Communication pubs.acs.org/crystal
Crystal Engineering Approach to Generate Crystalline Inclusion Compounds in Which 5‑Hydroxyisophthalic Acid Serves as a Host Krishna S. Peraka, Matteo Lusi, Alankriti Bajpai, and Michael J. Zaworotko* Department of Chemical Sciences and Bernal Institute, University of Limerick, Castletroy, Co. Limerick, Ireland S Supporting Information *
ABSTRACT: A series of seven crystalline inclusion compounds (CICs) in which 5-hydroxyisophthalic acid (HPA) serves as a host has been isolated. HPA self-assembles to consistently form 2-D hydrogen bonded networks with distorted honeycomb (hcb) topology when crystallized in the presence of guest molecules that contain a carbonyl moiety. In each of the seven structures, the hcb networks formed by HPA contain a phenolic OH group that is exposed to the walls of channels formed by stacking of the hcb networks. These phenolic moieties interact with the carbonyl group of guest molecules via OH (phenol)···O (carbonyl) hydrogen bonding. The consistency of the crystal packing herein is in contrast to the promiscuity in previously reported HPA crystal structures and suggests that HPA can serve as a host for a wide range of guests.
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An already established crystal engineering strategy to generate CICs is to form open 2-D networks from host compounds that exhibit three or more coplanar supramolecular homosynthons, e.g., tri-,26−30 tetra-,31 or hexasubstituted benzenes.32,33 Another crystal engineering strategy is to form multicomponent networks through association of two or more molecules or ions.21,34−36 Trimesic acid (TMA) is prototypal for both approaches thanks to its three carboxylic acid moieties that favor formation of honeycomb (hcb) networks through self-assembly,26,27 as salts with primary amines,37 or as cocrystals with bipyridines.38,39 5-Hydroxyisophthalic acid (HPA) is an analog of TMA that could have the propensity to form CICs through the formation of hcb networks. However, although HPA has been quite well-studied in the solid state in terms of cocrystals,40−43 solvates,44,45 and its hydrate,46 the crystal structure of the anhydrate remains unreported and only two of these structures exist as TMAlike hcb networks.41,42 Herein we demonstrate that hcb networks of HPA can indeed form consistently when HPA is crystallized with the right guest molecules. Eight crystal structures that involve HPA as a neutral molecule are archived in the Cambridge Structural Database (CSD).47,48 Analysis of the crystal packing in these entries reveals eight different sets of hydrogen bonding motifs, indicating that HPA is highly promiscuous and difficult to control in terms of its crystal packing (Figure 1). Six of these structures, refcodes PUPHAK, DAVSAW, XIVLOD, WAYSEW, XIVLUJ, and HUSGOT, exhibit a variety of supramolecular homosynthons and supramolecular heterosynthons49
rystalline inclusion compounds (CICs) contain voids, typically in the form of channels, in which a second chemical species (the guest) is located.1 The formation of such structures is a long-recognized phenomenon as exemplified by Dianin’s compound,2,3 Werner clathrates,4 and urea channel compounds.5 CICs are of fundamental interest because they are controlled by molecular recognition events both in terms of the crystal packing of the CIC (the host) and how they interact with guest molecules. Nassimbeni has classified CICs based on the nature of the voids therein:6 (i) host molecules such as crown ethers, calixarenes, cryptands and cyclodextrins exhibit intrinsic porosity that also exists when the host is dissolved;7 (ii) host molecules that are not inherently porous but selfassemble via intermolecular interactions to form networks that contain void spaces (extrinsic porosity).8 Whereas the first generation of CICs were largely a result of serendipity, it is becoming recognized that CICs are quite commonly observed for larger molecules such as those used in pharmaceuticals (often in the form of solvates or hydrates),9,10 and certain compounds are amenable to crystal engineering.11−14 With respect to crystal engineering, the directional nature of hydrogen bonds15,16 and an understanding of hydrogen bonded supramolecular synthons17 can enable the design of new CICs from first principles. Adamantane tetracarboxylic acid forms diamondoid networks with large voids that are filled by interpenetration.18 Related tetrahedral molecules with peripheral carboxylic acid moieties, for which the term “tecton” was coined,19 form CICs.20 2-D networks that form CICs can also be designed from first principles.21,22 CICs are no longer just a scientific curiosity since they are seen as relevant to molecular electronic devices and molecular motors and to enabling surface conductivity.23−25 © XXXX American Chemical Society
Received: December 30, 2016 Revised: January 30, 2017 Published: January 30, 2017 A
DOI: 10.1021/acs.cgd.6b01904 Cryst. Growth Des. XXXX, XXX, XXX−XXX
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Figure 1. Different modes of crystal packing and supramolecular synthons in previously reported structures of HPA: (a) PUPHAK; (b) DAVSAW; (c) XIVLOD; (d) WAYSEW; (e) XIVLUJ; (f) HUSGOT; (g) NUHMIM; and (h) XIVMIY.
