Halogen Bond Effect for Single-Crystal-to-Single-Crystal

Note pubs.acs.org/Macromolecules

Halogen Bond Effect for Single-Crystal-to-Single-Crystal Transformation: Topochemical Polymerization of Substituted Quinodimethane Takahito Itoh,*,† Shinji Nomura,† Hirofumi Nakasho,† Takahiro Uno,† Masataka Kubo,† Norimitsu Tohnai,‡ and Mikiji Miyata‡ †

Division of Chemistry for Materials, Graduate School of Engineering, Mie University, 1577 Kurima Machiya-cho, Tsu, Mie 514-8507, Japan ‡ Department of Material and Life Science, Graduate School of Engineering, Osaka University, 2-1 Yamadaoka, Suita, Osaka 565-0871, Japan S Supporting Information *

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INTRODUCTION Topochemical polymerization is counted as a specific case in the solid-state polymerizations, and it proceeds with no movement of a center of gravity of the monomer molecule and only slight rotation of the monomer molecule around the center of gravity, indicating that the crystallographic position and symmetry of the monomer crystals are retained in the resulting polymer crystals. X-ray single-crystal structure determination of monomer and especially polymer crystals can provide directly an experimental evidence to prove the topochemical polymerization. Such direct proofs are available for the polymerizations of derivatives of diacetylenes,1 distyrylpyrazines and phenylene diacrylates,2 trienes and triacetylenes,3 muconic and sorbic acids,4 and [2,2′-bi-1Hindene]-1,1′-dione-3,5-diyldialkyl carboxylate.5 Previously, we investigated photochemical and thermal polymerizations of 7,7,8,8-tetrakis(alkoxycarbonyl)quinodimethanes with various alkoxy groups in the solid state6 and found that 7,7,8,8tetrakis(alkoxycarbonyl)quinodimethane monomers with methoxy, chloroethoxy, and bromoethoxy groups afford their highly crystalline polymers (Scheme 1).

might proceed via a topochemical reaction mechanism. On photochemical polymerization in the solid state, UV light is absorbed mainly by the molecules present at the surface of the crystals, and the strength of the UV light is weakened rapidly inside of the crystals. That is, polymerization rate is significantly different between the surface and inside of the crystals. It is therefore considered that the difference in the polymerization rate induces defects or cracks in the crystals, resulting in polymer single crystals with poor quality. Moreover, when the crystals of these quinodimethanes were kept at room temperature in dark for 6 months, their solid-state polymerizations took place slowly, and white polymer crystals, indicating the presence of cracks, were formed. It is considered that it is difficult to obtain their polymer crystals suitable for Xray single-crystal structure analysis on the thermal polymerization. To obtain the polymer crystals suitable for X-ray singlecrystal determination, polymerization reaction is required to proceed rapidly and homogeneously in the crystal. As X-ray, γray, and electron beam have generally excellent penetration ability, solid-state polymerization reactions under their irradiation are expected to take place rapidly and homogeneously and to provide polymer crystals with fewer defects. It is reported that the solid-state polymerizations of diacetylene and diene derivatives proceeded rapidly and smoothly under the irradiation of an X-ray beam.1,4 In addition, halogen atoms are expected stabilizing crystal structures by halogen bonds.6 In order to obtain polymer crystal with high quality through the rapid and homogeneous polymerization reaction, we attempted the polymerization of 7,7,8,8-tetrakis(bromoethoxycarbonyl)quinodimethane (1) in the solid state under the irradiation of a γ-ray beam using 60Co as the high-energy source. Successfully, we obtained single polymer crystal of 1 with fewer defects or fewer cracks suitable for the structure determination. In this Note, we describe the result of the crystal structure analysis of 1 and its polymer crystal (poly-1) obtained by exposure to 60Co γ-irradiation and provide the first example with clear-cut evidence of a topochemical polymerization for substituted quinodimethane-type monomer.

