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Hydrogen bonded 2-fold interpenetrated diamondoid networks for the solid state [2+2] polymerizations of crisscrossed olefins: Molecular connections vs Supramolecular connections Rajorshi Mandal, Mousumi Garai, and Kumar Biradha Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.7b01114 • Publication Date (Web): 11 Sep 2017 Downloaded from http://pubs.acs.org on September 11, 2017
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Hydrogen bonded 2-fold interpenetrated diamondoid networks for the solid state [2+2] polymerizations of crisscrossed olefins: Molecular connections vs Supramolecular connections Rajorshi Mandal, Mousumi Garai and Kumar Biradha* Department of Chemistry, Indian Institute of Technology, Kharagpur-721302, India. KEYWORDS: Criss-crossed olefins, [2+2] photopolymerization, organic polymer. ABSTRACT: The molecular isomers of a diene containing amide and pyridine moieties revealed that they exhibit different crystal structures and reactivities. One of the isomer forms 2-fold interpenetrated diamonded network via N-H···N hydrogen bonds and undergoes [2+2] photo-polymerization in the solid state despite of having unfavorable criss-cross aligned double bonds.
The importance of organic polymers is ever growing given their light weight compared to the ceramic based polymers. In general, polyamides are an essential class of organic materials that possess excellent physical properties such as high thermal and mechanical stability, chemical resistance and flexibility.1-9 Development of ecofriendly and biodegradable polymers is indispensable for sustainable society.10-13 The solid state [2+2] photopolymerization offers solvent free and high yield synthesis of polymers from the appropriate monomers containing multiple olefin moieties. The polymerization of such molecules heavily depends on the arrangement of monomers in the crystal lattice. The recent surge in the literature of solid state pericyclic reactions such as [2+2], [4+4], [4+2] and [1,4] for the creation of organic polymers with potential applications owes to the advancements in the understanding of molecular packing forces.14-24 Recently, we have shown how the N-H···O hydrogen bonded 2D-layers, that are formed by bis-pyridine-acryl amido alkanes, template single-crystal-to-single-crystal polymerization in the crystal lattice to produce crystalline organic polymers.25 Further, the design of coordination polymers of the monomers has also been shown as one of the methodology to template such [2+2] polymerization reactions.26-30 The organic polymers synthesized by us using these methodologies contain amide and cyclobutane moieties in the main chain and pyridine moieties as pendants and have shown excellent ability to form plastic sheets in their salt form (HCOOH, HCl).25 However, all the previous studies concentrate on molecules containing aliphatic spacers or flexible p-xylyl spacers that exhibit linear geometries.25,30,31 The bis-pyridylacrylamido derivatives containing rigid aromatic spacers have not been explored to date given their low solubility. Here, we would like to present photochemical reactivity of two molecular isomers 1 and 2 that have the similar shape and functional groups but differ in their covalent connectivity. Both the
molecules have rigid phenyl spacer that is connected in (1,3) angular fashion (Scheme 1). Given the covalent similarities of 1 and 2, it is interesting to explore how similar or different their crystal structures. Our studies reveal that both the molecules form entirely different structures and therefore exhibit different reactivity. Remarkably, one of these materials exhibited solid-state [2+2] polymerization between criss-cross aligned double bonds which is a first example of its kind to the best of our knowledge. Scheme 1. Chemical diagrams of 1 and 2.
