Sonogashira–Hagihara and Mizoroki–Heck Coupling Polymerizations

Note pubs.acs.org/Macromolecules

Sonogashira−Hagihara and Mizoroki−Heck Coupling Polymerizations Catalyzed by Pd Nanoclusters Shizuka Asada, Ayaka Nito, Yu Miyagi, Junya Ishida, Yasushi Obora, and Fumio Sanda* Department of Chemistry and Materials Engineering, Faculty of Chemistry, Materials and Bioengineering, Kansai University, 3-3-35 Yamate-cho, Suita, Osaka 564-8680, Japan S Supporting Information *

M

and polymerizations. These Pd catalysts ligated with phosphines are initially active, but most of them gradually undergo degradation in air. It is desirable to develop a further simple and versatile catalytic system to widen the possibility of coupling polymerizations as a tool for preparing functional conjugated polymers. The present research investigated the Sonogashira−Hagihara and Mizoroki−Heck coupling polymerizations using DMF-protected stable Pd NCs for the first time as well as the optical properties of the formed polymers. The Sonogashira−Hagihara coupling polymerization of pand m-diethynylbenzenes 1p and 1m with 1,4-diiodo-2,5-bis[2(2-methoxyethoxy)ethoxy]benzene 2 (Scheme 1A) was carried

etal nanoclusters (NCs) with sizes less than 2 nm feature high surface areas and corner/edge sites as well as high surface-to-volume ratio, resulting in excellent catalytic activity compared with metal NCs with larger sizes (3−100 nm). Metal NCs have been synthesized by various methods,1 but most of them are accompanied by aggregation to bulk metal, thereby lowering the versatility as catalysts for organic reactions. A wide variety of attempts have been made to stabilize Pd NCs, e.g., NCs supported with naturally derived porous inorganic materials such as diatomite2 and montmorillonite;3 NCs immobilized on TiO2 surfaces,4 N-doped active carbon5 and carbon nanotubes;6 NCs ligated with oligosilsesquioxane,7 aminophosphines,8 phosphines,9−12 thiolate,13 and aromatic ligand with Pd−C bond;14 and NCs contained in ionic liquid microemulsions.15,16 Pd NCs are also stabilized with polymers containing heteroatoms and/or aromatic rings, which serve as ligands for Pd. The examples of such polymers are polystyrene derivatives bearing oligoethylene glycol units,17 poly(ethylene glycol)-block-poly(propylene glycol)-block-poly(ethylene glycol),18 poly(4-styrenesulfonic acid-co-maleic acid),19 polysilane,20 diamine-functionalized mesoporous polymers,21 polystyrene,22 polyvinylpyrrolidone,23 dendrimers based on polyarylamine,24 poly(arylene−ether−disulfide),25 1,2,3-triazole,26 and poly(propyleneimine).27 Obora and co-workers have recently developed an innovative preparation method for surfactant-free and DMF-protected stable Pd NCs with sizes of 1−1.5 nm,28 wherein DMF molecules efficiently stabilize Pd NCs to prevent precipitation and aggregation. This simple but powerful method for preparation of Pd NCs is expected to expand the possibility in catalysis remarkably, leading to rare metal resource innovation in synthetic chemistry. In a manner similar to common Pd complex catalysts, Pd NCs catalyze a variety of reactions including hydrogenation,5,6,15,16,20 dehydrogenation,23 oxygen reduction,13 oxidation,4,7,18 allylic substitution,3 cyclotrimerization of iodonorbornenes,29 Mizoroki−Heck coupling,2,6,8,16,17,21,25,28 Sonogashira−Hagihara coupling,20 Suzuki−Miyaura coupling,2,9−12,14,19,20,22 Suzuki−Miyaura coupling polymerization,30−33 and Migita−Kosugi−Stille coupling.34 The Sonogashira−Hagihara and Mizoroki−Heck coupling polymerizations are widely employed for preparing poly(aryleneethynylene)s and poly(arylenevinylene)s applicable to photoelectrically functional materials,35,36 including lightemitting diodes,37 photovoltaic materials,38,39 and bio/chemo sensors.40,41 Homogenous Pd catalysts such as Pd(PPh3)2Cl2, Pd(PPh3)4, and their derivatives are commonly used for coupling reactions © XXXX American Chemical Society

