Molecular Pedal Motion Influences Thermal Expansion Properties

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Molecular Pedal Motion Influences Thermal Expansion Properties within Isostructural Hydrogen-Bonded Co-crystals Kristin M. Hutchins, Daniel K. Unruh, Frank Verdu, and Ryan H. Groeneman Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.7b01386 • Publication Date (Web): 04 Jan 2018 Downloaded from http://pubs.acs.org on January 8, 2018

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Crystal Growth & Design

Molecular Pedal Motion Influences Thermal Expansion Properties within Isostructural HydrogenBonded Co-crystals Kristin M. Hutchins,*,† Daniel K. Unruh,† Frank A. Verdu,‡ and Ryan H. Groeneman*,‡ †

Department of Chemistry and Biochemistry, Texas Tech University, Lubbock, TX, 79409 USA.

Email: [email protected]; Tel: (+1) 806-834-2744 ‡

Department of Biological Sciences, Webster University, St. Louis, MO, 63119, USA. E-mail:

[email protected]; Tel: (+1) 314-246-7466

ABSTRACT

The influence of molecular pedal motion on the thermal expansion properties of three isostructural hydrogen-bonded co-crystals based upon resorcinol is reported. The resulting cocrystals all exhibit discrete four-component assemblies held together by O-H···N hydrogen bonds and are comprised of resorcinol (res) with a series of isosteric bipyridines namely 4,4’azopyridine

(4,4’-AP),

trans-1,2-bis(4-pyridyl)ethylene

(4,4’-BPE),

and

1,2-bis(4-

pyridyl)acetylene (4,4’-BPA). The ability to change the core of the hydrogen bond acceptor molecules from an azo (N=N) to an ethylene (C=C) and finally an acetylene (C≡C) group affords co-crystals that differ in their tendency to undergo dynamic pedal motion in the organic solid

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state. All three co-crystals, 2(res)•2(4,4’-AP), 2(res)•2(4,4’-BPE), and 2(res)•2(4,4’-BPA), exhibit thermal expansions that correlate with the strength of the non-covalent interactions, as well as the propensity of the core to undergo pedal motion.

Co-crystallization of small organic molecules has been demonstrated to give rise to properties that are often absent in single component solids (e.g. solubility, bioavailability, reactivity).1-5 Incorporation of the second molecule, a co-crystal former (CCF), into the crystal lattice alters the crystal packing, and in many cases, the new arrangement imparts new functionality into the multicomponent material.6,7 There have been numerous examples in the chemical literature whereby introducing small, incremental changes to the molecular structure of either component of the co-crystal can significantly alter crystal packing, and, thus, the resulting crystal properties. In many of these examples, the use of non-covalent interactions (e.g. hydrogen and halogen bonding) has been exploited to control the overall packing of the multicomponent solid. To this end, it has been previously demonstrated that small modifications to the hydrogen-bond-donating CCF component of a co-crystal could be used to achieve both dramatic molecular motion as well as ‘colossal’ thermal expansion (TE) within isostructural co-crystals. In particular, the ability to achieve widespread molecular motion of aromatic rings and azo (N=N) groups within co-crystals based upon 4-phenylazopyridine (4-PAP) and 4,6-diXresorcinol (res) (where: X = Cl and Br) has been reported.8 The motion and the TE properties are based upon the tendencies of N=N to undergo dynamic pedal motion in the solid state; however, the CCF was required to unlock this motion since none was observed in the single-component


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solid. In a subsequent paper, it was reported that TE properties could also be varied via tuning the strength of halogen bonding interactions.9 In particular, three isosteric CCFs namely, 4,6diX-res (where: X = Cl, Br, I), were co-crystallized with 4,4’-azopyridine (4,4’-AP) to form discrete four-component hydrogen-bonded assemblies. By changing the halogen substituents on the res, the strength of the intermolecular interactions was tuned and, in turn, the TE properties. Here, we report how varying the linker between the pyridine rings of the bipyridine component affects dynamic motion and, thus, TE properties. In particular, we aimed to determine the role that molecular pedal motion plays in achieving TE in a series of isostructural hydrogen-bonded co-crystals. To this end, it is well known that the ability of molecules to undergo molecular motion (Scheme 1a) in the crystalline state has given rise to properties such as bulk actuation and TE.10,11 Here, we discuss three isostructural co-crystals based upon resorcinol (res) and isosteric bipyridines namely 4,4’-azopyridine (4,4’-AP), trans-1,2-bis(4pyridyl)ethylene (4,4’-BPE), and 1,2-bis(4-pyridyl)acetylene (4,4’-BPA) (Scheme 1b). In all cases, a discrete four-component hydrogen-bonded assembly in co-crystals of 2(res)•2(4,4’-AP), 2(res)•2(4,4’-BPE), and 2(res)•2(4,4’-BPA) was realized. The choice of the linkers was based upon the propensity of molecular motion, wherein the N=N was expected to undergo molecular motion to a larger extent than the ethylene (C=C) group.12 The acetylene (C≡C) group was utilized as a control, since it is unable to undergo pedal motion. It is important to note that pedal motion is a product of the crystal packing environments and is not guaranteed to occur in either a single- or multi-component material. In all three co-crystals, the TE occurs along comparable crystallographic planes, where the type and strength of non-covalent interactions, as well as the effect of pedal motion, gives rise to the final thermal parameters. To the best of our knowledge,


