Pressure Effects on Crystallization, Polymorphism, and Solvation of 4

Article pubs.acs.org/crystal

Pressure Effects on Crystallization, Polymorphism, and Solvation of 4,4′-Bipyridinium Perchlorate Michalina Anioła and Andrzej Katrusiak* Faculty of Chemistry, Adam Mickiewicz University, Umultowska 89b, 61-614 Poznań, Poland S Supporting Information *

ABSTRACT: Pressure efficiently changes the crystallization preferences of three polymorphs and three solvates of 4,4′-bipyridinium perchlorate, [C10H9N2]+ClO4−. At ambient pressure a triclinic dihydrate [C10H9N2]+ClO4−·2H2O is formed of the solutions in water and in the water/methanol mixture, while the methanol solution in open vials yields concomitant monoclinic quarterhydrate 4([C10H9N2]+ClO4−)·H2O and triclinic anhydrate [C10H9N2]+ClO4−. At 0.30 GPa an orthorhombic anhydrate is formed, stable to 0.60 GPa, when it collapses into a monoclinic phase. The transition changes the conformation of cations from planar to twisted, differently than the analogue phase transition in [C10H9N2]+Br·H2O. Recrystallizations from the methanol solution above 0.55 GPa leads to methanol solvate [C10H9N2]+ClO4−·CH3OH. In the solvates, chains of the cations are aggregated through either NH+···N bonds (like in the neat polymorphs) or NH+···OH···N bonds, or both these bonds in one crystal. The pyridinium protons are disordered in all these structures, which implies disproportionation between neutral molecules, C10H8N2, monocations [C10H9N2]+ and dications [C10H10N2]2+. In quarterhydrate 4([C10H9N2]+ClO4−)·H2O the disorder of protons propagates through the hydrogen bonds mediated by disordered water molecules, also involved in weak H-bonds to perchlorate anions.

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INTRODUCTION High-pressure is known for its strong effects on the crystal structures and their transformations;1 hence it is increasingly used for obtaining new forms of organic compounds. Polymorphs,2 hydrates, and solvates in general can be useful for improving properties of versatile functional compounds and materials, such as active pharmaceutical ingredients (APIs), herbicides, and pesticides.3,4 For some substances even a moderate pressure can drastically change the crystallization preference. High-pressure methods can be particularly useful when solvents and high temperature treatment can deteriorate the product quality, and therefore expensive inert-gas protection is required. However, relatively little is known about the pressure effects on the crystallization of organic compounds.5,6 It was established that pressure can be used for obtaining new crystalline forms, unknown at ambient conditions. For example, at ambient pressure thiourea7 crystallizes exclusively as anhydrate of aqueous solution, but above 0.6 GPa a C(NH2)S·H2O hydrate is formed; above 0.7 GPa another hydrate 3C(NH2)S·2H2O and above 1.2 GPa the anhydrate thiourea is obtained again, albeit in another phase than that at normal conditions. The high-pressure preference for the formation of hydrates and other solvates was also reported for 1,4-diazabicyclo[2.2.2]octane hydroiodide, i.e., dabcoHI8 and dabcoHClO4.9 The pressure-promoted hydration is quite common in nature and includes the formation of methane hydrates10 and numerous other minerals.11 Presently, we have investigated the pressure effect for the crystallization of 4,4′bipyridine perchlorate (44′biPyHClO4). All 44′biPyHA monosalts (where A denotes anions: Br−, Cl−, I−, ClO4−, BF4−) investigated until now preferentially form © 2017 American Chemical Society

