Expanding the Scope of Molecular Mixed Crystals Enabled by Three

Article pubs.acs.org/crystal

Expanding the Scope of Molecular Mixed Crystals Enabled by Three Component Solid Solutions Matteo Lusi,*,† Inigo J. Vitorica-Yrezabal,‡ and Michael J. Zaworotko*,† †

Department of Chemical and Environmental Science, University of Limerick, Limerick, Ireland School of Chemistry, University of Manchester, Manchester M13 9PL, United Kingdom

‡

S Supporting Information *

ABSTRACT: Crystalline solid solutions (mixed crystals) formed from molecular compounds are a long-known class of multicomponent solids whose composition can be varied, at least in principle, in continuum. Should such control occur in mixed crystals over a wide range of compositions, then it follows that some structural and physical properties would also be tuned in continuum. However, the potential utility of mixed crystals as molecular materials has been hindered by their limited accessibility across a broad range of compositions and difficulties associated with their preparation. We address these matters herein through the use of mechanochemistry and solution crystallization to study three-component mixed crystals. In addition to expanding the composition range of previously known mixed crystals, we demonstrate that anthracene (ant) and phenazine (pnz), which tend to crystallize as separate phases, form mixed crystals with a third molecule which is miscible in both, namely, acridine (acr). The three-component mixed crystal is only accessible mechanochemically as solution methods afford phase separation. If this observation is general, then it could have broad implications for this understudied class of molecular crystals.

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applications.8 These phases are characterized by the fact that the different molecular constituents randomly occupy equivalent crystallographic sites. Importantly, their stoichiometry is not limited to integer values. Within certain limits, their composition can be controlled by simply varying the relative amount of the reagents to afford continuous (ideally linear) changes in the structural parameters and properties such as density, solubility, reactivity, and so forth.9 Mixed crystals have been exploited to tune a range of properties for coordination complexes,10 H-bonded11 and Xbonded12 networks, coordination polymers,13 and organic molecular crystals.14 Unfortunately, their practical development is hindered by obtaining a pure and homogeneous phase over a sufficiently large composition range. Moreover, in normal conditions, only a few groups of molecules are known to form substitutional solid solutions, so much so that crystallization has long been used as a purification technique.15 In our opinion, mixed phases in general, and solid solutions in particular, represent a challenge to crystal engineering and in this contribution we address a convenient and understudied method to extend the range of “mixable molecules” and their compositions by exploiting a third molecule. This approach builds up on previous work that revealed how solid-state reactions can afford homogeneous solid solutions of metallorganic salts, polymers, and complexes16 and a recent study of acridine and phenazine that offered new insights into the

INTRODUCTION The properties of a molecular material are determined by the chemical properties of the molecular species that comprise it, as well as their spatial arrangement or crystal packing.1 Indeed, even small variations in crystal packing of the same molecular compound (polymorphs) may in some circumstances have a significant enough effect on the bulk physicochemical properties of the solid to impact the end application.2 The development of new molecular materials with improved performances can therefore proceed either by synthesizing new molecules (the “hard way”) or by controlling the assembly of existing molecular components in the solid state to form new crystalline motifs. The latter, a supramolecular or crystal engineering3 approach, has evolved from its initial focus upon design of the structure into control over the bulk properties of solids as diverse as porous metal−organic materials4 and cocrystals containing pharmaceutical5 or explosive components.6 Multicomponent solids can be optimized by systematically varying one or more of the molecular components (metal centers, ligands, counterions, etc.). However, even though this creates platforms with compositional diversity, inherent discontinuity in the chemical space does not necessarily allow for a precise modulation of structural features or physicochemical properties. Nonstoichiometric substitutional mixed crystals (or crystalline solid solutions)7 could represent a viable alternative to better understand and control structure−property relationships and to precisely optimize a material’s properties for practical © XXXX American Chemical Society

Received: May 19, 2015

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DOI: 10.1021/acs.cgd.5b00685 Cryst. Growth Des. XXXX, XXX, XXX−XXX

