Article pubs.acs.org/crystal
Solid State Packing and Photoreactivity of Alkali Metal Salts of trans,trans-Muconate Goutam Kumar Kole,†,‡ Anjana Chanthapally,† Geok Kheng Tan,† and Jagadese J. Vittal*,† †
Department of Chemistry, National University of Singapore, 3 Science Drive 3, Singapore 117543 Chemistry Division, Bhabha Atomic Research Centre, Mumbai 400085, India
‡
S Supporting Information *
ABSTRACT: Three alkali-metal salts of trans,trans-muconate (muco) viz. Li2muco (1), Na2muco (2), and K2muco (3) have been prepared, and the influence of the crystal packing on the solid state photoreactivity has been investigated. Although the CC bonds of the muco ligands are oriented infinitely parallel in 1, it was found to be photoinert. In contrast, the muco ligands of 2 and 3 in the crystalline state undergo photodimerization yielding cycloocta-3,7-diene-1,2,5,6-tetracarboxylate which has been formed stepwise via the [2 + 2] cycloaddition reaction of a single pair of CC bonds and subsequent Cope rearrangement. This study demonstrates how the size of the metal ion can influence the crystal packing in metal organic salts.
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INTRODUCTION Research on the topochemical [2 + 2] cycloaddition reaction in the solid state has progressed far from where Schmidt left us with his “topochemical postulates”1 by the collective efforts of organic/inorganic crystal engineers, solid state chemists, and photochemists.2−6 The understanding of the molecular arrangement in the solid state by directional supramolecular interactions has become more developed, in which crystal engineering principles7 can be utilized to control solid state packing of a substrate as desired. Apart from the physical properties, the crystal packing has a tremendous effect on the chemical properties as well, in particular, on the photoreactivity. The way an unsymmetrical olefin or an organic molecule containing multiple CC bonds stacks in parallel arrangement is much more important in organic solid state synthesis, as different photodimers can be obtained from various possible ways of packing. For example, it is demonstrated how two stereoisomers can be obtained via the solid state [2 + 2] cycloaddition reaction from the head-to-head and head-to-tail parallel stacking of trans-3-(4′-pyridyl)acrylic acid (4-PA), an unsymmetrical olefin.8,9 For an organic molecule having multiple CC bonds, not only does different crystal packing lead to different products formation, but the same crystal packing often leads to the formation of different products; e.g. it may either lead to © XXXX American Chemical Society
photodimerization or photopolymerization/oligomerization when stacked in an infinite parallel arrangement.10−17 When the CC bonds are conjugated, the chemistry is very interesting and much more challenging as there are many possibilities viz [2 + 2] cycloaddition of a single CC bond pair, [2 + 2] cycloaddition of multiple CC bond pairs, [4 + 4] cycloaddition reaction18−20 and photopolymerization/ oligomerization. The [2 + 2] cycloaddition of a single CC bond pair leads to the formation of compounds containing one cyclobutane ring, whereas simultaneous [2 + 2] cycloaddition of multiple CC bond pairs leads to the formation of ladderane compounds.21−25 The [4 + 4] cycloaddition reaction leads to the formation of compounds containing a cyclooctadiene ring and so on. The cyclooctadiene-compounds can also be obtained via the [2 + 2] cycloaddition reaction of a single CC bond pair followed by Cope rearrangement.26 Therefore, there are ample opportunities remaining to further understand the reactivity of the diverse solid state chemistry of organic molecules containing consecutive CC bonds. Solid state photoreactivity of muconate has been reported before by Okada as organic salts26 and by Michaelides in Received: August 22, 2015 Revised: October 2, 2015
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DOI: 10.1021/acs.cgd.5b01213 Cryst. Growth Des. XXXX, XXX, XXX−XXX
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coordination polymers.27−29 Recently, we have shown the formation of the cyclooctadiene ring via concerted [4 + 4] cycloaddition reaction of muco in its gold(I) macrocyclic complexes.30 Although photoreactive CC bonds have been successively aligned parallel for exhibiting solid state photoreactivity in discrete metal complexes,30−35 coordination polymers,36−42 and various types of organic salts8,9,43−48 and cocrystals,22,49−54 the metal−organic salts have not been yet investigated extensively in this regard. The photoreactivity of two polymorphs of K2SDC (where H2SDC is trans-4,4′-stilbene dicarboxylic acid) was reported recently.55 The size of the alkali metal ions may also have a huge influence on the solid state structures of their carboxylate salts. In this contribution, we have employed trans,trans-muconic acid as olefin containing consecutive CC bonds, and herein, we report its solid state photoreactivity in alkali-metal organic salts.
Figure 1. Infinite, slip-stacked alignment of muco in 1 where each C C bond is in close proximity with another CC bond of other muco.