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this is the first report examining how pedal motion affects TE parameters within a hydrogenbonded co-crystal.

Scheme 1. Dynamic pedal motion in: (a) bipyridine-based molecules and (b) co-crystallization of bipyridines with res.

Co-crystallization of res with 4,4’-AP or 4,4’-BPE was achieved by dissolving the two components (1:1 molar ratio) separately in ethanol, combining the solutions into one vial, and allowing the solution to evaporate slowly. Similar methods have been used to produce hydrogenbonded co-crystals involving these components;13-15 however, the pedal motion at various temperatures and the thermal expansion parameters were not investigated. Crystals suitable for X-ray diffraction formed within 2 days. The formulas and structures were determined to be identical, namely, 2(res)•2(4,4’-AP) and 2(res)•2(4,4’-BPE). The formation of crystals of composition 2(res)•2(4,4’-BPA) was achieved as above; however, res was dissolved in ethanol and 4,4’-BPA was dissolved in toluene.


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Single-crystal analyses revealed all three co-crystals to be isostructural, crystallizing in the centrosymmetric triclinic space group Pī. Each co-crystal is based upon a discrete fourcomponent supramolecular assembly sustained by four O-H···N hydrogen bonds [O···N (Å): 4,4’-AP: 2.777(3), 2.796(3); 4,4’-BPE: 2.742(2), 2.758(2); 4,4’-BPA: 2.760(2), 2.765(2)] (Figure 1). The asymmetric unit for each co-crystal contains one res and one bipyridine molecule. The presence of an inversion center above the linker group produces the fourcomponent hydrogen-bonded assembly. The N=N core of 2(res)•2(4,4’-AP) and the C=C core of 2(res)•2(4,4’-BPE) were both found to be disordered at room temperature (290 K) (Figure 1a and 1b).12 The site occupancies of each orientation were allowed to freely refine in both cocrystals, and the sum of the two occupancies was constrained to a total of one. In order to determine if the disorder is dynamic or static, and to calculate the TE values, a variable temperature crystallographic study was performed on the original crystal, and additional data sets were collected at five lower temperatures: 270, 250, 230, 210, and 190 K. The first data set was collected at 290 K and additional data sets were collected as the crystal was cooled. After collection and refinement of the additional X-ray data, we determined that the disorder was indeed dynamic. Pedal motion occurred in both co-crystals as there were changes in the occupancies of the disordered sites throughout the cooling cycle (Table 1). At the lowest temperature, namely 190 K, the N=N core was still found to be disordered; however, the disorder in the C=C core was resolved at 210 K (Figure 1a and 1b). As expected, no disorder of the acetylene group in 2(res)•2(4,4’-BPA) was observed at any temperature (Figure 1c). A list of the occupancies of the disordered sites for 2(res)•2(4,4’-AP) and 2(res)•2(4,4’-BPE) are provided in Table 1.


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Figure 1. X-ray crystal structures at 290 and 190 K of: (a) 2(res)•2(4,4’-AP) showing the unresolved disorder of the azo core, (b) 2(res)•2(4,4’-BPE) showing the resolved disorder of the olefinic core, and (c) 2(res)•2(4,4’-BPA) showing no disorder of the acetylene core. Disorder in the aromatic rings is omitted for clarity.


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Table 1. Pedal motion in co-crystals of 2(res)•2(4,4’-AP) and 2(res)•2(4,4’-BPE) indicated by changes in the occupancies of the N=N or C=C crystallographic sites, respectively. 2(res)• 2(4,4’-AP) 2(res)• 2(4,4’-BPE)

Temperature 290 K (major / minor)

0.83 / 0.17

0.91 / 0.09

270 K (major / minor)

0.86 / 0.14

0.93 / 0.07


0.89 / 0.11

0.95 / 0.05


0.92 / 0.08

0.96 / 0.04


0.94 / 0.06

1.0 / 0.0


0.96 / 0.04

1.0 / 0.0

In all three co-crystals, the discrete four-component assemblies stack offset along the crystallographic a-axis and are held together by π-π interactions, wherein the pyridine ring of one assembly is positioned above the linker group in a neighboring assembly (Figure 2). All of the ππ stacking distances are comparable between the three co-crystals, and the distances within the discrete assembly (3.50-3.55 Å) are slightly longer than the distance between assemblies (3.333.50 Å).


