Observation of a Persistent Supramolecular Synthon Involving

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Observation of a Persistent Supramolecular Synthon Involving Carboxyl Groups and H2O That Guides the Formation of Polycatenated Co-crystals of a Tritopic Carboxylic Acid and Bis(pyridyls) Lian-Cheng Wang,† Junliang Sun,*,‡ Zhi-Tang Huang,† and Qi-Yu Zheng*,† †

Beijing National Laboratory for Molecular Sciences, CAS Key Laboratory of Molecular Recognition and Function, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China ‡ Beijing National Laboratory for Molecular Sciences, College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, China S Supporting Information *

ABSTRACT: In a nonprotic solvent, a (H2O)2(COOH)3(L) supramolecular synthon was found to guide the formation of five new crystal structures together with HPB-3a (1,3,5-tris(4carboxyphenyl)-2,4,6-tris(4-tert-butylphenyl)benzene). This synthon acted as a 3-connected trigonal planar node (L = acetone) in 1 [(HPB-3a)(H2O)2(acetone)3] or a 4-connected trigonal pyramidal node (L = pyridyl) in 2 [(HPB3a)(bipy) 0.5 (H 2O) 2], 3 [(HPB-3a)(azopy) 0.5 (H 2O) 2], 4 [(HPB-3a)(bipy-ete)0.5(H2O)2], and 5 [(HPB-3a)(bipy-eta)0.5(H2O)2] (bipy = 4,4′-bipyridine, azopy = azopyridine, bipy-ete = trans-1,2-bis(4-pyridyl)ethene, bipy-eta = 1,2-bis(4-pyridyl)ethane). The formation of this synthon could be attributed to the C3-symmetry of planar HPB-3a molecules and the hydrophobic interactions between tert-butyl groups. 1 represents a continuously interdigitated 63-hcb layer structure. 2−5 are with the same topology and display amazing 2D homochiral bilayers, which were penetrated in parallel by two others (“above” and “below”) with the opposite chirality to form overall 3D racemic networks. However, the synthon was not as robust in the presence of protic solvents. In 6 [(HPB-3a)(MeOH)3], carboxyl groups interact directly with hydroxyl groups of methanol to form 1D hydrogen bonding chains. The structure is a 3-fold interpenetrated 49·66-acs network.

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contrast to the widely studied interpenetrating networks, parallel polycatenation which leads to dimensionality increase has been less investigated. We have been interested in the construction of novel topological structures from delicate organic tectons.10 Hexaphenylbenzene (HPB), which exhibits a nonplanar propellerlike conformation because of steric hindrance of peripheral phenyl rings, has attracted considerable attention in recent years. This unique conformation could enable a sextuple accumulation of π···π interactions between peripheral aromatic rings facing each other.11 Derivatives of HPB have been widely used in areas such as construction of large molecular cages,12 preparation of porous organic polymer (POP) materials,13 crystal engineering,14 precursors for synthetic graphene fragments,15 and surface chemistry.16 The C3-symmetric HPB-3a (1,3,5-tris(4-carboxyphenyl)-2,4,6-tris(4-tert-butylphenyl)benzene) with three alternating tert-butyl and carboxyl groups at a HPB core (Scheme 1) displays a variety of chiral honeycomb structures assembled via van der Waals (vdW)

ydrogen bonding has played an important role in supramolecular chemistry to construct solids with specific applications, such as selective adsorption and catalysis,1,2 because of its strength and directionality.3 However, predictable control of molecular orientations in the outcoming structures is difficult and remains an elusive task because of multicompetitions with other nondirectional interactions, and the final frameworks with large cavities often tend to be selfinterpenetrated to fill the voids.4 Thus, some impressive structures were first found quite serendipitously. The intriguing features of molecular architectures and topologies are of great interest due to both the aesthetic appeal and encouraging properties.5 Analysis of the crystal structures is nondeductible for the application of gained knowledge to guide new structure construction.6 The structure classification by Wells7 and O’Keeffe and Hyde8 set the ground for our understanding and designing of new crystalline materials. A topological analysis of interpenetrated 3D networks of hydrogen-bonded organic species by Blatov and Proserpio9 revealed that 4connected nets took a major part in all 122 different motifs. They are generally dia, cds, and lvt types based on tetrahedral or square planar nodes. It is a rare case in which the 4connected node represents trigonal pyramidal geometry. And in © 2012 American Chemical Society

Received: July 19, 2012 Revised: November 19, 2012 Published: November 26, 2012 1

