Mechanochemical Synthesis of Amide Functionalized Porous Organic

Communication pubs.acs.org/crystal

Mechanochemical Synthesis of Amide Functionalized Porous Organic Polymers Lalit Rajput*,† and Rahul Banerjee*,‡ †

Solid State and Structural Chemistry Unit, Indian Institute of Science, Bangalore 560 012, India Physical/Materials Chemistry Division, CSIR-National Chemical Laboratory, Dr Homi Bhabha Road, Pune 411 008, India

‡

S Supporting Information *

ABSTRACT: Two porous organic polymers decorated with the amide functionality were synthesized mechanochemically and their properties were compared with the ones prepared by conventional solution mediated method. All the POPs were subjected to gas and water vapor sorption studies. The mechanochemically synthesized POPs have less surface area and show moderate adsorption properties compared to the solution mediated POPs. The amide based POPs show remarkable stability in water and concentrated acids.

P

Scheme 1. Mechanochemical Synthesis of Amide Based Porous Organic Polymers (POPs)

orous organic polymers (POPs) are an important class of amorphous porous materials because of their applications in gas storage, separation, and catalysis.1−3 POPs possess high chemical and thermal stabilities as they are constructed exclusively by covalent bonds. The porosity of the POPs can be easily tuned by increasing or decreasing the length of the monomer. Several organic reactions were adopted for the synthesis of POPs such as Sonogashira-Hagihara, Yamamoto, and Suzuki coupling, olefin metathesis, and cyclotrimerization.4 Most of these reactions involve drastic reaction conditions such as high temperature and pressure, aromatic solvents, and inert atmosphere. Recently, in order to avoid these drastic reaction conditions, researchers started using mechanochemical (MC) synthesis of MOFs and COFs as a good alternative to conventional solution-based synthesis due to its short, simple, economical, and environment friendly synthetic route.5 In mechanochemical synthesis, generally chemical reactions which require high temperature are activated by mechanical grinding.6 However, it is quite surprising to note that very few reactions were attempted for the mechanochemical synthesis of POPs.7 Herein, we demonstrate a rapid, solvent free, room temperature mechanochemical synthesis of two secondary amide linkage based POPs (Scheme 1). The simple route for the synthesis of amide is the coupling of acid chloride with the amine since the other precursors such as acid, ester involve solution mediated heat, catalyst, and/or strong base. The acid chloride−amine reaction is exothermic and generally carried out at low temperature in the presence of triethylamine (TEA) to capture the generated side product HCl, in situ. Among several microporous organic polymers, functional POPs, with carboxy (−COO), carbonyl (CO), hydroxyl (−OH), and amine (−NH2) functionality are of interest as they involve © 2014 American Chemical Society

polar sites which can strongly interact with the guest molecules via noncovalent interactions, especially hydrogen bonding.8 The secondary amide functionality is an ideal candidate for such study as it creates both a strong hydrogen bond donor as well as acceptor sites.9 Further, due to the flexible nature of the amide linkage, not only can it produce the dynamic frameworks but it can also significantly enhance CO2 uptake in organic or inorganic frameworks.10 Received: March 31, 2014 Revised: May 8, 2014 Published: May 9, 2014 2729

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Figure 1. Comparison of the FT-IR spectra of POPs with their starting materials (a) TMCPD and (b) TMCBD. SEM images of polyamide POP (c) TMCPD[MC], (d) TMCPD[SM], (e) TMCBD[MC], and (f) TMCBD[SM]. SEM images are shown on 200 nm scale.

