Microporous Diaminotriazine-Decorated Porphyrin-Based Hydrogen

Sep 12, 2016 - Beijing Key Laboratory for Green Catalysis and Separation and Department of Chemistry and Chemical Engineering, College of Environmenta...
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Microporous Diaminotriazine-Decorated Porphyrin-Based HydrogenBonded Organic Framework: Permanent Porosity and Proton Conduction Wei Yang,† Fan Yang,‡ Tong-Liang Hu,† Stephen Charles King,† Hailong Wang,*,† Hui Wu,§ Wei Zhou,§ Jian-Rong Li,‡ Hadi D. Arman,† and Banglin Chen*,† †

Department of Chemistry, University of Texas at San Antonio, San Antonio, Texas 78249-0698, United States Beijing Key Laboratory for Green Catalysis and Separation and Department of Chemistry and Chemical Engineering, College of Environmental and Energy Engineering, Beijing University of Technology, Beijing 100124, P. R. China § NIST Center for Neutron Research, National Institute of Standards & Technology, Gaithersburg, Maryland 20899-6102, United States ‡

S Supporting Information *

ABSTRACT: A diaminotriazine-decorated porphyrin-based microporous hydrogenbonded organic framework has been successfully prepared and characterized using single crystal X-ray diffraction analysis. Its activated phase exhibits permanent porosity, gas separation, and proton conductivity under humid conditions.

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figures of merit for small molecule separation of C2H2/C2H4,13 C2H2/CO2,14 fluorocarbon19 mixtures, and chiral secondary alcohols,15 suggesting a bright future for HOFs as an important category of highly porous functional materials. However, present research on the applications of HOFs mainly focuses on gas adsorption and separation, and there are very few reported examples of HOFs for their diverse applications in catalysis, sensing, or electron conduction.26 Porphyrins are well-known as important advanced molecular materials that fulfill various functions including catalysis and photon/electron-related utilizations.27,28 Recently, successful incorporation of the porphyrin moiety into crystalline porous MOFs and COFs has realized versatile applications, which has in turn stirred a great deal of interest in the chemistry of these porous porphyrin-based frameworks.29−31 With this in mind, we developed a diaminotriazine-decorated porphyrin-based hydrogen-bonded organic framework, HOF-6, assembled from 5,10,15,20-tetrakis(4-(2,4-diaminotriazinyl)phenyl)porphyrin (H2TDPP), Scheme 1. This HOF shows a porous three-dimensional (3D) supramolecular structure with pore sizes of about ∼6.4−7.5 Å2. Activated HOF-6 displays permanent porosity and selective gas adsorption. In addition, because of the inclusion of functional porphyrin and 2,4-

INTRODUCTION Hydrogen bonds, very important interactions in the selfassembly of biological and organic functional materials, can be directional.1,2 They also play important roles in protonassociated biological activities and organic catalysis.3,4 Hydrogen-bonded organic frameworks (HOFs), which are formed through such hydrogen bonding interactions to link discrete organic building blocks, started to attract extensive research interest in the early 1990s,5−13 although the first such framework of Dianin’s compound was established to have permanent porosity back in 1976.10 Numerous attempts at synthesizing and characterizing HOFs have been made, with attention given to structures and pore functional properties.6−14 The establishment of high permanent porosity for HOFs is still quite challenging,12 because the strength of hydrogen bonding interactions is very weak to stabilize the frameworks, compared with the coordination and covalent bonds in metal−organic frameworks (MOFs) and covalent organic frameworks (COFs), respectively. As a result, most structurally porous HOFs collapse after the removal of the guests. Recently, the advantages of HOFs, including their easy synthesis, straightforward processing, and facile recyclability (simply through recrystallization), have motivated renewed interest in engineering porous HOFs. Since 2010, a proliferating number of HOFs with permanent porosity, confirmed on the basis of vapor/gas sorption isotherms, have been reported.13−25 Some were even found to show excellent © XXXX American Chemical Society