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(H2O)2(COOH)3 formed. H-bonding interactions between carboxyl groups and vdW interactions between tert-butyl groups cannot be simultaneously satisfied for steric reasons. This leads to the formation of an H-bonded supramolecular synthon (with H2O), which allows for vdW interactions between tert-butyl groups. According to the different rotation directions, clockwise or anti-clockwise, HPB has two enantiomers M- or P-. In 1, the average dihedral angle between the peripheral phenyl rings and the central benzene ring is 66.82°. M- and P-enantiomers are alternatively distributed in each layer. The hydrogen-bonded acetone molecules are orientated alternatively to both sides and interacting with the adjacent layers via C−H···O and C−H···π interactions (Figure S1 of the Supporting Information). Thus, 1 represents a continuously interdigitated 63-hcb layer structure. From a design perspective, networks with higher dimensionality, such as bilayer or 3D pillared networks, will be generated if hydrogen bonded acetone molecules are replaced by pillared ligands. Thus, N-hydrogen bond acceptors 4,4′-bipyridine, 4,4′azopyridine, trans-1,2-bis(4-pyridyl)ethene, and 1,2-bis(4pyridyl)ethane were introduced respectively to co-crystallize with HPB-3a from acetone. Block single crystals 2−5 were obtained. The X-ray crystallographic studies reveal that 2−5 have the same topology, so we will choose 2 for detailed structural discussions. The crystallographic data are given in Table S1 of the Supporting Information. As shown in Figure 2, the honeycomb layers are maintained in 2. As expected, (H2O)2 units connected three carboxyl groups and one 4,4′-bipyridine molecule in a trigonal pyramidal fashion, acting as 4-connected nodes (Figure 2b). The O−H···N distance is 2.009 Å (1.860 Å for 3, 1.832 Å for 4, and 1.822 Å for 5). The average dihedral angle between peripheral phenyl rings and the central benzene ring is 67.13° (67.36° for 3, 68.63° for 4, and 75.26° for 5). Interestingly, the axial 4,4′-bipyridine pillars are at the same side of the honeycomb layer and connect two layers with the same chirality to form a homochiral bilayer. The bilayer has large channels along the a- and b-axes with the height about 15.87 Å, measured from the central benzene rings (16.32 Å for 3, 16.48 Å for 4, and 16.67 Å for 5). We are encouraged to see that the (H2O)2(COOH)3 unit is retained, meaning that, unlike the cases for other hydrogen bonding systems,17 the strongest proton donor (COOH) did not form hydrogen bonds with the strongest proton acceptor (pyridine) here. This unusual hydrogen bonding could be due to the unique conformation of HPB-3a molecules, the increased number of HBs due to water participation, and especially vdW interactions between tert-butyl groups. Thus, structures with the same topology are constructed in this system, which are usually sustained by strong noncovalent interactions, such as coordination bonds18 and charge-assisted hydrogen bonds.19 To bring two layers together, the pillars must have anti-parallel hydrogen bond acceptors, so 1,2-bis(4-pyridyl)ethane took the anti conformation in 5. This bilayer structure resembles the quasihexagonal guanidinium disulfonate (GS) bilayer structures.20 But, different from the case of GS sheets where the dense charge assisted hydrogen bonds prevent interpenetration, in 2 the large channels and open windows facilitate interpenetration. Each bilayer is penetrated by two others (“above” and “below”) of opposite chirality, leading to an overall racemic 3D entanglement (Figure 2c). The topology is 2D → 3D parallel polycatenation. The driving force for catenation is multiple vdW interactions generated through space filling, including C−

Scheme 1. Structures of HPB-3a and Bis(pyridyl) Pillars

interactions as well as dimeric hydrogen bonds (HBs) between carboxyl groups on Au(111) surfaces.16b We anticipate that the complex interaction modes between HPB-3a molecules would potentially lead to intriguing topological structures in crystal engineering. Here we report that in a nonprotic solvent an unprecedented (H2O)2(COOH)3(Py) unit was promoted to serve as 4-connected trigonal pyramidal node during the selfassembly of HPB-3a with four different bis(pyridyl) pillars. The topology of the resulting frameworks is parallel polycatenation of two-dimensional homochiral bilayers (2D → 3D parallel polycatenation). The crystal structure of 1 [(HPB-3a)(H2O)2(acetone)3] was shown in Figure 1. Instead of forming dimeric HBs between

Figure 1. One binodal honeycomb layer in 1: C, gray; O, red; H, white; acetone, green. Inset: The (H2O)2 aggregate connects three carboxyl groups in a trigonal planar fashion. Yellow indicates points of extension.