mediated synthetic counterparts. The absorption bands in between 1656 and 1661 cm−1 correspond to the amide CO stretching frequency, also known as amide-I band. The band observed in the range 1505−1516 cm−1 corresponds to NH bending vibration, called amide-II band, whereas the bands in between 1240 and 1250 cm−1 indicate the interaction between N−H bending and C−N stretching. However, the bands corresponding to the N−H stretching frequency between 3300 and 3500 cm−1 merged with the stretching frequency of −OH bands due to the moisture included in the pores. No acid chloride and amine bands corresponding to the starting compounds appear, demonstrating the complete transformation of starting material to polyamide POPs. It is observed that for all the POPs, the amide carbonyl frequency is shifted toward lower wavenumber which signifies that the amide groups are involved in hydrogen bonding within the framework with either each other or solvent molecules. 13 C cross-polarization magic-angle-spinning (CP-MAS) solid state NMR was collected for further characterization of the POPs. The spectra obtained for the mechanochemically synthesized POPs were compared with those of the conventionally synthesized POPs and the corresponding reference monomers M1 and M2 (Figure 2). The exact match of the solid-state NMR profiles indicates that the POPs obtained mechanochemically as well as solution mediated have the same local framework. All the POPs as well as the monomers display a signal at 168 ppm which corresponds to the amide carbonyl. The overlapping signals between 122 and 138 ppm correspond to the aromatic carbons of phenyl moieties of both the POPs. Scanning electron microscopy (SEM) images showed that the polymer particle size varies according to the reaction condition (Figure 1c−f). The polyamide POPs are spherical in

We have attempted the synthesis of two POPs, namely, TMCPD and TMCBD mechanochemically, in the presence of the required amount of triethyl amine (TEA). These POPs were prepared by coupling 1,3,5-benzenetricarbonyl chloride (TMC) with p-phenylenediamine (PD) or benzidine (BD), respectively. For comparison, we also synthesized both of these POPs by conventional solution based synthesis. The POPs reported in this paper were synthesized by mechanochemical grinding (MC) and solution mediated method (SM) at 0−5 °C. A total of four polyamide POPs TMCPD[MC], TMCBD[MC], TMCPD[SM], and TMCBD[SM] were synthesized which exhibit moderate sorption behavior with almost identical thermal and chemical stability. In a typical mechanochemical synthesis, TMC (0.2 g, 0.75 mmol) and either PD (0.1303 g, 1.21 mmol) or BZD (0.2227 g, 1.21 mmol) were mixed in a mortar and ground manually using a pestle in the presence of TEA. Within 5 min, the color of the mixture changed from brown to pale yellow for TMCPD[MC] and from dark brown to light brown for TMCBD[MC]. The manual grinding was continued for a further 40 min. The material was then thoroughly washed with MeOH, water, and acetone to remove hydrochloride salt and organic oligomeric impurities. The POPs were obtained in the form of fine powders having olive color. Both POPs are insoluble in water and common organic solvents, suggesting the polymeric nature of the compounds. Monomers of both POPs were prepared for the sake of comparison by coupling TMC with aniline (M1 for TMCPD[MC]) and 4-aminobiphenyl (M2 for TMCBD[MC]). All the materials were characterized using FT-IR spectroscopy (Figure 1a,b). Both POPs synthesized mechanochemically showed FT-IR spectra similar to those of their solution 2730

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TMCBD[MC]. This lower surface area might be due to the random orientation of the pores in the polymers. However, the POPs synthesized conventionally have comparatively higher surface area of 49 m2 g−1 for TMCPD[SM] and 40 m2 g−1 for TMCBD[SM]. All the N2 sorption isotherms rise above P/P0 = 0.9 suggesting the presence of mesopores in the network.11 Further, the narrow hysteresis indicates the presence of microand mesopores in the framework. The pore size distribution plots of the POPs show peak maxima in the range of 7−10 Å suggesting the microporous nature of the materials. The CO2 uptake of TMCPD[MC] and TMCPD[SM] are 14 and 18 cc g−1 at 273 K, respectively, whereas for TMCBD[MC] and TMCBD[SM] the value is 16 and 21 cc g−1. We collected the water vapor adsorption isotherms for all the POPs and found that at 293 K TMCPD[MC] and TMCPD[SM] take up ∼14% water vapor, whereas for TMCBD[MC] and TMCBD[SM] the value is ∼10% at P/Po = 0.9. Atomistic simulations were used to build the representative fragments for both the polymeric networks (Figure 4). They

Figure 2. Comparison of the 13C CP-MAS solid state NMR spectra of polyamide POPs (a) TMCPD and (b) TMCBD with their reference monomers. * indicates peaks arising from spinning side bands.