Scheme 1. Polymerizations of Substituted Quinodimethanes in the Solid State

Unfortunately, photochemical and thermal polymerizations of these quinodimethanes in the solid state did not afford the polymer crystals suitable for single-crystal structure determination. Therefore, on the basis of the indirect experimental evidence such as infrared, elemental analysis, and powder X-ray analysis for the monomers and the corresponding polymers, we concluded that solid-state polymerizations of these monomers © 2015 American Chemical Society

Received: May 19, 2015 Revised: July 9, 2015 Published: July 23, 2015 5450

DOI: 10.1021/acs.macromol.5b01078 Macromolecules 2015, 48, 5450−5455

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EXPERIMENTAL SECTION

Materials. 1 was prepared as described previously7 and recrystallized from chloroform to give the yellow platelet crystals. Crystal suitable for X-ray single-crystal structure determination was obtained by recrystallization from chloroform upon slow evaporation of the solvent. Polymerization Procedure. Yellow platelet crystal of 1 with various sizes and qualities (obtained by recrystallization) was scattered on a Petri dish and then exposed to 60Co γ-radiation, which was carried out at The Institute of Scientific and Industrial Research, Osaka University, at a dose of 5 kGy (1 kGy = 0.1 Mrad) at 30 °C and then kept for 1 week at room temperature to give colorless, transparent platelet crystal. X-ray Measurement. The single-crystal X-ray data were collected on a Rigaku RAXIS-RAPID imaging plate diffractometer using Cu Kα radiation (λ = 1.541 86 Å) monochromated with graphite. The structure was solved by the direct methods with the program SIR928 and refined by full-matrix least-squares procedures. All calculations were performed using the TEXSAN crystallographic software package of the Molecular Structure Corporation.9

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RESULTS AND DISCUSSION Crystal Structure of Monomer. The single-crystal of 1 suitable for X-ray structure determination was obtained successfully by recrystallization from chloroform upon slow evaporation of the solvent. The cell constants and other crystallographic parameters obtained by the single-crystal structure analysis of 1 at −180 °C are given in Table 1,

Figure 1. Crystal structures of 1 (a) viewed from the crystallographic a-axis, (b) viewed from the crystallographic b-axis, and (c) viewed from the crystallographic c-axis. The distances (in Å) of halogen bond, hydrogen bond, and halogen−hydrogen bond interactions are shown by numbers.

Table 1. Crystallographic Data for 1 and Poly-1 formula formula weight shape of crystal crystal system space group a (Å) b (Å) c (Å) α (deg) β (deg) γ (deg) V (Å3) Z Dcalc (g/cm3) no. unique reflecns no. obsd reflecns R1 (F2 < 2σ(F2)) R, Rw GOF 2θmax (deg) temp (°C)

1

poly-1

C10H10Br2O4 353.99 platelet triclinic P-1̅ (No. 2) 6.911(1) 7.727(1) 12.082(2) 101.66(1) 97.22(1) 106.59(1) 593.906 2 2.158 1993 5778 0.059 0.093, 0.161 1.85 136.5 −180

(C10H10Br2O4)n 353.99 platelet triclinic P-1̅ (No. 2) 6.795(2) 8.051(2) 11.928(3) 98.28(2) 103.43(2) 106.30(2) 593.692 2 1.980 1956 5060 0.132 0.213, 0.299 1.32 136.4 −180

exists in the same plane of the quinodimethane ring and another ester group at 7 and 8 positions has an upward or downward carbonyl conformation to the molecular plane of the quinodimethane ring. Monomer molecules stack along a-axis to form columns through weak hydrogen-bonding interaction (HC−H···OC, 2.63 Å). And also, the molecules construct monolayer sheet with weak hydrogen-bonding interactions (Ar−H···OC, 2.52 Å, and HC−H···OC, 2.67 Å) between parallel stacks in the direction extended to the b-axis. In addition, the sheets extended to the b-axis are connected through strong halogen bond (Br···Br, 3.27 Å) and weak halogen−hydrogen bonds (HC−H···Br, 2.99 and 3.03 Å) in parallel along the c-axis. Previously, we evaluated the stacking manner in a columnar structure by structural parameters (θ1, θ2, ds, and dcc) used for a planner structure molecule, where ds is the stacking distance between the adjacent monomers in a column, dcc is the distance between the reacting exo-methylene carbons, and θ1 and θ2 are the angles between the stacking axis and longer axis of the monomer molecule and the shorter axis of the molecule, respectively.6b The stacking parameters are summarized in Table 2. Crystal Structure of Polymer. When the platelet monomer single crystals obtained by recrystallization from chloroform were heated or irradiated with high-pressure mercury lamp, polymer crystals obtained by both polymerizations were not suitable for the single-crystal structure analysis because of poor quality. On the other hand, when the yellow platelet monomer crystals with various sizes and qualities were subjected to 60Co γ-radiation polymerization at 30 °C and kept for 1 week at room temperature, mixtures of the pink to white color polymer crystals and the crack-free colorless, transparent ones were obtained. We selected colorless and transparent platelet polymer crystal (poly-1) suitable for the crystal structure determination and performed the single-crystal

