Communication Cite This: Organometallics XXXX, XXX, XXX−XXX
Visible-Light-Sensitive Sulfonium Photoacid Generators Bearing a Ferrocenyl Chromophore Yukihiro Takahashi, Shintaro Kodama,*,† and Youichi Ishii* Department of Applied Chemistry, Faculty of Science and Engineering, Chuo University, 1-13-27 Kasuga, Bunkyo-ku, Tokyo 112-8551, Japan S Supporting Information *
ABSTRACT: A series of sulfonium salts bearing a ferrocenyl chromophore, [FcMeArS][X] (Ar = Ph, 4-CH3C6H4; X = BF4, PF6, BArF4), has been synthesized, which can generate an acid under 436 nm irradiation. The present study provides a rare example of ferrocene derivatives having a photoacid-generating function.
M
Scheme 1. Synthesis of Sulfonium Salts 1a−c
any efforts have been devoted to the development of photoacid-generating systems using UV light, mainly to realize faster curing of photosensitive resins and fine-pattern lithography with photoresists. Therefore, most of the commercially available photoacid generators (PAGs) are sensitive only to UV light.1 In contrast, photoacid generation with PAGs and visible-light sources has attracted considerable attention in recent years, not only because of safety and economic reasons but also because of increasing demands for visible-light-sensitive materials which can be applied to photocurable adhesives for UV-cut materials, the 3D printing system, the laser direct imaging process, and so forth.2 PAGs are composed of a chromophore and an acid precursor and are decomposed to generate an acid on absorbing a light of a specific wavelength. Therefore, visible-light-sensitive PAGs typically possess a broad π-conjugated structure in the chromophore constructed via a multistep synthesis. In this communication, we report a new simple approach for the synthesis of visible-light-sensitive sulfonium PAGs:3 introduction of a ferrocene moiety as a visible-light-absorbing chromophore. It is noteworthy that ferrocene-based PAGs are still limited4 in spite of the wide application of ferrocene derivatives in the chemical fields.5 Sulfonium salt [FcMePhS][BF4] (1a; Fc = ferrocenyl) was synthesized from the reaction of bromoferrocene with nBuLi and (PhS)2,6 followed by methylation of the resulting (phenylthio)ferrocene with [Me3O][BF4] (Scheme 1). The molecular structure of 1a was unambiguously confirmed by single-crystal X-ray analysis (Figure 1a). The sum of the bond angles around the S1 atom is 310.5°, which suggests sp3 hybridization of the sulfur atom. The PF6 and BArF4 (ArF = 3,5-(CF3)2C6H3) analogues [FcMePhS][X] (X = PF6 (1b), BArF4 (1c)) were also obtained by the anion metathesis of 1a with NaPF6 and NaBArF4 (Scheme 1 and Figure S1 in the Supporting Information). In addition, the p-tolyl analogue of 1a, [FcMeArS][BF4] (2; Ar = 4-CH3C6H4), was synthesized by a procedure similar to that for 1a (Figure 1b; see the Supporting Information for details). © XXXX American Chemical Society
Figure 1. ORTEP drawings for (a) 1a and (b) 2. Ellipsoids are shown at the 50% probability level. H atoms are omitted for clarity. Selected interatomic distances (Å) and angles (deg) are as follows. 1a: S1−C1, 1.794(2); S1−C2, 1.752(2); S1−C12, 1.784(2); C1−S1−C2, 103.52(10); C1−S1−C12, 104.31(10); C2−S1−C12, 102.65(9). 2: S1−C1, 1.805(7); S1−C2, 1.751(7); S1−C12, 1.806(7); C1−S1−C2, 102.5(3); C1−S1−C12, 101.7(3); C2−S1−C12, 103.6(3).
The UV−vis spectrum of 1a displays a relatively strong absorption band at 303 nm (ε = 1880 M−1 cm−1) in addition to an absorption band at 436 nm (ε = 290 M−1 cm−1), which are tentatively assigned to the metal to ligand charge transfer (MLCT) transition and the d−d transition of the ferrocenyl group, respectively (Figure 2).7,8 Sulfonium salts 1b,c and 2 Received: April 7, 2018
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DOI: 10.1021/acs.organomet.8b00203 Organometallics XXXX, XXX, XXX−XXX
Communication
Organometallics also exhibited similar absorption bands, and their optical properties are summarized in Table 1.
