1,4-Bis(phenylethynyl)-cyclohexane-1,4-diol - American Chemical

CRYSTAL GROWTH & DESIGN 2008 VOL. 8, NO. 12 4359–4366

Articles A Structural Investigation of Six Solvates of trans-1,4-Bis(phenylethynyl)-cyclohexane-1,4-diol Raju Mondal*,† and Judith A. K. Howard‡ Department of Inorganic Chemistry, Indian Association for the CultiVation of Science, Raja S. C. Mullick Road, JadaVpur, Kolkata 700032, India, and Department of Chemistry, UniVersity of Durham, South Road, Durham DH1 3LE, United Kingdom ReceiVed January 2, 2007; ReVised Manuscript ReceiVed May 6, 2008

ABSTRACT: The six different solvate structures of trans-1,4-bis(phenylethynyl)-cyclohexane-1,4-diol reported herein show the importance of weak C-H · · · O bonds and C-H · · · π interactions in directing crystal packing. The host molecule consists of a unique combination of strong hydrogen bonding (hydroxyl) and weak interactions (phenylethynyl). While strong hydrogen bonds play a central role, the combined influence of the weak interactions is also evident in the structures. It is apparent that the orientation of the phenylethynyl moieties influences the formation of C-H · · · O and C-H · · · π interactions. The interesting occurrence of a structure with two different conformers of the host molecule within the same crystal coexisting with ethanol and stabilized by several weak C-H · · · π interactions is also reported. Introduction With the ever-increasing interest in crystal engineering, solvated structures are attracting more and more attention owing to the diversity of the interactions involved.1 During the past decade or so, the major part of crystal engineering studies associated with topics such as pseudopolymorphism involve designing host molecules with different shapes, steric bulk, and functionalities.2 Weber suggested that a successful host molecule would be bulky and rigid and preferably contain host-guest interaction-specific functional groups, because molecular recognition lies at the heart of inclusion compound chemistry.3 Conventional strong hydrogen bonds (O-H · · · O, N-H · · · O, etc.) have long been recognized as being of fundamental importance in the studies of molecular recognition and crystal engineering involving organic systems. Besides these conventional strong hydrogen bonds, varieties of unconventional intermolecular interactions or contacts have also been found to be instrumental in determining the supramolecular structure of organic solids.4 Although these interactions are individually weaker and geometrically less well-defined, their combined effect can be equally important as stronger interactions.5 Among these weaker hydrogen bonds, an increased interest in C-H · · · X (where X ) N, O, π) interactions can be identified readily.6 However, many questions have been raised about the consistency and reliability of C-H · · · X interactions because of their weakness and electrostatic and therefore nondirectional * Corresponding author. E-mail: [email protected]. † Indian Association for the Cultivation of Science. ‡ University of Durham.

nature.7 From a crystal engineering point of view, the major obstacle preventing C-H · · · X interactions from being of fundamental importance in determining supramolecular structure of organic solids is that the presence of these weak forces is not predictable. Consequently, the anomalous behavior frequently observed within a series of molecular structures makes these weaker interactions less suitable for the tuning, or prediction, of crystal structures.7,8 Notwithstanding these difficulties, it has been observed that weak hydrogen bonds are of great importance for other kind of crystallographic studies, e.g., conformational changes in small organic molecules in the solid state, especially for host-guest types of molecular systems where the host molecule is an organic molecule with a strong hydrogen bonding group (-OH, -NH2) but also possesses a large acyclic hydrocarbon chain, extended aromatic ring moiety, or some other multiatomic group that extends from the molecule in an antenna-like fashion.9 Crystal packing, or more specifically structure-determining interactions, of these types of molecules consist primarily of hard acid-hard base (O/N-H · · · N/O) interactions. However, these contacts have little control over the orientation of the extended part of the molecule. This is because these parts of the molecules often reside a relatively long distance away from the hard hydrogen bond donor-acceptor sites. Results and Discussion Changes in molecular conformations due to weak hydrogen bonding are very common in solvates and host-guest systems of molecules, where molecular distortions often appear in the

10.1021/cg070002w CCC: $40.75  2008 American Chemical Society Published on Web 09/30/2008

