Authors: Shuli Chen | Yan Mi | Fei-Long Hu | David James Young | Jian‐Ping Lang
Source: CCS Chemistry (article, 2022-05-26)
Field: Chemistry — Inorganic Chemistry

Pore-Directed Solid-State Selective Photoinduced <i>E</i> – <i>Z</i> Isomerization and Dimerization within Metal–Organic Frameworks Open AccessCCS ChemistryRESEARCH ARTICLES21 Jun 2022Pore-Directed Solid-State Selective Photoinduced E–Z Isomerization and Dimerization within Metal–Organic Frameworks Shu-Li Chen†, Yan Mi†, Fei-Long Hu, David James Young and Jianping Lang Shu-Li Chen† Key Laboratory of Chemistry and Engineering of Forest Products, State Ethnic Affairs Commission, Guangxi Minzu University, Nanning 530006, Guangxi College of Chemistry, Chemical Engineering and Materials Science, Soochow University, Suzhou 215123, Jiangsu , Yan Mi† Key Laboratory of Chemistry and Engineering of Forest Products, State Ethnic Affairs Commission, Guangxi Minzu University, Nanning 530006, Guangxi , Fei-Long Hu *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] Key Laboratory of Chemistry and Engineering of Forest Products, State Ethnic Affairs Commission, Guangxi Minzu University, Nanning 530006, Guangxi , David James Young College of Engineering, IT and Environment, Charles Darwin University, Darwin 0909, Northern Territory and Jianping Lang *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] College of Chemistry, Chemical Engineering and Materials Science, Soochow University, Suzhou 215123, Jiangsu https://doi.org/10.31635/ccschem.022.202202115 SectionsSupplemental MaterialAboutAbstractPDF ToolsAdd to favoritesDownload CitationsTrack Citations ShareFacebookTwitterLinked InEmail Design and construction of suitable pore microenvironments for selective catalytic reactions of small guest molecules is a major goal for chemists. Herein, we report control of competitive E–Z photoisomerization and photodimerization within porous metal–organic frameworks (MOFs) by fine-tuning the pore microenvironments using different dicarboxylate linkers. MOFs with small pores ( ( E )-X⊂MOF1 and ( E )-X⊂MOF1′) favor the photoinduced E–Z isomerization of one encapsulated diaryl alkene substrate while those with large pores ( (( E )-X)2⊂MOF2) prefer the photodimerization of two encapsulated alkene substrates. Both reactions show broad functional group compatibility and proceed stereospecifically in good yields under mild conditions. High local concentration of diaryl alkene ligands and their preorientation within pores facilitate stereoselective dimerization. This pore engineering strategy is applicable to control and create pore microenvironemnts for other photoinduced organic reactions within porous MOFs. Download figure Download PowerPoint Introduction The regulation of chemical reactions in the solid state via supramolecular confinement has attracted considerable interest.1–4 Confining reactive guest molecules in porous materials limits the opportunity for their conformational changes, thus promoting regio- and stereo-selective processes.5 Construction of an environment that brings guest molecules into close contact and imposes physical constraints offers the enticing prospect of stereocontrol by restricting conformations in the excited state.6 Designing host motifs that can be used as templates or vessels to efficiently control the selectivity is clearly desirable but is also very challenging.1,2,4,7–9 Artificial molecular containers provide relatively rigid and hydrophobic central cavities that approximate the binding pockets of enzymes.10 They can recognize and orient guest substrates in elaborate cavities and induce energy transfer, photosensitization, or asymmetry, providing stereocontrol or selectivity.11,12 Metal–organic frameworks (MOFs) have many interesting applications resulting from their tunable structures and properties.13–17 The regular arrangement of linkers and metal nodes allows precise construction of target porous frameworks with specific pore environments to reduce the degree of freedom of guest molecules and enhance reaction efficiency and stereocontrol.10,18 