Article pubs.acs.org/crystal
Cite This: Cryst. Growth Des. XXXX, XXX, XXX−XXX
Contrast Solid-State Photoreactive Behavior of Two TwoDimensional Zn(II) Coordination Polymers Hong Sheng Quah,† Jocelyn Li Xian Yap,† Uma Sambasivam,† Anjana Chanthapally,§ Bruno Donnadieu,†,# and Jagadese J. Vittal*,† †
Department of Chemistry, National University of Singapore, 3 Science Drive 3, Singapore 117543 Department of Science, M.A. College of Engineering, Kothamangalam 686666, Kerala, India
§
S Supporting Information *
ABSTRACT: A two-dimensional photoreactive coordination polymer (2D CP), [Zn2(Fumarate)2(H2O)2(2F-spy)4] (1) (2Fspy = 2′-fluoro-4-styrylpyridine), undergoes partial [2 + 2] cycloaddition reaction under UV light in a single-crystal-tosingle-crystal manner to a three-dimensional (3D) CP [Zn2(Fumarate)2(H2O)2(2F-spy)3(rctt-2F-ppcb)0.5] (2) (where rctt-2F-ppcb = 1,3-bis(4′-pyridyl)-2,4-bis(2′-fluoro-phenyl)cyclobutane), whereas another similar photoreactive 2D CP, [Zn2(Fumarate)2(3F-spy)2] (3) (3F-spy = 3′-fluoro-4-styrylpyridine), when exposed to UV light, forms a cyclobutane ring arising from the hetero-cross-coupling of 3F-spy and fumarate, in addition to the expected cyclobutane ring, rctt-3F-ppcb (rctt-3F-ppcb = 1,3bis(4′-pyridyl)-2,4-bis(3′-fluoro-phenyl)cyclobutane) by the [2 + 2] cycloaddition reaction of 3F-spy pairs aligned in a head-to-tail (HT) fashion. It is unusual to observe two different photoproducts in a solid-state [2 + 2] photo-cycloaddition reaction involving CPs. It is possible because the metal ions appear to support the reactions between two different types of ligands.
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tion is well-known10 in solution, it has rarely been observed in the solid state.11 Such an unusual photoreaction that is not topochemical has also been observed between 7-fluoro-4methylcoumarin and 6-fluoro-4-methylcoumarin in the solid state.12 Further, in a [2 + 2] cycloaddition reaction, the solids will either be photostable or photoreactive to furnish a rcttcyclobutane derivative exclusively. In some occasions, the rtctisomer has also been obtained from the crisscrossed alignment of the olefin pairs.13 Herein, we found a photoreactive two-dimensional coordination polymer (2D CP), [Zn2(Fumarate)2(H2O)2(2F-spy)4] (1) (2F-spy = 2′-fluoro-4-styrylpyridine) that undergoes partial [2 + 2] cycloaddition reaction under UV light in a singlecrystal-to-single-crystal (SCSC) manner to a 3D CP, [Zn 2 (Fumarate) 2 (H 2 O) 2 (2F-spy) 3 (rctt-2F-ppcb) 0.5 ] (2) (where rctt-2F-ppcb = 1,3-bis(4′-pyridyl)-2,4-bis(2′-fluorophenyl)cyclobutane). In contrast, another photoreactive 2D CP, [Zn2(Fumarate)2(3F-spy)2] (3) (3F-spy = 3′-fluoro-4styrylpyridine), on exposure to UV light, forms a cyclobutane ring arising from the hetero-cross-coupling of 3F-spy and fumarate in addition to the expected cyclobutane ring, rctt-3Fppcb (rctt-3F-ppcb = 1,3-bis(4′-pyridyl)-2,4-bis(3′-fluorophenyl)cyclobutane) by the [2 + 2] photocycloaddition
INTRODUCTION One of the ultimate goals of crystal engineering is to unravel our understanding of the structure−function-property relationships of the crystalline materials.1 In this context, the reactivity of solids can be successfully designed and controlled by molecular packing, employing various types of weak intermolecular interactions.2,3 The seminal work by Schmidt and his co-workers on [2 + 2] cycloaddition reactions of substituted trans-cinnamic acids and related derivatives have opened up solid-state synthesis of stereoisomers of cyclobutane derivatives.4 Since then, the solid-state photochemical reaction has been one of the key interests of solid-state organic chemists. A single-step solid-state synthesis of highly strained cyclobutane is undoubtedly attractive in organic synthesis, as it is clean and environmentally benign without the use of any solvents.5 The solid-state photo-cycloaddition between identical olefins (homocycloaddition) has been well