Competition Between Head-to-Head and Head-to-Tail

Article Cite This: Cryst. Growth Des. XXXX, XXX, XXX−XXX

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Competition Between Head-to-Head and Head-to-Tail Photocycloaddition Reaction in the Solid State: A Case Study Vaishali A. Sawant,†,‡ Jien Wu,† and Jagdese J. Vittal*,† †

Department of Chemistry, National University of Singapore, 3 Science Drive 3, 11753, Singapore Department of Technology, Shivaji University, Kolhapur, Vidyanagar, Kolhapur 416004, India

‡

S Supporting Information *

ABSTRACT: Solid state [2 + 2] cycloaddition photochemical reactions have recently emerged as a benign green method to synthesize cyclobutane derivatives. Synthesis of cyclobutane isomers are often controlled by the manner in which unsymmetrical olefin pairs have been aligned either in a head-to-head (HH) or head-to-tail (HT) manner using bridging ligands, organic templating agents, metalorganic clipping agents, and metallophilic interactions. Here we synthesized three Zn(II) one-dimensional coordination polymers (1D CPs) in which the terminal ligands having olefin groups are aligned in both a HH and HT fashion to undergo a photo-cycloaddition reaction. Of these, two CPs provided HH photoproducts, while the third CP gave a mixture of HH and HT photoproducts. These results hope to provide additional knowledge to design the solids to obtain the desired cyclobutane regioselectively.

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INTRODUCTION

Schmidt and co-workers published their seminal work on solid state [2 + 2] photocycloaddition reaction about 50 years ago.1−3 Later the designing principles for aligning a pair of olefin bonds extensively studied in organic crystals have been successfully extended to metal complexes, coordination polymers, and metal−organic framework structures.4−13 In the majority of cases, symmetrical organic molecules containing olefin bonds such as 4,4′-bipyridylethylene (bpe), 1,4-bis[2-(4pyridyl)ethenyl]benzene (bpeb), 4,4′-stilbenedicarboxylate (sdb), muconate, etc. have been extensively used to study their photoreactivity in the solid state.14−30 Similarly, unsymmetrical or terminal ligands containing olefin bonds have also been investigated.31−52 Unlike the olefin bearing symmetrical ligands, terminal ligands such as 4-styrylpridines (4-spy) can be aligned in two different ways as shown in Figure 1, namely, in a head-to-head (HH) and heat-to-tail (HT) manner, giving rise to two different isomeric products. Many strategies have been employed to align them in both ways. For example, a combination of π···π interactions and argentophilic interactions have been used to align the olefin pairs in a HH manner successfully. Even if the olefin pairs disposed in an antiparallel manner, a pedal motion brings them to parallel orientations prior to [2 + 2] cycloaddition reaction.31−34 In another interesting paper, MacGillivray et al. have shown that it is possible to align the pentafluorostyrylpyridine groups in a HH fashion using Ag···Ag interactions, despite the common wisdom that these groups should align in a HT manner due to donor−acceptor interactions between pentafluorophenyl and pyridyl groups.35 Such HH alignment © XXXX American Chemical Society

Figure 1. (Left) Head-to-head alignment of 4-spy ligand pairs leading to one isomer. (Right) Head-to-tail alignment of 4-spy ligand furnishes another isomer. H atoms are not shown.

was not present in other compounds due to the presence of anions, and hence HH or HT alignment cannot be predicted.36 Hydrogen bonding has also been used, in addition to these noncovalent interactions to orient the olefin bonds in a HH fashion.37,38 In the absence of metallophilic interactions, HT alignment dominates due to donor−acceptor interactions when 4-styrylpyridine (4-spy) and their derivatives39−43 or monoprotonated bpe ligands44−49 were used. When 4-styrylpyridine ligands were aligned in both HH and HT manner in a Ag(I) complex, C−H···π interaction plays a decisive role in promoting the intramolecular HH-photodimerization over HT intermolecular photoreaction.53 In the course of our studies on the photoreactivity of the Zn(II) Received: March 24, 2018 Revised: April 17, 2018

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DOI: 10.1021/acs.cgd.8b00442 Cryst. Growth Des. XXXX, XXX, XXX−XXX

Crystal Growth & Design

Article

Interestingly the 3F-4spy ligands in the adjacent 1D chains are interdigitated as shown in Figure 3. One of the 3F-4spy

coordination polymers (CPs), we encountered three solid state structures of Zn(II) CPs containing the 4-spy and its two fluoroderivatives aligned such that both HH and HT photocyclization are equally probable. Since these are the ideal systems to understand the competition between HH and HT photoreactivity which can give two different isomers of cyclobutane rings, we have investigated this further, and results are presented below.