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probably restricted due to nonavailability of “reaction cavity”, a concept that Cohen introduced long ago.57 The K2muco salt crystallized in the monoclinic P21/n space group with Z = 2, where each K+ is coordinated by six oxygen atoms from muco and each carboxylate oxygen is in turn coordinated to three K+ centers in μ1,1,1 and μ1,3 bridging and chelate modes. A detailed analysis of the crystal structure reveals that the muco are arranged in infinite parallel orientation with a center-to-center distance of 3.97 Å (Figure 2). As the
RESULTS AND DISCUSSION The alkali-metal organic salts of muconate were obtained by reacting alkali-metal hydroxides with trans,trans-muconic acid in water, and single crystals were obtained by slow evaporation. Single crystal X-ray crystallography has been used to characterize the solid state structures of Li2muco and K2muco successfully, while suitable single crystals were not obtained for Na2muco for this purpose. Although Li2muco (1) was found to be photostable, the higher congeners, Na2muco (2) and K2muco (3) (Scheme 1) were found to be photoreactive and furnished almost similar photoreactivity. Scheme 1. Structures of Li2muco (1), Na2muco (2), and K2muco (3)
Figure 2. Infinite parallel arrangement of muco in 3 where the adjacent CC bonds are stacked parallel.
arrangement is infinitely parallel on a plane, each CC bond is closely stacked with two more CC bonds of the nearest neighbors as shown in Figure 2. Such a “face-to-face” parallel arrangement of the adjacent double bonds has various possibilities for photoreaction including polymerization, dimerization with a single pair of CC bonds, dimerization with a single pair of CC bonds followed by Cope rearrangement leading to the formation of a cyclooctadiene ring, formation of the same cyclooctadiene ring via concerted [4 + 4] cycloaddition reaction and formation of a ladderane via dimerization with both pairs of CC bonds. Interestingly both 2 and 3 were found to be photoreactive in the solid state. However, the PXRD patterns of 2 and 3 do not match with each other (see SI). The details of our investigations are discussed below. While 1 was found to be photoinactive, both 2 and 3 were found to be photoreactive and exhibited almost similar behavior. Upon irradiation under UV light, the formation of a single cyclobutane ring and cycloocta-3,7-diene-1,2,5,6-tetracarboxylate (A and B respectively in Figure 3) were observed by 1H NMR spectroscopy (see SI). This result triggered the question whether the formation of cycloocta-3,7-diene-1,2,5,6-
The Li2muco salt crystallized in the monoclinic P21/c space group with Z = 2. The asymmetric unit contains half of the formula unit. Each Li+ is coordinated by four oxygen atoms from muco in distorted tetrahedral geometry where each carboxylate oxygen of muco coordinates two Li+ centers in μ1,1 and μ1,3 bridging modes. A thorough analysis to the crystal structure reveals that the muco are arranged in slipped stacked parallel orientation where only one olefin bond is closely situated with another olefin bond from the neighboring muco with a center-to-center distance of 3.98 Å. However, all the muco ligands are not coplanar, as shown in Figure 1. The infinitely parallel arrangement of the olefin bonds provide equal opportunity for the muco ligands to undergo photodimerization leading to polymerization. Unfortunately, this salt was not found to exhibit any photochemical reaction upon irradiation under UV light. Infinitely aligned olefin bonds are expected to show at least 82−87% photoreactivity.56 The reaction is B
DOI: 10.1021/acs.cgd.5b01213 Cryst. Growth Des. XXXX, XXX, XXX−XXX
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Figure 3. Schematic representation of the overall photoreactivity of Na+, and K+ salts of muco, 2 and 3 in the solid state.
tetracarboxylate was stepwise or concerted. To investigate this further, the courses of the photoreactions for both 2 and 3 have been monitored by 1H NMR spectroscopy with time, and the percent (%) conversions versus time (h) curves were plotted as shown in Figure 4. From the plot for 2, it can be observed that as the reaction proceeds, the percent composition of muco decreases gradually while B increases with time, whereas for A, it goes through a maximum and then gradually decreases (Figure 4a). This is analogous to the kinetics of consecutive reactions where the final product is formed through an intermediate.58−60 Therefore, it may be proposed that the formation of B from 2 follows stepwise mechanism during the photoreaction under UV light. It is also supported by the fact that ∼21% of A was observed to form after irradiation of 2 for 1 h when no B was detected by 1H NMR spectroscopy (Figure 5a and also see SI). On the other hand, the formation of B from 3 also follows stepwise mechanism; however, the maxima of A in % conversion vs time plot is not so prominent as compared to that for 2 (Figure 4b). It also can be noted that about 11% of B could be detected in 1H NMR spectroscopy even within an hour of UV irradiation which was not the case for 2. These observations can be attributed to the fact that k1 (rate constant for the first step) is much greater than the k2 (rate constant for second step) in the former case, whereas in the latter case, they are comparable.60 In both the cases, the dimer A undergoes Cope rearrangement to form B. In 2 the reaction is incomplete; about 12% of muco was left unreacted even after irradiation for longer time (30 h). However, in 3, all the muco was found to be consumed. The observed yields for cycloocta-3,7-diene-1,2,5,6tetracarboxylate (B) were 80% from 2 and 86% from 3. During the UV irradiation experiments with 2 and 3, in addition to A, some other dimers were also observed as minor products, C and D, which could not be converted to B even after heating. The proposed structures of C and D, as shown in the Figure 3, are in agreement with the observation of Michaelides et al. in coordination polymers of muco.28,29 The maximum percentages of the combined formation of C and D are about 5% and 10% respectively from 2 and 3. The formations of these minor products are inconsistent and could only be detected toward the end of the dimerization process (Figure 5), so their percentages are not accounted for in Figure
Figure 4. Percent (%) conversions versus time plots for (a) Na2muco and (b) K2muco.