where the molecule is determined by a half part of the molecular structure because the center of symmetry is present at the center of the molecule, and the molecular packing structure of 1 in the crystal is shown in Figure 1. 1 crystal belongs to a space group of P-1̅ (No. 2) and has a triclinic unit cell with a = 6.911(1) Å, b = 7.727(1) Å, c = 12.082(2) Å, α = 101.66(1)°, β = 97.22(1)°, and γ = 106.59(1)°, where one molecule is included. As shown in Figure 1, the monomer molecule is a highly planar as expected for the quinodimethane ring, where one carbonyl of the two ester moieties on exo-methylene carbons at 7 and 8 positions 5451

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radiation polymerization, that is, molecular structures of 1 and poly-1, is shown in Figure 3.

Table 2. Stacking Parameters for Crystals of 1 and Poly-1

1 poly-1

θ1 (deg)

θ2 (deg)

ds (Å)

dcc (Å)

31.3 12.4

89.4 87.6

6.91 6.80

3.73 1.55

structure analysis at −180 °C. The crystallographic data and the stacking parameters are summarized in Table 1 and Table 2, respectively, and also the polymer molecular packing structure in the crystal is shown in Figure 2.

Figure 3. Changes of the stacking parameters before and after γradiation polymerization.

Dynamical Mechanism for Topochemical Polymerization. The monomer molecule forms a columnar structure as pointed out already, and the polymerization takes place along this column structure (along a-axis), where almost no large displacement of the center of mass is observed before and after the polymerization reaction: the monomer molecules are rotated to approach each other to make a covalent bond (1.55 Å) between the exo-methylene carbons of the adjacent monomer molecules, which are separated by 3.73 Å. The angles (θ1 and θ2) between the column axis and the longer axis of the monomer molecules and between the column axis and the shorter axis of the monomer molecules are about 31.3° and 89.4°, respectively, which are bent to 12.4° and 87.6° upon the polymerization. It was reported in the topochemical polymerizations of diacetylene derivatives, triene and triacetylene derivatives, and diene derivatives that when the ds values in the crystals are matched to those for the corresponding polymer crystals, topochemical polymerization could occur easily and rapidly.1,3,4 In the polymerization of 1, the ds value (6.91 Å) in the monomer crystal is very close to that (6.80 Å) in the corresponding polymer crystal. Therefore, this matching in the ds value between the monomer crystal and the polymer one allowed 1 to polymerize in a topochemical mode and also to provide the polymer crystal with fewer defects or fewer cracks. Figure 4 shows molecular arrangements, conformations of side chains, and intermolecular interactions before and after the polymerization of 1. Their comparisons suggest sequential processes along the three crystallographic axes as follows. First, along the crystallographic a-axis (Figure 4a), the exo-methylene sp2 carbon atoms of 1 with a distance of 3.73 Å approach each other to form a covalent bond with a distance of 1.55 Å. Such a change from sp2 to sp3 carbon insists a bromoethoxycarbonyl group (A in Figure 4a), which is placed on an aromatic ring of a neighboring quinodimethane molecule, to move upward and the other group (B in Figure 4a) to move downward. This induces a conformational jumping of the bromoethoxycarbonyl group so as to form a halogen−hydrogen bond (HC−H···Br, 2.84 Å). Second, along the crystallographic b-axis (Figure 4b),

Figure 2. Crystal structures of poly-1 (a) viewed from the crystallographic a-axis, (b) viewed from the crystallographic b-axis, and (c) viewed from the crystallographic c-axis. The distances (in Å) of halogen bond, hydrogen bond, and halogen−hydrogen bond interactions are shown by numbers.