documented in the literature. There have been a few reports on the heterophoto-cycloaddition in the solution state.6,7 To demonstrate the hetero-cycloaddition in the solid state, Fujita and coworkers elegantly imprisoned acenaphtylenes and maleimides inside a palladium cage which precludes two large acenaphthylene molecules and is forced to align two heteroguest molecules instead to form a cyclobutane ring with an unusual stereoisomer.8 Clements et al. also showed that γ-cyclodextrin can facilitate the heterodimerization between cinnamic acid and coumarins.9 Although intramolecular arene-alkene cycloaddi© XXXX American Chemical Society
Received: April 11, 2018
A
DOI: 10.1021/acs.cgd.8b00540 Cryst. Growth Des. XXXX, XXX, XXX−XXX
Crystal Growth & Design
Article
reaction of 3F-spy pairs aligned in a head-to-tail (HT) fashion. The details are discussed below.
Interestingly, no stereoisomeric cyclobutane other than HT dimer was formed. We observed that the single crystal was maintained after 25% of photoconversion for 30 min of UV exposure. This led to the formation of 2 in an SCSC manner as predicted from the packing of 1. The single crystal X-ray structure of 2 is shown in Figure 2. Prolonged exposure to UV
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RESULTS AND DISCUSSION Light yellow block-shaped single crystals of 1 were obtained from Zn(NO3)2·6H2O, 2F-spy, and fumaric acid in a 1:1:1 ratio in 33% yield. Single crystal X-ray structure determination revealed that the asymmetric unit contains the formula units as shown in Figure 1a. The Zn1 atom is bonded to two fumarate
Figure 1. (a) A view of the atoms in the symmetric unit in 1. (b) A portion of the 2D structure in 1 is shown. 2F-spy ligands are omitted. (c) Alignment of a 2F-spy pair between the adjacent layers in 1 are shown. Hydrogen atoms are not shown.
anions in a monodentate fashion occupying cis positions, and two water molecules are also bonded in the plane, while two 2F-spy ligands occupy the axial positions to give a distorted octahedral geometry. The other carboxylates of these fumarate anions are chelated to two Zn2 atoms. These Zn2 atoms are further bridged by two more fumarate anions in a monodentate fashion to give a 14-membered ring. The axial positions are occupied by 2F-spy ligands. The 2D sheet is spread in the acplane as shown in Figure 1b. As analyzed by TOPOS,14 the topology of the 2D sheet is fes and Shubnikov plane net, {4.82} (Figure S1). The closest Zn···Zn distance is 4.934 Å, and hence the olefin bonds of the 2F-spy ligands in the same 2D layer are not close enough to undergo photo-cycloaddition reaction. Furthermore, the pyridyl and phenyl rings in the three 2F-spy ligands are twisted in the range 31.8−50.4°. Only in the fourth 2F-spy ligand (atoms N4−C52) they are roughly coplanar with an interplanar angle of 7.8°. The 2F-spy ligands from the adjacent 2D layers are interdigitated in such a way that the least-twisted 2F-spy ligand pairs (atoms N4−C52) are nicely aligned in parallel in a HT manner as shown in Figure 1c. The center-to-center olefin bond distance, 3.907 Å, is suitable to undergo [2 + 2] cycloaddition reaction. As such, it is expected that only 25% of the 2F-spy ligands will be dimerized upon exposure to UV light and able to connect the 2D layers to form a 3D structure. To test this prediction, the photoreactivity of 1 was monitored by 1H NMR spectroscopy by taking out UVirradiated solid at regular intervals of time and dissolve them in DMSO-d6 with a drop of trifluoroacetic acid. The presence of doublet of triplet peaks at 5−5.2 ppm indicates the formation of the cyclobutane ring (Figure S17). There is a plateau at about 16% of conversion and increased up to 29% of HT cycloaddition after 8 h of UV-exposure (Figure S11).15
Figure 2. (a) A fragment of the structure of 2. (b) A portion of the non-interpenetrated 3D structure of 2. No free 2F-4spy ligand was shown. Hydrogen atoms are not shown in both figures.