Scheme 1. (A) Sonogashira−Hagihara Coupling Polymerization of 1p and 1m with 2 and (B) Mizoroki−Heck Coupling Polymerization of 3p with 2

out using DMF-protected Pd NCs28 in the presence of CuI at 80−130 °C for 20−120 h under an Ar atmosphere to obtain the corresponding p- and m-linked poly(phenyleneethynylene)s [poly(1p−2) and poly(1m−2)] as summarized in Tables 1 and 2. Diiodophenylene monomer 2 tethering oligoethylene glycol chains was employed42,43 because nonsubstituted phenyleneethynylene and phenylenevinylene are poorly soluble, making molecular weight determination and absorption−emission spectroscopic measurement of the resulting polymers difficult.37 In the polymerization of 1p with 2, the polymer yield and Mn increased as the Pd NC content was increased from 0.01 to 0.10 mol % under the conditions of 100 °C for 72 h in DMF (runs 2, 4, and 9 in Table 1). When the polymerization was carried out using 0.10 mol % Pd NCs for 72 h, polymers insoluble in common organic solvents, including DMF, were obtained at 110 °C and above (runs 15 and 16 in Table 1). It is Received: April 14, 2017 Revised: April 19, 2017

A

DOI: 10.1021/acs.macromol.7b00779 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules Table 1. Sonogashira−Hagihara Coupling Polymerization of 1p with 2a yieldb (%) run

solvent

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16

Pd NCs (mol %)

DMF DMF DMF DMF DMF DMF DMF DMF DMF DMF/BuOH = 8/1 DMF/BuOH = 1/8 NMP DMA DMF DMF DMF

temp (°C)

0.01 0.01 0.05 0.05 0.10 0.10 0.10 0.10 0.10 0.10 0.10 0.10 0.10 0.10 0.10 0.10

100 100 100 100 80 90 100 100 100 100 100 100 100 100 110 120

time (h)

sol

insol

Mnd

Mtopd

Đd

48 72 48 72 72 72 20 48 72 72 72 72 72 96 72 72

− 1 8 23 1 25 −c 22 47 42 3 52 −c 1 1 −

− − − − − − − − − − − − − 66 58 76

− 2600 4300 6000 4600 6100 − 6400 10400 7200 7000 6400 − 5000 5100 −

− 6600 7400 8200 6900 8000 − 8500 29100 16500 11400 8000 − 9200 4000 −

− 7.11 2.00 2.28 2.33 1.83 − 1.71 4.89 4.23 6.23 13.88 − 15.23 6.91 −

c

a Conditions: [M]0 = 0.1 mol/L in solv/Et3N = 9/1, [Pd]/[CuI] = 2. bIsolated by precipitation with methanol and then separated into soluble and insoluble parts in DMF. cNo polymer was obtained by precipitation with methanol. dEstimated by SEC eluted with DMF.