4360 Crystal Growth & Design, Vol. 8, No. 12, 2008 Scheme 1

host molecule, resulting in cavities with a favorable size for, and thereby facilitating, the inclusion of guest molecules.10 Sometimes a particular portion of the host molecule twists, bends, or folds in order to accommodate the solvent, while there are instances where the host molecules reorient themselves to facilitate the formation of C-H · · · π interactions; both circumstances ultimately lead to some form of conformational change within the host molecule.11 The unsolvated crystal structure of the host molecule trans1,4-bis(phenylethynyl)-cyclohexane-1,4-diol (A), (Scheme 1) of the present study is quite unique.12 The two different concomitant polymorphs of A contain a total of 10 different molecular conformations; essentially these arise from small conformational variations of the phenyl rings. Our previous results show that gem-alkynol and its derivatives by virtue of their rigid geometry and good hydrogen bonding capabilities can act as good host molecules.13 With this in mind, we explore the possibility of a systematic study of pseudopolymorphs of A. The results are interesting, and we report herein six solvates of A, in which solvents with varying steric bulks and hydrogen bond forming capabilities are employed. Crystallization of A from diethylamine yields 1. Diethylamine, a base with strong hydrogen bond forming capabilities, is often found to form characteristic hydrogen bond motifs within solvates.13 The crystal packing of 1 shows a striking similarity with those of the unsolvated crystal structures of A,12 as illustrated in Figure 1. However, the asymmetric unit contains only one full molecule of A; this is in contrast to the multiple conformations observed in the stable polymorph of A. Interestingly, the diethylamine molecule is in the gauche conformation rather than the usual staggered conformation.13 The diethylamine molecule is disordered between two components in 70:30 ratio. However, both components form a similar type of hydrogen bonding, and the major component is considered for hydrogen bond analysis. In contrast to the O-H · · · O tetramer motif observed in the crystal structure of A, a centrosymmtric square motif comprising O-H · · · N (2.03(2) Å, 164.3(15)°) and N-H · · · O (2.12(3) Å, 171.3(12)°) bonds form the central core for the structure of 1. Subsequently, the replacement of the diol (one A molecule) at two of the juxtaposed square motif vertices by two amine molecules reduces the cross-linking of the molecular chain. A closer inspection of the structure reveals that the C-H · · · O and C-H · · · π interactions provide an extra source of stability. The crystal structure of 1 is a good example demonstrating how a particular portion of the host molecule may orient itself to facilitate the formation of C-H · · · X interactions. As illustrated in Figure 2, the pairwise arrangement

Mondal and Howard

of two A molecules is ideally situated for the formation of a C-H · · · O bond. On the other hand, almost mutual perpendicular arrangements of phenylethynyl groups lead to the better C-H · · · π interactions. Crystallization of A from methanol yields 2. The asymmetric unit contains only one molecule of A and one methanol molecule. From a hydrogen bonding point of view, instead of the square motif observed in the unsolvated structure of A, an infinite O-H · · · O-H · · · O hydrogen bonded helical chain appears to be the dominant interaction pattern in 2, and further support is provided by a C-H · · · O (2.599(14) Å, 134.7(11)°) bond involving a cyclohexane ring proton. The topological arrangement of A molecules in 2 are similar to those in 1. While strong O-H · · · O (1.865(15) Å, 173.0(16)°; 1.795(17) Å, 176.3(13)°) bonds dictate the crystal packing, the orientation of the phenyl rings, which remain the prime cause for conformational variance in A molecules, are mainly guided by several weak interactions consisting of a C-H · · · O bond (2.680(17) Å, 170.3(15)°), a C-H · · · π (ethynyl) (2.83 Å, 150.5°) interaction and a C-H · · · π (phenyl) (3.15 Å, 141.6°) interaction (Figure 3). Although the hard-hard hydrogen bond patterns are different for 1 and 2, there is similarity in patterns formed by the weak intermolecular interactions. For both the structures, the two extended nonpolar parts of the molecules that contain phenyl rings orient themselves in such a way as to facilitate the formation of weak bonds. These results were compelling and prompted us to crystallize other solvates of A. The crystal structure of the ethanol solvate of A, 3, is quite interesting from the perspective of “freezing out” a particular conformation. The crystal structure of 3 can be envisaged as a special case of conformational isomorphism;14 the term describes the occurrence of different conformers in the same crystal structure. The asymmetric unit of 3 contains two distinctly different conformers of A, one biaxial and one biequatorial conformer of A, along with two molecules of ethanol. An infinite chain of O-H · · · O-H · · · O bonds plays a central role in the crystal packing; however, the importance of the combined influence of weak hydrogen bonds and interactions is also evident. Both biaxial and biequatorial conformers obtain extra stability by forming several C-H · · · π interactions and C-H · · · O bonds, as evident in Figure 4, while the orientation of one of the ethanol molecules also facilitates the formation of C-H · · · π interactions involving the ethyl protons. It is noteworthy that the occurrence of two energetically different conformers of a molecule trapped in the same solvate crystal lattice is unusual. The only two structures known to demonstrate similar behavior, trans-cyclohexane1,4-diol (C) and trans-1,4-diethynylcyclohexane-1,4-diol (B) (Scheme 1), are essentially the parent molecules of A.14,15 However, there is a subtle difference; unlike 3, the compounds of B and C were obtained as unsolvated forms. The structure 3, thus, provides us with a nice example of a case where solvent can influence the conformation of the solute molecule, resulting in a crystalline state in which two conformers of the solute (A) are present. Encouraged by these results, we then investigated solvates of A containing higher molecular weight alcohols. Interestingly, both isopropyl (4) and n-propyl alcohol (5) solvates of A crystallize with similar unit cell parameters. For both crystal structures, as in the case of 2, a helical chain consisting of an infinite O-H · · · O-H · · · O hydrogen bond network is a recurrent theme, while C-H · · · O and C-H · · · π interactions play an important role in further stabilizing the crystal structures (Figure

