.84°, 33.41°, 63.19°, 14.77°, 16.65°, 16.77°,

21.02° 22.50° 26.87° 20.74° 23.22° 20.32° 85.53° 52.62° 86.32° 87.23° 89.56° 36.12°

π−π motifsb

H-bond motifs R22(8), R22(8), R22(8), R22(8), R22(8), R22(8), R12(6), R22(8), R22(8), R22(8), R22(8), R22(8),

3

R 4(12), R55(14), R57(16) R34(12), R55(14), R57(16) R34(12), R78(22) R34(12), R78(22) R24(8), R34(12), R66(18) R34(10), R44(14), R46(14) R22(8), R24(12), R44(12), R66(16), R86(20) R33(8), R24(9), R44(12), R44(14), R44(16) R24(12), R44(12) R23(8), R43(10), R68(20) R23(8), R43(10), R68(20) R34(12), R66(20)

EF: 2.987 Å, 3.010 EF: 3.083 Å, 3.016 EF: 2.652 Å, 3.240 EF: 2.909 Å, 3.016 EF: 3.136 Å EF: 2.796 Å EF: 3.124 Å EF: 2.845 Å EF: 3.144 Å OFF: 3.451 Å OFF: 3.435 Å

ϕc (°) Å Å Å Å

85.35 82.10 75.86 81.26 74.80 75.05 50.92 82.42 51.03 7.5 6.8

a

Dihedral angle between the mean planes of adjacent sulfonate and guanidinium involved in the H-bonding networks. bEF: H···centroid distance; OFF: centroid···centroid distance. cDihedral angle between the mean planes of adjacent naphthalene and/or N-containing heterocycle.

the bilayer, while the alkyl tails hide in the hydrophobic cavities (Figure 2). Water molecules are present in 1MA and 2EA as a compensation for the not well matching of the neighboring bilayers. The water molecules in 1MA and 2EA and the −NH2 groups of PA cations in 3PA protruding out of the H bond networks connect the neighboring layers to form bilayer structures of H bond networks. As a contrast, the −CH3 groups of DMA cations in 4DMA interrupt the bilayer structure to keep a discrete bilayer H-bonding network. It is notable that alkylammonium cations with longer alkyl chains such as butyl disfavor the formation of the (Cat)·G·NDS-type compounds. The reason can be simply given from the crystal packing

the H atom from one naphthalene group and the centroid of the C6 ring of the adjacent aryl ring (Figure 1d). It is supposed that these motifs may have a contribution to the formation of the 2D-like bilayer structure. As a group, the type I compounds have the following common features. They all crystallize in the bilayer architecture which is constructed by the G and NSG components in a little different way from the ones found in (Guest)·G2·NDS compounds. The formed cavity A is suitable for the accommodation of the protonated linear lower alkyl amines (not more than three carbon atoms). From the packing diagrams of the four compounds, it is found that the NH groups of the alkyl amines locate at the hydrophilic surfaces of 6572



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Figure 2. Space filling modes of (a) 1MA, (b) 2EA, (c) 3PA, and (d) 4DMA.

diagrams of the type I compounds. The cavity A cannot allow the filling of longer alkyl chains than propyl because of the limited space of the cavity and increasing repulsions between the chains (Figure 2). Type II compounds include 5iPA and 6tBA, in which the Cat ions are protonated branched alkyl amines, i.e., isopropylammonium and tert-butylammonium, which are strikingly different from the linear ones in 1MA−4DMA. It is reasonable to conclude that the larger volumes of the branched alkyl amines destabilize the bilayer structures shown in type I compounds. From the viewpoint of H-bonding network and π−π stacking, the difference between type I and II is clear. Taking 6tBA for example, the H-bonding network is not a flat plane as shown in 3PA but a wavy one (Figure 3a,b). Besides the H bond motif R22(8) which is almost always accompanied by the G and NDS ions, three new motifs, R34(10), R44(14), and R46(14), appear in 6tBA. Distances of the N···O H bonds are between 2.827 and 3.050 Å. The 2D π−π interaction and a herringbone-like arrangement of the naphthalene groups as shown in the type I compounds are broken in 6tPA (Figure 3c,d) because of the introduction of tert-butylammonium which has a larger volume than those in type I compounds. As a consequence, 1D π−π interaction remains among the naphthalene groups, propagating along the a axis. The EF motif has a contact of 2.796 Å. The dihedral angle ϕ between the mean planes of adjacent naphthalene groups is 75.05°. The transformation of structural packing from type I to type II compounds can be described as a result of a relative slide of 1D NDS arrays with an EF motif in the ab plane aroused by a larger guest cation and a merge of the separate cavities of A and B. Compounds 7IM, 8PZ, and 9PY belong to type III of the serial compounds, in which the cations are all simple Ncontaining heterocycles. Structural features of 9PY are shown in Figure 4. The H-bonding network is an assembly of onedimensional belts running along the a axis (Figure 4b). There are three types of H bond motifs, i.e., R22(8), R24(12), and R44(12), in the belt. Five out of the six hydrogen atoms of the G