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RESULTS AND DISCUSSION The compounds [Zn2(1,3-bdc)2(3F-4spy)4] (1), [Zn2(1,3bdc)2(2F-4spy)4]·0.5H2O (3), and [Zn2(1,3-bdc)2(4spy)4]· 2MeOH (5) (where 1,3-bdc = 1,3-benzenedicarboxylate) have been synthesized as single crystals in moderate yield by slow evaporation of the methanolic solution of Zn(NO3)2· 6H2O, 1,3-bdc and the respective 4-styrylpridine ligands in a 1:1:2 molar ratio. The crystal structures as determined by X-ray crystallography reveal that they belong to the triclinic space group P1̅ with Z = 1, and the asymmetric units contain half of the formula unit. They are isomorphous, but they are not strictly isostructural with small differences in their bond parameters and conformations. In 1, a crystallographic center of inversion is present in the Zn···Zn, each Zn(II) atom is bonded to two 3F-4spy ligands in a trans manner, and four oxygen atoms from three 1,3-bdc ligands occupy approximately coplanar giving rise to highly distorted octahedral geometry. Two carboxylate groups from two 1,3-bdc ligands bridge the two Zn(II) atoms, while one carboxylate from another 1,3-bdc ligand is chelating to the same Zn(II) atom. The 3F-4spy ligands are aligned parallel with an interplanar angle of 9° with a Zn···Zn separation of 4.147 Å. Two 3F-4spy ligands are not perfectly planar; the pyridyl and phenyl rings are twisted by 4.5 and 13.5°. The olefin bonds in these intramolecular 3F-4spy ligands are disposed in a HH manner in parallel and separated by 3.767 Å. In each 1,3-bdc ligand, one carboxylate bridges two Zn(II) atoms, while the other carboxylate is chelating. Due to the angular geometry of this dicarboxylate ligand, they form a one-dimensional coordination polymer (1D CP) with rings formed by [Zn2(1,3-bdc)2]. These chains are propagated approximately along the b-axis as shown in Figure 2.

Figure 3. In the packing of interstrands, the details of the intermolecular olefin pair alignments of 3F-4spy ligands in 1 are shown. Color codes: Zn green, O pink, N blue, C gold, and F aquamarine. The H atoms have been omitted for clarity.

ligands is aligned in a HT manner and stabilized by π···π interactions between pyridyl and 3-fluorophenyl groups. The olefin bonds in the intermolecular HT 3F-4spy ligands are nicely aligned in parallel and separated by 3.781 Å. But the second 3F-4spy ligand is spatially misplaced and not involved in such close alignment. On the basis of the crystal packing and the Schmidt’s topochemical criteria,1 one can expect that 50% of the [2 + 2] cycloaddition can occur in a HT manner, but 100% is expected to occur in a HH manner. Both are equally probable, and hence its photoreactivity was investigated. The photoreactivity of the powdered 1 under UV light was monitored using 1H NMR spectroscopy by taking out the UVirradiated samples at regular intervals of time and dissolved in DMSO-d6 solvent (Figure S5 in Supporting Information). Indeed, quantitative photoconversion was observed in less than 40 min (Figure S6 in Supporting Information). The disappearance of the olefin protons signals at 7.89 and 7.51 ppm, the shift of the signal for the pyridyl protons from 8.77 and 8.14 to 8.68 and 7.93 ppm, and the appearance of the cyclobutane protons at 5.08 and 4.76 ppm indicated 100% conversion of 3F-4spy ligands to cyclobutane rings. It is likely that the photoreaction occurs in a HH fashion. In the 1H NMR spectrum, the cyclobutane ring gives two doublets, and we thought of comparing this with the literature chemical shifts to confirm their stereochemistry. Dimerization of HH 4-spy ligand pair gives a single multiplet at 4.6 ppm.53−55 On the other hand, photocyclization of HT 4-spy ligand pairs and their fluoro derivatives also furnished a single multiplet or broad singlet around the same region40,56,57 as well as a doublet of multiplets at 4.9−5.1 ppm.58,59 Due to the ambiguity in the interpretations of the 1H NMR spectral data of the photoproduct [Zn2(1,3-bdc)2(rctt-3F-ppcb)2] 2, (where rctt-3F-ppcb