shape and agglomerated. The solution mediated POPs have bigger and more regular size and shape compared to those synthesized mechanochemically. As expected, the average particle size of the mechanochemically synthesized POPs is less (25−30 nm) compared to the solution mediated POPs (100−150 nm). The reason behind the better particle size for solution mediated POPs could be due to the slow and controlled reaction rate. The X-ray powder diffraction (XRPD) patterns revealed the amorphous nature of the materials (Figure S1). Thermogravimetric analysis (TGA) shows that the mechanochemically synthesized POPs show almost identical thermal behavior as their solution mediated synthesized counterparts (Figure S2). All the POPs are stable up to 500 °C confirming the polymeric nature of the compounds. The initial weight loss [∼8%] in all of them in TGA corresponds to the loss of trapped solvent as well as the moisture in the pores. The framework decomposition occurs above 500 °C with a gradual weight loss of 50−60% for all the POPs. To test the porosity of the POPs, N2 adsorption isotherms were collected at 77 K (Figure 3). The materials are microporous in nature and show type-III absorption isotherm. The POPs synthesized mechanochemically have lower surface area of 14 m2 g−1 for TMCPD[MC] and 18 m2 g−1 for

Figure 4. Optimized geometry of the basic structure directing models using Dmol3 and the expected three-dimensional polymeric structure of (a) TMCPD and (b) TMCBD.

adopt 3D porous architecture. The possibility of the 3D framework of the POPs can be attributed to the flexibility provided by the amide functionality which allows the bending and rotation of the struts out of the plane of the central benzene ring (three connected). Due to the longer struts there is significantly more catenation and entanglement of fragments in the actual samples, which might be another reason for the lower surface area of both the POPs. It should be noted that the models represent fragments of the networks and not the pore structure itself. To investigate the stability of the POPs, we submerged the materials in water for a month in a vial. FT-IR shows no change in the amide frequencies of the POPs (Figure S7). The high stability of the mechanochemically synthesized POPs in water motivated us to check their stability in acids. To our surprise the polyamide POPs show remarkable stability in concentrated acids (9 N). We monitored the acid stability of mechanochemically synthesized POPs in 9 N HCl and 9 N H2SO4 for 8 days, showing no change in the amide IR bands (Figure S8, S9, and S10). Further, the stability of the POPs was tested by N2 adsorption on the acid treated samples which shows not much

Figure 3. Sorption studied on POPs: (a) N2 adsorption isotherms at 77 K, (b) CO2 uptake at 273 K, and (c) water adsorption isotherms at P/Po = 0.9 at 293 K. (d) Photographs of the POPs. 2731

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change compared to the as synthesized materials (Figure S11). However, the POPs are not stable in base and the amide linkage gets hydrolyzed into the corresponding acid and amine. In summary, we have demonstrated for the first time that the POP materials with flexible amide linkage can be synthesized mechanochemically. Simple room temperature amide coupling is employed for the synthesis of two stable POPs. The studies also reveal that the physical properties such as size, shape, and physisorption of these materials are the function of the synthetic pathway. Although the surface area and sorption properties of these mechanochemically synthesized POPs are moderate, we believe that our strategy will encourage the new green methods for the synthesis of POPs in the near future. Further efforts toward the improvement of surface area and the mechanochemical synthesis of new functional POPs are currently underway in our laboratory.

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ASSOCIATED CONTENT

S Supporting Information *

Experimental procedures, complete sorption data, XRPD, TGA, FT-IR, SEM images and single crystal data of M2. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]; Tel: +91 80 22933311. *E-mail: [email protected]; Tel: +91 2025902535. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS L.R. thanks the DST for a Young Scientist Fellowship and Prof. Gautam R. Desiraju (IISc, Bangalore) for providing the laboratory facilities. R.B. thanks CSIR (CSC0122 and CSS0102) for funding. The authors are thankful to Arijit Mukherjee and Srinu Tothadi for the help to carry out the atomistic simulations.

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REFERENCES

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