4. The formations of these minor products might have stemmed from the structural change in 2 and 3 that might have taken place after most of the muco converted to dimer A or B in the solid state. The overall photoreactivity observed in this series of metal organic salts is summarized in Figure 3. In all three salts, muco ligands are aligned parallel within the required distance but in different ways. The arrangement of muco is perfectly parallel in 3 and presumably in 2; however, it is slip-stacked in Li-salt, 1. The dimer which could be theoretically obtained from the Lisalt is also different from the one that is obtained from Na- and K-salts. Unfortunately, the Li-salt was found to be photoinactive, yet the required alignment of muco in the solid state to produce stereoisomer E has been achieved by tuning the size of the alkali metal ions. Both the Na- and K-salts exhibited similar photoreactivity and yielded cycloocta-3,7-diene-1,2,5,6-tetracarboxylate (B) as the major product. The formation of B follows stepwise mechanism which goes through [2 + 2] cycloaddition reaction of a single pair of CC bonds and subsequent Cope rearrangement. The similarity in the observed photoreactivity of Na2- and K2muco also helps us to predict the structure of the former where both the CC bonds of muco are expected to align parallel within the distance of 4.2 Å apart. However, we C
DOI: 10.1021/acs.cgd.5b01213 Cryst. Growth Des. XXXX, XXX, XXX−XXX
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products of 2 and 3 have been investigated, and it was found that cycloocta-3,7-diene-1,2,5,6-tetracarboxylate could be obtained via Cope rearrangement of the dimerized product from a single pair of CC bonds. The size of the alkali metal ions might be the structure determining factors that control the difference in solid state packing of muco. This study emphasizes the importance of alkali metal organic salts in organic synthesis, and the concept can be employed to other conjugated olefins of a similar kind.
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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.5b01213. Experimental details, NMR, PXRD, TG analyses and crystal data (PDF) Crystallographic information files61 (CIF1 and CIF2)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Fax: +65 67791691. Tel: +65 65162975. Funding
Ministry of Education Singapore Notes
The authors declare no competing financial interest.
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Figure 5. 1H NMR spectra of (a) 2 after 1 h UV irradiation, (b) 2 after 3 h UV irradiation, (c) 2 after 18 h UV irradiation, and (d) 3 after 18 h of UV irradiation. The asterisked peaks represent the minor products.
ACKNOWLEDGMENTS We sincerely thank the Ministry of Education Singapore for financial support through NUS FRC Grant R-143-000-604-112. We also thank Yimian Hong for the X-ray data collection of the two salts.
cannot be sure that the alignment of muco in Na-salt is infinitely parallel as observed in K-salt because of mismatch of their PXRD patterns (see SI); it could be either infinite or discrete pairs. The formation of ladderane as the sole product has been reported with ester21 and pyridyl22 systems. In our previous work on gold(I) macrocyclic complexes, discretely aligned muco pairs underwent a [4 + 4] cycloaddition reaction in a concerted manner and yielded cycloocta-3,7-diene-1,2,5,6-tetracarboxylate as the sole product quantitatively.30 However, in this work, infinitely aligned muco furnished cycloocta-3,7-diene-1,2,5,6tetracarboxylate as the major product in a stepwise manner. Similarly, mixtures of products were observed from lanthanide coordination polymers of muco.27−29 It is still not clear why a single [2 + 2] cycloaddition followed by Cope rearrangement was preferred over a direct [4 + 4] cycloaddition reaction or the double [2 + 2] cycloaddition reaction which can furnish a ladderane structure. The parameters that control the stepwise and concerted pathway to yield ladderane or cyclooctadiene derivative (B) are yet to be established. Hence, more studies are needed to fully understand and control the photoreactivity of conjugated olefin bond systems. Further theoretical or computational studies may provide a better understanding of the reactivity of the muco anions.
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CONCLUSION The muco anion has been aligned parallel in two ways in the solid state as metal organic salts of alkali metal ions and their photoreactivity has been discussed. Two different products could have been obtained, if 1 were photoreactive, but the Li+ salt was surprisingly inert. The pathways of various photoD
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