Received: June 16, 2016 Revised: August 31, 2016

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asymmetric unit of HOF-6, there is only one-quarter of a H2TDPP molecule. The whole H2TDPP molecule is generated by means of the symmetry operation, in terms of the symmetric center and symmetric plane. There are two kinds of hydrogenbonding interactions between the 2,4-diaminotriazinyl groups (DAT) in this compound, namely, (i) N4−H4B···N7#1 (symmetric code: #1: −x, y, 1 − z) and (ii) N3−H3A··· N5#2 symmetric codes #2: −1/2 − x, 1/2 − y, −z, Figure 1b. These two interactions have similar strengths on the basis of their similar D···A distances [2.994(5) vs 3.001(4)] Å and D− H···A angles [161.7 vs 157.4]. By virtue of the type (i) hydrogen-bonding interaction, a self-assembled planar supramolecular layer is afforded with a very big cavity of about 24 × 26 Å, Figure 1c. This layer is bridged utilizing the type (ii) hydrogen bonds to connect with another two layers, Figures 1d and S3, forming a 3D supramolecular architecture with porosity sizes of ∼6.4 and 7.5 Å along the [101] and [100] direction, respectively. The pore sizes are slightly bigger than that (3.2− 6.7 Å) in a reported porphyrin-based structure made up of ZnTDPP ligand,33 but smaller than that (6.0−13.0 Å) in the structure comprised of the mixture of H2TDPP and ZnTDPP ligands.34 Ignoring the interpenetration, the isolated network of HOF-6 can be simplified as a (4,4)-connected bcu-x-4-P3212 topology by considering both the porphyrin core of H2TDPP and the type (i) hydrogen-bonding moiety as 4-connected nodes (Figure S5). The final structure of HOF-6 is composed of two interpenetrated 3D supramolecular structures. Moreover, the NMR spectrum of as-synthesized HOF-6 in DMSOd6 solution reveals the presence of DMF signals (7.94, 2.88, and 2.72) and THF proton signals (3.58 and 1.74) (Figure S2). The integrations of these protons, in combination with the 43.9% weight loss in the TGA curve for solvent release, disclose that there are six DMF molecules and five THF molecules per H2TDPP linker filled in the HOF-6 cavities to stabilize the porous structure. This is also very consistent with the elemental analysis result. As a result, the molecular formula is identified as H2TDPP·(DMF)6·(THF)5. The solvent-accessible volume of

Scheme 1. Schematic Molecular Structure of the HOF-6 Building Block

diaminotriazinyl (DAT) motifs as dangling group, the hydrated HOF-6a manifests a moderate proton conductivity of 3.4 × 10−6 S cm−1 at 300 K (∼97% RH).

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RESULTS AND DISCUSSION Synthesis of HOF-6 (H2TDPP·(DMF)6·(THF)5). The building block H2TDPP and its precursor of 5,10,15,20tetrakistetra(4-cyanophenyl)porphyrin have been synthesized according to previously reported methods.32−34 Slow diffusion of THF vapor into the DMF solution of H2TDPP leads to the formation of HOF-6. Single crystal X-ray diffraction reveals that HOF-6 is a 3D hydrogen-bonded supramolecular framework. In addition, the sample of HOF-6 has been also characterized with NMR spectroscopy (Figures S1 and S2), thermogravimetric analysis (TGA), and powder X-ray diffraction (XRD). The powder XRD analysis confirms the phase purity for the bulk material, Figures S3 and S4. The TGA curve shows that the solvent guests in HOF-6 are gradually lost in the temperature range from 25 to 215 °C (Figure S3), affording the desolvated framework. The material is decomposed in the region of 385−580 °C. Crystal Structure of HOF-6. This new HOF crystallizes in a monoclinic system and belongs to space group C2/m. In the