Meta-linked conjugated polymers are commonly more soluble in solvents than their para-linked counterparts. The polymer yield and Mn were increased by extension of the polymerization time in the polymerization of 1m with 2 (Scheme 1A) using 0.10 mol % of Pd NCs at 100 °C (runs 1 and 2 in Table 2) in a manner similar to the polymerization of 1p with 2 as mentioned above. The polymer yield and Mn were increased by raising the temperature, similar to the trend observed with polymerization of 1p with 2. A solvent-soluble polymer with an Mn as high as 288 300 was obtained by the polymerization at 120 °C for 72 h (run 4 in Table 2). The polymer obtained by the polymerization at 130 °C was insoluble (run 5 in Table 2), likely because the Mn became too high to dissolve in solvents. As expected, the molecular weight limit regarding solubility of poly(1m−2) was higher than that of poly(1p−2). The higher temperature (120 °C) was necessary for the polymerization of 1m with 2 compared to that of 1p with 2 (100 °C) to obtain the polymer with higher yield and Mn, presumably due to the larger steric hindrance of 1m during the polymerization. Thus, DMF-protected Pd NCs successfully catalyzed the Sonogashira−Hagihara coupling polymerization of 1p/1m with 2 in a manner similar to conventional catalysis by PdCl2(PPh3)2 and Pd(PPh3)4 (Tables S1 and S2). Further, the Mizoroki−Heck coupling polymerization of pdivinylbenzene 3p with 2 was carried out using DMF-protected Pd NCs as illustrated in Scheme 1B. The corresponding poly(phenylenevinylene), poly(3p−2)s with Mn’s ranging from 3300 to 5900 were obtained in 49−82% yields. The yield and Mn increased as the polymerization temperature was increased from 100 to 140 °C. Thus, DMF-protected Pd NCs successfully catalyzed the Mizoroki−Heck coupling polymerization. It is reported that trans- and cis-vinylene proton signals appear around 7.36 and 6.50 ppm in the 1H NMR spectrum.45 In the present study, the intensity of the 1H NMR signals of poly(3p−2) assignable to cis-vinylene proton around 6.50 ppm is negligibly small compared to the signals assignable to transvinylene and aromatic ring protons at 7.06−7.61 ppm (see Supporting Information, p S10). It is therefore considered that

Table 2. Sonogashira−Hagihara Coupling Polymerization of 1m with 2a yieldb (%) run

temp (°C)

time (h)

sol

1 2 3 4 5

100 100 100 120 130

72 96 120 72 72

27 53 31 60

insol

Mnc

Mtopc

Đc

3500 6700 5800 288300

6400 10500 8400 896300

2.04 2.43 1.75 2.56

71

a

Conditions: [M]0 = 0.1 mol/L in DMF/Et3N = 9/1, [Pd]/[CuI] = 2. b Isolated by precipitation with methanol, then separated into soluble and insoluble parts in DMF. cEstimated by SEC eluted with DMF.

likely that the solvent-insoluble polymers have high molecular weights because the solvent-soluble and -insoluble polymers exhibited almost the same IR spectroscopic patterns (see Supporting Information). When the polymerization was carried out using 0.10 mol % of Pd NCs at 100 °C, extension of the polymerization time increased both the polymer yield and Mn (runs 7−9 in Table 1), but the polymerization for 96 h gave a solvent-insoluble polymer presumably because the Mn became too high to dissolve (run 14 in Table 1). DMF gave the most satisfactory results among the solvents examined (DMF, DMF/ BuOH = 8/1, 1/8, NMP, DMA) when the polymerization was carried out using 0.10 mol % of Pd NCs at 100 °C (runs 9−13 in Table 1). Alcohols are sometimes used as solvents for PdNCs-catalyzed reactions, including Sonogashira−Hagihara coupling.20 In the present study, the addition of a small amount of BuOH did not increase the polymer yield or Mn (run 10 in Table 1); instead, addition of a large amount of BuOH decreased the polymer yield (run 11 in Table 1). This outcome was attributed to restructuring the DMF layers by partial liberation of DMF molecules, followed by penetration of alcohol molecules onto the Pd surface, resulting in unpredictable variations in catalytic activity of metal NCs.44 It was concluded that the conditions of run 9 were optimum among the conditions examined for the polymerization of 1p with 2 to obtain the polymer with high yield and Mn. B

DOI: 10.1021/acs.macromol.7b00779 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules

61% comparable to analogous alkyloxy-substituted poly(phenyleneethynylene) derivatives (Φ = 20−60%) that show light absorption around 450 nm and emission around 510 nm.45−48 In summary, we have demonstrated the first Sonogashira− Hagihara coupling polymerization and Mizoroki−Heck coupling polymerization using DMF-protected stable Pd NCs as catalyst to obtain poly(phenyleneethynylene)s and poly(phenylenevinylene) with moderate molecular weights. DMFprotected Pd NCs successfully catalyzed the coupling polymerizations with results similar to those achieved with conventional PdCl2(PPh3)2 and Pd(PPh3)4 catalysts. DMF-protected Pd NCs are prepared by a simple, nonsupported and surfactantfree method and very stable under ambient conditions. The present Pd NCs are solvent stabilized, so they potentially mitigate one problem of the Pd homogeneous catalysis for polymerization: the possibility for ligand incorporation into the growing polymer. We hope the present study opens a new methodology for synthesizing conjugated polymers using a simple catalytic system, leading to rare metal resource innovation.

the geometry of vinylene moieties of poly(3p−2) is predominantly trans. Table 3. Mizoroki−Heck Coupling Polymerization of 3p with 2a run

temp (°C)

time (h)

yieldb (%)

Mnd

Mtopd

Đd

1 2 3 4

100 120 140 140

96 72 72 96

−c 49 77 82

− 3300 4300 5900

− 5000 7200 7200

− 1.79 2.32 6.48

a Conditions: [M]0 = 0.1 mol/L in DMF/Et3N = 9/1, [Pd] = 0.10 mmol/L. bIsolated by precipitation with methanol and then separated into soluble and insoluble parts in DMF. cNo polymer was obtained by precipitation with methanol. dEstimated by SEC eluted with DMF.

Figure 1 shows the UV−vis spectra of the polymers obtained by the Sonogashira−Hagihara coupling polymerization of 1p

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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.macromol.7b00779. Experimental procedure; results of the Sonogashira− Hagihara coupling polymerization of 1p and 1m with 2 using conventional Pd catalysts and DMF-protected Pd NCs; pictures of polymer solutions under irradiation by UV light; 1H NMR and IR spectra of the monomers, precursors, and polymers; SEC traces of the polymers (PDF)

Figure 1. UV−vis spectra of poly(1p−2), poly(1m−2), and poly(3p− 2) measured in CHCl3 (c = 0.04 mM) at room temperature. Samples: run 8 in Table 1, run 2 in Table 2, and run 4 in Table 3.

and 1m with 2 and by Mizoroki−Heck coupling polymerization of 3p with 2 catalyzed by DMF-protected Pd NCs. Poly(1p−2) absorbed light at a wavelength region longer than that of poly(1m−2) as predicted by the higher conjugation efficiency of p-linkage compared to m-linkage. Poly(3p−2) absorbed light at a much longer wavelength with a clear λmax at 451 nm, indicating the long conjugation of the highly planar phenylenevinylene linkage compared with their phenyleneethynylene counterparts. Figure 2 shows the photoluminescence spectra of the polymers measured in CHCl3, together with photographs of the sample solutions. Poly(1p−2) and poly(1m−2) did not fluoresce at the concentration used (0.002 mM), while poly(3p−2) strongly fluoresced with a quantum yield (Φ) of

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (F.S.). ORCID

Fumio Sanda: 0000-0002-1113-4771 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was partly supported by a Grant-in-Aid for Scientific Research (B) (16H04158) and a Grant-in-Aid for Challenging Exploratory Research (16K14011). The authors are grateful to Prof. Kenneth B. Wagener and Dr. Kathryn R. Williams at the University of Florida for their helpful suggestions and comments.

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Figure 2. Photoluminescence spectra of poly(1p−2), poly(1m−2), and poly(3p−2) excited at 310, 297, and 450 nm, respectively, measured in CHCl3 (c = 0.002 mM), together with the photographs of the sample solutions under irradiation with UV light (λ = 365 nm). Samples: run 8 in Table 1, run 2 in Table 2, and run 4 in Table 3. C

DOI: 10.1021/acs.macromol.7b00779 Macromolecules XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.macromol.7b00779 Macromolecules XXXX, XXX, XXX−XXX