carboxyl groups, three carboxyl groups are joined by two water molecules. As far as we know, such a well-defined (H2O)2(COOH)3 supramolecular synthon has never been reported previously. H2O molecules have two hydrogen atoms and two lone electron pairs that enable them to potentially participate in four hydrogen bonds. In 1 the (H2O)2 unit is connected to three carboxyl groups in a trigonal planar fashion with one axial proton capped by an acetone molecule through hydrogen bonding. The HB distances are O2−H···O6 = 1.926 (1.950) Å, O2−H···O4 = 2.050 (1.978) Å, O8−H···O2 = 1.864 (1.782) Å, O5−H···O1 = 1.872 (1.931) Å, O3−H···O1 = 1.945 (1.874) Å, O1−H···O7 = 1.896 (1.957) Å, and O1−H···O9 = 1.931 (1.949) Å (two sets for their different orientations, vide inf ra). From the molecular structure information of HPB-3a, we can deduce why this unprecedented synthon 2

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Figure 2. (a) The chiral bilayer in 2. The moieties of the tert-butyl substituted phenyl in HPB-3a and solvent molecules have been omitted for clarity. (b) The 4-connected trigonal pyramidal (H2O)2(COOH)3(Py) synthon. C, gray; O, red; N, blue; H, white. Yellow indicates points of extension. (c) The parallel polycatenation of honeycomb bilayers in 2; the sequence is ···PMPM···, leading to a racemate.

H···π, π···π interpenetrations between 4,4′-bipyridine pillars and HPB-3a molecules (C−H···C = 2.820 Å, C···C = 3.309 Å) and multiple C−H···O interactions between adjacent layers (selected distances are 2.409 Å, 2.625 Å, 2.652 Å). Note that 2D → 3D parallel polycatenation is rare and the first example was reported in 1997.21 Due to the versatile coordination geometry and variable coordination numbers of metal ions, a series of 3D coordination polymers assembled via 2D → 3D parallel polycatenation have been obtained based on either an undulating layer motif22 or a “thick” motif.23 In bilayer structures, this 4-connected trigonal pyramidal node, which leads to a honeycomb, is even less observed than the 5connected square pyramidal node, which leads to a square grid. A recent report is a porous metal−organic framework (MOF) constructed from a rare Zn2(COO)3 SBU (second building unit).24 Compound with parallel polycatenation features constructed from the combination of hydrogen bonding and coordination interactions has been successfully obtained.25 But to the best of our knowledge, our finding is the first report in pure organic hydrogen bonding systems. From the above results, we can see that the (H2O)2(COOH)3 synthon takes an important role in structure

formation. It retains integrity even in the presence of pyridyl ligands; however, it is not as robust in the presence of protic solvents. From a mixture of acetone and methanol, a 3-fold interpenetrated 49·66-acs net 6 [(HPB-3a)(MeOH)3] formed. The carboxyl groups interact directly with the hydroxyl groups of methanol to form 1D hydrogen bonding chains (Figure 3b). The average dihedral angle between the peripheral phenyl rings and the central benzene ring is 59.74°. The whole structure is racemic because of the alternative distribution of M- and Penantiomers. Each HPB-3a connected six others, and the simplified net exhibits a 3D 6-connected acs topology with 3fold interpenetration (Figure 3a, c). In conclusion, we have constructed interesting topological structures based on HPB-3a and the (H2O)2(COOH)3(L) supramolecular synthon. The topology of parent HPB-3a from nonprotic solvent is continuously interdigitation of 63-hcb layers. Extension of the finding with four different bis(pyridyl) ligands leads to parallel polycatenated frameworks. But the synthon is not as robust in protic solvents. This work illustrates the significant influence of the intrinsic structure of organic tectons in the determination of solid-state crystal formation. Weak hydrophobic interactions adjust dimeric HBs between 3

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carboxyl groups to fulfill close packing of organic tectons. Water molecules could take an unanticipated vital role in the structure formation. The well-defined connection geometry and modest hydrogen bonding strength of the (H2O)2(COOH)3 synthon guided the self-assembly of HPB-3a in a nonprotic solvent. Our findings expand the interaction modes between carboxyl groups where the dimer or 1D chain prevailed previously. Construction of more complex structures with other HPB derivatives is continued in our lab.

ASSOCIATED CONTENT

S Supporting Information *

Synthesis and crystal data of 1−6, and TOPOS analysis and Xray crystallographic files in CIF format. CCDC numbers of 1−6 are 885786−885791. This material is available free of charge via the Internet at http://pubs.acs.org.

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Figure 3. (a) Hydrogen-bonding honeycomb fragments formed by HPB-3a and methanol in 6. (b) 1D COOH···OH hydrogen bonding chain. (c) 3-fold interpenetration of 49·66-acs nets in 6.

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AUTHOR INFORMATION

Corresponding Author

*Phone: +86-10-62652811. Fax: +86-10-62554449. E-mail: [email protected]; [email protected]. Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Major State Basic Research Development Program of China (No. 2011CB932501), the National Natural Science Foundation of China (No. 20932004), and the Chinese Academy of Sciences. 4

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