Dynamic Transformation and Reversible Guest Encapsulations of

K.; Okano , T.; Kachi-Terajima , C.; Sakaguchi , H. Eur. J. Inorg. Chem. 2011, 3059– 3066. [Crossref], [CAS]. 4. Transformation of a CuII Thiazo...
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Dynamic Transformation and Reversible Guest Encapsulations of Pseudopolymorphs of a Fully Fluorinated β‑Diketonate Pd(II) Complex Kyosuke Nakajima and Akiko Hori* Department of Chemistry, School of Science, Kitasato University, 1-15-1 Kitasato, Minami-ku, Sagamihara, Kanagawa 252-0373, Japan S Supporting Information *

ABSTRACT: Two types of pseudopolymorphs, 1·(2)3 and 1· (2)2, were obtained from Pd(II) complex (1) with mesitylene (2), and the quantitative transformation into 1·(2)2 from 1· (2)3 was observed over 6 h after shaking for approximately 10 s. The transformation was observed in both a mesitylene-rich solution and under powder conditions. The guest release processes of 1·(2)3 and 1·(2)2 occurred in hexane to yield 1, and the guest insertion processes occurred in 2 to reproduce 1· (2)3 and 1·(2)2, whereas each crystal was constructed from the discrete molecules of each type. These crystals also selectively and repeatedly encapsulated three benzenes or one toluene in the uniform cavities of 1·(2)3 and 1·(2)2. In contrast, the corresponding nonfluorinated derivative, [Pd(dbm)2] (dbm = dibenzoylmetanide), shows no guest encapsulation phenomena, indicating that the remarkable guest encapsulations are induced by fluorine substitutions. The dynamic crystal transformation and the reversible guest encapsulations of the two pseudopolymorphs were investigated by X-ray crystallographic and thermogravimetric studies for both single crystals and powder species. through arene−perfluoroarene interactions, the fluorination dramatically contributes to the host−guest chemistry for the nonporous crystalline materials. Such an idea prompted us to prepare the corresponding Pd(II) complex 1 for the creation of the pseudopolymorphs, and we discovered the transformation to 1·(2)2 from 1·(2)3 by external stimuli. The transformation was clearly characterized by X-ray crystallography and thermogravimetry (TG). The crystals of 1·(2)2 and 1·(2)3 were further investigated as a molecular-ordered host framework, and we discuss the reversible and quantitative guest encapsulations of the two pseudopolymorphs. The Pd(II) complex 1 was prepared from the corresponding ligand and freshly prepared Na2[PdCl4] with NaOH in a 74% yield by typical procedures.12 The results of the elemental analysis showed the following for the nonsolvated crystal of 1: calcd. for C30H2F20O4Pd (%): C 39.48, H 0.22; found: C 39.59, H 0.45. The single crystals of 1 were obtained as yellow needles in CH2Cl2 and were suitable for an X-ray crystallographic analysis.13 The complexes in the crystal were highly overlapped along the b axis to produce layered arrangements with the shortest intermolecular metal···metal distance being 3.621(1) Å. Crystal 1 was recrystallized using mesitylene (2) in CH2Cl2 to yield orange prismatic crystals of 1·(2)3 (Figure 1a-i), which

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ynamic crystal transformation, induced and progressed by external stimuli, e.g., light, heat, pressure, or vibration, has become a significant field of interest in crystal engineering and material science, and accordingly, the development of single crystal-to-crystal transformations has also been a crucial issue for many chemists.1,2 Such crystal transformations are roughly classified into two types: (i) a chemical reaction that occurs between atoms of the components to induce molecular rearrangements in the crystals3,4 and (ii) a replacement of parts of the crystal components by different molecules to produce different crystal states in host−guest chemistry.1,5 These phenomena can be artificially produced by chemical techniques, which have been highly developed during the last two decades. On the other hand, another challenging operation for crystal transformations is for polymorph and pseudopolymorph crystals.6,7 In particular, the handling of pseudopolymorphism, in which a number of crystal components are rearranged to give differential crystal states, is still a difficult subject because the systematic preparations of pseudopolymorphs having multiple stable crystal states are seldom performed, whereas accidental products of pseudopolymorphs are commonly obtained.6−8 In such research areas, we found a unique compound that is a fully fluorinated coordination complex suitable for the investigation of pseudopolymorphs.9−11 The crystals of the Cu and Ni derivatives of this compound encapsulated aromatic molecules to give several pseudopolymorphs. Because the aromatic guest encapsulation is induced by the pentafluorophenyl groups of the complex © 2014 American Chemical Society