Figure 3. Crystal structure of 6tBA (type II): (a) Packing diagram viewed along the a axis; (b) H bond motifs in the ab plane. H bonds are depicted as dotted lines; (c) EF motifs between adjacent naphthalene groups forming a chain-like structure extending along the a axis. The balls represent the centroids of the C6 rings of the aryl groups.

are involved in the H-bonding interaction. For the sulfonate ions, one oxygen atom does not join the H-bonding network. The pyridinium cation affords one H bond with one sulfonate hanging to the belt. Distances of the N···O H bonds are between 2.822 and 2.944 Å, indicating strong interactions between the donor and acceptor units. There are 1D chains formed by the naphthalene groups through the π−π interactions in 9PY, similar to the type II compounds such as 6tPA (Figure 4c). However, because of the different shapes between the branched amines and simple Ncontaining heterocycles, the packing modes of the 1D chains of the both types of compounds are not the same. It can be seen as a cubic packing in 9PY but a rhombic packing in 6tPA. The EF motif in 9PY shows a contact of 3.144 Å. The dihedral angle ϕ between the mean planes of adjacent naphthalene groups is 51.03°, both of which indicate weak π−π interactions among the naphthalene groups. Compounds 7IM and 8PZ show similar rhombic packings of the 1D chains formed by the naphthalene groups to 9PY. However, because the nature of the imidazolium, pyrazolium, and pyridinium cations is strikingly different from each other, so are the H-bonding networks of the three compounds. It is notable that the imidazolium and pyrazolium rings are bisdonors of H bonds and can form richer H bond motifs in the corresponding compounds than 9PY (Table 1). Furthermore, 6573



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Figure 4. Crystal structure of 9PY (type III): (a) Packing diagram viewed along the c axis, showing a layer structure; (b) H bond motifs in the one-dimensional belt. H bonds are depicted as dotted lines; (c) EF motifs between adjacent naphthalene groups. The balls represent the centroids of the C6 rings of the aryl groups.

the imidazolium cation can act as a bridge to link two neighboring H-bonding networks. Type IV compounds include two compounds, 10MIM and 11EIM, in which the 2-methylimidazolium and 2-ethylimidazolium are both 2-substituted imidazolium derivatives. They have very similar structural packings. Compound 11EIM shows a layer structure along the a axis, which is distinguished by the H-bonding network (Figure 5a,b). There are four types of H bond motifs, i.e., R22(8), R23(8), R43(10), and R68(20) in 11EIM. Distances of the N···O and O···O H bonds among the EIM, G, NDS, and water components are in the range of 2.682−2.990 Å. In 11EIM, the planes of the naphthalene and imidazolium groups are arranged alternately. All of them stack nearly perpendicularly to the b axis, with a dihedral angle ϕ between the mean planes of adjacent rings of about 6.8°. Thus, 1D π−π interaction with an OFF motif exists among the adjacent naphthalene and imidazolium rings, propagating along the b axis (Figure 5c,d). The distance is found to be 3.435 Å between

Figure 5. Crystal structure of 11EIM (type IV): (a) Packing diagram viewed along the c axis; (b) H bond motifs. H bonds are depicted as dotted lines; (c) Arrangement of the EIM, G, and NDS ions. Hydrogen atom and oxygen atoms of NDS ions are omitted for clarity; (d) OFF motifs between adjacent naphthalene and imidazolium groups along the b axis. The balls represent the centroids of the C6 rings of the aryl groups and the C3N2 rings of the imidazolium groups.