Figure 2. A perspective view of the coordination polymer 1. The centroids of the olefin groups of the intramolecular HH 3F-4spy pairs, shown as dotted lines, are separated by 3.767 Å. Color codes: Zn green, O red, N blue, C gold and F aquamarine. The H atoms have been omitted for clarity. B



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= 1,2-bis(4′-pyridyl)-3,4-bis(3′-fluoro-phenyl)cyclobutane), it was difficult to conclude whether the HH or HT photoreaction occurred in 1. Hence the solid state structure of the photoproduct becomes inevitable to resolve this problem. However, our attempts to perform single-crystal-to-singlecrystal (SCSC) structural transformation of 1 was not successful. Hence we synthesized similar coordination polymers with 2F-4spy and 4-spy ligands hoping to achieve a SCSC reaction. The description of the crystal structure of 3 is essentially the same as 1, but 0.25 lattice water was found in the asymmetric unit. Here also, although the pyridyl and 2F-phenyl groups are twisted by 16.1 and 6.2°, the HH 2F-4spy ligands are aligned in parallel (interplanar angle, 3.3°). The olefin pairs in this intraHH 2F-4spy terminal ligands are separated by 3.699 Å. In one of the well-aligned intermolecular HT 2F-4spy ligand pairs, the distance between the centers of the olefin pairs is 3.682 Å as shown in Figure 4. Here also 50% of HT dimerization is expected and 100% HH photodimerization is predictable using Schmidt’s criteria for [2 + 2] cycloaddition reaction.1,60

Figure 5. A view of the photodimerized 1D coordination polymer 4 showing the formation of HH-rctt-2F-ppcb ring. Color codes: Zn green, O red, N blue, C gold, and F aquamarine. The H atoms have been omitted for clarity.

data of 2 and 4, it may be concluded that the solid state structure of 2 should be similar to that of 4. In the meantime, we were able to separate the cyclobutane derivatives HH-rctt-3F-ppcb and HH-rctt-2F-ppcb from 2 and 4, respectively, recrystallized and determined their molecular structures as shown in Figure 6.

Figure 6. Molecular structures showing the stereochemistry of HHrctt-3F-ppcb isolated from 2 (left) and HH-rctt-2F-ppcb isolated from 4 (right).

Previously Liu et al. reported a similar photoreactive Cd(II) 1D CP, [Cd2(1,3-bdc)2(4spy)4], and studied their solid state photoreactivity.56 In this compound, the intramolecular HH4spy pairs were misaligned, while one of the intermolecular HT-4spy pairs was nicely aligned in parallel to undergo intermolecular partial [2 + 2] photo-cycloaddition to furnish an interesting two-dimensional (2D) CP. Hence we set out to synthesize a similar Zn(II) 1D CP with 4-spy to compare its photoreactivity with the Cd(II) analogue and the 2′ and 3′fluoro-substituted-4spy ligands. Although crystallized with similar crystal data in triclinic space group P1,̅ the crystal structure of the Zn(II) compound [Zn2(1,3-bdc)2(4spy)4]·2MeOH, 5 reveals that they are not strictly isostructural to 1 and 3. The repeating basic unit has center of inversion with Z = 1 (Figure S13 in Supporting Information). One of the 4-spy ligands was disordered with two independent orientations and a common occupancy factor of 0.613(5). Although the two six-membered rings in each disorder model is approximately coplanar (3.1° and 11.0°), the olefin bond is crisscrossed. In the un-disordered 4-spy ligand (N1−C12 atoms), the two six-membered rings are twisted by 34.2°. The intramolecular HH distances between the centers of the olefin bonds, 4.288 and 4.150 Å, are little more

Figure 4. A view of the packing in 3 showing the intermolecular alignments of the olefin bonds between the 2F-4spy ligands. Color codes: Zn green, O red, N blue, C gold, and F aquamarine. The H atoms have been omitted for clarity.