Figure 3. UV−vis spectral changes of the MeCN solution of TBPBNa by photogenerated acid from 1a upon irradiation at 436 nm. [TBPBNa] = 2.47 × 10−4 M (1 mL), [1a] = 4.94 × 10−4 M (2 mL).
Figure 2. UV−vis spectrum of 1a in MeCN. [1a] = 9.8 × 10−4 M.
was irradiated at 436 nm for an appropriate period of time, and the resulting reaction mixture was added to aqueous NH3/ MeOH to precipitate the poly(cyclohexene oxide) (PCHO) produced. The plots of weight yield of PCHO against irradiation time are presented in Figure 4. While the [BF4]−
Table 1. Optical Properties and Quantum Yields of Acid Generation of 1a−c and 2a sample
λmax (nm)
εmax (M−1 cm−1)
Φacidb
1a
303 436 304 436 304 436 303 437
1880 290 1510 290 1090 290 1530 270
0.094
1b 1c 2 a
0.103 0.092 0.059
In MeCN. b436 nm irradiation.
Solubilities of 1a−c, 2, and the commercially available sulfonium salt [MePh2S][BF4] (3), which is the Ph analogue of 1a,9 were measured and are given in Table S2 in the Supporting Information. Although the introduction of a ferrocenyl unit (3 → 1a) causes a decrease of the solubility10 in some polar solvents (EtOH, iPrOH, H2O), changing the anion to [BArF4]− (1c) dramatically improved the solubility in a variety of organic solvents. Next, acid generation from 1a in MeCN on irradiation with the g line from an ultrahigh-pressure Hg lamp (wavelength 436 nm),11 which coincides exactly with the absorption of 1a in the visible-light region, was evaluated using the sodium salt of tetrabromophenol blue (TBPBNa) as an indicator.12 TBPBNa reacts with acids to cause a decrease in the absorption band at 618 nm. A MeCN solution of sulfonium salt 1a (4.94 × 10−4 M; 2 mL) was irradiated at 436 nm for an appropriate period of time, and a TBPBNa solution (2.47 × 10−4 M; 1 mL) was added to the resultant solution to detect the acid generation. As a result, a constant decrease of the absorption band of TBPBNa was observed, which confirms that 1a generated an acid upon irradiation at 436 nm (Figure 3). Sulfonium salts 1b,c and 2 were also confirmed to generate an acid, although 2 was less sensitive to 436 nm light.13,14 The quantum yields of acid generation (Φacid) of 1a−c and 2 were determined to be 0.094, 0.103, 0.092, and 0.059, respectively.15,16 These values are comparable to those of some visible-light-sensitive PAGs (Table 1).2c,f In contrast, no photoinduced acid generation was observed with 3 and 436 nm light, which suggests that the ferrocenyl group plays an essential role in acid generation upon visible-light irradiation.17,18 With the novel visible light sensitive PAGs in hand, the activities of 1a−c as initiators for cationic photopolymerization of cyclohexene oxide (CHO) were evaluated.19 A CH2Cl2 solution (3 mL) of CHO (1.000 g) and 1a−c (0.5 mol %)
Figure 4. Plots of weight yield of PCHO against irradiation time. Light intensity at 436 nm: 2.4 mW/cm2.
salt 1a hardly afforded PCHO even after 50 h (0.6% yield), [PF6]− and [BArF4]− salts 1b,c turned out to be much more effective, and PCHO was obtained in 32.8 and 99.5% yields, respectively. These results are consistent with the strength of the generated acid.19 To our knowledge, there is no example of a sulfonium PAG having a ferrocenyl chromophore that is sensitive to 436 nm light. We consider that the present results provide a new approach toward the development of visible-light-sensitive sulfonium PAGs using a short-step synthetic procedure. Further studies on the detailed photoacid generation mechanism of 1a− c and 2, together with the synthesis of other PAGs having an organometallic chromophore and their application as photoinitiators and photocatalysts, are in progress.