Crystal Growth & Design

Article

form α, CSD refcode PHENAZ10, (PNZα), crystals was isolated as the products of the (co)crystallization.7 Two-Component Phases. Different results were observed when we subjected ant and acr to mechanochemical grinding with an agate mortar and pestle in 1:9, 3:7, 1:1, 7:3, and 9:1 ratios in the presence of toluene (see Experimental Section). Powder X-ray diffraction (PXRD) (Figure 1a) indicates that a

relationship between isostructurality and molecular size.17 Herein we complete that work and report how the anthracene:acridine and anthracene:phenazine binary systems can be expanded into a ternary anthracene:acridine:phenazine system (hereafter, ant, acr, and pnz, respectively) through the use of mechanochemical methods, which in recent years has come to the forefront as a method to facilitate the discovery and processing of multicomponent crystals such as cocrystals.18

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EXPERIMENTAL DETAILS

All reagents and solvents were purchased from either Sigma-Aldrich or Alpha Aesar and used without further purification. Mechanochemical Synthesis. Syntheses were performed by manual grinding of the reagents in stoichiometric amount with an agate mortar and pestle after adding few drops of toluene (see SI for details). Solution Growth of Single Crystals. A small amount (5−10 mg) of aromatic mixture in the desired stoichiometric ratio was prepared and dissolved with the minimal amount of methanol in a vial under gentle heat and recrystallized by solvent evaporation at room temperature. Powder X-ray Diffraction and Rietveld Refinement. All diffraction patterns were recorded on a PANalytical EMPYREAN diffractometer system using Bragg−Brentano geometry and an incident beam of Cu Kα radiation (λ = 1.5418 Å). Room temperature scans were performed on a spinning silicon sample holder. Rietveld refinement was performed with the PANalytical HIGHSCORE software package using the CSD .cif files ANTCEN14, ACRDIN04, ACRDIN07, PHENAZ10, and PHENAZ11 as starting models for anthracene, acridine form III, acridine form II, phenazine α, and phenazine β, respectively. The missing H atoms in the .cif files were added with SHELX HFIX command. Due to software architecture the phase for PNZβ was transformed into P21/c and kept as such for the unit cell and quantification refinement. The cell metrics used in the main text were obtained by reconverting the refined structures in P21/ n. Thermal Analysis. Thermogravimetric analysis (TGA) was performed on a TA Instrument Q50 under nitrogen stream (50 mL/min) at 20 °C/min heating rate on 3 to 5 mg of ground sample. Differential scanning calorimetry (DSC) measurements were performed on TA DSC Q-2000 under nitrogen stream (50 mL/ min) by cycling the sample, between room temperature and 175 °C, 3 to 5 mg of ground powder at 10 °C/min heating/cooling rates. NMR Spectroscopy. After the single crystal X-ray diffraction experiment selected crystals were transferred each in a separate NMR tube and dissolved in deuterated acetone. The NMR spectra were collected on a Joel 270 MHz NMR spectrometer operating at room temperature. Single Crystal X-ray Diffraction. Crystal structures were determined at 100 K by X-ray diffraction on either a Bruker Quest with Mo sealed tube source or on Oxford Supernova diffractometers. The data were integrated with APEX and CRYSALISPRO suites, respectively.

Figure 1. (a) Comparison of PXRD patterns for the ant:acr system. From top to bottom: calculated for ant (ANT) measured for the product of manual grinding of ant and acr mixtures with ant composition = 100%, 90%, 70%, 50%, 30%, 10%, and 0% respectively, calculated for acr (ACRII). (b) Comparison of PXRD patterns for the ant:pnz system. From top to bottom: calculated for ant (ANT) measured for the product of manual grinding of ant and pnz mixtures with ant composition = 100%, 90%, 70%, 50%, 30%, 10%, and 0%, respectively, calculated for pnz (PNZα).