hydrates when crystallized of aqueous and methanol solutions in open vials at ambient pressure.12 In their structures the 44′biPyH+ cations are linked by NH+···N hydrogen bonds into linear chains, analogues to those in 1,4-diazabicyclo[2.2.2]octane monosalts (dabcoHA), known for their ferroelectric and relaxor properties.13−16 However, at normal conditions the crystallization of 44′biPyHClO4 dissolved in methanol by evaporation in open vials (hence with some contest of H2O from the air) yielded the dihydrate only.12 The hydration can be disadvantageous for the stability of the sample at increased temperature and for its potential applications. Hence this present study aimed at applying high-pressure in order to obtain anhydrous 44′biPyHClO4 (Figure 1). The anhydrous 44′biPyHClO4 crystals can possess ferroelectric and relaxor properties similar to those in dabcoHI,17 and dabcoHBr,18 dabcoHClO4, and dabcoHReO4.19 We were also interested in conformational properties of the 44′biPyH+ cation. In all 44′biPyHA salts obtained at normal conditions the bipyridinium cations assume the twisted conformation, while in the disalts the planar conformation is favored. However, high pressure transforms the 4,4′-bipyridine hydrobromide monohydrate (44′biPyHBr·H2O) crystals and induces the planar conformation.20

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EXPERIMENTAL SECTION

Reiterated recrystallizations by evaporating the solution prepared by dissolving 44′biPyHClO4·2H2O in methanol at ambient pressure Received: January 19, 2017 Revised: April 5, 2017 Published: April 13, 2017 3134

DOI: 10.1021/acs.cgd.7b00085 Cryst. Growth Des. 2017, 17, 3134−3141

Crystal Growth & Design

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one perchlorate anion, Cl(2)O4, we have performed low-temperature measurements at 250, 220, 190, and 100 K. This series of additional measurements was not conclusive with regard to the crystal symmetry, because even when the crystal model was refined in space group Cc the protons and one perchlorate anion (of four symmetry-independent ClO4 anions in space group Cc) remained disordered, while the Rfactors were marginally reduced despite considerably increased number of refined parameters. Similarly, the disorder of protons and ClO4 anions could not be eliminated for other lower-symmetry structural models. Therefore, the C2/c symmetry of quarterhydrate has been assumed. The high-pressure crystallizations were performed in situ in a Merrill-Bassett diamond anvil-cell (DAC),21 modified by mounting the diamond anvils directly on steel backing plates. The gasket made of tungsten foil 0.2 mm thick with a 0.4 mm hole was used. The anvil culet size was 0.8 mm in diameter. Pressure in the DAC was gradually increased, in approximately 0.1 GPa steps, and it was calibrated by the ruby-fluorescence method,22,23 with a Photon Control spectrometer affording the precision of 0.02 GPa. The obtained samples were identified by X-ray measurements carried out on an Excalibur diffractometer, equipped with an EOS CCD detector and monochromated MoKα radiation (0.71073 Å). The structure was solved by direct methods and refined by full-matrix least-squares by SHELXL.24 The high-pressure in situ crystallizations of 44′biPyHClO4 were performed from methanol and ethanol solutions. The purity of solvents was 99.8%. The single crystals at high pressure were grown at isochoric conditions. The high-pressure crystallization of the methanol solution yielded a methanol solvate 44′biPyHClO4·CH3OH at 0.55 GPa (Figure 2, Table 2). The determined structure is consistent with that described previously at ambient pressure.29 In order to avoid the solvation of 44′biPyHClO4 with methanol, the ethanol solution was used for further high-pressure crystallizations. At 0.30 GPa another single crystal was obtained at isochoric conditions (Figure 3). It occurred that a new polymorph of neat 44′biPyHClO4 was formed, of orthorhombic space group Cmc21. It has been labeled as phase β. Then the pressure in the DAC was gradually increased, and X-ray diffraction measurements were recorded for the same phase at 0.30 and 0.40 GPa. At 0.60 GPa this crystal compressed in the DAC transformed to yet another phase, labeled γ, of monoclinic space group P21/c. Although the transition proceeds between symmetry classes mm2 and 1 2/m 1, which do not conform to any group−subgroup relations, the structures of phases β and γ are clearly connected. We tried to recrystallize 44′biPyHClO4 above 0.6 GPa, to improve the quality of the samples directly crystallized in the γ-phase; however all these tries resulted in the disalt 44′biPyHClO4. Therefore, all diffraction studies on the γphase were performed on the crystals obtained in the solid−solid transition from the β-phase. It was attempted to recover the single crystals of β-44′biPyHClO4 grown in the DAC to the ambient