Received: February 13, 2014 Revised: June 6, 2014 Published: June 23, 2014 3169

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recrystallization process again. Thus, the prismatic crystals of 1·(2)3 were generally obtained as the first crystallization product from the solution of 1 and 2. The crystal structures of 1·(2)3 and 1·(2)2 are shown in Figure 1b and c, respectively.13 The crystal parameters and the selected bond information on 1, 1·(2)3, and 1·(2)2 are summarized in Tables S1−S9 of the Supporting Information. In the crystal of 1·(2)3, the asymmetric unit contains two halves of complex 1 and three independent mesitylene molecules. The coordination environment of the metal in 1·(2)3 is very similar to that in the crystal of 1. Two complexes are alternately arranged along the b axis, and the intermolecular Pd···Pd distance is 6.8579(5) Å. Two types of mesitylenes (shown in blue and green in Figure 1b) were inserted between the complexes through arene−perfluoroarene interactions,16,17 and the average distance between the centroid of the aromatic rings in 2 and the centroid of the pentafluorophenyl groups in 1 is 3.593 Å along the b axis. Accordingly, the pentafluorophenyl group is highly twisted from the coordination plane to form the alternate structure (the corresponding torsion angle is −53.2°). The remaining one mesitylene (shown in pink) is sandwiched between two pentafluorophenyl groups of the complexes along the a axis and the average distance of the intermolecular stacking between the centroid of the aromatic rings in 2 and the centroid of pentafluorophenyl group is slightly long (3.671 Å). Accordingly, the mesitylene (pink) is linearly arranged in the columnar cavity along the b axis. Thus, the strength of the arene−perfluoroarene interaction of the mesitylene (pink) is weaker than that of the above two mesitylene molecules (blue and green), which suggests that the mesitylene (pink) most likely contributes to the crystal transformation. On the other hand, the crystal of 1·(2)2 is composed of onehalf of complex 1 and one mesitylene molecule 2 in the asymmetric unit. The coordination structures around the metal in the crystal of 1·(2)2 are almost the same as those in crystals 1 and 1·(2)3. The guest of 2 closely interacts with the pentafluorophenyl groups of 1 through arene−perfluoroarene interactions; the shortest intermolecular distance between the centroid of the aromatic ring in 2 and the centroid of the pentafluorophenyl group in 1 is clearly short (3.499 Å). The pentafluorophenyl group of the adjacent complex also interacts with 2 (the corresponding distance is 3.662 Å) to form sandwich structures. Both pentafluorophenyl groups are highly twisted from the coordination planes. Accordingly, the guest molecules of 2 linearly arrange in the columnar cavity along the a axis, as shown in Figure 1c. For the pseudopolymorphs 1·(2)3 and 1·(2)2, TG studies distinguished two different eliminations for the crystals (Figure 2) at a scanning rate of 5 °C min−1.18 The total elimination of

Figure 1. (a) Photographs of the transformation to block crystals 1· (2)2 from prismatic crystals 1·(2)3. The crystals were calmly maintained for 2, 4, and 6 h after shaking the bottle containing the crystallizing solution.14 The crystal packing structures of (b) 1·(2)3 projected along the c axis and of (c) 1·(2)2 projected along the a axis are also shown. The graphics with stick and CPK models show complex 1 and mesitylene 2, respectively; color schemes: Pd, dark blue; O, red.

were composed of the complex and three mesitylene molecules.14 Notably, the prismatic crystals were completely transformed into new orange block crystals over the course of 6 h after shaking the bottle containing the crystals of 1·(2)3 and the remaining solution (Figure 1a). A light shaking by hand for approximately 10 s was sufficient.15 Figure 1a indicates that the prismatic crystals were dissolved at the crystal interface and were reorganized as block crystals. The small block crystals grew larger during the time period from 2 to 6 h. These new block crystals were proven to be 1·(2)2 (Figure 1a-iv). The following results of the elemental analyses clearly showed the compositions of 1·(2)3 and 1·(2)2: calcd. for C57H38F20O4Pd (%) for 1·(2)3: C 53.77, H 3.01; found: C 53.49, H 3.04; calcd. for C48H26F20O4Pd (%) for 1·(2)2: C 50.00, H 2.27; found: C 50.02, H 2.57. The crystal of 1·(2)2 was stable in solution with external stimuli; however, the solution mixture again gave prismatic crystals of 1·(2)3 when the block crystals of 1·(2)2 were completely dissolved upon heating or adding halogenated organic solvents, which caused the solution to start the