mixture of two crystalline phases is produced from these experiments and that their relative abundance varies depending on the relative ratio of the reagents. One phase is isomorphous to pure ANT whereas the second phase is isomorphous to ACRII. Quantitative Rietveld analysis revealed that the proportion of ACRII in the product is higher than that added as a reagent, which can be explained by the dissolution of up to 20% of ant into the ACRII phase (Figure 2). Moreover, the unit cell dimensions for the latter phase were observed to change with composition whereas the metrics for ANT remain constant, validating the hypothesis that a solid solution isomorphous to ACRII is indeed generated (SI). When equivalent experiments were conducted for the ant:pnz system, the amount of crystalline phases isomorphous to ANT and PNZα found in the products is equal to that of the reagents (Figure 2). In these products the cell metrics do not vary, suggesting that no (co)recrystallization had occurred and that no mixed phase was formed (Figure 1b). However, in principle, structural variation between different compositions can be too small to be detected and a lack of variation in structural

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RESULTS AND DISCUSSION Kitaigorodski reported that ant and acr form two distinct solid solutions. On one side of the phase diagram, acr serves as host, CSD refcode ACRDIN07, (ACRII),19 and dissolves20 up to 5% of ant. The other end of the phase diagram is represented by a mixed crystal form isostructural to ant, CSD refcode ANTCEN, (ANT). One crystal of this form was found to be composed of 20% ant and 80% acr.21 Repeating this experiment under controlled conditions revealed that the limit for the composition in the solid solution isomorphous to ANT can have a limit as low as 55% ant.22 Equivalent crystallization experiments for the ant:pnz system did not afford solid solutions, but rather a physical mixture of pure ANT and pnz B



Article

Figure 4. Short interactions between the phenazine molecule and the hydrogen of hypothetical acr (or ant) molecules in PNZβ. The reported distances are calculated by adding dummy hydrogen atoms to the nitrogen atoms at neutron normalized C−H bond distances.

Figure 2. Amount of ANT measured in the product as a function of reagent stoichiometry.

parameters does not preclude that a mixed phase had been formed. Three-Component Phases. Synthesis of the threecomponent mixed crystals was attempted by solvent evaporation (see SI). Crystals suitable for SCXRD were obtained within several days. In each vial multiple crystal forms could be visually distinguished by their morphology (Figure 3). SCXRD

Figure 5. (a) Linear dependence between the occupancy of C1 and β angle for the solid solution of PNZβ (top). (b) Quadratic dependence between the occupancy of C1 and β angle for the solid solution of PNZα (bottom).

Figure 3. Microscopic images taken at 20× magnification for each of the crystal forms characterized by SCXRD.

measurements revealed that the crystals so obtained are isomorphous to one of the forms of ANT, PNZα, PNZβ, and, surprisingly, acr form III, CSD refcode ACRDIN04, (ACRIII), instead of the expected ACRII. In each vial, for the same crystal form samples with varied metrics were found showing that solution methods produce inhomogeneous product. Large metric variation is observed for the samples isomorphous to PNZα and PNZβ. For these systems, good quality SXRD data enabled full structure refinement inclusive of the C/N occupancy (Figure 4). The unit cell of PNZβ varies linearly with the amount of carbon in position C1 (Figure 5a), while the carbon occupancy in position 8 is more homogeneous and limited to about 10% (SI). This can be understood by considering that the hypothetical replacement of the nitrogen N8 with a CH group would create a short H−H contact (shorter that 2 Å) with a hydrogen atom from the adjacent molecule (Figure 4). Similar behavior is observed whether or not ant was present in the crystallization solution, suggesting that it could be due to the random orientation of the acr molecule in the solid solution. Notably, those samples that were

observed to be isomorphous to PNZα also exhibit metric and composition variation, although to a limited extent (Figure 5b). This variation was not detected in the analysis of the mechanochemical products, probably due to its smaller magnitude. The cell metrics of the samples isomorphous to ANT exhibit only minor variation (within two or three ESD values) and free refinement of the occupancy was possible only in a few cases. In other words, the ANT crystals appear to be composed of pure anthracene (see SI). ACRIII was produced as small colorless plates for which X-ray diffraction analysis revealed a small cell metric variation, but poor quality data did not allow adequate refinement of the relative C/N occupancy (Table 1). The larger crystals among those used for SCXRD were selected, transferred individually into nuclear magnetic resonance (NMR) tubes, and dissolved in deuterated acetone. NMR spectra revealed that the crystals isomorphous to PNZα and PNZβ comprise pnz and acr exclusively, while those isomorphous to ANT exhibit the characteristic peaks of ant plus peaks of an unknown molecule, probably due to C