On the other hand, photoinduced E–Z isomerization and dimerization of aryl arenes are well-known competing reactions.19 Geometric isomerization induced by UV light irradiation always complicates the stereochemical control of cycloadditions.19–23 For example, photoirradiation of a dilute solution of trans-4-styryl-pyridine affords only the E-isomer24 but produces a mixture of stereo- and regio-isomeric cyclobutanes in a concentrated solution.25–27 Conditions that favor close contact of alkene pairs lead to photodimerization while the opposite prefers E–Z photoisomerization if space permits.28,29 Thus, designing and creating suitable microenvironments of porous materials for selectively initialing the two reactions remains a big challenge. In this work, inspired by nature, which exploits tailored microenvironments within enzymes to promote and stereoselect biological reactions, we intend to design pore microenvironments of porous MOFs by using different dicarboxylic acids to achieve the desired regio- and stereo-selectivity. Three photoactive MOFs ( ( E )-X⊂MOF1, ( E )-X⊂MOF1′, (( E )-X)2⊂MOF2) with different pore sizes were prepared that produce either a wide range of substituted cis-isomers of 4-styrylpyridine or substituted cyclobutanes through stereoselective E–Z photoisomerization or photodimerization reactions (Figure 1). Such a pore engineering strategy for control of photoisomerization versus photodimerization with a wide range of substituted aryl alkenes has been reported for the first time. Figure 1 | Selective photoinduced E–Z isomerization and dimerization achieved by pore engineering strategy. Download figure Download PowerPoint Experimental Methods Preparation of photoisomerization products (Z)-X (X = 1a–1ab) Taking ( Z )-1a as an example: a petri dish containing 500 mg of ( E )-1a⊂MOF1 was placed in a UV-light box (λ = 365 nm) and irradiated for 6 h. The reaction conversion rate was determined by 1H NMR spectroscopy. After the photoreaction, the product was transferred into a 50 mL beaker and NaOH solution (4 mol/L, 40 mL) was added with stirring at room temperature for 16 h. After this period, the aqueous solution was extracted with CH2Cl2 (50 mL × 3) and the product isolated and purified by column chromatography to obtain ( Z )-1a. Preparation of photodimerization products Dimer-X (X = 1a–1w, 1ac–1ai) Taking Dimer-1a as an example: a petri dish containing 500 mg of (( E )-1a)2⊂MOF2 was placed in a UV-light box (λ = 365 nm) and irradiated for 5 h. The reaction conversion rate was determined by 1H NMR spectroscopy. After the photoreaction, the mixture was transferred to a 50 mL beaker, and NaOH solution (4 mol/L, 40 mL) was added with stirring at room temperature for 24 h. The aqueous solution was then extracted with CH2Cl2 (50 mL × 3) and the product isolated and purified by column chromatography to obtain a white solid Dimer-1a. Results and Discussion Colorless crystals of [Zn2(SDB)2((E)-X)2]n ( ( E )-X⊂MOF1) were obtained from solvothermal reactions of Zn(II) salt with V-shaped ligand 4,4′-sulfonyldibenzoic acid (H2SDB) and ( E )-X ( X = 1a– 1ab, Supporting Information Schemes S1–S2 and Table S1). Taking ( E )-1p⊂MOF1 as an example, a chain-like motif composed of square-like window units is formed by bridging pairs of Zn(II) ions with SDB2− ligands (Figure 2a and Supporting Information Figure S1). Each ( E )-1p ligand binds at Zn(II) center through the pyridyl group and threads into a square-like window of an adjacent 1D chain, forming a polythread arrangement. Each square-like window has an approximate dimension of 8.59 × 8.53 Å and can accommodate only one monomer ( E )-X with a size ranging from 12.70 × 7.40 × 3.20 Å to 12.87 × 8.92 × 4.14 Å ( Supporting Information Figure S2). Figure 2 | (a) The polythread motif of [Zn2(SDB)2((E)-1p)2]n chains showing the monomer encapsulation; (b) the interdigitate network of [Zn(BDC)((E)-1h)]n layers showing the dimer encapsulation. Download figure Download PowerPoint The above preorganization led to the formation of cis substituted 4-styrylpyridines after 365 nm