Nanotubular Hydrogen-Bonded Organic Framework Architecture of 1

Communication pubs.acs.org/crystal

Nanotubular Hydrogen-Bonded Organic Framework Architecture of 1,2-Phenylenediboronic Acid Hosting Ice Clusters Krzysztof Durka,*,† Katarzyna N. Jarzembska,*,‡ Radosław Kamiński,‡ Sergiusz Luliński,† Janusz Serwatowski,† and Krzysztof Woźniak‡ †

Department of Chemistry, Warsaw University of Technology, Noakowskiego 3, 00-664 Warszawa, Poland Department of Chemistry, University of Warsaw, Pasteura 1, 02-093 Warszawa, Poland

‡

S Supporting Information *

ABSTRACT: A nanotubular crystal network of a smallmolecule compound, 1,2-phenylenediboronic acid, constituting a hydrogen-bonded organic framework (HOF) architecture is presented. In contrast to the carbon nanotubes, the intermolecular connections are based strictly on hydrogenbonding interactions. Its uniqueness is additionally enhanced by the existence of the other two crystal phases characterized by more compact structures. Furthermore, the experiment and computational analysis show that such channels may host water clusters, such as water-based polymers similar to the ones found in the hexagonal structures of ice. Water molecules incorporated into pores interact weakly with the boronic framework and, thus, are not responsible for the framework templation.

T

Scheme 1. Synthesis of 1,2-Phenylenediboronic Acid (1) and an Equilibrium Between 1 and Its Semi-Anhydride (1-sanh) in Solution

he application of arylboronic acids in supramolecular chemistry is of growing importance. 1 Specifically, compounds with two or more boronic groups were found to be especially suitable for the construction of extended Hbonded crystal networks.2,3 Although it is difficult to stabilize the hydrogen-bonded organic framework,2,4 the unique features of such networks (i.e., high flexibility, easy purification, and regeneration) may dominate over the traditional porous materials such as zeolites5 or metal−organic frameworks (MOFs).6 Here we present the nanotubular network of 1,2phenylenediboronic acid (1), constituting a hydrogen-bonded organic framework architecture (1a). In contrast to the carbon nanotubes, the intermolecular connections in 1a are based strictly on the hydrogen-bonding interactions. Its uniqueness is additionally enhanced by the existence of the other two polymorphic forms, 1b and 1c, characterized by more compact structures. Furthermore, the experimental and computational analysis show that channels of 1a may host water clusters, such as water-based polymers similar to the ones found in the hexagonal structures of ice (Ice Ih and Ice II). 1,2-Phenylenediboronic acid was obtained from the Br/Li exchange reaction of 2-(2′-bromophenyl)butyl[1,3,6,2]dioxazaborocane (previously obtained from 2-bromophenylboronic acid and N-butyldiethanolamine), followed by addition of B(OMe)3 and subsequent hydrolysis (Scheme 1). Our synthetic protocol offers a significant advantage over the earlier published method,7 as it does not require the use of toxic organomercury precursors and hazardous boron halides. The 1 H and 13C NMR studies revealed that molecules of 1 coexist in © 2013 American Chemical Society

solutions in equilibrium with cyclic semianhydride form (1sanh), possessing the structure of benzoxadiborole. All forms of 1 are characterized by completely different supramolecular patterns.8 In 1a, diboronic acid moieties form hexameric rings, which are further connected by lateral hydrogen interactions. This results in the unprecedented nanotubular network formation (Figure 1a), the first one encountered among phenylenediboronic acids, with the van der Waals diameter of channels calculated at the narrowest point Received: July 18, 2013 Revised: August 8, 2013 Published: August 22, 2013 4181

dx.doi.org/10.1021/cg401087j | Cryst. Growth Des. 2013, 13, 4181−4185

Crystal Growth & Design

Communication

Figure 1. Packing diagrams showing the nanotubular architecture in (a) the crystal structure of 1a, (b) water channels in 1b, and (c) a molecular layer in 1c.

Figure 2. Unit cell packing diagrams for (a) 1a and (b) 1b with void surfaces generated in CRYSTALEXPLORER (ϱ = 0.0003 au).10,11

framework are created through the lateral hydrogen bonds formed via B(OH)2 groups. In the case of the anhydrous form, 1c, the structure is based on classical hydrogen-bonded sheets, commonly observed in structures of diboronic acids.1,2 In contrast to the 1b channels, the nanotubular walls in 1a are not functionalized with hydroxyl groups and do not interact via hydrogen-bonding interactions with the inserted water molecules. The absence of such interactions constitutes the main reason justifying the presence of the disorder. This also suggests that solvent molecules are not responsible for templating the self-assembly of diboronic acid molecules, leading to the channel nature of 1a. Calculations of the void surfaces with the Spackman approach10 show that both types of

approximately equal to 8.8 Å. The channels host highly disordered water molecules confined during crystal growth. The calculation of solvent accessible volume performed with the PLATON package9 shows that the amount of residual electron density in a channel corresponds to one solvent molecule per molecule of acid on average, which is also confirmed by a thermogravimetric analysis (TGA). In the case of 1b, the molecules form hydrogen-bonded “zigzags” which further assemble by weak C−H···π and O···B interactions into a 3-dimensional channel array (Figure 1b). The channels are filled with water molecules linked together via hydrogen bonds forming a 1D ribbon. The connections between water chains and polar walls of the diboronic acid 4182