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Table 1. Summary of the Results of SCXRD Experiments R

Xtal

a

b

c

β

C1:N1

esd

N8:C8

esd

ratio

#

morphology

%

Form

Å

Å

Å

°

%

%

%

%

311 111 131 011 131 011 131 101 101 101 113 113 113 111 111 101 311 110 311 110 131 130 131

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23

prism yellow prism yellow prism yellow prism yellow prism yellow prism yellow prism yellow needle yellow needle yellow needle yellow needle yellow needle yellow needle yellow needle yellow prism col.less prism col.less prism col.less prism col.less prism col.less prism col.less plate colorless plate colorless plate colorless

7.3 5.2 7.9 7.2 8.3 7.9 5.5 5.7 5.0 7.6 5.8 5.8 6.8 8.4 5.2 4.8 6.3 9.5 7.2 6.2 7.9 14.3 21.0

PNZβ PNZβ PNZβ PNZβ PNZβ PNZβ PNZβ PNZα PNZα PNZα PNZα PNZα PNZα PNZα ANT ANT ANT ANT ANT ANT ACRIII ACRIII ACRIII

6.737(1) 6.727(1) 6.721(1) 6.705(1) 6.685(1) 6.683(1) 6.67(1) 7.077(1) 7.073(1) 7.084(1) 7.083(1) 7.115(1) 7.106(1) 7.146(1) 9.269(1) 9.284(1) 9.263(1) 9.298(2) 9.276(1) 9.284(1) 11.175(1) 11.165(2) 11.183(5)

11.612(2) 11.687(1) 11.713(1) 11.852(1) 11.856(1) 11.802(1) 11.83(2) 4.990(1) 4.976(1) 4.982(1) 4.988(1) 4.987(5) 4.993(1) 4.993(1) 5.995(3) 5.999(1) 5.992(1) 5.994(2) 5.988(1) 5.993(1) 5.929(1) 5.922(1) 5.934(1)

11.455(1) 11.489(1) 11.492(1) 11.556(1) 11.516(1) 11.522(1) 11.52(1) 12.600(1) 12.616(1) 12.617(1) 12.565(3) 12.622(1) 12.597(2) 12.643(1) 8.410(1) 8.421(1) 8.409(1) 8.408(3) 8.403(1) 8.422(1) 13.567(1) 13.563(3) 13.589(1)

96.01(1) 95.37(1) 95.00(1) 94.11(1) 93.87(1) 93.95(1) 93.86(3) 103.05(1) 103.03(1) 103.04(2) 103.03(1) 102.96(1) 102.77(2) 102.74(1) 102.48(1) 102.49(1) 102.61(1) 102.50(3) 102.54(2) 102.53(1) 99.79(1) 100.02(1) 99.84(4)