Figure 1. Cation and anion of 44′biPyHClO4, as present in phase β at 296 K/0.30 GPa, with generally applied atomic labels. resulted in two different concomitant crystals: anhydrate 44′biPyHClO4, of triclinic space group P1̅, hereafter labeled as phase α (Table 1), and monoclinic quarterhydrate

Table 1. Selected Crystal Data of Neat 44′biPyHClO4 Polymorphs α, β, and γ at 296 K α-44′biPyHClO4 pressure formula crystal system space group a/Å b/Å c/Å α/° β/° γ/° V/Å3 Dx/g·cm−3 Z/Z′ final R1

0.1 MPa triclinic P1̅ 8.9242(4) 9.7535(4) 14.1061(7) 97.751(4) 100.577(4) 111.020(4) 1099.19(9) 1.551 4/2 0.0759

β-44′biPyHClO4 0.30 GPa [C10H9N2]+ClO4− orthorhombic Cmc21 6.423(3) 16.122(4) 9.8159(11) 90 90 90 1034.7(6) 1.648 4/0.5 0.0224

γ-44′biPyHClO4 0.60 GPa monoclinic P21/c 6.431(10) 15.976(2) 9.734(6) 90 90.13(8) 90 1000.1(17) 1.704 4/1 0.0772

44′biPyHClO4·1/4H2O. The quarterhydrate crystal symmetry at normal conditions was determined as monoclinic space group C2/c (Table 2). However, because of a considerable disorder of the protons protons in hydrogen bonds NH+···N and N···H2O···H+N, as well as of

Table 2. Selected Crystal Data of Hydrates and Solvate of 44′biPyHClO4 at 296 K pressure formula crystal system space group a/Å b/Å c/Å α/° β/° γ/° V/Å3 Dx/g·cm−3 Z/Z′ final R1 a

44′biPyHClO4·1/4H2O

44′biPyHClO4·2H2Oa

44′biPyHClO4·CH3OH

0.1 MPa 4[C10H9N2]+4ClO4−·H2O monoclinic C2/c 26.6265(7) 9.41030(10) 19.3793(4) 90 113.993(2) 90 4436.18(16) 1.563 4/0.5 0.0759

0.1 MPa [C10H9N2]+·ClO4−·2H2O triclinic P1̅ 8.1537(7) 9.765(2) 17.709(2) 78.14(3) 82.87(3) 67.43(3) 1272.6(4) 1.528 4/2 0.0698

0.55 GPa [C10H9N2]+ClO4−·CH3OH monoclinic P21/c 6.6597(12) 15.142(5) 12.1936(17) 90 92.454(13) 90 1228.5(5) 1.558 4/1 0.0937