Solvates of trans-1,4-Bis(phenylethynyl)-cyclohexane-1,4-diol

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Figure 1. (a) Packing diagram for 1, notice the square synthon shown in dashed black lines, (b) (O-H · · · O)4 square motif and 4.82 network observed for unsolvated A molecules,12 and (c) network and square motif observed for 1.

Figure 2. Crystal packing of 1. Notice the role of the C-H · · · O and C-H · · · π interactions. Some hydrogen atoms are omitted for clarity.

4362 Crystal Growth & Design, Vol. 8, No. 12, 2008

Mondal and Howard

Figure 3. Cooperative intermolecular hydrogen bonding in 2 showing weak C-H · · · O bonds and C-H · · · π interactions.

Figure 4. Weak intermolecular C-H · · · O and C-H · · · π interactions observed in 3 with (a) biaxial and (b) biequatorial conformers of A.

5). However, the effect of the higher steric requirement of the solvents is clearly manifested in the mutual orientation of the phenyl rings within the A molecules. A comparative depiction of the packing diagrams for 2, 4, and 5 places this in perspective (Figure 6). The interdigitation of pairs of phenylethynyl groups belonging to two A molecules is the main feature of the crystal packing. The solvent molecules, which form the helical

O-H · · · O-H · · · O chains, are located in the cavities created by this interdigitation. However, for 4 and 5, due to greater steric bulk of the propyl and isopropyl groups, the interdigitation is clearly restricted in their respective crystal packing arrangements. In order to accommodate the bulkier isopropyl groups in 4, the phenyl rings, although interdigitated in a similar fashion to the phenyl rings



Figure 5. (a) C-H · · · π interactions observed among the juxtaposed phenyl rings for 4 and (b) cooperative intermolecular hydrogen bonding in 5 showing weak C-H · · · O bonds and C-H · · · π interactions in bold, while strong O-H · · · O bonds are displayed as dashed lines.