49 54 58 65 67 70 71 5 13 13 15 20 20 24 100 100 100 99 99 96 5 2 100

3 2 3 2 3 2 2 2 2 3 3 2 4 4 NA NA NA 4 3 3 3 1 NA

89 90 88 85 83 85 86 NA NA NA NA NA NA NA NA NA NA NA NA NA NA NA NA

3 2 3 3 3 2 2 NA NA NA NA NA NA NA NA NA NA NA NA NA NA NA NA

reacting ant:acr:pnz

phases as a consequence of grinding. However, the experimental PXRD patterns indicate no selective peak broadening for the different phases and between the twocomponent and three-component reactions. In order to exclude the hypothesis of selective amorphization, a 1:1:1 mixture of the three aromatics, individually ground with toluene, was prepare and analyzed showing no compositional change (see SI). In other words, the presence of acr facilitates the dissolution of ant into PNZβ, which is precluded when only ant and pnz are present as a binary system. Furthermore, if compared to the variability observed in the single crystal experiment, the narrow width of the diffraction peaks measured in the PXRDs indicate that each phase produced mechanochemically is extremely homogeneous. The analysis of the unit cell metrics for the PNZβ phase indicates that the monoclinic angle changes regularly with composition but that different relationships exist. A linear correspondence with composition is found for the acr:pnz system and a second correlation occurs for the threecomponent ant:acr:pnz system. Remarkably, the presence of ant reduces structure deformation as indicated by the decreased slope for the β angle variation (Figure 7). Differential scanning calorimetry (DSC) experiments confirm that the change in stoichiometry affects the melting point of the product mixture in a gradual manner (Figure 6). These results suggest a need to further study substitutional mixed crystals and offer new possibilities for modifying structure and physical properties of multicomponent crystalline phases by exploiting the presence of a “solid solvent”. We will test the generality of this approach in different threecomponent systems.

photodimerization of ant. NMR therefore validates the SCXRD data obtained for ANT, PNZα, and PNZβ. Single crystals of ACR were too small for accurate SCXRD and NMR analysis. Mechanochemical mixing of all three aromatic compounds in 3:1:1, 1:3:1, 1:1:3, and 1:1:1 ratios resulted in a phase mixture including ANT ACRII PNZα and PNZβ. Quantitative phase analysis by the Rietveld method indicated that the relative amount of ANT is reduced upon grinding. In part this is expected since ant can dissolve into ACRII (see above). Nonetheless, the magnitude of ANT that disappeared is larger than the amount of ACRII. The PNZβ phase produced from mechanochemistry is present in an amount that is more than the sum of acr and pnz present in the reagents, suggesting that mechanochemical synthesis enables the formation of a threecomponent crystalline phase (Figure 6). This effect could also be the result of selective amorphization of the ant and acr

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CONCLUSIONS As mentioned in the Introduction, crystal engineering efforts to make new crystal forms are mainly justified by the motivation

Figure 6. (a) Quantitative composition analysis of the reacting (red) and the produced (green) mixture by phase. For each powder mixture the melting point (expressed in °C) is reported as calculated by DSC. D



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Article

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENTS

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REFERENCES

The authors thank the Science Foundation of Ireland for Award 13/RP/B2549. We also acknowledge the curiosity of Professor Joel Bernstein that inspired this work and discussions with Dr. Einat Schur and Dr. Elisa Nauha.

Figure 7. Relationship between the estimated composition of PNZβ and the monoclinic angle for three-component phase (blue) and twocomponent crystallization (red).