According to ref 12. 3135



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The pyridine H atoms were generally located from the molecular geometry, and the Uiso’s were constrained to 1.2·Ueq of their carriers. The pyridinium protons appeared in the difference Fourier maps, but the geometrical constraints were applied for their location, too. However, the site occupation factors (SOF) for the protons possibly disordered in the N(1)H+···N(2) hydrogen bonds were refined with the free-variable condition SOF(N1) + SOF(N2) = 1. Such refinements were performed for two symmetry-independent protons in α-44′biPyHClO4 (in both H-bonds the protons refined at disordered positionssee the Discussion), for one symmetryindependent proton in β-44′biPyHClO4 , for one symmetryindependent proton in γ-44′biPyHClO 4, as well as for two independent hydrogen bonds N(1)H+···N(1) and N(1′)H+···N(11) in quarterhydrate 44′biPyHClO4·1/4H2O. In α-44′biPyHClO4 in both H-bonds the protons refined at disordered positions (see Discussion), while the arbitrary assignment of the proton at one of their sites or their 50:50 disorder in both their sites resulted in some increase of the R factor; the SOF refinements in β-44′biPyHClO4 and in γ44′biPyHClO4 were not conclusive, and we assigned the H-site arbitrarily at one of the N atomsthis has been done merely for completing the model, and no reliable information on the proton site or its disorder has been obtained. In hydrate 44′biPyHClO4·1/4H2O, according to the difference Fourier maps the protons are located at N(1) in molecule A, at N(11) of molecule B and N(21) of molecule C, while no peak at N(1′) was found. However, taking into account the disorder of the perchlorate anions and relatively high residue factors (Tables 2 and S3 in Supporting Information), we have assumed the structural model with all pyridinium protons and water H atoms disordered in half-occupied sites, as explained in Discussion. In the water-mediated independent hydrogen bond N(21)H+··· O(1w) in the quarterhydrate, the proton disorder is implied by the symmetry, and two proton sites each with the SOF equal to 0.5 were included. In this hydrogen bond the orientationally disordered water molecule was assigned, with four half-occupied H positions preserving the geometrical dimensions (O−H bond distances of 0.97 Å and H− O−H angles of 109.3°) and “aiming” at the closest pyridine N and perchlorate O atoms H-acceptors. In 44′biPyHClO4·CH3OH the pyrimidinium proton and hydroxyl H atom were located in the difference Fourier map, and then they were constrained in the idealized positions. The selected crystal data of the 44′biPyHClO4 polymorphs and solvates are listed in Tables 1 and 2, respectively. The crystallographic information about these experiments and structures have been deposited in CIF format in the Cambridge Database Centre as supplementary publications CCDC 1527637−1527645. Their copies can be obtained free of charge from www.ccdc.cam.ac.uk/cif. Apart from regular single-crystal X-ray diffraction measurements, also short diffraction experiments aimed at quick identification of 44′biPyHClO4 phases and its solvates obtained at various thermodynamic conditions were performed. These short measurements were continued until the unit-cell dimensions were sufficiently accurate for unequivocal identification of the phase under study.

Figure 2. Crystallization of 44′biPyHClO4·CH3OH from the saturated solution of 44′biPyHClO4·2H2O in methanol in the DAC chamber in isochoric conditions: (a) one seed of 44′biPyHClO4·2H2O at 343 K (at the bottom-left edge of the chamber); (b) this crystal after 10 min at the same temperature; (c) at 296 K and 0.55 GPa; and (d) after 24 h the 44′biPyHClO4·2H2O crystal dissolved and a new crystal of 44′biPyHClO4·CH3OH grew instead (the microscope is focused on the upper face of the sample crystal, close to the top culet, and off the bottom culet). A ruby chip for pressure calibration lies on the top-right edge of the gasket, and another irregular ruby lies close to the sample (a−c), and then it moves to the bottom-right part of the chamber (d); several other small rubies are scattered in the chamber.

Figure 3. Isochoric growth of anhydrous β-44′biPyHClO4 crystal (at the button edge) from 44′biPyHClO4·2H2O dissolved in ethanol: (a) one seed at 383 K; and (b) at 0.30 GPa/296 K. Many small rubies are scattered in the chamber and a big one lies by the upper edge of the gasket.

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DISCUSSION The crystallization preference of 44′biPyHClO4 is sensitive to the solvent and thermodynamic conditions. At ambient pressure the triclinic dihydrate 44′biPyHClO4·2H2O is formed of solutions containing water. Of dry methanol solution in open vials (thus absorbing some moisture from air) at ambient pressure two concomitant forms precipitated: anhydrate α44′biPyHClO4 and quarterhydrate 44′biPyHClO4·1/4H2O (Table 2). This anhydrate has been labeled as α-polymorph because later we have also obtained at high-pressure two other polymorphs. The performed crystallizations show that the sufficient presence of water in the solution favors the dihydrate, a smaller water content affords the quarterhydrate and the dry anhydrous solution results in anhydrate α. High-pressure

pressure, but the samples broke into very small pieces, which indicated that the crystal either transforms to phase α or to another unknown phase. The high-pressure X-ray diffraction studies generally are subject to some experimental difficulties, such as limited access to the reciprocal space, incomplete data, high background of the radiation scattered on the DAC elements, their absorption, and the small crystal sample. For these reasons the results of the high-pressure measurements for the samples enclosed in a DAC are usually considerably lower compared to those performed on the bare crystals. 3136