The solid state photoreactivity of 3 is very similar to that of 1 and furnished 100% conversion of olefin groups to cyclobutane rings in the photoproduct 4 as monitored by the 1H NMR spectral data (Figures S7 and S8 in Supporting Information). The observation of two doublets at 5.14 and 4.92 ppm in the spectrum of the complete photodimerized product similar to 2 further requires the need for the direct structural evidence in the solid state. Hence this prompted us to attempt the SCSC transformation. We have succeeded in getting suitable single crystals of 4 at the end of the UV experiment for intensity data collection, and the crystal structure analysis revealed the formation of the HH dimer, 1,2-bis(4′-pyridyl)-3,4-bis(2′fluoro-phenyl)cyclobutane (HH-rctt-2F-ppcb) as shown in Figure 5. This leads to the formation of another 1D CP. From the solid state structure of 4 and the 1H NMR spectral C



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photoreaction is more favorable over the intermolecular reaction in all the three crystals containing Zn(II). In the event of the photoreaction, it is likely that the geometry can be modified intramolecularly with ease to adjust the packing effects. The Zn···Zn distance increased from 4.131 in 3 to 4.335 to 4 to adjust the strain created by the formation of the cyclobutane ring. On the other hand, the Cd···Cd distance decreased slightly from 4.002 to 3.996 Å in the same photochemical process.56 In our previous work also we found that intramolecular photocyclization has been favored due to the presence of C−H···π, which prevents intermolecular [2 + 2] photo-cycloaddition, which was equally possible. The differences in the geometry and the photochemical behavior of the crystals containing the fluoro derivatives in 1 and 3 as compared to 5 could be attributed to the effect of fluorine substitution which has been recognized recently.43,61,62 Overall, the results described in this paper highlight the complications due to the influence of a small variation in the backbone of the terminal ligands on the packing and the resulting photoreactivity. Further, this provided additional knowledge toward designing of the solids to synthesize cyclobutane regioselectively.

than the Schmidt’s distance criterion to facilitate [2 + 2] cycloaddition reaction under UV light. On the contrary, the intermolecular olefin bonds from the adjacent strands that are disposed in a HT fashion. But only 50% of the 4spy ligands are disposed well within the Schmidt’s criteria (3.776 and 3.685 Å respectively in the major and minor disordered components, Figure S14 in Supporting Information). On the basis of analysis of packing, one can expect that only 50% photoreaction would occur in a HT fashion under normal conditions without pedal motion or any other molecular movements. The 1H NMR spectra were recorded in DMSO-d6 after the powdered 5 was irradiated under UV light at regular intervals of time (Figure S10 in Supporting Information). Only 90% of the photoconversion could be observed after 150 min of UV irradiation. Two new chemical shifts were observed for pyridyl protons at 8.67 and 8.64 ppm in addition 5 at 8.74 ppm. Further, complex chemical shifts in the region of 5.15−4.65 ppm indicate the formation probably two different types of cyclobutane rings. By comparing with the two doublets observed in 4 (Figures 7 and S9), it may be concluded that

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CONCLUSION Aligning olefin bond pairs in a HH and HT manner in the solid state go back to the classical work of Schmidt where different polymorphic modifications of cinnamic acid and its derivatives have been used to achieve different stereoisomers.1−5,12,13 In this work, we have investigated the photoreactivity of three Zn(II) 1D CPs in which the 4-spy and its derivatives have been aligned intramolecularly and intermolecularly in both HH and HT manner in the same compounds. Interestingly 1 and 3 furnished clean HH products 2 and 4 respectively, while 3 reacts in an SCSC manner furnishing 4 (Figure 8). In this

Figure 7. 1H NMR spectra of (a) photodimerized compound 6 and (b) 4 with the selected peak assignments.