(1) (a) Guo, C.; Hickey, M. B.; Guggenheim, E. R.; Enkelmann, V.; Foxman, B. M. Chem. Commun. 2005, 2220. (b) Xiao, J.; Yang, M.; Lauher, J. W.; Fowler, F. W. Angew. Chem., Int. Ed. 2000, 39, 2132. (2) (a) Aguiar, A. J.; Krc, J.; Kinkel, A. W.; Samyn, J. C. J. Pharm. Sci. 1967, 56, 847. (b) Bauer, J.; Spanton, S.; Henry, R.; Quick, J.; Dziki, W.; Porter, W.; Morris, J. Pharm. Res. 2001, 18, 859. (c) Chernyshov, D.; Hostettler, M.; Törnroos, K. W.; Bürgi, H.-B. Angew. Chem., Int. Ed. 2003, 42, 3825. (d) Bekö, S. L.; Hammer, S. M.; Schmidt, M. U. Angew. Chem., Int. Ed. 2012, 51, 4735. (3) (a) Desiraju, G. R. Crystal Engineering. The Design of Organic Solids; Elsevier: Amsterdam, 1989. (b) Etter, M. C. Acc. Chem. Res. 1990, 23, 120. (c) Aakeroy, C. B.; Seddon, K. R. Chem. Soc. Rev. 1993, 22, 397. (d) Bernstein, J. In Supramolecular engineering of synthetic metallic materials: conductors and magnets; Veciana, J., Rovira, C., Amabilino, D. B., Eds.; Kluwer Academic Publishers: Dordrecht, 1999; p 23. (e) Moulton, B.; Zaworotko, M. J. Chem. Rev. 2001, 101, 1629. (4) Perry, J. J., IV; Perman, J. A.; Zaworotko, M. J. Chem. Soc. Rev. 2009, 38, 1400. (5) (a) Almarsson, O.; Zaworotko, M. J. Chem. Commun. 2004, 1889. (b) Trask, A. V.; Motherwell, W. D. S.; Jones, W. Cryst. Growth Des. 2005, 5, 1013. (c) Jones, W.; Motherwell, W. D. S.; Trask, A. V. MRS Bull. 2006, 31, 875. (d) Vishweshwar, P.; McMahon, J. A.; Bis, J. A.; Zaworotko, M. J. J. Pharm. Sci. 2006, 95, 499. (6) Bolton, O.; Simke, L. R.; Pagoria, P. F.; Matzger, A. J. Cryst. Growth Des. 2012, 12, 4311. (7) Kitaigorodsky, A. I. Mixed crystals; Springer-Verlag: Berlin, 1984; Vol. 33. (8) (a) Lin, Z.-G.; Tang, L.-C.; Chou, C.-P. Inorg. Chem. 2008, 47, 2362. (b) Lee, J.; Zhang, Q.; Saito, F. J. Solid State Chem. 2001, 160, 469. (c) Endo, K.; Yamamoto, K.; Deguchi, K. J. Phys. Chem. Solids 1993, 54, 357. (d) Mishra, M. K.; Ramamurty, U.; Desiraju, G. R. J. Am. Chem. Soc. 2015, 137, 1794. (9) (a) Denton, A. R.; Ashcroft, N. W. Phys. Rev. A 1991, 43, 3161. (b) Barth, T. F. W. Am. J. Sci. 1930, 19 (Series 5), 135. (10) (a) Ward, M. D. Organometallics 1987, 6, 754. (b) Vithana, C.; Uekusa, H.; Sekine, A.; Ohashi, Y. Acta Crystallogr., Sect. B 2002, 58, 227. (11) Willett, R. D.; Butcher, R. E.; Landee, C. P.; Twamley, B. Polyhedron 2006, 25, 2093. (12) Mínguez Espallargas, G.; van de Streek, J.; Fernandes, P.; Florence, A. J.; Brunelli, M.; Shankland, K.; Brammer, L. Angew. Chem., Int. Ed. 2010, 49, 8892. (13) Fukushima, T.; Horike, S.; Inubushi, Y.; Nakagawa, K.; Kubota, Y.; Takata, M.; Kitagawa, S. Angew. Chem. 2010, 122, 4930. (14) (a) Nie, Q.; Gong, J. B.; Wang, J. K.; Wang, S. Ind. Eng. Chem. Res. 2005, 45, 432. (b) Hulliger, J.; Roth, S. W.; Quintel, A.; Bebie, H. J. Solid State Chem. 2000, 152, 49. (c) Morimoto, M.; Kobatake, S.; Irie, M. J. Am. Chem. Soc. 2003, 125, 11080. (d) Delori, A.; Maclure, P.; Bhardwaj, R. M.; Johnston, A.; Florence, A. J.; Sutcliffe, O. B.; Oswald, I. D. H. CrystEngComm 2014, 16, 5827. (15) Wilcox, W. R.; Friedenberg, R.; Back, N. Chem. Rev. 1964, 64, 187.