the centroids of the C6 ring of the aryl group and the C3N2 ring of the imidazolium cation. Compound 12MP is classified as type V. The (R,S)-3methylpiperidium is an aliphatic cyclic amine with a stable chair conformation. It has a relatively larger shape than the other cations used in the series of (Cat)·G·NDS compounds and is even comparable with the naphthalene ring. From the packing 6574



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diagram of the compound, it is found that 12MP has a layer structure organized by the H-bonding network in the ab plane (Figure 6a,b). Three types of H bond motifs, i.e., R22(8),

Figure 7. (a) DSC and (b) dielectric constant curves of 9PY.

found by studying the partially and completely deuterated samples 9PY-d5 and 9PY-d12, indicating the phase transition has no direct relationship with the proton motions (Figure S2, Supporting Information). The phase transition in 9PY is further verified by a variabletemperature dielectric constant measurement (Figure 7b). It is known that dielectric constant (ε = ε′ − iε″) of a solid-state material is sensitive to structural phase transitions because the corresponding microscopic rearrangements of atoms, ions, and molecules usually arouse changes of the electric polarity in the system. For 9PY, the real part (ε′) of the dielectric constant shows a step-like change at about 210 K, corresponding to the phase transition as shown in the DSC curve. The value of the change of the ε′ around the Tc is about 1.5, a relatively small value. It indicates the motional changes, especially the pyridinium cation, is largely restricted. Otherwise, there should be a large dielectric change (e.g., Δε′ ≈ 10), as seen in other dynamic molecular crystals.41−43 Another striking feature of the ε′ is a gradual increase at temperatures below the Tc. Such a fluctuation may be ascribed to a pretransitional effect because of molecular motions such as reorientation.42 The observed peaks and step-like changes in the DSC and dielectric constant measurements are a reflection of structural phase transition. In order to deeply understand the origin of the phase transition, detailed information on the structure changes is needed. Therefore, temperature-dependent X-ray crystallographic experiments on 9PY were performed at every 5 K in the temperature range 153−283 K besides a room-temperature (293 K) structure analysis as illustrated above. The detailed structural analysis sheds light on the understanding of the origin of the structural phase transition in 9PY. Compound 9PY always crystallizes in the triclinic space group P1̅ in the temperature range 153−293 K. The asymmetric unit contains two half NDS anions, one pyridinium cation, and one G cation (Figure 8a). For convenience, the

Figure 6. Crystal structure of 12MP (type V): (a) Packing diagram viewed along the b axis; (b) H bond motifs in the layer plane. H bonds are depicted as dotted lines.

R34(12), and R66(20), are found in this compound. Distances of the N···O H bonds are in the range 2.840−2.944 Å. The −NH2 parts of the MP cations are assembled in the hydrophilic Hbonding networks, while the alkyl parts hide in the hydrophobic cavities formed by the naphthalene groups. Because of the large volume of the MP cation, neighboring NDS anions interdigitated by the cations have no π−π interactions which are widespread in the other (Cat)·G·NDS compounds. Phase Transition in 9PY. Among the 12 compounds reported here, only 9PY is found to show a structural phase transition. The phase transition in 9PY is first disclosed by differential scanning calorimetry (DSC) in the measured temperature range 193−283 K (Figure 7a). There is a pair of peaks appearing at 210 K on cooling and 212 K on heating with a small thermal hysteresis of 2 K. The phase transition point (Tc) is fairly consistent with a variable-temperature X-ray single crystal structure analysis discussed in detail below. The enthalpy change is 724 J mol−1, and the corresponding entropy change ΔS is 3.43 J mol−1 K−1. Thus, the degree of the disorder state of the phase transition can be roughly estimated by using Boltzmann equation, ΔS = R ln(N), where the R is the gas constant and the N is the ratio of the maximum number of possible configurations per molecule in the disordered phase over the one in the ordered phase. The resulting value of N is 1.51, indicating a slight disorder of the 9PY above the Tc against a typical order−disorder transition with reorientations over two sites (N = 2). No obvious deuterium isotope effect of 9PY is 6575



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Figure 9. Temperature-dependent cell parameters of 9PY.