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ASSOCIATED CONTENT
* Supporting Information S
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.organomet.8b00203. Experimental details and characterization data (PDF) Accession Codes
CCDC 1833116, 1833118, 1833161, and 1835401 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]. uk, or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033. B
DOI: 10.1021/acs.organomet.8b00203 Organometallics XXXX, XXX, XXX−XXX
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Organometallics
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(8) We assume that the absorptions at ca. 300 nm assigned as MLCT bands7 are tailing into the visible region and are overlapped with the d−d transition band. (9) In the case of 3, there is no absorption in the visible region. (10) The thermal decomposition temperature (Td) of 1a (192 °C) is higher than that of 3 (150 °C), which suggests that the ferrocenyl moiety improves the thermal stability, although the BArF4 analogue of 1a (1c) exhibited a lower Td value (96 °C) (for details, see Table S3 in the Supporting Information). (11) A combination of BLF-50S-440B and SCF-50S-42L filters (Sigma Koki Co. Ltd.) was used for 436 nm light without UV light, and the monochromaticity of the light used was confirmed by means of a UV radiometer (UV-M03A). (12) Shah, M.; Allen, N. S.; Salleh, N. G.; Corrales, T.; Egde, M.; Catalina, F.; Bosch, P.; Green, A. J. Photochem. Photobiol., A 1997, 111, 229−232. (13) We consider that introduction of the electron-donating Me group into the phenyl ring stabilizes the positive charge of the sulfonium moiety to reduce the photodegradation efficiency. (14) We further examined the synthesis of a p-ClC6H4 analogue of 1a, [FcMeArS][BF4] (4; Ar = p-ClC6H4), which was expected to show improved photodegradation efficiency. Unfortunately, however, this compound was isolated as a brown oil, and analytically pure samples were not obtained, although the 1H NMR (CDCl3) spectrum suggested formation of 4: δ 7.70 (d, J = 9.0 Hz, 2H), 7.54 (d, J = 8.5 Hz, 2H), 5.04 (br, 1H), 4.79 (br, 1H), 4.75 (br, 1H), 4.71 (br, 1H), 4.51 (s, 5H), and 3.55 (s, 3H) (Figure S3 in the Supporting Information). In addition, samples of 4 dissolved in MeCN caused a disappearance of the TBPBNa absorption at 618 nm without irradiation of 436 nm light, which is considered to suggest partial decomposition of 4 by room light during sample preparation or storage. (15) The quantum yields of acid generation were determined according to the procedure in ref 16 (for details, see the Supporting Information). (16) Miyashige, R.; Tanaka, H.; Shimaguchi, T. (Toyo Kasei Kogyo Co., Ltd., Japan). New Sulfonium Salt Compound, Method for Producting It, and Use of It. Jpn. Kokai Tokkyo Koho JP 2003246774A2 20030902, 2003. (17) At present, we assume that C−S bond cleavage of sulfonium salts 1a−c and 2 is caused by charge transfer from the Fe center to the sulfonium moiety under 436 nm irradiation, leading to generation of the acid corresponding to the counteranion of 1a−c and 2. For an article concerning the reductive cleavage of sulfonium salts, see: Wang, X.; Saeva, F. D.; Kampmeier, J. A. J. Am. Chem. Soc. 1999, 121, 4364− 4368. (18) There was no significant UV−vis absorption change of the mixture of TBPBNa and the sulfonium salt (1a−c or 2) in MeCN in the absence of light. (19) Zhou, W.; Kuebler, S. M.; Carrig, D.; Perry, J. W.; Marder, S. R. J. Am. Chem. Soc. 2002, 124, 1897−1901.
AUTHOR INFORMATION
Corresponding Authors
*E-mail for S.K.:
[email protected]. *E-mail for Y.I.:
[email protected]. ORCID
Shintaro Kodama: 0000-0003-4190-9539 Youichi Ishii: 0000-0002-1914-7147 Present Address †
S.K.: Department of Applied Chemistry, Graduate School of Engineering, Osaka Prefecture University, 1-1 Gakuen-cho, Nakaku, Sakai, Osaka 599-8531, Japan. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported in part by JST Adaptable and Seamless Technology Transfer Program through Target-Driven R&D (A-STEP) Feasibility Study (FS) Stage Exploratory Research (AS242Z00246M, AS251Z01402M). The authors acknowledge Prof. Tamejiro Hiyama at Chuo University for providing a single-crystal X-ray diffractometer and Prof. HoChol Chang at Chuo University for TG-DTA measurements. The authors also thank Mr. Kousuke Sakajiri at Chuo University for his experimental support.
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REFERENCES
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DOI: 10.1021/acs.organomet.8b00203 Organometallics XXXX, XXX, XXX−XXX