The compound 1 was synthesized by the condensation reaction of m-phenylenediamine and (E)-3-(pyridin-3-yl)acrylic acid. The compound 2 was synthesized by the reaction of 3-aminopyridine with m-phenylenediacrylic acid, that is in turn prepared by the reaction of isophthaldehyde and malonic acid. The single crystals of 1 and 2 suitable for single crystal X-ray diffraction were obtained from MeOH-DMF (2:1) and MeOH-CH3CN (1:1) respectively. The crystal structure analyses reveals that 1 and 2 are not iso-structural and crystallize in P21/n and Pnn2 space groups respectively. The asymmetric units of 1 and 2 are constituted by one full molecule and half molecules respectively. In case of 2, the asymmetric unit also includes half molecule of disordered MeOH. Although both molecules exhibit planarity, the molecular geometries
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found to be very different: 1 exhibits angular geometry while 2 exhibits linear geometry (Figure 1a & 2a). In 1, the molecules assemble through the combination of N-H···O and N-H···N hydrogen bonds to form highly corrugated wavy 2D-layers. The N-H···O (H···O: 2.048 Å, N···O: 2.896 Å, N-H···O: 168°) hydrogen bonds found to be somewhat shorter than the N-H···N (H···N: 2.268 Å, N···N: 3.127 Å, N-H···N: 176°) hydrogen bonds (Figure 1b). The double bonds of adjacent molecules found to exhibit larger separation distance of 4.957 Å preventing the solid state [2+2] reaction. Therefore, the crystals of 1 were found to be photostable even after prolonged irradiation. Whereas in 2, the assembling of molecules occurs exclusively through N-H···N (H···N: 2.121 Å; N···N: 2.977 Å, N-H···N 174°) hydrogen bonds. Further analysis of the crystal structure reveals that such hydrogen bonding leads to the formation of 3D-network as each molecule hydrogen bonds with four of its neighbours in near tetrahedral geometry (Figure 2b) with four of its neighbours in near tetrahedral geometry (Figure 2b).
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(a)
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4.957 Å
Figure 1. Illustrations for the crystal structure 1: (a) angular geometry of 1; (b) 2D corrugated wavy layer via N-H···O and N-H···N hydrogen bonds.
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Figure 2. Illustrations for the crystal structure of 2: (a) the molecular geometry of 2, notice the plane of symmetry; (b) tetrahedral node via N-H···N hydrogen bonds; (c) portion of the diamondoid network; (d) 2-fold interpenetrated diamondoid networks through amide···π interactions, notice the proximity of double bonds for [2+2] polymerization; (e) the criss-cross alignment (circled) of double bonds.
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The reduction of the molecules as nodes and hydrogen bonds as node connections illustrates that the crystal lattice is constituted by doubly interpenetrated diamondoid networks with each arm length of 10.2 Å (Figure 2c). It is interesting to note here that the molecules containing bis-amido pyridine groups separated by aliphatic spacer – (CH2)5- was shown to form 4-fold interpenetrated diamondoid networks in the similar fashion.32 In 2, the interpenetration of the networks occurs through the amide···π (3.792 Å) interactions that brings the double bonds together with criss-cross alignment (3.442 Å) (Figure 2d & 2e). Such non-parallel or near perpendicular alignment of double bonds was usually known as highly unfavourable for [2+2] reaction, however in few cases they were shown to react via pedal motion to lead to discrete dimers containing cyclobutane moieties.33-35 Hitherto, [2+2] photopolymerization reactions between the criss-crossed olefins have not been reported, the crystal structure of 2 provided such unique opportunity. Accordingly, the crystalline material of 2 was irradiated under UV-light and the reaction progress was monitored by recording 1H-NMR in DMSO-D6 at various time intervals. As the reaction progresses, gradual appearance of cyclobutane protons at δ= 3.965 and 4.524 ppm and shifting of amide proton from 10.49 to 10.14 ppm were observed. It also shows shifting of pyridine protons from 8.84, 8.29, 8.17 and 7.69 ppm to 8.67, 8.22, 8.01 and 7.30 ppm. The central phenyl protons were found to exhibit an up-field shift from 7.88, 7.55 and 7.39 to 6.90, 6.73 and 6.65 ppm. Upon 40 hours of irradiation the overall yield of the reaction is found to be 73% after which the progress of the reaction found to be very sluggish (Figure S7).