Figure 10. A packing overlay diagram of 9PY at 203 K (pale gray) and 223 K (dark gray). Figure 8. Packing diagrams of 9PY at 293 K: (a) Asymmetric unit, showing the A, B, C, and D components; (b) space-filling model of the pyridinium cations in the lattice; (c) space-filling model of the environment around the pyridinium cation. Dotted lines denote H bonds.

The relative displacements of local structure are evaluated by dihedral angles of A−C, B−C, and A−B of the rings of the cationic and anionic components in the measured temperature range (Figure 11). Curves of the temperature-dependent dihedral angles of A−B, A−C, and B−C all show striking changes around 211 K. With the decrease of the temperature, the dihedral angles of A−B and B−C continuously decrease and increase, respectively, accompanied by abrupt changes around 211 K. In particular, the value of A−C gradually decreases above the Tc with a change from 7° to 5.5° and then

mean planes of the four components are named as A, B, C, and D, respectively. It is found that, at 293 K, the A and C planes have a dihedral angle of 7.89°, B−C of 45.81°, and A−B of 51.18°. Focusing on the pyridinium cation C, it locates in a cavity wrapped by the A, B, and D parts and anchors through H bonds between the NH group and O atom of the sulfonate (Figure 8b,c). The environment around the pyridinium cation consists mainly of the rigid naphthalenyl rings which readjust themselves to fit the size of the pyridyl ring. It is reasonable to infer that the delicate matching of the pyridinium cation to the cavity leaves enough room for possible motions of the pyridyl ring in the crystal lattice upon temperature change. Detailed structure changes are discussed below through comparing the crystal structures of 9PY measured at different temperatures. The temperature-dependent values of the cell parameters (a, b, c, α, β, γ) are shown in Figure 9. With the decrease of the temperature, the cell parameters show sudden decreases in the temperature range 207−218 K except that the γ shows a linear increase. The β undergoes the largest change of 2.7° across 211 K, corresponding to a 2.8% change. This point is recognized as an inflection point, being consistent with the Tc verified by DSC and dielectric constant measurements. Two packing diagrams of the unit cell above and below the transition temperatures (223 and 203 K) were superposed based on the O point viewed along the b axis (Figure 10). It found that the rings of the components undergo deflexions.

Figure 11. Temperature-dependent dihedral angles of 9PY. The A and B denote planes of the naphthalenyl rings, the C the plane of the pyridyl ring, and the D the plane of the G ion, as shown in Figure 8. 6576



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noticeably increases below the Tc with a change from 5.5° to 10°. These observations imply distinct twists of the rings of the anions and cations during the phase transition. As to the cationic and anionic components of 9PY, the pyridinium cation is particularly unique. Below the Tc, the assignments of the N (a site) and C (b and 1−4 sites) atoms of the six-atom ring are definite (Figure 12). All of the atom

Figure 12. Atom displacements of the assigned C and N atoms of the six-atom pyridinium ring below and above the Tc.

displacements show normal changes with the temperature. These results indicate the pyridinium ring has a fixed position in the crystal lattice. However, above the Tc, the assignments of the a and b sites encounter a problem. The a and b sites assigned either as C and N atoms (Figure 12) or as N and C atoms (Figure S3) lead to a bifurcation of the atom displacements above the Tc. The former assignment is adopted in the following H bonds analysis because it shows a smaller divergence than the latter. A better solution to the bifurcation is to refine the a and b sites with mixed site occupancies of N and C atoms, implying an disorder of the pyridinium. The corresponding two sites are then designated as a′ and b′, respectively (Figure 12). It is clear that this model eliminates the bifurcation of the atom displacements of the two sites above the Tc. The occupancy of C atom over the a′ site or the N atom over the b′ site is in the range from 0.47 to 0.63 upon warming (Figure S4). Such a preference of the N atom for the b site is largely determined by the competition between the N−H···O and C−H···O H bonds as discussed below. These facts strongly support the idea that the pyridinium cation undergoes a disorder over at least two sites, albeit unequally. Analysis of the H bonds between the pyridinium cation and the adjacent sulfonate O atoms of the B-type NDS anions is more informative, considering both of the N−H···O and C− H···O bonds (Figure 13). Temperature dependence of the D··· A distances shows inflections around 211 K. Below the Tc, the N···O and C···O distances increase and decrease, respectively, with the increase of the temperature. They meet each other at the Tc with a value around 3.0 Å. This value is, in general, taken as the boundary between the N−H···O and C−H···O H bonds, i.e., N···O < 3.0 Å and C···O > 3.0 Å.1,44 Above the Tc, the N··· O and C···O distances exhibit opposite changes with the increase of the temperature. The similar trend is also shown in the temperature-dependence of the H-bond angles. The N− H···O bond angles decrease quickly from 152° at 153 K to 143° at 213 K and then increase slowly from 140° at 218 K to 144° at 283 K. Meanwhile, the C−H···O bond angles increase from 124° at 153 K to 131° at 213 K and then decrease from 133° at