UV light irradiation over ( E )-X⊂MOF1 crystals for 6 h. Using ( E )-1p⊂MOF1 as an example, the olefin protons got shifted from 7.65 and 7.16 ppm to 6.89 and 6.70 ppm. The resulting coupling constant (J = 12 Hz) suggested cis C=C configuration, in contrast to that of the trans monomer (J = 16 Hz). Signals for pyridyl protons of ( E )-1p⊂MOF1 also got shifted upfield from 8.58 and 7.66 ppm to 8.41 and 7.03 ppm. The changes to the 1H NMR signals were consistent with an E to Z isomerization upon UV irradiation ( Supporting Information Figure S3). Substituted Z-4-styrylpyridine products were also obtained upon UV irradiation over other ( E )-X⊂MOF1 crystals for 6 h. The 1H NMR spectrum of each product (Figure 3, Supporting Information Figure S4) indicated the absence of the trans isomer.6,30,31 Detailed 2D NMR spectral analysis confirmed the stereochemistry of the representative product ( Z )-1s ( Supporting Information Figure S5). Such an E–Z transformation had a broad substrate scope without much limit to the substituent groups and their positions. Substituted ( E )-X ( X = 1d– 1ab) containing para-, meta-, and ortho-functional groups were all suitable reactants. Notably, some cyclobutane derivatives were also obtained during the photoisomerization of ( E )-X⊂MOF1, ( X = 1a– 1o, 1q, 1s– 1w) because the nonencapsulated ( E )-X ligand pairs between chains were locating at an ideal C=C bond contact distance and orientation for photodimerization ( Supporting Information Figures S6 and S7). Figure 3 | Synthesis of (Z)-X (X = 1a–1ab) derivatives. Download figure Download PowerPoint Next, we attempted to prepare larger pore ( E )-X⊂MOFs by employing another V-shaped ligand 4,4′-methylenedibenzoic acid (H2MDB). According to the literature,32,33 compounds prepared from this ligand hold their Ar-CH2-Ar angles in a wide range from 103.80° to 121.48°. For the resulting ( E )-X⊂MOF1′ ( X = 1l, 1n, 1p– 1q, 1t, 1x– 1y, 1aa), the size of each rhomboid window in their chain structures, comparable to that of ( E )-X⊂MOF1, has the approximate dimensions of 8.36 × 8.36 Å ( Supporting Information Figure S8), which encloses only one guest ( E )-X molecule. As expected, only a photoinduced E–Z transformation was observed for one encapsulated ( E )-X ( X = 1l, 1n, 1p– 1q, 1t, 1x– 1y, 1aa) molecule within the window. Taking ( E )-1p⊂MOF1′ as an example, similar changes to the 1H NMR signals were observed for irradiated ( E )-X⊂MOF1′, indicating the E–Z transformation ( Supporting Information Figure S9). Pairs of two encapsulated substrates were achieved by employing a linear ligand terephthalic acid (1,4-H2BDC). Each [Zn8(1,4-BDC)4((E)-X)8] unit (denoted as (( E )-X)2⊂MOF2, where X = 1a– 1w, 1ac– 1ai) was formed by combining Zn(II) centers with the 1,4-BDC2− linker. Close inspection revealed that pairs of ( E )-X monomers oriented in a head-to-tail fashion within the square-like [Zn8(1,4-BDC)4] unit (Figure 2b). Taking (( E )-1h)2⊂MOF2 as an example, the relevant distance between the C=C bonds from an encapsulated pair of ( E )-1h was 3.89 Å ( Supporting Information Figure S10), fulfilling Schmidt’s criteria for [2+2] photodimerization.28 Intermolecular photodimerization of preorganized olefinic substrates produced the head-to-tail rctt-1,3-bis(4-pyridyl)-2,4-bis(phenyl)cyclo-butane (HT-ppcb) in a good yield and an excellent stereo- and regio-selectivity. Taking (( E )-1h)2⊂MOF2 as an example, the pyridyl proton signals got shifted from 8.54 and 7.54 ppm to 8.35 and 7.26 ppm, respectively, consistent with the formation of the substituted cyclobutane ( Supporting Information Figure S11).6,30,31 Signals associated with the olefinic protons were absent from the 1H NMR spectra. Cyclobutane derivatives were precipitated by destruction of this MOF in a basic solution. By addition of acid to the filtrate solution, the resulting H2BDC was precipitated and recycled ( Supporting Information Figure S12).34 Other substituted cyclobutane products ( Dimer-X, X = 1a– 1w, 1ac– 1ai) (Figure 4) were obtained upon 365 nm UV irradiation over (( E )-X)2⊂MOF2 crystals