CL-170656
Received: July 13, 2017 | Accepted: July 31, 2017 | Web Released: August 5, 2017
Double Chiral Hybrid Materials: Formation of Chiral Phenolic Resins on Polyamine-associated Chiral Silica
R.-H. Jin
X.-L. Liu
S. Tsunega
T. Ito
M. Takanashi
M. Saito
K. Kaikake
Xin-Ling Liu, Seiji Tsunega, Takumi Ito, Maho Takanashi, Miwa Saito, Katsuya Kaikake, and Ren-Hua Jin* Department of Material and Life Chemistry, Kanagawa University, 3-27-1 Rokkakubashi, Kanagawa-ku, Yokohama, Kanagawa 221-8686 (E-mail:
[email protected]) Chiral SiO2 nanofibers associated with polyamines on their surfaces could promote polymerization between resorcinol and formaldehyde on silica surface to give phenolic resins (RF). In this process, the chiral information was effectively transferred from SiO2 to the final phenolic resins forming double chiral hybrid. Keywords: Double chiral hybrid | Chiral SiO2 nanofiber | Chiral phenolic resin
The synthesis of chiral polymers with optical activity is of major importance because of their great demand in many areas like life sciences, pharmaceuticals, and optics.13 To achieve chirality in polymers during the polymerization process, various chiral sources have been employed for chiral transfer, which includes chiral organometallic complexes catalysts, chiral organic solvents, chiral polymeric templates, and even circular polarized light.4 On the other hand, some inorganic materials (e.g., porous SiO2, metal nanoparticles) have been widely used as supports for the load of chiral components to construct various heterogeneous catalysts, which possess advantages like easy separation and recycling of catalysts; however, most of these inorganic supports are achiral.57 Based on the best of our understanding, to date, there are no reports on chiral polymerization using achiral catalyst supported by chiral inorganic supports. For a long time, the chiral inorganics are less of a concern compared with organics, despite the existence of natural inorganic crystals with intrinsically chiral crystal structures (e.g., quartz) or surfaces (e.g., Pt(643)).8,9 However, in the past two decades, the research on the chiral transfer between organics and inorganics has gained increasing attention, and a series of synthetic inorganics have been endowed with chirality by molecular imprinting from small chiral organic molecules (on the molecular scale) or by copying the helical shapes (with helical pitch sizes over tens of nanometers) of organic templates.10,11 Interestingly, without the combination of external chiral components, it has been demonstrated that some natural and synthetic chiral inorganics could be directly applied to the enantioselective separation and synthesis of chiral organic small molecules (e.g., amino acids, glucoses).9,1215 Consequently, these achievements have opened a new avenue for chirality transfer from inorganics to organics. Recently, we reported a successful approach for the preparation of carbon nanotubes via carbonization of hybrid nanofibers of phenolic resins-coated
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silica which were synthesized by polymerization of resorcinol and formaldehyde on the polyamine (catalyst)-associated silica (template and support) nanofibers.16 On the other hand, we also succeeded in the synthesis of chiral silica promoted by crystalline complexes self-assembled from polyethyleneimine (PEI) and enantiomeric tartaric acid (tart).17 By combining the above two achievements, herein, we attempted the synthesis of chiral phenolic resins (RF) by polymerization of resorcinol (R) and formaldehyde (F), employing the SiO2/PEI chiral hybrid, in which silica is the chiral source and PEI is a catalyst. The RF isolated after removing chiral silica via treatment with HF (3% aq) showed remarkable optical activity but without helical outward on the length-scale over tens nm. Moreover, the optical activity was still present on the carbonaceous products obtained from the pyrolysis of chiral RF at 500 °C in inert atmosphere. The general synthetic procedures are shown in Scheme 1. First, SiO2 was deposited on the chiral complexes of PEI/tart to form PEI/tart@SiO2. Then, from PEI/tart@SiO2, we prepared two types (I and II) of PEI-associated chiral silica (SiO2/PEI) for the synthesis of phenolic resins. Type I (SiO2/PEI-I) was obtained by treatment of PEI/tart@SiO2 with HCl (aq) and ammonia alternatively to remove the tart component. Type II (SiO2/PEI-II) was obtained via calcination of PEI/tart@SiO2 at 600 °C for 3 h to remove all the organic components (PEI/tart) and then re-adsorbing PEI onto the remaining silica. Notably, in our earlier work, PEI/tart@SiO2 was used in the preparation of chiral PEI/tart@SiO2@RF and their direct carbonization to obtain chiral carbonaceous products. However, because of the existence of tartaric acid residues in PEI/tart@SiO2, it was still not clear whether RF itself was intrinsically optical active due to no isolation of RF from SiO2. Herein, we performed polymerization between R and F in water at 60 °C for 24 h using D- and L-forms of SiO2/PEI-I (SiO2/PEI-I-D and SiO2/PEI-I-L) as catalytic/chiral templates (details seeing the Supporting Information), producing orange-colored powders of SiO2/PEI@RFI-D and SiO2/PEI@RF-I-L, respectively. Similar reaction was also performed using SiO2/PEI-II-D and -L. To address the concern mentioned above, we treated all the pairs of SiO2/PEI@RFI-D and -L, as well as SiO2/PEI@RF-II-D and -L, with 3% HF solution to remove the SiO2 and PEI components. This resulted in a pair of RF-I-D and RF-I-L and a pair of RF-II-D and RF-II-L. To understand some details of the products of the type I series, characterizations were performed with TG-DTA, FT-IR, 13 C NMR, and SEM on the D-form products. According to the mass loss in the temperature range of 150800 °C on the TG-
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Scheme 1. Synthetic procedures of chiral phenolic resins (RF) on the surfaces of two types (I and II) of SiO2/PEI.