Figure 13. Temperature dependence of H bond lengths and angles between the pyridinium cation and the B-type NDS anions. Lines are shown to guide the eye and the dashed lines indicate the crossover of the H-bonding interactions between the pyridinium and the sulfonates. (below) Packing diagrams below (203 K) and above (223 K) the Tc are shown to illustrate the changes of the H bonds (dotted lines).

218 K to 126° at 283 K. The asymmetries of the curves below and above the Tc correspond to relatively strong and weak Hbonding interactions, respectively, between the pyridinium cation and the sulfonates, which makes it possible that the pyridinium is ordered below the Tc and disordered above the Tc. This corresponds to a reorientation of the pyridinium ring as reflected by the dielectric constant measurement. However, additional proofs for the dynamic disorder of the pyridinium cation above the Tc are still needed such as those from solidstate NMR techniques and theoretical calculations, which will be further investigated. By the end, we would like to try to give a possible explanation for the question: why does 9PY undergo a structure phase transition in the series of (Cat)·G·NDS compounds? Generally, the structural phase transition of a molecular compound is definitely a crystal packing-determined property, which is ultimately determined by the sizes, shapes, and electrical properties of all of the components in the crystal lattice. For the 12 compounds, the shapes of the cations are obviously a key factor to determine the occurrence of a phase transition. Hirshfeld surface analysis is thus utilized to visualize 6577



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been successfully synthesized. These serial compounds constitute a new family of guanidinium organo(di)sulfonate compounds by incorporating the Cat into the G·NDS network instead of the second G cation. The 12 compounds of (Cat)·G· NDS can be classified into five types based on the H bond networks and NDS stacks: (I) 1MA−4DMA; (II) 5iPA−6tBA; (III) 7IM−9PY; (IV) 10MIM−11EIM; and (V) 12MP. In these compounds, the shapes, sizes, and electrostatic properties of the Cat ions play key roles in the crystal packings. By the introduction of the Cat ions, the relatively stable H bond network formed by G and S is interrupted and reorganized, and new H bond motifs are formed in the complicated 1D and 2D H-bonding networks. The π−π interactions between the naphthalene rings and/or between the naphthalene-heterocycle rings adjust the intermolecular interactions, along with the Hbonding interactions, among individual Cat and NDS ions and make contributions to the stabilization of various types of structures. Compound 9PY is unique among the 12 (Cat)·G·NDS compounds by exhibiting a structural phase transition at 211 K. Besides proofs from the DSC and dielectric measurements, temperature-dependent X-ray crystallographic experiments disclose striking changes of the crystal structure around 211 K as shown in cell parameters, dihedral angles, atom displacements, and H bonds. It is found that competing Hbonding interactions among the pyridinium and sulfonates are the driving force for the structural phase transition. This fact indicates that molecular compounds are a promising reservoir of new phase transition compounds, e.g., switchable dielectric and ferroelectric compounds, by taking advantage of structural versatility and rational design. It is expected that full understanding of intermolecular interactions by exploration of more concrete phase transition compounds would pave the way for reaching the ultimate goal of crystal engineering, i.e., to design a crystal with desired properties.