in 2, do not penetrate quite so far into the cavity formed by the phenyl rings of the neighboring chain of A molecules (see the orange and green groups in Figure 6). Additionally, in comparison to the C-H · · · O and C-H · · · π (ethynyl) interactions in 2, the interdigitated rings are held together by C-H · · · π (phenyl) interactions. Since the infinite O-H · · · O-H · · · O chain is the main stabilizing factor for these solvates, the phenylethynyl groups are of secondary importance, and reorient themselves to create more space to accommodate the bulkier solvents. For 4 and 5, in order to create bigger cavities with respect to those observed in the structure of 2, one pair phenylethynyl groups are separated further (green molecules in Figure 6), while the second pair of phenylethynyl groups, which are situated between the phenylethynyl groups of the first pair, moved closer to each other (orange groups in Figure 6), and this results in the formation of strong C-H · · · π interactions (2.74 Å) (Figure 6). We made several attempts to prepare solvates with higher molecular weight alcohols but without much success. The steric bulk of higher alcohols would require the generation of cavities that would be too big to allow for favorable orientations of the molecules of A necessary for forming weak hydrogen bonds to be maintained. To bypass this unexpected stumbling block, other solvents with structural similarity to the alcohols we have used successfully were tried, and consequently we report herein the interesting structure of the DMF solvate of A (6). Despite being an aprotic solvent, our reason for choosing DMF for solvate studies of A is that it is closely resembles the alcohols that have been utilized successfully to create solvates of A. The fact that the protons of DMF have greater relative acidity with respect to the alcohols studied is bound to increase the perspective hydrogen bond donor capability of the solvent. This should compensate for the absence of extra stability that results from the hydrogen bonds formed by the hydroxyl or amine group of the solvents used to create the solvates of A reported so far. Indeed, six out of the seven hydrogen atoms of DMF take part in weak hydrogen bonding as shown in Figure 7. One of the methyl protons sits just above the phenyl ring of A and

forms a C-H · · · π interaction, whereas five other hydrogen atoms of DMF form C-H · · · O bonds, three with the oxygen atoms of A and two with other DMF molecules, while A forms a string of O-H · · · O bonds with DMF. The most interesting interaction is a very strong offset π-π interaction4 (3.26 Å) between two parallel phenyl rings of adjacent A molecules. Interestingly, unlike the other four structures we have reported so far, the aromatic protons do not form any C-H · · · π interactions; instead the presence of two additional C-H · · · O bonds involving aromatic protons provide further stability and lead to a packing arrangement containing a molecular “thread”. Conclusion The conformations of the A molecules in the compounds reported herein, or more specifically the orientations of the phenyl groups, are influenced by the nature of the included guests and weak hydrogen bonds. The greater degree of hydrogen bonding exhibited by methanol and DMF and their smaller size with respect to the other solvents used in this study resulted in 1:2 host-guest inclusion compounds, but the larger n-propanol and isopropanol molecules played a role in the packing of their solvates that crystallized in 1:1 ratio. Compound 3 represents an unusual crystal structure in which two different conformers of A crystallized together, as a solvate, in a single crystal. This structure highlights the ongoing debate regarding pseudopolymorph nomenclature.15,16 Without wishing to enter into this debate, we are still left with an important question: shall we call 3 a 1:1, 2:2, or ternary 1:1:2 host-guest inclusion compound? Clearly, unlike the other solvates reported, 3 is not a pseudopolymorph of A. The biequatorial conformer of A was not observed in other structures, not even within the structures of the two unsolvated polymorphs of A. Biaxial and biequatorial conformers can be considered as two distinct species, rather than two easily equilibrating conformers. And henceforth 3 can be viewed as a ternary 1:1:2 molecular complex and, unlike


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Figure 6. Crystal packing of 2, 4, and 5 showing the interdigitation and orientation adopted by molecules of A in order to accommodate the solvent molecules.



Figure 7. (a) Cooperative intermolecular hydrogen bonding in 6, notice five out of the seven protons of the DMF molecule taking part in C-H · · · O interactions, while one proton forms a C-H · · · π interaction and (b) packing diagram of 6, showing how four C-H · · · O bonds involving aromatic protons help the formation of an offset π-π stacking interaction Table 1. Crystallographic Data and Structure Refinement Parameters for 1-6

empirical formula formula wt crystal system space group a [Å] b [Å] c [Å] R [deg] β [deg] γ [deg] volume [Å3] Dcalcd [g/cm3] µ [mm-1] θ [deg] range h range k range l reflns collected unique reflns obsd reflns R1 [I > 2σ(I)] wR2 [all] GOF CCDC number

1

2

3

4

5

6

C30H42N2O2 462.66 monoclinic P21/m 9.6774(5) 13.1531(8) 10.9959(6) 90 92.127(2) 90 1398.68(14) 1.099 0.068 2.4-26.1 -12 to 12 -17 to 17 -14 to 14 15873 3359 1946 0.0584 0.1484 0.962 632032

C24H28O4 380.46 orthorhombic Pbca 14.2235(2) 7.6409(1) 20.0319(3) 90 90 90 2177.07(5) 1.161 0.078 2.48-27.09 -18 to 18 -9 to 9 -26 to 26 31418 2490 1961 0.0390 0.1025 1.030 632035