Figure 1. Solid-state 13C NMR spectra of RF-I-D (insets, the structural units in RF). DTA curves (Figure S1), the mass ratio of organic components and temperature of exothermal peaks are listed as follows: 29.7% (270, 345 °C) for PEI/D-tart@SiO2, 10.8% (239 °C) for SiO2/PEI-I-D, 45.8% (270, 460 °C) for SiO2/PEI@RF-I-D. The decrease in mass ratio of ca. 18.9% from PEI/D-tart@SiO2 to SiO2/PEI-I-D indicated the removal of tartaric acid residues, while the increase of ca. 35% from SiO2/PEI-I-D to SiO2/ PEI@RF-I-D suggested the deposition of RF. The FT-IR spectra (Figure S2) also monitored these steps in a remote mode. From the PEI/D-tart@SiO2 to SiO2/PEI@RF-I-D, the characteristic peaks for SiOSi bond stretching (ca. 1078 cm¹1), bending (ca. 800 cm¹1), and rocking modes (ca. 455 cm¹1) were observed. However, these peaks decreased greatly or disappeared on the final RF-I-D treated by 3% HF (aq). Moreover, the removal of SiO2 was also supported by the energy-dispersive X-ray (EDX) spectra of RF products because the characteristic peaks for the Si element were hardly detected (Figure S3). The peak around 1626 cm¹1 for C=O of tartaric acid disappeared with the transformation of PEI/D-tart@SiO2 to SiO2/PEI-I-D, which further confirmed the removal of tartaric acid components. For the SiO2/PEI@RF-I-D and RF-I-D, peaks between 1440 and 1630 cm¹1 are assigned to the aromatic C=C of RF. As shown in the solid-state 13C NMR spectra of RF-I-D (Figure 1), six peaks appeared within the chemical shifts from 20 to 160 ppm, which were assigned to methylene (24 and 29 ppm) and aromatic carbons (102, 119, 129, and 151 ppm).18 There is no characterChem. Lett. 2017, 46, 1518–1521 | doi:10.1246/cl.170656
Figure 2. SEM (ad) and TEM (ef ) images for the samples of type I series: a) PEI/D-tart@SiO2, b) SiO2/PEI-I-D, c) SiO2/ PEI@RF-I-D, d) and e) RF-I-D, and f ) RF-I-L. istic peak around 43 ppm for the methylene carbon (CH2N) of PEI, indicating the removal of PEI in the step of HF treatment (Figure S4). The SEM images of the products are shown in Figure 2. In the present work, the morphologies of the D- and L-products are similar. Therefore, only the SEM images of the D-form series are shown here. This is quite different from the general phenomenon with helical outwards that show definite opposite images for P- and M-helical morphologies transferred from the D- and Lform enantiomers. Both the PEI/D-tart@SiO2 and SiO2/PEI-I-D (Figures 2a and 2b) exhibited nanofibrous morphologies with an average diameter of ca. 18 nm, indicating that the removal of tartaric acids residues by HCl (aq) did not damage the morphologies. In comparison, the SiO2/PEI@RF-I-D (Figure 2c)
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also appeared as nanofibers, but with an enhanced average diameter of ca. 22 nm, indicating selective deposition of RF on the surface of SiO2/PEI-I-D. However, after removal of PEI and SiO2 components from SiO2/PEI@RF-I-D by 3% HF, the nanofibers could not be maintained on the as-obtained RF-I-D (Figure 2d). The TEM images of the RF-I-D (Figure 2e) and RFI-L (Figure 2f ) further revealed that the nanofibers were broken somewhat and then coalesced during the etching by HF (aq). Apparently, the hard silica acts as props to support fibrous morphology, although the soft RF itself can sustain the freestanding feature. To check the optical activity of the products from the type I series, the powders were subjected to the solid-state diffused reflectance circular dichroism (DRCD) spectra. As shown in Figure 3, the mirror-image relationship of the CD curves is found on all the D-form products and their corresponding L-form ones. From Figure 3a, for PEI/tart@SiO2, we can see a pair of peaks with strong intensities located in the 200235-nm range and another one with weak intensity around 250 nm. Since a CD signal in 200240 nm is due to the pure D-tartaric acid
Figure 3. DRCD (left) and UVvis absorption (right) spectra for the D- and L-form samples of type I series: a) PEI/ tart@SiO2, b) SiO2/PEI-I, c) SiO2/PEI@RF-I, d) RF-I (black lines for D-form samples and red lines for L-form ones). 1520 | Chem. Lett. 2017, 46, 1518–1521 | doi:10.1246/cl.170656