the shapes of the cations and qualify the interactions, including Hirshfeld surfaces and fingerprint plots.45−47 This tool is very useful to directly “see” all close contacts (location and strength) of a molecule within its environment. So it is expected to be powerful in analyzing intermolecular interactions in crystals that undergo structural changes (e.g., motional changes of components in the crystal lattice).48 By comparing the Hirshfeld surfaces of the cations in 1MA−12MP, it is found that only the methylammonium in 1MA, ethylammonium in 2EA, and pyridinium in 9PY show relatively regular shapes in the cavities enveloped by G and NDS components (Figure S5). Other cations with more irregular shapes fit their cavities like a key fitting to a lock that makes motional changes impossible. As to the 1MA, 2EA, and 9PY, the former two develop multiple H bonds of the −NH3 heads with sulfonates which tightly fix the cations in the cavities. In contrast, the pyridinium has a medium (below the Tc) and weak (above the Tc) N−H···O bond and two weak C−H···O bonds (below and above the Tc) (Figure 14). The weakening of the interactions makes the pyridinium partly free in its cavity to eventually result in the phase transition.

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CONCLUSION In summary, a series of organic salts (Cat)·G·NDS based on protonated amines or N-containing heterocycles (Cat), guanidinium (G), and 1,5-naphthalenedisulfonate (NDS) has

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EXPERIMENTAL SECTION

General. All chemicals in the syntheses were of reagent grade and used as received without further purification. Infrared (IR) spectra were recorded using KBr plates on a Shimadzu IRPrestige-21 spectrometer. Elemental analyses were measured on a vario MICRO analyzer. Differential scanning calorimetry (DSC) measurements were performed on a TA Instruments Q2000 in the temperature range 193−283 K under nitrogen at atmospheric pressure in aluminum crucibles with a heating or cooling rate of 10 K·min−1. Dielectric measurement of powder samples was performed on a TongHui 2828 impedance analyzer over the frequency range from 1 kHz to 1 MHz with an applied electric field of 0.5 V in the temperature range 120− 270 K. X-ray Diffraction. Single-crystal X-ray diffraction data of 1MA− 12MP were collected on a Rigaku SCXmini CCD diffractometer equipped with graphite-monochromated MoKα radiation at 293 K. Temperature-dependent single-crystal X-ray diffraction data of 9PY were collected on Rigaku Saturn 924 from 153 to 283 K, and total of 26 sets of data were obtained. Data processing was performed using the CrystalClear software package (Rigaku, 2005). The structures were solved by direct methods and successive Fourier synthesis and then refined by full-matrix least-squares refinements on F2 using the SHELXLTL software package (Sheldrick, 2008). All non-hydrogen atoms were refined anisotropically and the positions of all hydrogen atoms were generated geometrically. Summary of crystallographic data and details of hydrogen bonding interactions for the compounds are given in Tables S1−S2, respectively. CCDC 1026880−1026891 (1MA−12MP) and 1026892−1026917 (9PY) contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via http://www.ccdc.cam.ac.uk/conts/

Figure 14. Hirshfeld surfaces and fingerprint plots of the pyridinium cation in 9PY below and above the Tc. The red, white, and blue regions of the surfaces correspond to positive (close contact), neutral, and negative isoenergy, respectively. The fingerprint plots highlight the N(C)−H···O interactions on the background of all intermolecular contacts. 6578