carbonyl moiety (aldehydes, esters, and ketones). These guests are identified in Scheme 1 along with their 3-letter codes: citral (CTL), 2-phenyl-2-butenal (2PB), (+)-camphor ((+)CMP), (±)-camphor ((±)CMP), isoamyl acetate (IAA), maltol (MTL), and vanillin (VNL).
involving the acid and phenol moieties, whereas NUHMIM and XIVMIY exhibit carboxylic acid supramolecular heterosynthons. In the hydrated cocrystal of caffeine with HPA (PUPHAK) and the ethanol solvate of HPA (DAVSAW), HPA dimers form through the formation of acid dimer supramolecular homosynthons. These HPA dimers are linked by the coformer (PUPHAK) or another HPA molecule (DAVSAW) to form 2D networks as presented in Figure 1a and b, respectively. HPA also forms structures comprising acid dimer supramolecular homosynthons as follows: discrete hexameric units in XIVLOD (Figure 1c); zigzag chains in WAYSEW (Figure 1d); distorted hcb networks sustained by acid−acid and phenol−phenol supramolecular homosynthons in XIVLUJ (Figure 1e); acid− acid supramolecular homosynthons and phenol−chloride− phenol supramolecular heterosynthons in HUSGOT (Figure 1f). XIVLUJ is perhaps the most relevant structure to this study since HPA molecules self-assemble into hcb layers that pack to form distorted hexagonal channels with large voids (Figure 1e). These channels contain 18-crown-6 ether and EtOH as guest molecules. In HUSGOT, the crystal packing is similar to that of XIVLUJ with the exception that −OHAr···Cl···OHAr supramolecular heterosynthons sustain the hcb networks and the hexagonal channels are occupied by charge balancing trimethylammonium cations (Figure 1f). In NUHMIM; HPA forms discrete units sustained by acid−pyridyl and acid− pyrimidyl supramolecular heterosynthons and the free OH group is hydrogen bonded to dioxane (Figure 1g). In the hydrate of HPA (XIVMIY), multiple supramolecular heterosynthons lead to a 3-D structure (Figure 1h). Overall, these crystal structures suggest that the need to satisfy the hydrogen bonding of the phenolic OH of HPA could be responsible for such a high level of variability of crystal packing patterns. While attempting to prepare cocrystals of HPA and citral (CTL), a 2:1 CIC in which HPA serves as a host for CTL was isolated. This CIC exhibits distorted hcb topology much like that of XIVLUJ with CTL serving as a guest. In this structure, the phenolic OH group of HPA forms a hydrogen bond with the carbonyl oxygen of the guest (dO−H···O = 1.780(1) Å, D O−H···O = 2.600(1) Å, ∠ O−H···O = 164.88(1)°). The complementarity of the host network and guest in terms of hydrogen bonding prompted us to crystallize HPA in the presence of a library of potential guests that also contain a
Scheme 1. HPA and the Guests Used in the Present Study with the Corresponding Three Letter Codes
Single crystals were isolated by slow evaporation of a saturated solution of HPA and the carbonyl compound in a 1:1 v/v mixture of MeOH and MeCN. Single crystal X-ray diffraction studies revealed that CTLHPA, 2PBHPA, and IAAHPA crystallize in the triclinic space group P1̅ whereas (+)CMPHPA, (±)CMPHPA, VNLHPA·MeOH, and MTLHPA·MeCN crystallize in P21, P21/n, P1, and P21/n, respectively. All seven structures exhibit the same distorted hcb networks as seen in XIVLUJ (Figures 2 and 3). Detailed accounts of the syntheses are provided in the Supporting Information (SI). All structures also exhibit O−H···O hydrogen bonds between the phenolic OH groups of HPA molecules and the carbonyl moiety of the guest molecules that occupy the channels formed by stacking of hcb networks. In the CICs presented herein, guest molecules are offset with respect to the plane of the hcb layers, which stack in an ABAB fashion (Figure 3a), with the exception of VNLHPA·MeOH, in which VNL guests lie coplanar with respect to the host network (Figure 3b). In MTLHPA·MeCN and VNLHPA·MeOH, solvent was also B
