Discrete, Hexagonal Boronate Ester-Linked Macrocycles Related to

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Discrete, Hexagonal Boronate Ester-Linked Macrocycles Related to Two-Dimensional Covalent Organic Frameworks Anton D. Chavez, Brian J. Smith, Merry K. Smith, Peter A. Beaucage, Brian H. Northrop, and William R. Dichtel Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.6b01831 • Publication Date (Web): 24 Jun 2016 Downloaded from http://pubs.acs.org on June 25, 2016

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Discrete, Hexagonal Boronate Ester-Linked Macrocycles Related to Two-Dimensional Covalent Organic Frameworks Anton D. Chavez,† Brian J. Smith,† Merry K. Smith,‡ Peter A. Beaucage,§ Brian H. Northrop‡* and William R. Dichtel†* †

Department of Chemistry and Chemical Biology, Cornell University, Ithaca, New York 14853 USA

‡

Department of Chemistry, Wesleyan University, Middletown, Connecticut 06459 USA

§

Department of Materials Science and Engineering, Cornell University, Ithaca, New York 14853 USA

Supporting Information ABSTRACT: The limited processability of two-dimensional covalent organic frameworks (2D COFs), which are typically isolated as insoluble microcrystalline powders, detracts from their utility. Here we apply similar design criteria to access hexagonal boronate ester-linked macrocycles that represent discrete analogues of 2D COFs. The macrocycles readily pack into hexagonal, layered solid-state structures whose spacing is determined by the size of the macrocycle and its side chains. In contrast to 2D COFs, the assembled macrocycles undergo reversible swelling upon exposure to solvents that interact favorably with the side chains. Incorporating hexa(ethylene glycol) monomethyl ether groups leads to dispersible macrocycles that cofacially assemble both in solution and the solid state. This approach expands the directional bonding approach to processable, boronate ester-linked macrocycles with designed size, shape, and functional group organization.

The dynamic integration of small molecules into supramolecular and macromolecular systems is a powerful approach that enables the controlled design of advanced materials. For example, the reversible condensation of boronic acid and catechol derivatives into boronate esters1 provides rationally designed 2D and threedimensional COFs.2-5 Interest in COFs has increased dramatically since their discovery6 in 2005 because their permanent porosity, high internal surface area, precise placement of functionality, and modular design provide desirable properties for gas storage,7-10 optoelectronics,11-13 charge transport,14 and electrical energy storage devices.15 Realizing the full potential of these properties is ham-

pered in part because 2D COFs are typically isolated as insoluble, microcrystalline powders, complicating their incorporation into devices or otherwise controlling their final form. Here we expand the COF design strategy to access large, discrete macrocycles in a single synthetic step.16,17 We also show, for the first time, that the macrocycles assemble into layered structures reminiscent of 2D COFs, yet are dynamic and remain solution processable.18-23 The formation and assembly of these macrocycles also provides important insight into COF formation that will be necessary to improve materials quality.24,25

Figure 1. Comparison of the modular synthesis of periodic 2D COFs (left-facing reaction) and discrete boronate ester macrocycles (right-facing reaction)