Figure 1. X-ray crystal structure of HOF-6 indicating (a) building block of H2TDDP, (b) hydrogen-bonding interaction between DAT moieties (core protons omitted for clarity), (c) three neighboring 2D supramolecular grids in different colors, (d and e) 3D packing supramolecular structure along the [1,0,1] and [1,0,0] directions with a channel size of ∼6.4 and 7.5 Å, respectively. B

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the hydrogen-bonded network is calculated as 3343.2 Å3, corresponding to the 63.4% solvent-accessible space. Gas Sorption and Gas Selectivities. For the present HOF, all DAT moieties are used to form the hydrogen-bonded supported framework, which, in cooperation with the structural interpenetration, helps to provide a stable porous HOF. Driven by this interesting porous HOF architecture, we evaluated the permanent porosity and also the gas separation potential. Before the gas sorption measurement, the activation of acetoneexchanged crystals in a high vacuum provided an activated sample (HOF-6a) at room temperature. The crystalline nature of HOF-6a was demonstrated with a powder XRD investigation (Figure S6). However, the elimination of solvent guests from the pores gave rise to the activated HOF-6a whose phase is different from that of the originally synthesized species. The CO2 gas sorption isotherms at 196 K clearly confirm the permanent porosity of HOF-6a (Figure 2). Moreover, two Figure 3. (a) CO2 (red and green) and N2 (blue and black) sorption isotherms of HOF-6a at 273 and 296 K (solid symbols: adsorption, open symbols: desorption). Mixture adsorption selectivities (b) and isotherms (c and d) predicted by IAST of HOF-6a for CO2/N2 (15%:85%) at 273 and 296 K, respectively.

the simulated selectivities of this HOF material for CO2 over N2 were also studied. Though some HOFs have been confirmed to exhibit permanent porosity, functional HOFs for gas separation have been rarely investigated.7 The ideal adsorbed solution theory (IAST) calculation is a well-known way to predict selectivity for binary gas mixture separation in zeolites, MOFs, and porous polymers. Mixture adsorption selectivities and isotherms of our activated HOF under different pressures at 273 K were evaluated by the IAST method for mixed CO2/N2 (CO2/N2 = 15:85) as a function of total bulk pressure, Figure 3b−d. The IAST selectivity of HOF-6a for CO2/N2 is 21.9 at 273 K at 1 atm, confirming the gas separation potential for this HOF material, which is comparable to ZnTDPP HOF (∼35),33 Ni-MOF-74 (∼30),37 and a py-CF3 modified MOF (25−45)38 under similar conditions. Proton Conductivity. Compared with the well-established MOF and COF materials,39−43 functional HOFs have been much less explored. Encouraged by the discovery of high proton conductivity for MOF nanofilm by introducing metalfree porphyrins as effective proton donors,44 we speculated that the porphyrin core protons and DAT groups in HOF-6 might be able to serve as Lewis acids to effectively provide protons as well and thus afford special proton-conducting pathways. In addition, those porous organic frameworks including porous organic polymers and COFs exhibiting high proton conductivity also inspire us to develop new proton conducting materials.45,46 The proton conductivity behavior of the solid samples was examined utilizing AC impedance spectroscopy. The proton conduction measurement was carried out on a compacted pellet of HOF-6a. As shown in the Figure 4, the Nyquist plots exhibit circular arcs at high frequency regions. Determined from the semicircle in Nyquist plots, the proton conductivity of hydrated HOF-6a at 27 °C was calculated to be 3.4 × 10−6 S cm−1 (97% RH). Notably, the PXRD pattern of HOF-6a after exposing in water vapor reveals that the treated sample still is crystalline (Figure S6). This indicates that HOF6a is still stable in the proton conduction measurement. This proton conductivity value is comparable to those reported in an inorganic polyoxometalate-based nanotube (4.4 × 10−6 S cm−1, 298 K, 97% RH)47 and an aluminophosphate Ag-JU103 (3.1 ×

Figure 2. CO2 sorption isotherms of HOF-6a at 196 K (solid symbols: adsorption, open symbols: desorption).