DOI: 10.1021/acs.cgd.6b01904 Cryst. Growth Des. XXXX, XXX, XXX−XXX
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because of the coplanar and parallel orientation of guest molecules within the hcb layers, whereas a center of inversion between the hcb layers cannot occur because of the AAAA-type stacking of layers (Figure 3b). It should be noted that in MTLHPA·MeCN and VNLHPA· MeOH, despite the presence of both carbonyl and phenolic OH groups in the guests, the phenolic group of HPA still hydrogen bonds with carbonyl groups. The persistence of structure and composition observed in these seven CICs indicates that a hydrogen bond acceptor such as a carbonyl group can be used to control the supramolecular arrangement of HPA. However, we note that the acetone solvate of HPA (WAYSEW) is an exception, presumably due to relatively small size of acetone. In summary, crystallization of HPA with a diverse range of organic guests containing a carbonyl group reliably produces crystal packing in the form of a 2-D distorted hcb network. The ability of HPA to selectively form CICs with carbonyl guests could be of interest for separation and purification of aldehydes, ketones, or esters from mixtures containing other competing hydrogen bonding moieties such as carboxylic acids or alcohols. This work also demonstrates how a crystal engineering strategy aids in controlling the otherwise promiscuous behavior of HPA.
Figure 2. HPA as a distorted hcb network with CTL guests. CTLHPA is provided as a representative example.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.cgd.6b01904. Experimental details, single crystal data, PXRD, TGA, and FTIR for the 5-hydroxyisophthalic acid (HPA) channel inclusion compounds (PDF) Accession Codes
CCDC 1524966−1524972 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
Figure 3. (a) ABAB-type stacking in CTLHPA observed down the aand b-axes, respectively. The citral guest molecules are offset with respect to the plane of the hcb layers. (b) Crystal packing of VNLHPA·MeOH showing AAAA-type stacking of hcb layers and the coplanar orientation of vanillin and MeOH guest molecules.
Krishna S. Peraka: 0000-0003-2061-7721 Alankriti Bajpai: 0000-0002-7139-8031 Michael J. Zaworotko: 0000-0002-1360-540X Notes
The authors declare no competing financial interest.
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included in the host cavity. The presence of the solvent could be attributable to additional phenolic OH groups on MTL and VNL that would otherwise exhibit unsatisfied OH moieties. In MTLHPA·MeCN, the free OH group of MTL hydrogen bonds to the CN group of MeCN (dO−H···O = 2.116(1) Å, DO···N = 2.857(1) Å, ∠O−H···O = 146.83(1)°). In VNLHPA·MeOH, the OH group of VNL is hydrogen bonded with MeOH (dO−H···O = 1.788(1) Å, DO···O = 2.538(1) Å, ∠O−H···O = 147.77(1)°). Interestingly, VNLHPA·MeOH crystallizes in a chiral space group despite being composed of three achiral molecular components. The absence of centers of inversion in the molecular components and the nature of the crystal packing precludes a center of inversion. With respect to the crystal packing, a center of inversion within the hcb layer is prevented
ACKNOWLEDGMENTS We gratefully acknowledge the support of Science Foundation Ireland for Award 13/RP/B2549. REFERENCES
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DOI: 10.1021/acs.cgd.6b01904 Cryst. Growth Des. XXXX, XXX, XXX−XXX