did not retain its single crystallinity. The structure of 2 revealed a non-interpenetrated 3D structure by the formation of 1,3bis(4′-pyridyl)-2,4-bis(2′-fluoro-phenyl)cyclobutane (rctt-2Fppcb) in 25% which connects the adjacent layers, while the rest of 2F-spy remained unreacted. Overall, this 3D new metal− organic framework (MOF) structure has the topology of dmc, {4.82}{4.85} (Figure S2).14 The corresponding 3F-spy ligand furnished [Zn2(Fumarate)2(3F-spy)2] (3) as colorless platy single crystals in 31% yield. Single crystal X-ray structure determination revealed that 3 is made of a paddlewheel building block. All the atoms except the two Zn(II) atoms are disordered with a common occupancy factor refined to 0.515(4). The fumarate anions connecting the paddlewheel units generate a 2D structure in the ab-plane, and the 3F-spy ligands are projecting along the c-axis. Here also the 3F-spy ligands from the adjacent layers are interdigitated in a HT manner. Of the two 3F-spy ligands, only one is aligned with another 3Fspy from the adjacent layer (Figure 3). The center-to-center distance between these olefin pair is 3.605 Å congenial for photocycloaddition reaction. On the basis of the packing, 50% formation of 1,3-bis(4′-pyridyl)-2,4-bis(3′-fluoro-phenyl)cyclobutane (rctt-3F-ppcb) is predicted from the 3F-spy pair B
DOI: 10.1021/acs.cgd.8b00540 Cryst. Growth Des. XXXX, XXX, XXX−XXX
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Figure 4. (a) Photo-cycloaddition reaction showing the formation of rctt-3F-ppcb. (b) Photo-cycloaddition reaction between 3F-spy and formate anion forming the photoproduct, pfcb.
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CONCLUSION Incomplete cycloaddition reaction will usually result in severe disorder, which will be difficult to resolve in a SCSC reaction.16 Here, 25% photoreaction of 1 with a 2D structure led to the transformation of a 3D structure, 2. There are not many such examples reported without severe disorder.17 Furthermore, solid-state photo-cycloaddition reactions will always yield single photoproducts cleanly and do not yield a mixture of completely different cyclobutane photoproducts, as usually observed in solution for intermolecular olefin pairs.1−5 Solid-state photocycloaddition reaction between olefin bond and phenyl ring has rarely been reported, where it has been found to occur up to only 66%, as characterized by NMR spectroscopy.11a Further, MacGillivray et al. have cleverly designed cocrystals to align two different molecules containing photoreactive olefin bonds to undergo cross-photo-cycloaddition reaction.18 Here, the photochemical reaction of 3 gives two different cyclobutane products, namely, rctt-3F-ppcb and the heterodimer pfcb. This is because of the close alignments between a pair of 3F-spy ligands, as well as between another 3F-spy ligand and fumarate anion in the 2D structure of 3. Such a solution-like behavior of a solid-state reaction appears not to have been observed before, to the best of our knowledge. It appears that the metal ion supports the reaction between two different types of ligands in the polymeric structure.