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monomers of 2 and lower oligomers (if any) from the insoluble portion that is expected to be a higher oligomer. From the 1H-NMR studies, it was found that the soluble portion indeed contains 1:1 mixture of monomer 2 and oligomer (2LO) whereas the insoluble portion contains only polymer (2HO) (Figure 3). The purity of polymer 2HO is also confirmed by 13C-NMR spectra, which shows a peak at 170.81 ppm corresponding to carbonyl carbon (164.09 ppm in 2) and new peaks at 42.14 and 46.58 ppm for cyclobutane ring (Figure S6). The molecular weights of 2HO and 2LO were determined by MALDI-TOF analysis with 2,5-dihydroxy benzoic acid (DHB) as a matrix. The highest molecular peak for 2HO was found to be 4417 (i.e., 12-mer units) (Figure 4) whereas for 2LO it is found to be 2224 indicating hexamer (Figure S15).
Figure 4. MALDI-TOF mass spectra for 2HO.
The DRS measurement for 2 and 2HO indicates that they exhibit absorption edges at 415 and 389 nm respectively. Further, they exhibit solid state fluorescence emission maximum at 422 and 515 nm respectively with the excitation wavelength of 350 nm. The emission of 2HO is red shifted compared to 2 due to the fact that 2HO lacks olefin moieties in conjugation with pyridyl/phenylene and also due to higher aggregation (J-type) of the polymers (Figure 5). The TGA analyses of 2 and 2HO show that they are stable up to 489°C and 763°C respectively, indicating higher thermal stability of polyamide 2HO (Figure S21). Further, we note here that m-phenylenediacrylic acid was shown earlier to form dimer and hexamer from the usual parallel alignment of double bonds.36
1
Figure 3. H NMR spectra in DMSO-D6: (a) as crystallized material of 2; (b) MeOH soluble portion (2+ 2LO); (c) MeOH insoluble portion (2HO).
The light yellow colored crystals of 2 are turned to dark yellow amorphous material (Figure S18), the major portion of which is found to be insoluble in common organic solvents such as MeOH, EtOH, MeCN, DMF and DMA, but found to be soluble in DMSO upon heating. The irradiated material washed with hot MeOH to separate the
Figure 5. Solid state fluorescence spectrum for 2 and 2HO.
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In conclusion, the dissimilarities of molecular geometries of isomers resulted in the markedly different supramolecular aggregations and solid state reactivity. Although very few reports exist in the literature about solidstate [2+2] polymerization of pure organic solids, 37-39 there is no report that illustrates the purely strong hydrogen bonded 3D-network solids. Further, the compound 2 illustrates first example of [2+2] polymerization of crisscross aligned double bonds. We note here that the 2-fold interpenetration of diamondoid networks facilitated very short separation of double bonds (3.442 Å) which in turn facilitated the [2+2] polymerization through pedal motion. Also the reaction was shown to form two types of oligomers: hexamers and do-decamers which are separated based on their solubility and characterized by 1HNMR and MALDI-TOF.
ASSOCIATED CONTENT Supporting Information. Experimental details, NMR spectra, FTIR spectra, PXRD patterns, MALDI-TOF data, photo-physical study, TGA analysis and other experimental data and calculations on conformations of 1.
AUTHOR INFORMATION Corresponding Author * E-mail:
[email protected]. Fax: +91-3222282252. Tel: +91-3222-283346.
ORCID Kumar Biradha: 0000-0001-5464-1952
Notes The authors declare no competing financial interest.
ACKNOWLEDGMENT We gratefully acknowledge financial support from SERB (DST) and DST-FIST for single crystal X-ray facility. RM thanks DST-INSPIRE for research fellowship
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Hydrogen bonded 2-fold interpenetrated diamondoid networks for the solid state [2+2] polymerizations of crisscrossed olefins: Molecular connections vs Supramolecular connections Rajorshi Mandal, Mousumi Garai and Kumar Biradha*
Two isomers containing diene and amides were found to exhibit different structural geometries, crystal structures and reactivity. One of the isomer self-assembles to form 2-fold interpenetrated diamondoid network via N-H···N hydrogen bonds and undergoes [2+2] photopolymerization reaction despite of having unfavorable criss-cross aligned double bonds.
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