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recrystallizations of methanol solution favors the quarterhydrate, but above 0.5 GPa the methanol solvate is formed. The recrystallizations of ethanol solution above 0.30 GPa led to anhydrate phase β, which above 0.75 GPa transforms (with no need of recrystallization) to another anhydrate phase γ. In all three polymorphs of 44′biPyHClO4 and its dihydrate, the cations are NH+···N hydrogen-bonded into chains, as shown in Figures 4 and 5; in 44′biPyHClO4·1/4H2O, apart from

Figure 5. Average crystal structure of quarterhydrate 44′biPyHClO4· 1/4H2O with disordered protons and water H atoms: (a) projected down [010] with independent cations colored green (A), blue (B), and pink (C); (b) the NH+···OH···N and NH+···N hydrogen-bonded chains separately projected down [100]; and (c) labels of symmetryindependent non-H atoms (perchlorate anion and pyridinium protons are shown in their disordered sites); the inset shows schematically the water molecule and its surrounding hydrogen bonds in one of possible configurations; the symmetry elements indicated in red concern the average crystal structure.

Figure 4. Autostereographic projections25 of NH+···N hydrogen bonded chains in the structures of polymorphs (a) α-44′biPyHClO4 at 0.1 MPa with independent cations A and B marked purple and blue, respectively; (b) β-44′biPyHClO4 at 0.30 GPa; and (c) γ44′biPyHClO4 at 0.60 GPa. Blue dotted lines mark hydrogen bonds NH+···N. Displacements Δ of the bipyridinium cations along the close chains (marked in red): in phase α for independent chains ΔA = 2.6 Å and ΔB = 2.8 Å; in phase β equal 4.9 Å; and in phase γ 4.1 Å; the displacements in the opposite direction is equal to (b-Δ) in phase α; c/2 in phase β; and (c-Δ) in phase γ (b and c are the unit-cell dimensions).

NH+···N bonded chains, there are also independent chains of NH+···OH···N bonded 44′biPyH+ cations and water molecules (Figure 5). In the methanol solvate 44′biPyHClO4·CH3OH only NH+···OH···N bonded chains are present (Figure 6). 44′biPyHClO4 Polymorphs. In the structure of α44′biPyHClO4 two symmetry-independent cations A and B (Figures 1 and 4a) have their pyridine rings twisted about the central C4−C4′ bond by 32.8° and 29.8°, respectively.

Figure 6. Hydrogen-bonded chains in the structure of 44′biPyHClO4· CH3OH at 0.55 GPa. 3137



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Hydrogen bonds NH+···N link the cations into chains along direction [010]. In this structure the disordered protons refined to 0.81(6):0.19(6) in the H1′/H1 sites between cations A and to 0.54(7):0.46(7) in sites H11′/H11 between cations B. In orthorhombic polymorph β-44′biPyHClO4, recrystallized from the ethanol solution above 0.30 GPa, the bipyridine cations assume the planar conformation (Figure 4b), energetically unfavored by 10.5 kJ/mol.25 The planar conformation of cations is consistent with their mirror-plane symmetry in the crystal: all atoms of the cation lie on the mirror plane perpendicular to [100]. The chains of NH+···N bonded cations run along direction [001]. The β phase of 44′biPyHClO4 is stable to 0.40 GPa, when the crystal transforms to monoclinic phase γ. The unit-cell dimensions of phases β and γ are consistent (Table 1, Figure 7), except for the monoclinic shear strain of just over 1° reducing the orthorhombic symmetry of phase β to monoclinic phase γ.