for 5 h. The 1H NMR spectra displayed new signals at 4–6 ppm attributed to the cyclobutane protons ( Supporting Information Figure S13). Again, the corresponding reaction of fluorinated substrates could be monitored by changes in 19F NMR chemical shifts. Signals at −112.89, −116.91, −109.53, −110.74, −119.00, −112.94, and −112.19 ppm got moved to −112.53, −115.26, −108.88, −110.07, −117.07, −112.29, and −112.23 ppm for compounds Dimer-1e, Dimer-1j, Dimer-1m, Dimer-1o, Dimer-1v, Dimer-1w, and Dimer-1ah, respectively ( Supporting Information Figure S13). Detailed 2D NMR spectral analysis of Dimer-1s confirmed the stereochemistry of cyclobutane ( Supporting Information Figure S14). The 1H NMR spectrum of the recovered H2BDC showed no change, indicating the square-like windows were photoinert ( Supporting Information Figure S15). Figure 4 | Synthesis of cyclobutane derivatives of Dimer-X (X = 1a–1w, 1ac–1ai). Download figure Download PowerPoint All the substrates used in this work gave high yields of photodimerization products (Figure 4). Good regio- and stereo-selectivity were achieved from functionalized ( E )-X starting materials without limitation to the substituted group position ( Dimer-X, X = 1a– 1w, 1ac– 1ai). ( E )-X containing para-, meta-, and orthomethyl groups reacted efficiently in these photoinduced coupling reactions. Substrates bearing halogen substituents like chloro and bromo reacted likewise. Bioactive indanone derivatives and fluoro-containing compounds were appropriate substrates for this protocol ( ( E )-1e, ( E )-1j, ( E )-1m, ( E )-1o, ( E )-1v, ( E )-1w, ( E )-1ah, and ( E )-1ai). Likewise, thiophene-containing products of pharmaceutical interest were generated smoothly and selectively ( ( E )-1b and ( E )-1ae). Disubstituted substrates were also suitable candidates for this photoreaction. The relatively low concentration of UV radiant energy present in sunlight was nevertheless sufficient to complete the dimerization ( Supporting Information Figure S16). Time-dependent 1H NMR spectra showed that cyclobutane generated by sunlight was at a slower rate than the corresponding reaction facilitated by artificial UV light with the first-order rate constants of 2.3 × 10−3, 1.0 × 10−2, 3.2 × 10−3, 4.5 × 10−3, 5.2 × 10−3, 2.6 × 10−2, and 1.9 × 10−3 min−1 for (( E )-1a)2⊂MOF2, (( E )-1d)2⊂MOF2, (( E )-1f)2⊂MOF2, (( E )-1h)2⊂MOF2, (( E )-1p)2⊂MOF2, (( E )-1s)2⊂MOF2, and (( E )-1ac)2⊂MOF2, respectively ( Supporting Information Figure S16). However, these sunlight-driven reactions seem mild and clean and need no extra energy, which meets the green and sustainable chemistry synthesis criteria.34 Container molecules, like enzymes, if their local concentrations are increased, can be made more selective by incorporating recognition functionality.35 The competition of photoinduced isomerization vs photodimerization can be governed by monomer or dimer encapsulation, which is, in turn, pore size-dependent. In this work, square-like windows of different dimensions were formed by employing SDB2−, MDB2−, and BDC2− linkers (Figure 5). The Ar-SO2-Ar angles ranged from 97.57° to 115.44° in the SDB-based compounds, while the Ar-CH2-Ar angle ranged from 103.80° to 121.48° in the MDB-based compounds according to the literature.32,33,36 In a sense, it is the flexibility of carboxylate ligands that allows them to form conformation-adapted arrangements of the guest molecules. The Ar-SO2-Ar angles were 98.18° and 101.53° in the SDB-based compounds ( E )-1m⊂MOF1 and ( E )-1p⊂MOF1 while the Ar-CH2-Ar angles were 106.52° and 107.70° in MDB-based compounds ( E )-1p⊂MOF1′ and ( E )-1y⊂MOF1′, respectively. The resulting window sizes were 8.59 × 8.53 Å and 8.36 × 8.36 Å for ( E )-1p⊂MOF1 and ( E )-1p⊂MOF1′, which formed excellent host–guest (one monomer) compatibility (Figure 5). For (( E )-X)2⊂MOF2, although BDC2− is a linear ligand, the resulting [Zn2(BDC)2] unit acted as a bent connector with an angle of 98.90°, and the window had a size of 10.87 × 10.96 Å. Therefore, (( E )-X)2⊂MOF2 could accommodate one pair of ( E )-X ( X = 1a– 1w, 