the photoproduct 6 has two cyclobutane rings arising from both HH and HT photoreaction. From the 1H NMR data, the ratio of HH and HT products was estimated to be 25:50. NOESY experimental details and complete assignments of peaks are given in Figures S11 and S12, and Tables S2−S4 in Supporting Information. Here, the photoreactive behavior of 5 is different from that of Cd(II) analogue56 as well as from 1 and 3. Despite a smaller ionic radii (Zn(II), 0.83 Å versus Cd(II), 0.99 Å), the Zn···Zn distances in 1, 3, and 5 respectively are 4.147, 4.131, and 4.003 Å, which are longer than that observed in Cd(II) analogue, 4.002 Å. This could be attributed to the geometrical differences in the chelating carboxylates. Interestingly, the distance between the centers of the intradimer olefin bonds in the Cd(II) CP is 4.276 Å due to twisting of the two six-membered rings in the two 4-spy ligands (33 and 80.5°). Furthermore, the double bonds are crisscrossed. Hence there was no possibility of a HH reaction. Similar arrangements were also found in 5, and the intraolefin pairs are separated by 4.288 and 4.150 Å (major and minor disordered 4-spy ligands). It is likely that the rotation of the 4spy ligands around Zn−N bonds brings the olefin bonds in the HH pair closer under UV light and thus competes with the HT reaction. It appears that intramolecular

Figure 8. SCSC photochemical structural transformation of 3 to 4 via HH photo-cycloaddition reaction. A similar non-SCSC structural conversion is proposed for the photochemical reaction of 1 to 2.

process both 1 and 3 are structurally transformed from 1D CP to another 1D CP. The cyclobutane derivatives were separated, and their molecular structures have been established unequivocally by X-ray crystallography. On the contrary, the 4-spy ligands in 5 yielded 6, which has furnished a mixture of HH and HT cyclobutane derivatives when exposed to UV light. The formation of butane rings from the HT [2 + 2] cycloaddition reaction is expected to provide a layer CP as it stitches the neighboring strands together. In the case of 1 and 3, the photoproducts have been obtained cleanly by the photoreaction of the intramolecular HH dimer, while a mixture of products for 5. We believe that the structural and photoreactive insights provided here help in designing photoreactive CPs to furnish new regioselective cyclobutane rings. D



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100.875(1)°; V = 1350.05(9) Å3; Z = 1; ρcalc = 1.545 g·cm−3; μ = 0.969 mm−1; GOF = 1.060; final R1 = 0.0291; wR2 = 0.0682 [for 5994 data I > 2σ(I)]. Crys tal Da ta for 3 at 10 0(2) K ( CCDC182 3677) . C68H49F4N4O8.5Zn2, M = 1264.85; triclinic, P1̅; a = 9.2000(3), b = 10.1079(3), c = 15.7346(5) Å, α = 103.734(1), β = 96.230(1), γ = 104.484(1)°; V = 1354.30(7) Å3; Z = 1; ρcalc = 1.551 g·cm−3; μ = 0.967 mm−1; GOF = 1.077; final R1 = 0.0451; wR2 = 0.1274 [for 6816 data I > 2σ(I)]. Crystal Data for 4 at 100(2) K (CCDC 1823678). C68H50F4N4O9Zn2, M = 1273.86, triclinic, P1̅; a = 9.9446(4), b = 10.1403(5), c = 15.9535(9) Å; α= 74.905(2), β = 86.344(2), γ = 76.449(2)°; V = 1358.2(1) Å3; Z = 1; ρcalc = 1.547 g·cm−3; μ = 0.965 mm−1; GOF = 1.070; final R1 = 0.0617; wR2 = 0.1605 [for 4632 data I > 2σ(I)]. Crystal Data for 5 at 100(2) K (CCDC 1823679). C70H60F4N4O10Zn2, M = 1247.96; triclinic, P1̅; a = 9.9674(4), b = 11.8891(4), c = 13.2552(5) Å; α = 108.350(1), β = 78.486(1), γ = 89.154(2)°; V = 1455.82(1) Å3; Z = 1; ρcalc = 1.423 g·cm−3; μ = 0.891 mm−1; GOF = 1.070; final R1 = 0.0335; wR2 = 0.0639 [for 6178 data I > 2σ(I)]. Crystal Data for rctt-3F-ppcb at 100(2) K (CCDC 1823680). C26H22F2N2O, M = 416.46; monoclinic, P21/c; a = 15.7883(7), b = 11.3190(5), c = 11.9889(5) Å; β = 100.057(2)°; V = 2109.6(2) Å3; Z = 4; ρcalc = 1.311 g·cm−3; μ = 0.758 mm−1; GOF = 1.026; final R1 = 0.0467; wR2 = 0.1054 [for 3015 data I > 2σ(I)]. Crystal Data for rctt-2F-ppcb at 100(2) K (CCDC 1823681). C26H22F2N2O, M = 416.46; Monoclinic, P21/c; a = 15.7371(3), b = 11.1048(2), c = 12.2800(2) Å; β = 100.785(2)°; V = 2108.11(7) Å3; Z = 4; ρcalc = 1.312 g·.cm−3; μ = 0.759 mm−1; GOF = 1.057; final R1 = 0.0353; wR2 = 0.0868 [for 3476 data I > 2σ(I)].