Figure 8. Distances of bonds NH+···N (blue) and (two shortest, red and black) CH···O in 44′biPyHClO4 polymorphs α, β, and γ. The vertical dashed lines indicate the preference pressure Pp for the crystallization of phase β and the critical pressure Pc of the transition between phases β and γ. The shortest CH···O bonds in phase α do not correspond to those in phase β and γ; the lines joining the points are for guiding the eye only.

changes of the unit-cell dimensions at the transition correspond to the NH+···N shortening (shortening of c) and to the conformation twisting (shortening of b and lengthening of a). In phase β the positions of anions ClO4− are restricted by symmetry (anion ClO4− lies on the mirror plane) and transition to phase γ removes this restriction. Thus, the changed orientation of the anions contributes to the increased distances CH···O in phase γ, too. In phase γ the NH+···N bonded chains of cations are shifted by about 0.75 Å relative to their neighbors (cf. displacements Δ indicated in Figure 4), which breaks the mirror symmetry of chains and their surroundings. The pyridine rings slide into the asymmetric voids, which reduces the crystal volume and results in the conformational twisting of the rings in phase γ. The positions of protons in two symmetry independent hydrogen bonds could be reliably refined for the polymorph α only, whereas for the high-pressure polymorphs β and γ the position of the protons were assigned arbitrarily (cf. Experimental). According to the refinement of the structure of polymorphs α, the protons are disordered, and it can be assumed that the protons can be disordered in polymorphs β and γ, too. The disorder of protons in hydrogen bonds NH+···N implies the disproportionation of 44′biPy molecules, monocations and dications, as illustrated in Figure 9a. Quarterhydrate 44′biPyHClO4·1/4H2O. In the structure of quarterhydrate 44′biPyHClO4·1/4H2O at ambient-pressure (Figure 5) there are three independent bipyridine cations (two of them located at the inversion centers), two independent perchlorate anions at general positions, and one water molecule (located on a 2-fold axis). Bipyridine cation A (atomic labels 1−6′) is located at the general position and it is NH+···N bonded to another cation A on one side and to cation B (atomic labels 11−16) on the other; cation B is located at the

Figure 7. Unit-cell dimensions of phases β and γ of 44′biPyHClO4 related to the unit-cell dimensions of phase β at 0.30 GPa (indicated by subscript “o”). The lines joining the points are for guiding the eye only.

Phases β and γ of 44′biPyHClO4 are similar in the aggregation and arrangement of NH+···N bonded chains of cations (Figure 4b,c). However, at the transition the planar conformation of the cations in phase β becomes twisted by about 15° in phase γ. This conformational change is connected to the broken mirror-planes symmetry in phase γ and to intermolecular interactions between cations and anions. The NH+···N bonds become shorter by over 0.1 Å in phase γ, while the shortest of CH···O contacts become longer by about 0.2 Å (Figure 8). The elongation of the CH···O distances shows that the twisted conformation of cations in phase γ releases the strains generated between the cations and anions by the increased pressure. The twisted conformation compensates the changes in interionic distances. The smallest monotonic strain in the compressed crystal is along the NH+···N bonded chains (along the [001] direction), and the strongest monotonic strain is perpendicular to the rings along direction [100]. The abrupt 3138