to optimize physicochemical properties of a material for practical application. In the system reported herein we have shown a clear relationship between composition, thermal properties, and structure that suggests it is possible to precisely tune the structure and melting point of a solid solution. A broad range of compositions can be accessed by mechanochemical means. The discrepancies observed between the microcrystalline powders obtained by grinding and the single crystals grown from ethanol are likely due to the different reaction conditions. This different behavior confirms the importance of the preparation conditions used to form a crystalline material. Mixed crystals have the presently unrealized potential to afford tunable phases over a wide range of compositions. However, due to their complex liquid−solid phase diagrams, homogeneous mixed crystalline products can be difficult to obtain by traditional synthetic methods. This limitation has hindered the widespread implementation of mixed crystals in applications such as pharmaceutical science. Herein, we have confirmed that solvent assisted mechanochemical synthesis affords homogeneous solid solutions of organic molecules, although the reaction is not complete and some starting material remains present among the products. Most importantly we have observed that in a three-component system, the presence of acr facilitates the dissolution of ant into the pnz structure in a manner that had not been achieved directly in the two-component system. In this sense, acr acts as a “Trojan horse” for the inclusion of ant. We note that similar behavior was reported in solid solutions of cocrystals, although in those cases miscibility is enabled by the interactions between the coformers that allows miscibility in a manner that is crystallographically (and chemically) different from the solute components studied herein.23 In the present case, any of the three molecules can occupy the same crystallographic site and are held within the crystal lattice by the same type of weak interactions.

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ASSOCIATED CONTENT

S Supporting Information *

Synthesis and analysis. CCDC 1047869−1047891 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_ request/cif. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/ acs.cgd.5b00685. E



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(16) (a) Adams, C. J.; Gillon, A. L.; Lusi, M.; Orpen, A. G. CrystEngComm 2010, 12, 4403. (b) Adams, C. J.; Haddow, M. F.; Lusi, M.; Orpen, A. G. Proc. Natl. Acad. Sci. U.S.A. 2010, 107, 16033. (c) Lusi, M.; Atwood, J. L.; MacGillivray, L. R.; Barbour, L. J. CrystEngComm 2011, 13, 4311. (d) Batisai, E.; Lusi, M.; Jacobs, T.; Barbour, L. J. Chem. Commun. 2012, 48, 12171. (17) Schur, E.; Nauha, E.; Lusi, M.; Bernstein, J. Chem.Eur. J. 2015, 21, 1735. (18) (a) Wöhler, F. Justus Liebigs Ann. Chem. 1844, 51, 145. (b) Etter, M. C.; Urbanczyk-Lipkowska, Z.; Zia-Ebrahimi, M.; Panunto, T. W. J. Am. Chem. Soc. 1990, 112, 8415. (c) Boldyrev, V. V.; Tkácǒ vá, K. J. Mater. Synth. Process. 2000, 8, 121. (d) Shan, N.; Toda, F.; Jones, W. Chem. Commun. 2002, 2372. (e) Friscic, T.; James, S. L.; Boldyreva, E. V.; Bolm, C.; Jones, W.; Mack, J.; Steed, J. W.; Suslick, K. S. Chem. Commun. 2015, 51, 6248. (19) Through the years the nomenclature of the acridine polymorphs has not always been consistent. In this work the original nomenclature introduced by Lowde in 1953 is adopted. (20) The terminology used in the field has not always been consistent. In some cases the major component is referred to as the solvent or the host and the minor component(s) is referred to as the solute(s) or guest(s). In other instances, especially in metallurgy, the solvent and the host have been defined as the component whose structure is maintained in the solid solution. The two definitions often coincide but there are examples in which the major component adopts the crystalline structure of the minor component. To avoid confusion herein we will refer to hosts as the crystal form adopted by the product and guests as the molecule(s) dissolved in it regardless of which is the major component. (21) Myasnikova, R. M.; Kitaigorodskii, A. I. Kristallografiâ 1958, 3, 160. (22) Radomska, M.; Radomski, R.; Pigoń, K. Mol. Cryst. Liq. Cryst. 1972, 18, 75. (23) (a) Oliveira, M. A.; Peterson, M. L.; Klein, D. Cryst. Growth Des. 2008, 8, 4487. (b) Dabros, M.; Emery, P. R.; Thalladi, V. R. Angew. Chem., Int. Ed. Engl. 2007, 46, 4132.

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Expanding the Scope of Molecular Mixed Crystals Enabled by Three

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