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Figure 2. X-ray crystal structures at 290 K highlighting extended packing of: (a) 2(res)•2(4,4’AP), (b) 2(res)•2(4,4’-BPE), and (c) 2(res)•2(4,4’-BPA). Thermal expansion axes X1 and X3 denoted. Disorder in the aromatic rings is omitted for clarity.


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The stacks of four-component assemblies further self-assemble into layered structures, similar to co-crystals that incorporate 4,6-diX-res as the CCF (where: X = Cl, Br, I).8 Unlike cocrystals based upon 4,6-diX-res, however, the co-crystals described here are not sustained by strong O···X and C-H···X interactions, due to the lack of a halogen atom on the res CCF. Instead, the layers of 2(res)•2(4,4’-AP), 2(res)•2(4,4’-BPE), and 2(res)•2(4,4’-BPA) are held together by weaker C-H(pyridine)···O and C-H(pyridine)···π(res) forces [C-H(α)···O (Å): 4,4’AP: 3.65, 3.71; 4,4’-BPE: 3.61, 3.72; 4,4’-BPA: 3.54, 3.63; C-H(β)···O (Å): 4,4’-AP: 3.32, 3.35; 4,4’-BPE: 3.29, 3.39; 4,4’-BPA: 3.28, 3.29; C···centroid π(res) (Å): 4,4’-AP: 3.61, 3.68; 4,4’-BPE: 3.66, 3.68; 4,4’-BPA: 3.56, 3.60] (Table S9, Figure S4, S5). In addition to changes in the occupancies of the N=N and C=C sites upon cooling, the lengths of the crystallographic axes changed in all three co-crystals over the temperature range of 290-190 K (Figure S1-S3). For 2(res)•2(4,4’-AP), the a, b, and c-axes decreased by 1.2, 0.5, and 0.4%, respectively, while the a, b, and c-axes for 2(res)•2(4,4’-BPE) decreased by 1.0, 0.7, and 0.04%, respectively. For 2(res)•2(4,4’-BPA), the a, b, and c-axes decreased by 0.7, 0.6, and 0.2%, respectively. The volume of the unit cell of each co-crystal decreased over the range of 290-190 K by 2.3, 1.4, and 1.7% for 2(res)•2(4,4’-AP), 2(res)•2(4,4’-BPE), and 2(res)•2(4,4’BPA), respectively. Next, PASCal16 was used to calculate the principal axes and TE coefficients for 2(res)•2(4,4’-AP), 2(res)•2(4,4’-BPE), and 2(res)•2(4,4’-BPA) over the temperature range of 290 to 190 K. The thermal expansion coefficients ranged from negative (NTE) to ‘colossal’ positive (PTE) (Table 2). Stronger interactions are known to be less affected by temperature, while weaker interactions are more affected by temperature changes.17 The least expansion occurs along the X1 axis and coincides with the strongest interaction, which is the O-H···N


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hydrogen-bonding direction in all three co-crystals (Figure 2). The O-H···N hydrogen bond lengths in all three co-crystals decreased by ca. 0.01 Å or less over the temperature range of 290190 K (Table S8). Table 2. Thermal expansion coefficients (290-190 K) for 2(res)•2(4,4’-AP), 2(res)•2(4,4’-BPE), and 2(res)•2(4,4’-BPA) with approximate crystallographic axes denoted in brackets.

αX1 MK-1)

αX2 (MK-1)

αX3 (MK-1)

[axis]

[axis]

[axis]

-12 [-1 -4 -4]

87 [1 -2 2]

151 [-3 -1 0]

228

2(res)•2(4,4’-BPE)

-36 [1 -2 2]

52 [-2 2 1]

113 [-5 -2 1]

132

2(res)•2(4,4’-BPA)

-7 [1 -1 1]

72 [-1 1 1]

103 [-2 -1 1]

169

Co-crystal

2(res)•2(4,4’-AP)

αV (MK-1)

Due to the lack of the strong O···X halogen bonds, the next strongest interactions occur along the X2 axis and involve both C-H(pyridine)···O and C-H(pyridine)···π(res) interactions between

‘layers’

(Figure

3,

S4,

S5).