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Discrete 2D COF analogues are obtained by truncating polyfunctional nodes to contain only two reactive sites while maintaining the molecular shape and relative orientations of the remaining reactive moieties. For example, hexagonal boronate-ester linked macrocycles are obtained by functionalizing one of the catechol groups of the 2,3,6,7,10,11-hexahydroxytriphenylene (HHTP) monomer (Figure 1). This modification introduces an additional tunable chemical handle, the blocking groups, which modulate the macrocycle’s dispersability, and/or confer additional functions. Nevertheless, few examples of discrete analogues of COFs have been reported,26-29 and the higher-order assembly of such macrocycles in solution or the solid-state has not been characterized. Varying the blocking groups confers additional solubility, and the macrocycles assemble in solution into dispersible nanotubes. These studies complement and extend the chemistry of infinitely periodic COFs by providing new routes to designed molecular assemblies in solution and the solid state. Macrocycles 1, 2, and 3 were prepared by reacting a dihexyloxy tetrahydroxytriphenylene (C6THTP) with 1,4phenylenebis(boronic acid) (PBBA), 4,4′-biphenylbis(boronic acid) (BPBBA), or 2,7-pyrenebis(boronic acid) (PyBBA), under initially homogenous conditions that we recently developed for HHTP-based 2D COFs.24 The macrocycles precipitated from the reaction mixture as nanocrystalline powders (Figure S1) after reaction times of a few minutes. FTIR analysis of 1 revealed bands at 1240, 1077, and 654 cm–1 that correspond to C–O stretching, B–O stretching, and out-of-plane displacements of boron atoms diagnostic of boronate ester formation (Figure S2).30 None of the observed IR bands were attributable to residual unreacted starting materials PBBA or C6THTP. Powder X-ray diffraction (PXRD) analysis (Figure 2A) of the nanocrystalline powder revealed a pattern comparable to that of COF-5 that is consistent with the predicted structure of cofacially stacked macrocycle nanotubes packed in a hexagonal fashion with P6/mmm symmetry. Notably, the peak at 26.4° (001) in the PXRD spectrum of 1 suggests that the macrocycles stack cofacially. No evidence of residual crystalline monomers was observed and digestion of the precipitated material (5 mg/mL in DMSO with 5% H2O) yields the expected 1:1 stoichiometry of C6THTP:PBBA, as characterized by 1H NMR spectroscopy. The hexagonal packing of the macrocycles supports their hexagonal structure. However, MALDI and electrospray mass spectrometry of the macrocycles was unsuccessful, such that we cannot eliminate the possibility that minor amounts of macrocycles of other sizes are also formed. Although most samples of 1 did not appear mesoporous by N2 adsorption, one sample provided a Brunauer–Emmett– Teller surface area (SBET) of ca. 82 m2 g-1 as well as a pore size distribution centered at 25 Å (Figure S3). This observation indiscates that the macrocycles might support permanent porosity, but this property is more sensitive to sample preparation and activation protocols than the correpsonding COFs. Although these macrocycles crystallize into structures reminsicent of COF-5, noncovalent interactions between adjacent stacks provide a more dynamic structure. The macrocycles display a stimuli-responsive change in crystal unit cell upon exposure to the growth solvent (4:1 v/v dioxane:mesitylene, Figure 2B). This unit cell expansion is observed by PXRD as a shift in the peak positions and intensities that correspond with an increase in the intermacrocycle distance as a result of swelling by the solvent. In contrast, soaking COF-5 under identical conditions shows no shift in

Figure 2. (A) Experimental (black) and calculated (red) PXRD patterns of macrocycle 1 for the packing arrangement shown in the inset. (B) Partial PXRD patterns obtained of 1 after soaking in dioxane:mesitylene (4:1 v/v, blue) and dried under vacuum overnight (red). (C) The changes in the PXRD patterns shown in panel B correspond to the expansion of the unit cell associated with solvation and are reversible over several cycles.

crystal spacing or relative intensities of the peaks (Figure S4). Upon drying overnight, the macrocycles return to the more compressed state, as observed through a corresponding peak shift. This process is reversible (Figure 2C), as solvation and desolvation repeatedly interconverts between the two states. Overall the non-covalent nature of the macrocycle packing arrangement gives rise to a dynamic supramolecular system wherein solvent can be used to influence aggregation behavior, suggesting new possibilities for

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Figure 4. Experimental and calculated powder X-ray diffraction of 4 (unit cell length = 60 Å) compared to 1 (unit cell length = 45 Å). Figure 3. (A) Emission spectra (λex = 325 nm) for 1 (0.02 mg/mL in NMP) with increasing amounts of H2O added (B) Hydrolytic digestion of 1 suspensions in 4:1 dioxane:mesitylene (v/v) (1.5 M H2O at t=0) compared to COF-5, monitored by turbidity.