consecutive adsorption steps observed in the CO2 isotherm of this HOF material indicate the framework flexibility. The Brunauer−Emmett−Teller (BET) surface area of HOF-6a was calculated to be 130.0 m2 g−1, which is comparable to previously reported two-dimensional (2D) layered microporous porphyrin-based ZnTDPP HOF and MOFs.33,35,36 Inspired by the above results that HOF-6a is one of only a few HOFs with structural small pores and permanent porosity, we performed further measurements of the gas sorption isotherms to assess the gas storage and separation capacity of this HOF. The CO2 and N2 sorption measurements indicate a type I isotherm, as shown in Figure 3a. HOF-6a takes up carbon dioxide in moderate amounts of 23.2 and 12.0 cm3 g−1 at 273 and 296 K, respectively, and 1 atm. This is also similar to the reported ZnTDPP HOF.33 Much lower amounts of nitrogen for HOF6a (2.1 and 1.0 cm3 g−1) are taken up at 273 and 296 K, respectively, and 1 atm. The different CO2 and N2 uptakes in the gas sorption isotherms of HOF-6a hint at its potential in gas separation. On the basis of the temperature-dependent gas sorption isotherms, the coverage-dependent enthalpies of adsorption of CO2 and N2 on HOF-6a have been calculated based on the Virial method. The fitted enthalpies on HOF-6a at zero coverage are 21.5 and 14.2 kJ/mol for CO2 and N2 adsorption, respectively. The calculated Qst value of CO2 adsorption for the HOF-6a is consistent with those of some other reported HOFs.12,16 This indicates the presence of a strong interaction between the CO2 substrates and the framework. Aside from the investigation of the gas uptakes and sorption enthalpies based on single component gas sorption isotherms, C

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Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the reviewers for their constructive comments which have helped us to improve the quality of the work. This work was supported by the NSF Award DMR-1606826 and Welch Foundation Grant AX-1730 (B.C.).

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Figure 4. Nyquist plots of HOF-6a at 27 °C (black), 30 °C (red), 35 °C (green), and 40 °C (blue) in a relative humidity (RH) of ∼97%.

10−6 S cm−1, 293 K, 98% RH)48 as well as organic−inorganic hybrid materials such as Ca-SBBA MOF (8.6 × 10−6 S cm−1, 298 K, 98% RH),49 a tetracarboxylic acid−based Tb(L) MOF (1.0 × 10−7 S cm−1, 298 K, 97% RH)50 and UiO-66 (7.6 × 10−6 S cm−1, 303 K, 97% RH),51 indicating that HOFs hold promise as proton conduction materials. In order to obtain further insight about the proton-transport mechanism, measurements were performed at different temperatures. Following an increase in temperature, the proton conductivity is decreased to 3.2 × 10−6 S cm−1 at 30 °C, 2.4 × 10−6 S cm−1 at 35 °C, and 1.9 × 10−6 S cm−1 at 40 °C with a relative humidity of 97%. The exact reason for the decrease of proton conductivity with temperature is still not clear.

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CONCLUSIONS In summary, a multifunctional hydrogen-bonded organic framework HOF-6 has been successfully assembled from a metal-free porphyrin building block. The activated HOF sample exhibits permanent porosity and a moderately high selectivity for CO2/N2 gas separation. Furthermore, the hydrated sample displays moderately high proton conductivity. Given the fact that research on HOFs is still in its infancy, it is expected that the present results will help to arouse more extensive interest in exploiting the functionalities and unique characteristics of these new types of microporous materials.

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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.6b00924. Experimental details, TGA curve of HOF-6, powder Xray diffraction patterns of HOF-6 and HOF-6a, topology of HOF-6 (PDF) Accession Codes

CCDC 1044788 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 data_ [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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REFERENCES

AUTHOR INFORMATION

Corresponding Authors

*(H.W.) E-mail: [email protected]. *(B.C.) E-mail: [email protected]. D

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