EXPERIMENTAL SECTION

Preparation of [Zn2(1,3-bdc)2(3F-4spy)4], 1. Colorless blockshaped single crystals were obtained from the slow evaporation of the methanolic solution of Zn(NO3)2·6H20 (29 mg, 0.1 mmol), 1.3benzenedicarboxylic acid (16.6 mg, 0.1 mmol), and 3F-4spy (39.6 mg, 0.2 mmol) (yield: 42 mg, 67%). 1H NMR (300 MHz, DMSO-d6/TFA, 298 K): δ = 8.77 (d, 8H, Py-H of 3F-4spy), 8.49 (s, 2H, bdc), 8.14 (d, 8H, Py-H of 3F-4spy), 8.12 (dd, 4H, bdc), 7.89 (d, 4H, HCCH of 3F-4spy), 7.51 (d, 4H, HCCH of 3F-4spy), 7.51−7.37 (m, 14H, PhH of 3F-4spy and 2H of bdc) 7.13 (bdd, 4H, Ph-H of 3F-4spy); elemental analysis calcd (%) for C68H48F4N4O8Zn2: C 65.03, H 3.85, N 4.46; found: C 65.12, H 3.88, N 4.33. Preparation of [Zn2(1,3-bdc)2(rctt-3F-ppcb)2]·H2O, 2. Single crystals of 1 were ground finely and packed between the slides and exposed to UV for more than 1 h to form 2 in an almost quantitative yield (based on 1). These glass slides were flipped back at regular intervals of time to maintain the uniform exposure of UV irradiation. 1 H NMR (300 MHz, DMSO-d6/TFA, 298 K): δ = 8.68 (d, 8H, Py-H of rctt-3F-ppcb), 8.48 (t, 2H, bdc), 8.11 (dd, 4H, bdc), 7.93 (d, 8H, Py-H of rctt-3F-ppcb), 7.48 (t, 2H, bdc), 7.14 (bdd 4H, Ph-H of rctt3F-ppcb), 7.04 (bd, 4H, Ph-H of rctt-3F-ppcb), 7.00 (bd, 4H, Ph-H of rctt-3F-ppcb), 6.82 (bdd, 4H, Ph-H of rctt-3F-ppcb), 5.08 (d, 4H, cyclobutane proton of rctt-3F-ppcb), 4.76 (d, 4H, cyclobutane protons of rctt-3F-ppcb); elemental analysis calcd (%) for C68H50F4N4O9Zn2: C 64.11, H 3.96, N 4.40; found: C 64.13, H 3.84, N 4.58. Preparation of [Zn2(1,3-bdc)2(2F-4spy)4]·0.5H2O, 3. This compound was synthesized by using a similar synthetic procedure, except by using 2F-4spy (39.6 mg, 0.2 mmol) instead of 3F-4spy, and colorless block-shaped single crystals were formed within a few days (yield: 40 mg, 64%). 1H NMR (300 MHz, DMSO-d6/TFA, 298 K): δ = 8.81 (d, 8H, Py-H of 2F-4spy), 8.49 (s, 2H, bdc), 8.23 (d, 8H, Py-H of 2F-4spy), 8.14 (dd, 4H, bdc), 7.96 (d, 4H, HCCH of 2F-4spy), 7.86 (d, 4H, Ph-H of 2F-4spy), 7.56 (d, 4H, HCCH of 2F-4spy), 7.52 (t, 2H, bdc), 7.45 (m, 4H, Ph-H of 2F-4spy), 7.26 (bdd, 8H, PhH of 2F-4spy); elemental analysis calcd (%) for C68H48F4N4O8Zn2: C 65.03, H 3.85, N 4.46; found: C 64.87, H 3.91, N 4.29. Preparation of [Zn2(1,3-bdc)2(rctt-2F-ppcb)2], 4. Compound 4 was obtained upon UV irradiation of 3 for more than 1h. 1H NMR (300 MHz, DMSO-d6/TFA, 298 K): δ = 8.66 (d, 8H, Py-H of rctt-2Fppcb), 8.49 (t, 2H, bdc), 8.12 (dd, 4H, bdc), 7.93 (d, 8H, Py-H of rctt2F-ppcb), 7.53 (d, 2H, bdc), 7.35 (bdd, 4H, Ph-H of rctt-2F-ppcb), 7.09 (m, 4H, Ph-H of rctt-2F-ppcb), 6.97 (bdd, 4H, Ph-H of rctt-2Fppcb), 6.88 (bdd, 