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pyridinium proton, and each NH+···OH···N bond involves one pyridinium proton, too. For the water-mediated H-bond, its two possible configurations, NH+···OH···N and N··· HO···+HN, implies that each of the H-sites is half-occupied (Figure 5). The disorder of protons in the NH+···N and NH+··· OH···N bonds can also involve disproportionation of 44′biPy molecules, monocations and dications, as illustrated in Figure 9. One of H atoms of the water molecule forms a bifurcated hydrogen bond O1wH···O3/O8 to two oxygen atoms of two perchlorate anions (Figure 5), and according to the symmetry of the 2-fold axis passing through the water molecule, two other perchlorate anions can be H-bonded to the water molecule, too. The water molecule is also involved in the NH+···OH···N bond. Thus, for all four H-bonds formed by one water molecule to two 44′biPy cations and to the ClO4 anions, there are only three H atoms available. Thus, the two H atoms of each water molecule are disordered at four half-occupied sites. In this averaged arrangement of cations, anions and one water molecule can be described as H-deficient hydrogen bonding, where several disordered H atoms are involved in a larger number of H-bonds. However, when specific configurations of the H-sites are considered, the number of hydrogen bonds (NH+···N, NH+···O, OH···N and OH···O) is equal to the number of pyridinium protons and water H atoms, and there are no other contacts between oxygen atoms shorter than the sum of van der Waals radii. This H-bonding scheme in the quarterhydrate is very likely to involve a tumbling motion of the H2O molecule and also some disorder of the ClO4− anions. In the NH+···OH···N bonded chains the disproportionation defects would require H2O-mediated H+ transfers. It was shown for dabcoHA monosalts that high-pressure destabilizes the NH+···N hydrogen bonds and either bonds NH+···A or NH+···OH···N involving the solvate molecules are favored.27 It is very likely that, depending on the H-sites, polar nanoregions are formed in the crystal (Figure 9), analogous to those in pyrazine monosalts.28 The high polarizability of NH+···N bonds in dabcoHBF4, dabcoHBr, and dabcoHI leads to relaxor ferroelectric properties of these crystals.17 Methanol Solvate 44′biPyHClO4·CH3OH. The methanol solvate, 44′biPyHClO4·CH3OH, previously obtained by Gao and Wu at 0.1 MPa,29 was crystallized by us at isochoric conditions above 0.55 GPa of the methanol solution. All our recrystallizations at high-pressure from methanol solution yielded exclusively the methanol solvate, whereas the neat 44′biPyHClO4 can be recrystallized at high-pressure only from the ethanol solution. The structure at 0.55 GPa (Table 2) is fully ordered, while in the same structure at ambient pressure three oxygen atoms of ClO4− anion are disordered each in two sites.7 The methanol solvate 44′biPyHClO4·CH3OH is the only structure in this series of 44′biPyHClO4 salts, where the cations are linked exclusively by the NH+···OH···N bonds and no NH+···N bonds are formed. The methanol molecule, joining the 44′biPyH+ cations into chains along crystal direction [001], interacts also with the perchlorate anion, as illustrated in the Figure 6. The perchlorate anion is also involved in a hydrogen bond NH+···O to the cation. Thus, all the hydrogen bonds are bifurcated. The 44′biPyHClO4·CH3OH crystal is compressed similarly to those of anhydrous 44′biPyHClO4 phases β and γ in this respect that the least compressed is parameter c along the chains, somewhat more compressed is the orthogonal direction b along the pyridine-ring planes, and the most compressed is the direction perpendicular to the rings (Figure 10). In order to resolve the

Figure 9. Possible H-sites and disproportionation of 44′biPy molecules (blue), monocations (black), and dications (red) in (a) NH+···N; and (b) NH+···OH···N hydrogen-bonded chains. The highlighted arrows indicate the polarization of the chain domains.

inversion center. This NH+···N bonded chain in the sequence ···A···A···B···A···A···B··· (letters “A” and “B” denote disproportionated neutral molecules, cations and dications, as well as alternatively the 44′biPy units, in their A and B sites, as explained below) extends along the [102] direction (Figure 5). According to the difference Fourier map, the protons appears at the N1′ and N2 atoms (the latter in the other chain involving H2O-mediated H-bonds), suggesting that there is a dication + HBH + , and accordingly to the charge balance and stoichiometry, there should be a neutral molecule, too, for example, in the following configuration: ··· AH+ ··· A···+ HBH+ ··· AH+ ··· A···+ HBH+ ·· AH+ ··· A···+ HBH+ ···

Consistently with the symmetry requirements, in this average chain of cations AABAAB the proton in bond AH+···A is disordered. However, they can be ordered between cations A and B if the following disproportionations takes place: ···+ HAH+ ··· AH+ ··· B···+ HAH+ ··· AH+ ··· B···+ HAH+ ··· AH+ ··· B ··· ···+ HA···+ HAH+ ··· B···+ HA···+ HAH+ ··· B···+ HA···+ HAH+ ··· B ··· ···+ HAH+ ··· AH+ ··· B···+ HA···+ HAH+ ··· B···+ HAH+ ··· AH+ ··· B ···

etc. Thus, in all these configurations there would be neutral molecules, monocations, and dications in the chain, while the centrosymmetric B-site can be occupied either by the neutral molecule or the dication. Cations C (atomic labels 21−26), located at inversion centers, together with water molecules, located on 2-fold axes, are NH+···OH···N bonded into another chain along direction [001]. In this chain the protons are disordered, as required by the symmetry, and their disorder implies that the water H atoms are disordered, too. One of the water H atoms is involved in the hydrogen bond OH···N, and the other H atom forms a weak OH···O bonds to one of two close perchlorate anions (the relevant intermolecular O···O distances are 3.238 and 3.248 Å). In quarterhydrate 44′biPyHClO4·1/4H2O each NH+···N bond engages one 3139