post-synthetic assembly and manipulation that are unavailable to COFs. The design of these systems also allows for the preparation of dispersed macrocycle aggregates. Macrocycle 1 exhibits slight dispersibility in polar aprotic solvents such as NMP and DMF. The soluble species in NMP (0.02 mg/mL) was characterized by fluorescence emission spectroscopy after filtering through a 0.45 μm filter (Figure 3A). The fluorescence of 1 is both red shifted (λmax = 407 nm) and quenched relative to an independently prepared solution of the free monomer C6THTP (λmax = 383 nm), consistent with the cofacial stacking of macrocycles in solution. The macrocycles are hydrolyzed upon addition of H2O to the NMP solution, after which the fluorescence intensifies and blue-shifts to resemble that of the free monomers. Model studies confirmed that this shift is not just associated with boronate ester hydrolysis (Figure S9). These combined observations suggest that aggregation of the discrete boronate ester macrocycles is not limited to the solid state but extends to the solution phase as well. The hydrolytic stability of microcrystalline assemblies of 1 was also compared to COF-5 using turbidity analysis.25 The incorporation of alkyl chains within the pores of COFs has been previously shown to increase hydrolytic stability.31 H2O was added to a rapidly stirring suspension of 1 in dioxane:mesitylene (4:1 v/v) and the dissolution was monitored over time (Figure 3B). Despite its increased reliance on non-covalent interactions, 1 is hydrolyzed to soluble species more slowly than COF-5. The addition of H2O (to obtain an estimated [H2O] of 1.5M) to a suspension of 1 results in 50% dissolution at 14 min, with full dissolution requiring greater than one hour. In contrast, COF-5 undergoes 50% dissolution within 1 min under identical conditions. The increased aqueous stability of 1 likely stems from the increased hydrophobicity of the macrocycle assemblies conferred by the alkyl chains, as compared to the extended boronate ester linked COF-5 network.

In addition to the modularity of macrocycle design, as exemplified by the different boronate esters incorporated into 2 and 3, the side chains are easily varied. To further improve the solvent processability of these macrocycles, we also synthesized a THTP analogue with hexa(ethylene glycol) monomethyl ether chains (HxgTHTP), anticipating that the resulting macrocycle 4 would be more soluble (Figure 1). Indeed, the condensation of HxgTHTP and PBBA does not yield a precipitate until a solid reaction product is isolated by precipitation into hexanes. Wide-angle X-ray scattering (WAXS) and PXRD of the resulting solid suggests a similar structure of cofacially stacked, hexagonally packed macrocycles (Figure 4). The location of the d100 peak at 0.120 Å-1 matches very closely to the d100 at 0.122 Å-1 of a simulated macrocycle packing arrangement (Figure 4), corresponding to a unit cell length of 60 Å. Compared to 1, which exhibits only modest dispersibility in select solvents, the assemblies of 4 are readily dispersed in many common laboratory solvents (e.g., acetone, THF, CHCl3). Fluorescence analysis of 4 shows similar features as 1, in which the emission of dispersed macrocycle assemblies in solution is red-shifted and quenched compared to its free monomers, again suggesting cofacial stacking of macrocycles (Figure S10). H2O addition induces an intensified blue-shifted fluorescence signal corresponding to the hydrolysis of the assembly to monomers. 1H NMR spectra of solutions of 4 exhibit no apparent aromatic resonances, consistent with the formation of aggregates in solution. In contrast, spectra obtained after the addition of D2O, which hydrolyzes the macrocycles, show the expected aromatic resonances of the two monomers in a 1:1 molar ratio (Figure S11). The dispersed aggregates of 4 were amenable to gel permeation chromatography (GPC), which indicated their stability to dilution (Figure S12). The dispersed aggregates of 4 were assigned to a broad peak that eluted between 10-14 minutes. This peak is first observed in chromatograms of the crude reaction mixture after 4h, and it grows in intensity and appears at shorter retention times as the reaction time is increased to 10 days. Several observations make it more likely that this peak corresponds to aggregated macrocycles rather than the formation of a linear polymer. First, aggregates of 4 were isolated by precipitation into hexanes and redispersed in

