Chem. Mater. 2010, 22, 1367–1375 1367 DOI:10.1021/cm902039r
Comparative Study of Linear Poly(alkylarylsilane)s as Reducing Agents toward Ag(I) and Pd(II) Ions-Synthesis of Polymer-Metal Nanocomposites with Variable Size Domains of Metal Nanoparticles Ravi Shankar,* Vandana Shahi, and Usharani Sahoo Department of Chemistry, Indian Institute of Technology, Hauz Khas, New Delhi 110016, India Received July 7, 2009. Revised Manuscript Received November 14, 2009
Several new poly(alkylarylsilane)s, [Et3Si(CH2)2Si(C6H4R)]n [R = H (1), p-Me (2), p-OMe (3), pNMe2 (4)] with Mw = 2.9-4.6 103 and PDI = 1.41-1.64, have been synthesized by dehalocondensation of appropriate dichlorodiorganosilanes using Na dispersion in refluxing toluene. The σ-σ* electronic transitions associated with the silicon backbone of 1-4 exhibit a progressive red shift (λmax: 320-342 nm) as a result of increasing electron donating ability of the substituent on the phenyl ring. This intrinsic optimization of band gap energy has been considered as a measure of oxidation potential of these polysilanes which varies in the following order: 1 > 2 > 3 > 4. The result obtained from electronic spectra is further confirmed by electrochemical studies using cyclic voltammetry which reveals a lowering of the oxidation potential (Vpeak) from 0.95 to 0.49 V in the polysilanes 1-4. These polysilanes act as reducing agents of variable strengths toward Ag(I) and Pd(II) ions and afford the formation of metal nanoparticles with optimized size domains [21.0-4.8 nm for Ag; 31.616.6 nm for Pd], the smaller size nanoparticles being formed by the use of polysilanes with lower oxidation potential (or higher λmax). The polymer-metal nanocomposites have been characterized by NMR and UV-vis spectroscopy as well as powder X-ray diffraction, TEM, and DLS studies. Introduction Interest in linear poly(dialkyl/alkylarylsilane)s has emerged from their unusual electronic, optical, and photophysical properties and the potential applications derived therefrom.1,2 By virtue of σ-delocalized electrons on the silicon backbone, these polymers have been investigated for their affinity to act as reducing agents toward noble metal ions such as Ag(I), Au(III), Pd(II), and Pt(II), with an emphasis on exploring various applications of the polymer composites incorporating in situ generated metal nanoparticles. Fukushima et al.3,4 have reported the synthesis of Au, Ag, and Pd colloids immobilized on polysilane thin films, [R(R1)Si]n (R = Me or H, R1 = Ph; R = R1 = n-Hex) without the aid of an external reducing agent and their application in the electroless deposition of Cu on the polymer matrix. Kobayashi et al. have demonstrated that poly(methylphenylsilane)-supported Pd and Pt nanoparticles serve as potential catalysts for various organic transformations involving hydrogenation, Suzuki, Sonagashira coupling, *Corresponding author e-mail:
[email protected].
(1) Michl, J.; West, R. In Silicon-Containing Polymers: The Science and Technology of Their Synthesis and Applications; Jones, R. G., Ando, W., Chojnowski, J., Eds.; Kluwer: Dordrecht, The Netherlands, 2000; p 499. (2) Miller, R. D.; Michl, J. Chem. Rev. 1989, 89, 1359. (3) Fukushima, M.; Hamada, Y.; Tabei, E.; Aramata, M.; Mori, S.; Yamamoto, Y. Chem. Lett. 1998, 347. (4) Fukushima, M.; Noguchi, N.; Aramata, M.; Hamada, Y.; Tabei, E.; Mori, S.; Yamamoto, Y. Synth. Met. 1998, 97, 273. r 2010 American Chemical Society
and hydrosilylation reactions.5-7 Sanji and Sakurai have synthesized a shell cross-linked miscellar-like structure from a combination of poly(dimethyl-co-di-n-hexylsilane) and poly(methacrylic acid).8,9 The polysilane core of this assembly acts as the reducing agent for Au(III) and Pd(II) ions to afford metal nanoparticles, while the cross-linked PMA functions as a stabilizer to protect the metal nanoparticles from agglomeration and also maintains solubility in water. The catalytic activity of the composite containing Pd nanoparticles has been studied for hydrogenation reaction of alkenes in aqueous solution. Another typical example of the use of polysilane as a reducing agent is the generation of metal nanoparticles (Au, Ag, Pd) on the surface of polystyrene.10 The method involves the synthesis of polystyrene emulsions incorporating poly(methylphenylsilane) and their reactions with appropriate metal salts under UV irradiation. Our interest in this area relates to expanding the scope of functional polysilanes bearing donor substituents on the side chain groups for the generation and stabilization (5) Ueno, M.; Suzuki, T.; Naito, T.; Oyamada, H.; Kobayashi, S. Chem. Commun. 2008, 1647. (6) Oyamada, H.; Akiyama, R.; Hagio, H.; Naito, T.; Kobayashi, S. Chem. Commun. 2006, 4297. (7) Oyamada, H.; Naito, T.; Miyamoto, S.; Akiyama, R.; Hagio, H.; Kobayashi, S. Org. Biomol. Chem. 2008, 6, 61. (8) Sakurai, H. Proc. Jpn. Acad. Ser. B 2006, 82, 257. (9) Sanji, T.; Ogawa, Y.; Nakatsuka, Y.; Tanaka, M.; Sakurai, H. Chem. Lett. 2003, 980. (10) Tamai, T.; Watanabe, M.; Hatanaka, Y.; Tsujiwaki, H.; Nishioka, N.; Matsukawa, K. Langmuir 2008, 24, 14203.
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Chem. Mater., Vol. 22, No. 4, 2010 Scheme 1. Synthesis of Palladium Nanoparticles
of metal nanoparticles. Toward this end, we have reported earlier11,12 the synthesis of polysilanes with appended 2-thienyl/2-furyl-substituted carbosilyl side chains and their reactivity toward Ag(I) and Pd(II) ions (Scheme 1). This approach provides the synthesis of polymer-metal nanocomposites containing monodispersed, spherical metal nanoparticles of