Upon

cooling,

the

C-H(pyridine)···O,

C-

H(pyridine)···π(res), and inter-layer distances decreased in all three co-crystals (Table S9). The largest changes occurred in 2(res)•2(4,4’-AP) (average ∆C···O: 0.06 Å; average ∆C···centroid π(res): 0.05 Å; ∆ layer distance: 0.127 Å), resulting in the largest TE coefficient along X2. The changes in the C-H(pyridine)···O and inter-layer distances were similar for 2(res)•2(4,4’-BPE) and 2(res)•2(4,4’-BPA) (BPE: average ∆C···O: 0.04 Å; ∆ layer distance: 0.075 Å; BPA: average ∆C···O: 0.04 Å; ∆ layer distance: 0.069 Å). The co-crystal involving BPE, however, exhibited a smaller change in C-H(pyridine)···π(res) distance than the co-crystal with BPA (∆C···centroid π(res): 0.03 Å and 0.04 Å, respectively) (Figure 3). Due to smaller changes in the


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intermolecular interaction distances, the TE coefficients along X2 are smaller for 2(res)•2(4,4’BPE) and 2(res)•2(4,4’-BPA).

Figure 3. X-ray crystal structures at 290 and 190 K highlighting changes in inter-layer distances in: (a) 2(res)•2(4,4’-AP), (b) 2(res)•2(4,4’-BPE) and (c) 2(res)•2(4,4’-BPA). The left column is at 290 K, and the right column is at 190 K. Thermal expansion axis X2 denoted. Disorder in the aromatic rings is omitted for clarity.


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Finally, the greatest expansion occurs along X3, which is dominated by π-π forces and pedal motion. For each co-crystal, the π-π stacking distance within an assembly decreases (∆ π-π (Å): 4,4’-AP: 0.042; 4,4’-BPE: 0.033; 4,4’-BPA: 0.035), and the π-π stacking distance between assemblies also decreases (average ∆ π-π (Å): 4,4’-AP: 0.045; 4,4’-BPE: 0.038; 4,4’-BPA: 0.035) (Table S10). Moreover, the largest TE was observed in 2(res)•2(4,4’-AP), which exhibited the largest changes in π-π stacking distance and underwent pedal motion in the azo core over the entire temperature range. The middle TE coefficient was observed for 2(res)•2(4,4’-BPE), which exhibited pedal motion from 290-230 K, but resolved at 210 K. Finally, the smallest TE was observed in 2(res)•2(4,4’-BPA), where no pedal motion was observed. TE coefficients of comparable magnitude (i.e. ‘colossal’) to the co-crystals reported here, as well as mechanisms of TE have been explored for other organic-based crystals. For example, Barbour et. al. reported the largest PTE and NTE for a single-component crystal under atmospheric pressure. The crystal was based on a diyn-diol molecule, wherein the stacking angle changes as a function of temperature, giving rise to large TE coefficients of -204 x 10-6 K-1 and 515 x 10-6 K-1.11,18 Naumov, et. al. have recently reported thermosalient crystals of N′-2propylydene-4-hydroxybenzohydrazide with large positive and negative TE coefficients of 225.9 x 10−6 K−1, 238.8 x 10−6 K−1, −290.0 x 10−6 K−1, as well as self-propelling crystals of Lpyroglutamic acid with a large PTE coefficient of 303 x 10−6 K-1.19,20 Saha et. al. have also recently reported a variety of organic crystals and co-crystals that exhibit ‘colossal’ TE. For example, a one-dimensional hydrogen-bonded co-crystal of 1,2,3,4-cyclobutanetetracarboxylic acid and 4,4’-BPE exhibits two polymorphs with TE coefficients of 147 x 10-6 K-1 and 136 x 10-6 K-1, wherein the largest expansions occur in the direction of the weaker interactions, similar to


12

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the co-crystals reported here.21 In such organic crystalline materials where TE often arises from changes in non-covalent interactions (e.g. hydrogen/halogen bonding, weak intermolecular forces) studies that investigate both the degree and mechanism of TE will greatly aid in designing materials with tunable and controllable TE. In this report, we have described the role that weak intermolecular interactions and molecular pedal motion plays in TE within a series of isostructural hydrogen-bonded co-crystals. It has been established that both hydrogen and halogen bonds can be exploited to modify TE properties. Here, we add an additional tool for altering properties in organic solids via modification of a linker group that exhibits or lacks an ability to undergo dynamic motion. We are continuing to investigate the role that pedal motion has on TE properties in other organic and inorganic materials.

ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge on the ACS Publications website at DOI:. Experimental details, X-ray crystallographic data, intermolecular distances, and thermal expansion data (PDF). Accession Codes. CCDC 1577137−1577154 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 *Email: [email protected]. Tel: +1 806-834-2744 ORCID: Kristin M. Hutchins: 0000-0001-8792-2830 *Email: [email protected]. Tel: +1 314-246-7466. ORCID: Ryan H. Groeneman: 0000-0002-4602-6287 Notes The authors declare no competing financial interest. Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. ACKNOWLEDGMENT K.M.H gratefully acknowledges financial support from Texas Tech University in the form of startup funding. R.H.G. gratefully acknowledges financial support from Webster University in the form of various Faculty Research Grants.

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3. Tóth, R.; Walliser, R. M.; Murray, N. S.; Bora, D. K.; Braun, A.; Fortunato, G.; Housecroft, C. E.; Constable, E. C. Chem. Commun., 2016, 52, 2940. 4. McNamara, D. P.; Childs, S. L.; Giordano, J.; Iarriccio, A.; Cassidy, J.; Shet, M. S.; Mannion, R.; O'Donnell, E.; Park, A. Pharm Res, 2006, 23, 1888. 5. MacGillivray, L. R.; Papaefstathiou, G. S.; Friščić, T.; Hamilton, T. D.; Bučar, D.-K.; Chu, Q.; Varshney, D. B.; Georgiev, I. G. Acc. Chem. Res. 2008, 41, 280. 6. Aakeröy, C. B.; Wijethunga, T. K.; Desper, J. Chem. Eur. J., 2015, 21, 11029. 7. Bushuyev, O.S.; Friščić, T.; Barrett, C. J. Cryst. Growth Des., 2016, 16, 541. 8. Hutchins, K. M.; Groeneman, R. H.; Reinheimer, E. W.; Swenson, D. C.; MacGillivray, L. R. Chem. Sci., 2015, 6, 4717. 9. Hutchins, K. M.; Kummer, K. A.; Groeneman, R. H.; Reinheimer, E. W.; Sinnwell, M. A.; Swenson, D. C.; MacGillivray, L. R. CrystEngComm, 2016, 18, 8354. 10. Bushuyev, O.S.; Tomberg, A.; Vinden, J. R.; Moitessier, N.; Barrett, C. J.; Friščić, T. Chem. Commun., 2016, 52, 2103. 11. Das, D.; Jacobs, T.; Barbour, L. J. Nature Materials, 2010, 9, 36. 12. Harada, J.; Ogawa, K. Chem. Soc. Rev. 2009, 38, 2244. 13. MacGillivray, L. R.; Reid, J. L.; Ripmeester, J. A. J. Am. Chem. Soc. 2000, 122, 7817. 14. Khan, M.; Enkelmann, V.; Brunklaus, G. CrystEngComm, 2009, 11, 1001.


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15. Ravat, P.; SeethaLekshmi, S.; Biswas, S. N.; Nandy, P.; Varughese, S. Cryst. Growth Des. 2015, 15, 2389. 16. Cliffe, M. J.; Goodwin, A. L. J. Appl. Crystallogr., 2012, 45, 1321. 17. Saha, B. K. J. Indian Inst. Sci. 2017, 97, 177. 18. Das, D.; Jacobs, T.; Pietraszko, A.; Barbour, L. J. Chem. Commun., 2011, 47, 6009. 19. Panda, M. K.; Centore, R.; Causa, M.; Tuzi, A.; Borbone, F.; Naumov, P. Sci. Rep. 2016, 6, 29610. 20. Panda, M. K.; Runčevski, T.; Husain, A.; Dinnebier, R. E.; Naumov, P. J. Am. Chem. Soc. 2015, 137, 1895. 21. Bhattacharya, S.; Saha, B. K. Cryst. Growth Des. 2013, 13, 3299.


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For Table of Contents Use Only Molecular Pedal Motion Influences Thermal Expansion Properties within Isostructural Hydrogen-Bonded Co-crystals Kristin M. Hutchins,*,† Daniel K. Unruh,† Frank A. Verdu,‡ and Ryan H. Groeneman*,‡ SYNOPSIS: (60 words or less) The influence of molecular pedal motion on thermal expansion properties is investigated for a series of isostructural hydrogen-bonded co-crystals. The core of the hydrogen-bond acceptor molecule is modified using groups with different propensities for undergoing pedal motion in the solid state, namely azo, ethylene, and acetylene. The thermal expansion is related to pedal motion and strength of the non-covalent interactions.


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Molecular Pedal Motion Influences Thermal Expansion Properties

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