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channels are characterized by similar volumes (25.5% and 24.3% of the unit cell volume for 1a and 1b, respectively; see the Supporting Information). Despite similar dimensions, both kinds of channels significantly differ in their shapes (Figure 2). The 1b channels are oriented along the Y direction and also adjusted to the water ribbon insertion, whereas the 1a channels are hexameric. To investigate thermal stability and the possibility of extracting water from pores, thermogravimetric analysis was performed for all the studied crystal forms of 1. The TGA scans of 1a and 1c show significant mass decrease in the range of 75− 95 °C (Figure 3). In the case of 1a, the mass loss corresponds

Table 1. Stabilization Energies of Water Substructures and Cohesive Energy Values of the Optimized Crystal Structures of Ice (Ice II and Ice Ih), the 1a Structure Enclosing Various Water Contents and Substructures, 1b, and 1ca compound Ice II Ice Ih 1a1wopt[a] 1a1wexp[b] 1a2wopt[c] 1b 1c

H2O content

water polymeric substructure

anhydrous structure

full structure

2 2 1

−98.3 −107.6 −36.2

− − −175.5

−144.6 −154.2 −223.9

1

−8.34

−175.5

−192.5

2

−114.6

−175.5

−305.3

2 0

−88.6 −

−107.2 −151.8

−308.5 −151.8

All energy values are given in kJ mol−1 and are related to the asymmetric part of the unit cell appropriate to a given case. bFully optimized water positions. cWater oxygen atom located at the “experimental” position; water hydrogen atom positions are optimized. d Fully optimized structure containing two water molecules per diboronic acid moiety. a

In accordance with the computations, the anhydrous diboronic acid network is energetically most favored in the case of 1a (Table 1), which indicates the stiffness of the structure and the negligibility of the water-filling contribution to the crystal stability. Form 1c is characterized by the cohesive energy about 20 kJ·mol−1 less advantageous than form 1a. Indeed, it is much easier to crystallize the 1a form. This is surely the combined effect of energetics, entropy, and the crystallization kinetics. Naturally, 1b is the least advantageous form, when solely comparing the anhydrous networks, as the water molecules incorporated in this structure participate actively in its overall stabilization. 1a appeared to be the most interesting case due to its unique crystal architecture and the difficulties with determining the location of water molecules captured inside the diboronic acid tubes. Therefore, besides the experimental estimation of the water content, we decided to conduct a series of computations for different hypothetical solvent substructures, with the previous optimization of various structural models. The computational results are presented in Table 1 and in the Supporting Information. In accordance with the residual density analysis and to the aforementioned TGA experiment, the 1a crystal structure should contain one water molecule per each diboronic acid moiety. In such a case, when the global R3̅ symmetry is preserved, in the optimized structure, water molecules form 6membered rings held by hydrogen-bonding interactions. There are no significant interactions with the boronic acid tube walls. The interaction energy of about −36.2 kJ·mol−1 is attributed to every water molecule in the final crystal structure, whereas −223.4 kJ·mol−1 corresponds to the whole asymmetric unit (1a1w-opt). However, when the water oxygen atom position is fixed at the experimentally indicated location (see the Supporting Information), with the hydrogen atoms optimized, no proper hydrogen bond is formed between water molecules due to the long interatomic distances (1a1w-exp). This may suggest that, in reality, water aggregates are locally formed in the interior of the channel. Such water domains may contain higher water content being counterbalanced by the local empty or less occupied spaces (Figure 4, panels a and b). Thus, we

Figure 3. TGA plot showing the weight percentage and number of water molecules per molecule of diboronic acid corresponding to the observed mass loss.