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Figure 5. GPC traces of an aliquot of the reaction between HxgTHTP and PBBA with various equiv. of TCAT heated at 90 °C for 10 days. either fresh dioxane:mesitylene (4:1 v/v) or THF at room temperature. Sonication at low power (100W, 40 kHz, 100 min) disrupts the aggregates, reducing the intensity of the aggregate peak. However, the peak recovers in intensity and shifts to even higher molecular weight upon standing at room temperature over one week (Figure S13). These mild conditions are unlikely to involve covalent bond rupture or reformation.32 Furthermore, we also monitored aggregate formation from reaction mixtures containing 4tert-butylcatechol (TCAT), which serves as a monofunctional competitor to macrocycle or polymer formation. Reactions containing either 0.1 or 1.0 equiv TCAT exhibited shorter retention times than those in the absence of TCAT (Figures 5 and S14). This observation is inconsistent with a step-growth polymerization which should provide dramatically reduced molecular weights in the presence of large stoichiometric imbalances of reacting functional groups. The observation of larger aggregates in the presence of TCAT may implicate a nucleation/elongation mechanism for macrocycle assembly. Atomic force microscopy (AFM) of a dilute solution of 4 in THF deposited onto freshly cleaved mica indicated the presence of high aspect ratio rods with uniform widths, heights of 2.6 ± 0.1 nm, and lengths ranging from 100–500 nm (Figure 6). These nanotubes are disrupted by sonication, and AFM images obtained from solution just after this stimulus showed no high aspect ratio structures. These assemblies reform spontaneously over time, as shown in the AFM image obtained of a solution 12 h after sonication. Collectively, these experiments demonstrate that the soluble COF-like macrocycles display many of the same stacking tendencies as 2D COFs, yet the resulting assemblies are more dynamic.

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In conclusion we report new boronate ester-linked macrocycles that represent discrete analogues of 2D COFs and form similar layered, hexagonally packed solid-state structures. The macrocycle assemblies are more dynamic than COFs, as they swell reversibly upon exposure to solvent and exhibit solution-phase fluorescence consistent with aggregated macrocycles. The alkyl-functionalized assemblies are also more hydrolytically stable than the corresponding 2D COF because of their hydrophobicity. In addition to retaining the modular design of COF synthesis, the side chain length and identity impart additional chemical properties. Incorporating hexa(ethylene glycol) monomethyl ether chains provides dispersible macrocycles that cofacially assemble both in solution and the solid state. The solution structures are 1D COF nanotubes that are disrupted by sonication but reassemble spontaneously under ambient conditions. This approach expands the directional bonding approach to processable, covalently bonded macrocycles. Future studies of macrocycle formation and assembly will improve mechanistic understanding of the corresponding processes in 2D COF polymerizations. Finally, the directed assembly of COF-like macrocycles offers a potential route towards single-layer 2D polymers with modular design strategies

ASSOCIATED CONTENT Supporting Information Synthetic procedures and additional characterization data. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Authors [email protected] [email protected] Notes The authors declare no competing financial interests.

ACKNOWLEDGMENT This work was funded by the Army Research Office through the Multidisciplinary University Research Initiative (MURI, W911NF-15-10447, to W.R.D.) and Wesleyan University (B.H.N.). A.D.C. was supported through a National Defense Science and Engineering Graduate Fellowship (NDSEG). This work made use of the Cornell Center for Materials Research Shared Facilities, which are supported through the NSF MRSEC program (DMR-1120296). The Cornell High Energy Synchrotron Source (CHESS) is supported by the NSF & NIH/NIGMS via NSF award DMR-1332208.

Figure 6. (A) AFM image of a solution of 1 drop cast onto mica. (B) AFM image of the same solution sonicated for 1 hour before deposition. (C) AFM image of the same solution deposited after standing for 12 hours after sonication.

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