(Figure S5) and an absorption peak around 250 nm is assigned to PEI (Figure S6), the CD signals of PEI/tart@SiO2 around 200 235 and 250 nm correspond to the CD signals of tartaric acids and induced CD of PEI, respectively. However, after treating by HCl, the former signals disappeared while the latter ones remained for both SiO2/PEI-I-D and -L (Figure 3b). Together with the TG-DTA and FT-IR results shown above, this change in the CD signals further corroborated that the tartaric acids have been removed by HCl, while PEI remained in SiO2. In the CD spectra of SiO2/PEI@RF-I-D and -L (Figure 3c), broad peaks with strong intensities across 300600 nm, together with small peaks around 260 nm, were observed. These peaks are attributed to the phenolic resins of RF where the broad signals extending to visible light are due to the conjugation units in resorcinol residues. Surprisingly, when the PEI and chiral SiO2 components were removed completely by HF, the isolated RF (RF-I, Figure 3d) still showed the CD activity with mirror relations, similar to those of SiO2/PEI@RF-I-D and -L. Yang et al. reported helical phenolic resins of RF with helical pitches of hundreds of nm, which were prepared using helical supramolecular templates in the presence of ammonia.19 Using cellulose nanocrystals as templates, Khan et al. also prepared chiral nematic mesoporous phenolic resin with CD activity.20 These helical or nematic phenolic resins showed CD signals across 200700 nm, similar to the CD signals obtained in this work. Therefore, these results clarified the above-mentioned concerns that the RF itself, formed in the presence of SiO2/PEI, is intrinsically chiral, indicating that the organic polymer chirality is successfully transferred from inorganic chiral SiO2 to form double chiral organic inorganic hybrid materials (“double chiral” means that both the SiO2 and RF have chirality). The type II catalytic templates (SiO2/PEI-II) were also used in polymerization between R and F to further probe the specific roles of PEI and SiO2. In the control experiment using calcined silica without polyamine association, no polymerization took place at 60 °C for 24 h. Indeed, the amine groups of PEI catalyze the polymerization.21 Interestingly, the SiO2/PEI-II-L also effectively promoted the formation of chiral RF on SiO2 to form hybrid SiO2/PEI@RF-II-L, and the final RF-II-L product showed similar DRCD spectra (Figure S7) to that of RF-I-L (Figure 3d). For type I, PEI was in situ hybridized with SiO2 framework during the formation of SiO2; for type II, PEI was post-absorbed onto the calcined SiO2. Despite this difference in the combination way between PEI and SiO2, the final RF products in both cases were optically active, which strongly suggested that chiral SiO2 should be the chiral sources in the formation of chiral phenolic resin. We found that without PEI, the polymerization between R and F could be achieved by increasing the reaction temperature to 80 °C. Here, two reaction systems were employed: one is the reaction only containing R and F to give RF products, and the other is the reaction containing R, F, and chiral calcined SiO2. In the former one, the RF products appeared as spheres, as seen in the SEM images (Figure S8a); in the latter one, spheres were also observed, separated from SiO2 nanofibers (Figure S8b). In both, no obvious CD signals were observed in the DRCD spectra of the spherical RF resins (Figure S9). These results implied two points: i) the polymer of PEI is a key to promote the preferable deposition of RF on the surfaces of SiO2, and ii) the chiral transfer occurs mainly on the domains of PEI-associated SiO2.
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Different from the most frequently reported general chiral SiO2, which have definite helical morphologies with different pitches, there are no helical outwards in our SiO2 nanofibers on the size-scale over 10 nm. We recently revealed that cluster-like sub-10-nm chiral domains are present in our chiral silica.22,23 In other words, the chirality would arise from the asymmetric arrangement of Si and O atoms through the nanofibrous silica frames (