C24H26O3 362.45 monoclinic Cc 25.075(3) 12.7258(14) 12.6398(14) 90 91.437(4) 90 4032.1(8) 1.194 0.077 2.4-27.56 -32 to 30 -16 to 15 -16 to 16 23644 9071 7642 0.0374 0.0836 0.982 632033

C25 H28 O3 376.47 monoclinic P21/c 5.9578(3) 27.9297(17) 13.1767(7) 90 91.098(2) 90 2192.2(2) 1.141 0.073 2.67-27.42 -7 to 7 -36 to 36 -17 to 17 24217 5027 3538 0.0673 0.1047 1.108 632034

C25 H28 O3 376.47 monoclinic P21/c 6.0444(2) 28.5804(9) 12.6039(4) 90 94.719(1) 90 2169.96(12) 1.152 0.074 2.68 -27.44 -7 to 7 -35 to 35 -15 to 15 21229 4279 3303 0.0620 0.1450 1.059 632036

C28 H34 N2O4 462.57 monoclinic P21/n 5.9265(3) 9.0822(4) 23.5705(10) 90 94.983(2) 90 1263.90(10) 1.215 0.081 2.4-27.48 -7 to 7 -11 to 11 -30 to 30 13690 2902 2151 0.0415 0.0922 1.021 632031


the other structures reported herein, is certainly not a pseudopolymorph of A. X-ray Crystallography Single-crystal X-ray diffraction data were collected at 120 K using a Bruker SMART 6000 CCD area detector and Mo KR radiation with a wavelength of 0.71073 Å. Structure solution was via direct methods, and full-matrix anisotropic least-squares refinement of all non-hydrogen atoms was carried out with SHELX-97.17 All hydrogen atoms were located in the electron density difference maps, and their positional and isotropic thermal parameters were refined. Crystallographic data and refinement details are given in Table 1.

Acknowledgment. We thank Dr. Elinor C. Spencer for her careful reading of the manuscript.

References (1) Steed, J. W.; Atwood, J. L. Supramolecular Chemistry; Wiley: Chichester, U.K., 2000. Nassimbeni, L. R. In Crystal Engineering. From Molecules and Crystals to Materials; Braga, D., Grepioni, F., Orpen, A. G. Eds.; Kluwer Academic Publishers: Dordrecht, The Netherlands, 1999; pp 163-179. (2) Bernstein, J. Polymorphism in Molecular Crystals; Clarendon: Oxford, U.K., 2002. McCrone, W. C. In Physics and Chemistry of the Organic Solid State; Fox, D., Labes, M. M., Weissberger, A. Eds.; WileyInterscience: New York, 1965; Vol. 2. Threlfall, T. L. Analyst 1995, 120, 2435. (3) Weber, E. In Inclusion Compounds; Atwood, J. L., Davis, J. E. D., Macnicol, D. D. Eds.; Oxford University Press: Oxford, U.K., 1991; Vol 4. Weber, E. In Kirk-Othmer Encyclopedia of Chemical Technology, 4th ed.; Kroschwitz, J. Ed.; Wiley: NewYork, 1995; Vol 14. (4) Jeffrey, G. A. An Introduction to Hydrogen Bonding; Oxford University Press: Oxford, U.K., 1997. Armstrong, D. R.; Davidson, M. G.; Moncrief, D. Angew. Chem., Int. Ed. Engl. 1995, 34, 478. Desiraju, G. R.; Steiner, T. The Weak Hydrogen Bond in Structural Chemistry and Biology; Oxford University Press: Oxford, U.K., 1999. NishioM. Weak Hydrogen Bonds. In Encyclopedia of Supramolecular Chemistry Atwood, J. L., Steed, J. W., Eds.; Marcel Dekker Inc.: New York, 2004. Janiak, C. J. Chem. Soc., Dalton Trans. 2000, 3885–3896. Planas, J. G.; Masalles, C.; Sillanpa¨a¨, R.; Kiveka¨s, R.; Teixidor, F.; Vin˜as, C. CrystEngComm 2006, 8, 75–83. Sokolov, A. N.; Friscic, T.; Blais, S.; Ripmeester, J. A.; MacGillivray, L. R. Cryst. Growth Des. 2006, 6, 2427–2428.