1ac– 1ai molecules, with sizes ranging from 12.92 × 8.76 × 6.77 Å to 13.44 × 8.92 × 10.17 Å, Supporting Information Figure S17). The regiospecificity of the photodimerization of ( E )-X substrates is governed by the balance of factors related to size compatibility, coordination, electronic, and steric effects, and the thermodynamic stability of the host–guest system in the crystalline state.37 The formation of head-to-tail (HT) versus head-to-head (HH) regioisomers is highly dependent on coordination interactions.38 The construction of artificial containers with a size and shape comparable to cucurbit[8]ril24,39–41 can be viewed as molecular flasks for preorganization, which facilitate the regiospecific formation of cyclobutanes. Notably, no photodimeric products were detected upon UV irradiation of crystalline monomer samples, taking ( E )-1s as an example. This failure to achieve dimerization of unencapsulated substrates may be attributed to the poor alignment of neighbouring molecules in the solid state ( Supporting Information Figure S18).42,43 Figure 5 | Geometric parameters of different square-like windows and views showing the compatibility of one or two (E)-X (X = 1p, 1h) molecules within the windows. Download figure Download PowerPoint Conclusion We demonstrated that by using different dicarboxylate linkers, our pore engineering strategy can tune and control the competition between photoinduced E–Z isomerization and dimerization reactions within porous MOFs. Modifying the pore microenvironment led to the preorientation and selective encapsulation of monomer or dimer ( E )-X substrates within MOFs. The artificial molecular reactors of three MOFs, ( E )-X⊂MOF1, ( E )-X⊂MOF1′, and (( E )-X)2⊂MOF2, which have different pore sizes, were distinguished by their high catalytic efficiency with a wide scope of diaryl alkene substrates and excellent tolerance for substituent groups. This strategy provides a promising protocol for the solid-state synthesis of cyclobutane and cis arene alkene derivatives with excellent selectivity, that cannot be replicated in solution reactions. Supporting Information Supporting Information is available and includes X-ray crystographic data for structures ( E )-1m⊂MOF1, ( E )-1p⊂MOF1, ( E )-1p⊂MOF1′, crystographic data for structures E )-1h)2⊂MOF2, ( E )-1s, and Dimer-1k and the copy of 1H-, 13C-NMR spectra of products of photoisomerization and photodimerized product. CCDC 2165749, 2165750, 2165751, 2165752, 2165753, and 2165754 contain the supplementary crystallographic data for this paper. Conflict of Interest There is no conflict of interest to report. Funding Information The authors gratefully acknowledge the financial support of the National Natural Science Foundation of China (NSFC; grant nos. 21961004, 52002089, 21531006, and 21773163) and the China Postdoctoral Science Foundation (grant no. 2020M670525). References 1. Hutchins K. M.; Sumrak J. C.; MacGillivray L. R.Resorcinol-Templated Head-to-Head Photodimerization of a Thiophene in the Solid State and Unusual Edge-to-Face Stacking in a Discrete Hydrogen-Bonded Assembly.Org. Lett.2014, 16, 1052–1055. Google Scholar 2. Michito Y.; Yoshihisa T.; Takahiro K.; Fujita M.Cavity-Directed, Highly Stereoselective [2+2] Photodimerization of Olefins within Self-Assembled Coordination Cages.Angew. Chem. Int. Ed.2002, 41, 1347–1349. Google Scholar 3. Telmesani R.; Park S. H.; Lynch-Colameta T.; Beeler A. B.[2+2] Photocycloaddition of Cinnamates in Flow and Development of a Thiourea Catalyst.Angew. Chem. Int. Ed.2015, 54, 11521–11525. Google Scholar 4. Wei Y. S.; Zhang M.; Liao P. 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A.; and of Substituted and Google Scholar Information Chemical Chen ; Commission ; European commission ; Isomerization ; Library science ; Chemistry ; State (computer science) ; Political science ; Computer science ; Law ; Ecology ; European union ; Organic chemistry ; Business ; Catalysis Metal-Organic Frameworks: Synthesis and Applications ; Covalent Organic Framework Applications ; Mesoporous Materials and Catalysis