Figure 2. TG curves of 1·(2)3 (solid line) and 1·(2)2 (dashed line); the scan rate was 5.0 °C min−1. 3170

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1·(2)3 was observed to be −28.4% (solid line), which was close to the calculated value of −28.3%. The weight loss of 1·(2)3 indicated that the elimination occurred in two steps; the first and second steps were observed to be −9.4% at approximately 48−65 °C and −19.0% at approximately 65−110 °C, respectively. Another slight step was observed at approximately 90 °C, indicating that the three mesitylene molecules were restrained by different environments in the crystal, which was also demonstrated by X-ray crystallographic studies, as discussed above. The mesitylene (pink) in 1·(2)3 was most likely removed in the first step due to its position in the columnar cavity. For 1·(2)2, the weight loss was observed to be −21.6% in a one-step elimination at approximately 67−129 °C (dashed line), which was close to the calculated value of −20.8%. These elimination behaviors indicated that the release process is promoted at a lower temperature as the number of guest molecules increases because the existence of release pathways is determined by the guests in the crystal. To understand the exchange conditions, the crystals of 1·(2)3 were filtered off and replaced in small amounts of several solutions. Typically, the prismatic crystals of 1·(2)3 changed to block crystals of 1·(2)2 in 2 (Figure 3a), indicating that the

Both crystals of 1·(2)3 and 1·(2)2 were quickly added to hexane to give light yellow guest-free crystals of 1, while maintaining the original forms of the prismatic and block crystals, respectively. The colors of the two crystals were very similar to that of 1, which was prepared from the elimination of the guest molecules upon heating in the TG analyzer. No releases of guest molecules were observed in the TG for the crystals, indicating that the mesitylene molecules were extracted in hexane and no encapsulation of hexane was observed. XRD patterns of both guest-free crystals are completely the same for 1 (Figure 4). After the elimination of 2 from 1·(2)3 in hexane,

Figure 4. Powder X-ray analysis indicates the reversible mesitylene release/insertion processes for 1·(2)3 and 1·(2)2.

the remaining guest-free prismatic crystals of 1 were placed in 2 to reversibly produce 1·(2)3 without shaking.20 The TG study of the 1·(2)3 generated crystals clearly indicated a stepwise elimination similar to the above TG curves of 1·(2)3 (Figure 2). However, the guest-free block crystals of 1, prepared from 1· (2)2 in hexane, selectively and quantitatively reproduced 1·(2)2 in 2. Thus, the number of reinserted mesitylene molecules precisely depended on the host crystal structures along with the reproducibly and both XRD patterns show no conversion mixtures (Figure 4). Of note is that the guest reinsertion of 1· (2)3 and 1·(2)2 from each dry crystal of the prismatic and block crystals, respectively, occurred by only sprinkling one drop of 2 onto the crystal. Selectivity of the number of guest molecules inserted disappeared, and only 1·(2)2 was obtained when the mesitylene molecules were removed from 1·(2)3 and 1·(2)2 upon heating (∼140 °C) by TG. Both XRD patterns of the new guest-free crystals after TG were different with lower crystallinities from the hexane-extracted guest-free crystal of 1 (Figures 4 and 5a). Furthermore, the aromatic molecules were stoichiometrically and reversibly inserted under vapor conditions, when the mesitylene molecules were removed from the crystals. In this case, no relationship to the number of guest molecules inserted was observed for any of the conditions of the starting crystals 1· (2)3 or 1·(2)2. Typically, the guest-free and dry prismatic crystals of 1 from 1·(2)3 were stored under benzene (C6H6) vapor for approximately 10 min at rt. The crystal maintained its original form in the benzene vapor; however, XRD and TG studies clearly confirmed the reversible guest insertion and release processes between 1 and 1·3C6H6 (Figure 5). The guest-free and dry block crystals of 1 from 1·(2)2 show the same guest encapsulation behaviors (see Figures S9 and S11 of the Supporting Information). The XRD patterns of 1 from 1· (2)3 and 1·(2)2 were completely changed in the benzene vapor (Figure 5a),21 and new patterns were very similar to the single crystal of 1·3C6H6, which was independently prepared.13 During the elimination process of the benzene-encapsulating

Figure 3. (a) Single crystals of 1·(2)3 transform into the mesitylenepoor crystals of 1·(2)2 in mesitylene 2. (b) Powder X-ray analysis shows the transformation of the crystals.