to two simultaneous processes: solvent removal and dehydration of diboronic acid, which result in degradation of the crystal. The differential scanning calorimetry (DSC) analysis shows that the solvent goes off a little bit earlier than the dehydation occurs (see the Supporting Information); however, the stability range is very narrow and, in practice, both processes cannot be separated. A distinctive thermal behavior is observed for 1b. In this case, the solvent starts to be driven off at significantly lower temperature (ca. 60 °C), and dehydration takes place separately when the sample is heated above 95 °C. Therefore, it seems that for the 1b form, it is possible to remove the water content from the structure after heating it to about 80 °C (a mass loss corresponding to two water molecules per one diboronic acid molecule). This finding is a little unexpected, as the water molecules incorporated inside the 1b channels are held by strong O−H···O interactions with the diboronic acid walls. However, the stability range of such a waterless structure is very narrow, and even though the crystal morphology is preserved, the X-ray diffraction experiments showed that the sample loses its crystallinity. This is probably due to the fact that the solvent removal and dehydration processes could not be entirely separated. Finally, computational characterization of the studied compounds in terms of cohesive and molecular motif stabilization energies provides information about the relative crystal enthalpies and also enables, to some extent, modeling of the water content. The cohesive energy was therefore calculated for all the analyzed crystals (Table 1 of the Supporting Information). 4183



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organic frameworks, as such discrete molecules tend to pack in the most efficient manner. Recent works show that several known porous molecular networks are built by units, the socalled “molecular tectons”, where multiple hydrogen-bonded sites are attached to a designed core with the shape intrinsically resistant to close packing.2,4 Interestingly, even though 1 is a rather simple molecule and can form closely packed structures, such as 1b and 1c, the nanotubular hydrogen-bonded organic framework, 1a, architecture additionally occurs. Both, the X-ray diffraction experiment and theoretical computations, suggest that water molecules incorporated in the pores may form stable icelike clusters, which interact weakly with the boronic framework and, thus, are not responsible for the framework templation. Since the boronic group is structurally related to the carboxylic or amide groups, the described herein ice polymeric motifs may represent one of the possible structures found in biological systems. This work shall stimulate further searching for other small systems that can possess hydrogenbonded organic framework architecture.

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Figure 4. Schematic representation of two models for the association of water molecules inside the nanotubular channels: (a) random distribution and (b) domain structure. (c) The space-filling diagram viewed along the Z direction presenting the location of water clusters inside the channels. Water-based polymeric motifs with 3-fold symmetry: (d) Ice Ih (P63/mmc, the disordered hydrogen atoms were removed), (e) Ice II (R3̅),12 and (f) a potential water motif fitted to the 1a boronic acid tube (R3̅). Different colors indicate symmetryindependent water molecules.

ASSOCIATED CONTENT

S Supporting Information *

CCDC numbers: 930704 (1a), 931215 (1b) and 931216 (1c). The synthetic procedure, product characterization, differential scanning calorimetric analysis, details of X-ray crystallographic, TAAM refinement procedure, computational details, and solution multinuclear NMR studies. This material is available free of charge via the Internet at http://pubs.acs.org.

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decided to additionally analyze hypothetical nanotubular structures with various numbers of water molecules falling on one diboronic acid moiety. The most interesting water-based substructure is created when two water molecules per boronic acid fragment are introduced (1a2w-opt). The most advantageous water optimized substructure geometry (with R3̅ symmetry imposed) resembles the Ice II polymeric building blocks illustrated in Figure 4 (panels e and f). The boronic acid tubes could equally well host a polymeric motif extracted from the hexagonal ice structure, which is comparable in diameter to the mentioned Ice II case (Figure 4f). Due to the directionality of hydrogen bonds suitable specifically for the polymeric structure and not for the 3D hydrogen-bonded network, the stabilization energy of water molecules is most favorable for the in-tube 1a2w-opt water substructure. This result emphasizes the low strength of water molecule interactions with the boronic acid tube walls. When the symmetry of the water content is limited to the p1 rod group and the structure is optimized, water molecules form aggregates or some kind of 1D chains; however, they do not significantly interact with the boronic acid moieties. It should be also noted that formation of any water cluster is accompanied with some entropy loss, which should be overcome by the energetic gain. Despite the fact that the nanotubular 1a structure possesses higher solvent-accessible volumes (14.4%) than 1b (7.4%), in the presence of at least 2 equivalents of water molecules, only the 1b crystals appear. In turn, when a solution is waterdeficient, solely other forms of 1 are obtained. This is due to the different character of interactions between the water content and boronic acid framework in the case of 1a and 1b structures (Table 1). However, the mechanism of the crystallization process is still unclear. The number of purely molecular porous systems is significantly lower than that of metal−organic or covalent−

AUTHOR INFORMATION

Corresponding Author

*K.D.: e-mail, [email protected]. K.N.J.: e-mail, katarzyna. [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the grant from the National Science Centre (DEC-2011/01/N/ST5/05592). K.N.J. would like to thank the Foundation for Polish Science for financial support within the START program. Authors thank the Wrocław Centre for Networking and Supercomputing for providing computational facilities. We gratefully acknowledge the Sigma-Aldrich Corporation, Milwaukee, WI, for a long-term collaboration.

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REFERENCES

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Nanotubular Hydrogen-Bonded Organic Framework Architecture of 1

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