Article

Author Contributions

retrieving.html (or from the Cambridge Crystallographic Data Centre, 12, Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033). Synthesis of (Cat)·G·NDS. The serial compounds 1MA−12MP were synthesized by very similar methods. An aqueous solution (30 mL) of 1,5-naphthalenedisulfonic acid (1.824 g, 8 mmol) and guanidinium chloride (0.382 g, 4 mmol) was combined with a solution of alkylamine or N-containing heterocycle (4.3 mmol) used in 1MA−12MP. The mixture solution was stirred for 30 min and filtered. Colorless crystals were obtained by slow evaporation of the solution after several days. 1MA. Yield: 46% based on guanidinium. Selected IR peaks (KBr, cm−1): 3421 (m), 2964 (w), 2731 (w), 1666 (s), 1406 (s), 1209 (s), 1048 (w). Elemental analysis calcd (%) for C12H20N4O7S2 (396.44): C 36.26, H 5.08, N 14.13; found: C 36.18, H 4.81, N 14.29. 2EA. Yield: 53% based on guanidinium. Selected IR peaks (KBr, cm−1): 3421 (s), 3197 (s), 1666 (s), 1191 (s), 1040 (m). Elemental analysis calcd (%) for C13H22N4O7S2 (410.47): C 38.04, H 5.40, N 13.65; found: C 38.01, H 5.36, N 13.98. 3PA. Yield: 49% based on guanidinium. Selected IR peaks (KBr, cm−1): 3380 (s), 3178 (s), 1670 (s), 1198 (s), 1032 (s). Elemental analysis calcd (%) for C14H22N4O6S2 (406.48): C 41.37, H 5.46, N 13.78; found: C 40.74, H 5.32, N 13.91. 4DMA. Yield: 52% based on guanidinium. Selected IR peaks (KBr, cm−1): 3442 (w), 2951 (w), 1654 (s), 1401 (s), 1015 (m). Elemental analysis calcd (%) for C13H20N4O6S2 (392.45): C 39.79, H 5.14, N 14.28; found: C 38.81, H 5.20, N 14.38. 5iPA. Yield: 40% based on guanidinium. Selected IR peaks (KBr, cm−1): 2955(w), 2726(w), 1648(s), 1404(s), 1023(s). Elemental analysis calcd (%) for C14H22N4O6S2 (406.48): C 41.37, H 5.46, N 13.78; found: C 41.25, H 5.21, N 13.97. 6tBA. Yield: 32% based on guanidinium. Selected IR peaks (KBr, cm−1): 3380 (s), 3178 (s), 1670 (s), 1210 (s), 1032 (s). Elemental analysis calcd (%) for C15H24N4O6S2 (420.50): C 42.84, H 5.75, N 13.32; found: C 42.66, H 5.61, N 13.46. 7IM. Yield: 53% based on guanidinium. Selected IR peaks (KBr, cm−1): 2937 (w), 2632 (w), 1646 (s), 1413 (s), 1206 (m), 1045 (m). Elemental analysis calcd (%) for C14H17N5O6S2 (415.44): C 40.47, H 4.12, N 16.86; found: C 40.03, H 3.95, N 17.42. 8PZ. Yield: 48% based on guanidinium. Selected IR peaks (KBr, cm−1): 2945 (w), 2730 (w), 1660 (s), 1408 (s), 1211 (s), 1041 (m). Elemental analysis calcd (%) for C14H17N5O6S2 (415.44): C 40.47, H 4.12, N 16.86; found: C 40.43, H 3.98, N 17.13. 9PY. Yield: 35% based on guanidinium. Selected IR peaks (KBr, cm−1): 3350 (s), 3178 (s), 1666 (m), 1202 (s), 1036 (s). Elemental analysis calcd (%) for C16H18N4O6S2 (426.47): C 45.06, H 4.25, N 13.14; found: C 44.59, H 4.16, N 13.29. 10MIM. Yield: 54% based on guanidinium. Selected IR peaks (KBr, cm−1): 3365 (s), 3185 (s), 2842 (w), 1671 (s), 1214 (s), 1042 (s). Elemental analysis calcd (%) for C15H21N5O7S2 (447.49): C 40.26, H 4.73, N 15.65; found: C 40.07, H 4.52, N 15.89. 11EIM. Yield: 44% based on guanidinium. Selected IR peaks (KBr, cm−1): 3369 (m), 2948 (m), 2722 (m), 1650 (s), 1399 (s), 1007 (m). Elemental analysis calcd (%) for C16H23N5O7S2 (461.51): C 41.64, H 5.02, N 15.17; found: C 41.58, H 4.79, N 15.46. 12MP. Yield: 36% based on guanidinium. Selected IR peaks (KBr, cm−1): 3388 (m), 3155 (m), 2957 (m), 2733 (w), 1657 (s), 1406 (s), 1209 (m), 1012 (m). Elemental analysis calcd (%) for C17H26N4O6S2 (446.54): C 45.73, H 5.87, N 12.55; found: C 45.72, H 5.62, N 12.62.

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†

C.S. and B.W. contributed equally to the work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (Grant No. 21225102).

ASSOCIATED CONTENT

S Supporting Information *

Additional crystallographic data and figures and Hirshfeld surfaces and fingerprint plots for 1MA−12MP. This material is available free of charge via the Internet at http://pubs.acs.org.

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REFERENCES

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