4H, Ph-H of rctt-2F-ppcb), 5.14 (d, 4H, cyclobutane protons of rctt-2F-ppcb), 4.92 (d, 4H, cyclobutane protons of rctt-2Fppcb); elemental analysis calcd (%) for C68H50F4N4O9Zn2: C 64.11, H 3.96, N 4.40; found: C 64.42, H 3.90, N 4.26. Preparation of [Zn2(1,3-bdc)2(4-spy)4]·2CH3OH, 5. Pale yellow, block-shaped single crystals were obtained by using similar synthetic procedures mentioned above, except using 4spy (36.4 mg, 0.2 mmol) instead of 3F-4spy (yield: 35 mg, 70%). 1H NMR (300 MHz, DMSOd6/TFA, 298 K): δ = 8.74 (d, 8H, Py-H of 4spy), 8.49 (bs, 2H, bdc), 8.13 (d, 8H, Py-H of 4spy), 8.10 (dd, 4H, bdc), 7.90 (d, 4H, HC CH), 7.70 (d, 8H, Ph-H of 4spy), 7.51 (t, 2H, bdc), 7.47−7.25 (m, 16H, HCCH and Ph-H of 4spy); elemental analysis calcd (%) for C68H52N4O8Zn2: C 68.98, H 4.43, N 4.73; found: C 68.87, H 4.49, N 4.66. Preparation of 6. Compound 6 was obtained upon UV irradiation of 5 for more than 3 h. 1H NMR (300 MHz, DMSO-d6/TFA, 298 K): δ = 8.67 (d, Py-H of HH-isomer), 8.64 (d, Py-H of HT-isomer), 8.49 (t, bdc), 8.13 (dd, bdc), 7.94 (d, Py-H of HH-isomer), 7.87 (d, Py-H of HT-isomer), 7.51 (d, bdc), 7.23−7.00 (m, Ph-H of HH-isomer and HT-isomer), 5.06 (d, cyclobutane protons of HH-isomer), 4.98 (dd, cyclobutane protons of HT-isomer), 4.88 (dd, cyclobutane protons of HT-isomer), 4.73 (d, cyclobutane protons of HH-isomer); elemental analysis calcd (%) for C68H54N4O9Zn2: C 67.95, H 4.53, N 4.66; found: C 68.29, H 4.33, N 4.25. Crystal Data for 1 at 100(2) K (CCDC 1823676). C34H24F2N2O4Zn1, M = 1255.84; triclinic, P1̅; a = 8.5288(3), b = 10.1194(4), c = 16.2886(6) Å; α = 101.033(1), β = 83.176(1), γ =

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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.8b00442. Materials and general methods, isolation of cyclobutane derivatives, crystallographic data, PXRD patterns, TGA curves, 1H NMR spectra, NOESY spectra of 2 and 6 (PDF) Accession Codes

CCDC 1823676−1823681 (for 1, 3, 4, 5, rctt-3F-ppcb, and rctt2F-ppcb, respectively) 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] (J.J.V.). ORCID

Jagdese J. Vittal: 0000-0001-8302-0733 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the Ministry of Education, Singapore, for financial support through NUS FRC grant Tier 1 No. R-143-000-678114 and R-143-000-A12-114. We thank Ms. Geok Kheng Tan of X-ray diffraction laboratory, CMMAC at NUS for the X-ray intensity data collection and data processing. V.A.S. is thankful E



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to SERB, Department of Science and Technology, Government of India, for Overseas Postdoctoral Fellowship.

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