phenomena did not depend upon the basic principal of LeChatelier. The crystal transformation was also observed in a 1:1 solution of 2 and several organic solvents, e.g., hexane and EtOH. For the powder sample of 1, XRD and TG studies clearly probed that the following two unique phenomena were induced by 2 (Figure 3b): (1) the powder of 1 (Figure 3b-i), which has an XRD pattern that is the same as that of a single crystal of 1,13 quickly changed to 1·(2)3 (Figure 3b-ii) by soaking in 2 for a few seconds; (2) the powder of 1·(2)3 dramatically and completely changed to 1·(2)2 (Figure 3b-vi) during the XRD experiment.19 The results indicate that 1·(2)3 is kinetically obtained as a metastable product and that 1·(2)2 is a thermodynamically stable product. 3171

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ing nonfluorinated complex [Pd(dbm)2] shows no guest encapsulation in both single crystal and powder conditions. Only [Pd(dbm)2] was crystallized as a single crystal23 from a solution including the guests, and no releases of guest molecules were observed in the TG for [Pd(dbm)2] when maintained in a solution with the guest molecules (SI Figure S13). The results indicate that the fluorination clearly contributes and predicts guest encapsulations for these nonporous systems, which is relatively rare. The reversible guest release and insertion processes also occurred in 2 for both of the 1·(2)3 and 1·(2)2 crystals reproducibly, whereas each crystal was constructed from one discrete type of molecule. These results indicate that the fully fluorinated complex 1 produces a sufficient guest encapsulation system similar to the MOF (metal−organic framework) system. In particular, the crystals produce flexibly and dynamically transformed host cavities and retain memory of the original pseudopolymorph structure.

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ASSOCIATED CONTENT

S Supporting Information *

Experimental procedures, crystallographic structural analyses, and TG results. This material is available free of charge via the Internet at http://pubs.acs.org.

Figure 5. (a) Powder X-ray analysis shows the reversible benzene release/insertion processes for 1·(2)3 and 1·(2)2. (b) TG curves showing three repeated release processes of benzene and toluene molecules in the prismatic crystals 1, which were prepared from 1·(2)3. Benzene releases are shown in blue and toluene releases are shown in red. TG was performed at a scan rate of 5.0 °C min−1.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected].

prismatic crystal that slowly started at rt, a remarkable weight loss occurred at approximately 30−45 °C to reproduce 1. The weight loss was observed to be −18.2%, which was close to the calculated value of 1·3C6H6 (−20.4%). The block crystals of 1 from 1·(2)2 also reversibly recognized three benzene molecules to produce 1·3C6H6 while maintaining their original forms (SI Figure S11). The appearances of both crystals slightly decayed during the experiments, but these reversible guest encapsulations could be continuously repeated. The driving interaction of the guest encapsulation for one benzene is estimated as a metal···π interaction, and for two benzenes it is an arene− perfluoroarene interaction, which is similar to the case of the single crystals 1·3C6H613 and the corresponding Cu derivative.9 Toluene (C7H8) was also reversibly encapsulated into both types of crystals 1 to produce 1·C7H8,21,22 and the elimination process of the crystal 1·C7H8 started at approximately 50−86 °C to reproduce 1. The weight loss was observed to be −9.2%, which is the same as the calculated value (Figure 5b and Figure S10 and S12 of the Supporting Information). Based on our interpretation of the crystallographic and TG studies, the reversible guest insertion/release processes indicate that complex 1 recognizes the guest molecules at the molecular level not only by the cavity effect, but also by the strong intermolecular electrostatic interactions, which are induced by the fluorination of 1. In conclusion, we detected the pseudopolymorphism of 1 with mesitylene 2 to produce the prismatic crystal of 1·(2)3 and the block crystal of 1·(2)2. The transformation from 1·(2)3 to 1·(2)2 occurred in 2 by merely shaking the solution as a general phenomenon using several solutions. The two stable pseudopolymorphs and the several guest encapsulations were established by the fluorinated complex 1, revealing that the flexibe, twisted, and interacting pentafluorophenyl groups of 1 highly recognized the guest molecules through metal···π and arene−perfluoroarene interactions. In addition, the correspond-

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by a JST, PRESTO program (2008− 2012), and in part by a Kitasato University Research Grant for Young Researchers. We thank Prof. Hidetaka Yuge of Kitasato University for valuable advises. Thanks are also due to Prof. Yoko Sugawara of Kitasato University for the opportunity of the XRD measurements.

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REFERENCES

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