Mondal and Howard (5) Banerjee, R.; Desiraju, G. R.; Mondal, R.; Howard, J. A. K. Chem.sEur. J. 2004, 10, 3373–3383. (6) Steiner, T. Chem. Commun. 1997, 727. Thallapally, P. K.; Katz, A. K.; Carrell, H. L.; Desiraju, G. R. CrystEngComm 2003, 5, 87–92. Choudhury, A. R.; Nagarajan, K.; Row, T. N. G. CrystEngComm 2006, 8, 482–488. Glidewell, C.; Low, J. N.; Shakle, J. M. S.; Wardell, S. M. S. V.; Wardell, J. L. Acta Crystallogr. 2005, B61, 227–237. Nishio, M. CrysEngComm 2004, 6, 130–158. Lai, C. S.; Mohr, F.; Tiekink, E. R. T. CrystEngComm 2006, 8, 909–915. (7) Broder, C. K.; Davidson, M. G.; Forsyth, V. T.; Howard, J. A. K.; Lamb, S.; Mason, S. Cryst. Growth Des. 2002, 2, 163–169. (8) Desiraju, G. R. Chem. Commun. 2005, 2995–3001. (9) Brock, C. P.; Duncan, L. L. Chem. Mater. 1994, 6, 1307–1312. Cso¨regh, I.; Finge, S.; Weber, E. Struct. Chem. 2003, 14, 241–246. Kim, E.-I.; Paliwal, S.; Wilcox, C. S. J. Am. Chem. Soc. 1998, 120, 11192–11193. Kishikawa, K.; Yoshizaki, K.; Kohmoto, S.; Yamamoto, M.; Yamaguchi, K.; Yamada, K. J. Chem. Soc., Perkin Trans.1 1997, 1233–1239. (10) Mei, X.; Wolf, C. CrystEngComm 2006, 8, 377–380. Kla¨rner, F.-G.; Benkhoff, J.; Boese, R.; Burkert, U.; Kamicth, M.; Naatz, U. Angew. Chem., Int. Ed. Engl. 1996, 35, 1130–1132. Kla¨rner, F.-G.; Kahlert, B. Acc. Chem. Res. 2003, 36, 919–932. Biradha, K.; Dennis, D.; MacKinnonn, V. A.; Sharma, C. V. K.; Zaworotko, M. J. J. Am. Chem. Soc. 1998, 120, 11894–11903. (11) Guo, F.; Guo, W. S.; Toda, F. CrystEngComm 2003, 5, 45–47. Suezawa, H.; Yoshida, T.; Hirota, M.; Takahashi, H.; Umezawa, Y.; Honda, K.; Tsuboyama, S.; Nishio, M. J. Chem. Soc., Perkin Trans. 2 2001, 2053–2058. O’Leary, B. M.; Grotzfeld, R. M.; Rebek, J. J. Am. Chem. Soc. 1997, 119, 11701–11702. Endo, E.; Ezuhara, T.; Koyanagi, M.; Masuda, H.; Aoyama, Y. J. Am. Chem. Soc. 1997, 119, 499. (12) Das, D.; Banerjee, R.; Mondal, R.; Howard, J. A. K.; Boese, R.; Desiraju, G. R. Chem. Commun. 2006, 555–557, and references therein. (13) Mondal, R.; Howard, J. A. K. CrystEngComm 2005, 7, 462–464. Mondal, R.; Howard, J. A. K.; Banerjee, R.; Desiraju, G. R. Chem. Commun. 2004, 644–645. Mondal, R.; Howard, J. A. K.; Banerjee, R.; Desiraju, G. R. Cryst. Growth Des. 2006, 6, 2507–2516. (14) Bilton, C.; Howard, J. A. K.; Madhavi, N. N. L.; Nangia, A.; Desiraju, G. R.; Allen, F. H.; Wilson, C. C. Chem. Commun. 1999, 1675–1676. (15) Steiner, T.; Saenger, W. J. Chem. Soc. Perkin Trans. 2 1998, 371– 377. (16) Desiraju, G. R. CrystEngComm 2003, 5, 466–467. Sedon, K. R. Cryst. Growth Des. 2004, 4, 1087. Nangia, A. Cryst. Growth Des. 2006, 6, 2–4. (17) Sheldrick, G. M. 2008 Acta Crystallogr. A64, 112–122.

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1,4-Bis(phenylethynyl)-cyclohexane-1,4-diol - American Chemical

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