Poly-1 crystal belongs to a space group of P-1̅ (No. 2), which is the same with that of 1 crystal, and has a triclinic unit cell with a = 6.795(2) Å, b = 8.051(2) Å, c = 11.928(3) Å, α = 98.28(2)°, β = 103.43(2)°, and γ = 106.30(2)°, where one molecule is included. Polymerization reaction took place along a-axis to form trans-type polymer. During the polymerization, a change in the lattice length is −1.7 and −1.3% for the a- and caxes, respectively, and +4.2% for the b-axis. Changes in the lattice lengths between the monomer and polymer crystals are very small. And also, the unit-cell volume of 1 crystal does not change upon polymerization. These findings observed before and after polymerization reaction support strongly that this polymerization proceeds via a topochemical reaction mechanism. The change of the stacking parameters before and after γ5452

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Figure 4. Comparisons of molecular arrangements, conformations of side chains as well as intermolecular interactions before and after the solid-state polymerization of the substituted quinodimethane: (a) one-dimensional molecular arrangements along the crystallographic a-axis direction, (b) sheet structures with concavity spaces (hatched line) along the b-axis direction, and (c) stacked layered structures along the c-axis direction. A indicates the bromoethoxycarbonyl group present in the same plane of the quinodimethane ring, and B shows upward or downward bromoethoxycarbonyl group relative to the molecular plane of the quinodimethane ring.

the quinodimethane molecules form a two-dimensional sheet through hydrogen bonds (CO···H−CH, 2.59 and 2.67 Å; Ar−H···OC, 2.52 Å), while the resulting polymer chains

form the sheet through different hydrogen bonding (CO··· H−CH, 2.47 and 2.58 Å). After the polymerization, the concavity space (area indicated with hatched line) on the 5453

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surface is expanded due to the above-mentioned jumping of the bromoethoxycarbonyl group through the halogen−hydrogen bond (HC−H···Br, 2.84 Å). Third, along the c-axis (Figure 4c), the two-dimensional sheets of the monomer molecules are stacked through strong halogen bond (Br···Br, 3.27 Å) and halogen−hydrogen bonds (Br···H−CH, 2.99 and 3.03 Å). The resulting layered structure has a convex part which is inserted by the concavity space between the neighboring sheets like a lock-and-key relationship. The polymer crystal has a similar layered structure through weakened halogen bonds (Br···Br, 3.79 Å) and halogen−hydrogen bonds (Ar−H···Br, 2.98 Å). A convex part of the neighboring sheet is inserted into the expanded concavity space in the upper sheet, which is attributable to smooth and appropriate changes of the network and space size through halogen bonds. Such a layered structure would release crystal strains generated by the polymerization, allowing the topochemical polymerization to proceed successfully.

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CONCLUSION On the basis of the X-ray single-crystal structure determination of 1 and poly-1 crystals obtained by the irradiation of a γ-ray beam using 60Co, it was explicitly confirmed that the polymerization of the substituted quinodimethane occurs via a topochemical reaction mechanism. This is the first clear-cut experimental evidence of the topochemical polymerization for quinodimethane-type monomers. Successful topochemical polymerization is attributed to the halogen bond effect on the layered structures.

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ASSOCIATED CONTENT

S Supporting Information *

Crystallographic information on 1 and poly-1 in CIF format. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.5b01078.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (T.I.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by Grant-in-Aid for Scientific Research (No. 22550110) from the Ministry of Education, Culture, Sports, Science and Technology, Japan. The authors thank the Radiation Laboratory, the Institute of Scientific and Industrial Research, Osaka University, for the use of 60Co facilities.

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REFERENCES

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Macromolecules (9) TEXSAN, X-ray Structure Analysis Package; Molecular Structure Corporation, The Woodlands, TX, 1985.

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DOI: 10.1021/acs.macromol.5b01078 Macromolecules 2015, 48, 5450−5455