Figure 3. (a) A view of 3 showing the HT alignment between a pair of intermolecular 3F-spy ligands and orthogonal alignment olefins between a fumarate anion and a 3F-spy ligand. (b) A view of the packing of 3 showing the photoreactive olefin pairs.
[2 + 2] cycloaddition reaction under UV light. Interestingly, the olefin bond of the other 3F-spy is aligned in a crisscrossed manner with the olefin bond of the fumarate ligand from the adjacent layer with a distance of 3.580 Å. This complementary crisscrossed alignment of the 3F-spy and fumarate olefin pairs is expected to generate an unusual cross-coupling reaction under UV light. When the photoreactivity of 3 was tested in UV experiments, there were no SCSC reactions observed. However, the 1H NMR spectroscopic data provided clear evidence for the formation two different cyclobutane rings. Reduction in intensity of the doublet at 8.86 ppm due to 3F-spy monomer and appearance of the doublet at 8.76 ppm and multiplet at 5.00 ppm are an indication the cyclobutane formation, namely, 1,3-bis(4′-pyridyl)-2,4-bis(3′-fluoro-phenyl)cyclobutane (rctt3F-ppcb). Furthermore, new peaks observed at 4.06, 3.80, 3.48, and 3.37 ppm appear to indicate the formation of cyclobutane that is possibly less symmetrical and aliphatic due to the shielded chemical shift values compared to the HT dimer peak at 5.00 ppm. As mentioned, the cross-coupling arising from the orthogonal alignment of the olefin bonds of fumarate and 3F-spy leads to the formation of such a heterodimer in the solid state, namely, 1-(4′-pyridyl)-2,3-bis(carboxylic acid)-4(3′-fluoro benzene)cyclobutane (pfcb). From the time dependent 1H NMR plot of 3 (shown in Figure S20), it was found that the formation of pfcb is concurrent with the homodimer, rctt3F-ppcb after 2 h of UV irradiation. The percentage of heterodimer pfcb that formed appears to plateau off at 16% after 6 h of irradiation. On the other hand, the homodimer, rctt3F-ppcb formation reached 20% conversion. The 19F-NMR spectrum of 3F-spy in 3 showed a singlet at −112.65 ppm, while 4 obtained after UV irradiation for 6 h gives three singlet peaks at −112.65, −112.75, −112.84 ppm (Figure S21). They have been assigned to the F atoms in 3, rctt-3F-ppcb and the heterodimer pfcb, respectively. Furthermore, the irradiated compound was digested with nitric acid and dissolved in DMSO before ESI-MS analysis was performed. The characterization shows the monomer peak at 200.2, the heterodimer peak at 316.1, and the homodimer peak at 399.1 m/z (+) (Figure S26).
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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.8b00540. Materials and methods, syntheses, characterization, topology, PXRD patterns, TGA, time cycloaddition plots, 1H NMR spectra, solid-state reflectance UV and PL description, crystallographic packing figures (PDF) Accession Codes
CCDC 1825339−1825341 contain 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
[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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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Fax: (+65) 6779-1691. ORCID
Jagadese J. Vittal: 0000-0001-8302-0733 C
DOI: 10.1021/acs.cgd.8b00540 Cryst. Growth Des. XXXX, XXX, XXX−XXX
Crystal Growth & Design
Article
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(B.D.) Department of Chemistry, Mississippi State University, Mississippi State, MS 39762−9573, USA, Phone: 1-(662) 3253584. E-mail:
[email protected] Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS J.J.V. thanks the Ministry of Education, Singapore, for funding through National University of Singapore (Grant No. R-143000-A12-114). We thank Dr. Sini George for helping with the time-dimerization reaction.
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DOI: 10.1021/acs.cgd.8b00540 Cryst. Growth Des. XXXX, XXX, XXX−XXX