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Figure 10. Relative changes of the unit-cell dimensions in 44′biPyHClO4·CH3OH. The lines joining the points are for guiding the eye only.

preference for the dihydrate and methanol solvates formation, we have performed a series of recrystallizations of 44′biPyHClO4 dissolved in water and 1:1 H2O/methanol mixture. From the water solution up to 1.0 GPa exclusively the dihydrate crystals were obtained and above 1.0 GPa the quarterhydrate crystals precipitated. From the H2O/methanol mixture only the methanol solvate was obtained. The conformation of 4,4′-bipirydine cations in 44′biPyHClO4 neat and solvated compounds is either flat (τ = 0°) or considerably twisted. The strongest twisting occurs in the ambient-pressure methanol solvate (τ = 35°); in the dihydrate τ is 23° and in the quarterhydrate τ is 8° in cation A, while two other cations are planar. In the high-pressure structures of 44′biPyHClO4 the torsion angle τ is equal to 0° in phase β, and it is twisted to 18° in phase γ (Figure 11). It appears that the 44′biPyH+ conformation is controlled by intermolecular interactions in the crystal, similarly as it was explained for the transformation between the flat and twisted conformation observed in the phases of crystal 44′biPyHBr·H2O.20 A strong preference for the planar conformation in 4,4′-bipyridinium disalts, with only one exception of 44′biPy2HCl (τ = 26.4°) known so far, can be connected to the effect of crystal environment, too, in connection to the higher symmetry of those disalt crystals. The location of the cation on the inversion center implies the planar, albeit energetically unfavorable conformation.26

Figure 11. Dihedral torsion angle τ between pyridine rings in monosalts 44′biPyHA and their solvates (mainly hydrates) plotted as a function of pressure. Points for 44′biPyHCl, 44′biPyHBF4, 44′biPyHI, and 44′biPyHBr have been plotted according to refs 12 and 20. The lines joining the points are for guiding the eye only.

intermolecular and interionic interactions, strongly enhanced by pressure. The water-mediated NH+···OH···N and OH···O bonding to pyridinium, pyridine, and perchlorate anions in quarterhydrate 44′biPyHClO4·1/4H2O is intriguing in this respect that there are more H bonds than the H atoms involved, which are disordered at several partly occupied sites. It appears that such H-deficient H-bonding systems can be quite common in bio-organic systems.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.cgd.7b00085. Structure-refinements details for 44′biPyHClO4 polymorphs α, β at 296 K (Table S1), structure-refinements details for 44′biPyHClO4 polymorphs γ at 296 K (Table S2), and structure-refinements details for quarterhydrate and methanol solvate (Table S3) (PDF)

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Accession Codes

CONCLUSIONS It has been shown that the crystallization of 44′biPyHClO4 is sensitive to the crystallization method and conditions. The high-pressure equipment can be used for precisely controlling all thermodynamic conditions of crystallization, and therefore it provides a convenient environment for obtaining new forms of chemical compounds. Depending on the solvent, solution concentration, the water contents, temperature and pressure, three anhydrous phases of 44′biPyHClO4, two hydrates, and a methanol solvate 44′biPyHClO4·CH3OH have been obtained. The conformational transition in 44′biPyHClO4 proceeds in the opposite direction than that in 44′biPyHBr·H2O, where in high-pressure phase β the cations assume the planar conformation. The conformation of cations 44′biPyH+ depends mainly on their crystal environment and the effects of

CCDC 1527637−1527645 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12, Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Andrzej Katrusiak: 0000-0002-1439-7278 Notes

The authors declare no competing financial interest. 3140



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Article

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Pressure Effects on Crystallization, Polymorphism, and Solvation of 4

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