Langmuir 2005, 21, 12093-12095
12093
New Method for Facile Synthesis of Amphiphilic Thiol-Stabilized Ruthenium Nanoparticles and Their Redox-Active Ruthenium Nanocomposite Takashi Tsukatani and Hisashi Fujihara* Department of Applied Chemistry and Molecular Engineering Institute, Kinki University, Kowakae, Higashi-Osaka 577-8502, Japan Received August 26, 2005. In Final Form: October 27, 2005 The one-phase reduction of RuCl3 with lithium triethylborohydride as a reductant in tetrahydrofuran in the presence of 1-octanethiol, 1-octadecanethiol, 1,1′-binaphthalene-2,2′-dithiol, or oligoethyleneoxythiol gave organic solvent- and water-soluble thiol-stabilized ruthenium nanoparticles. The oligoethyleneoxythiolstabilized ruthenium nanoparticles were soluble in both water and organic solvents. The ruthenium nanoparticles were stable in the solid state and did not aggregate in solution. Transmission electron microscope images of the ruthenium nanoparticles reveal small dispersed particles with a narrow size distribution. The ligand-exchange reaction of octadecanethiol-stabilized ruthenium nanoparticles (2.0 nm) with phenothiazine-linked decanethiol afforded redox-active phenothiazine-functionalized ruthenium nanoparticles (1.9 nm) that showed a reversible redox peak at +0.51 V (vs Ag/0.1 M AgNO3) in the cyclic voltammogram.
Metal nanoparticles have been extensively studied owing to their unique physical properties, chemical reactivity, and potential applications in electronics, catalysis, and biochemistry.1 Recently, there has been a remarkably increase in interest in the preparation and properties of metal nanoparticles modified with functional molecules. The surface properties of metal nanoparticles can be controlled by organic functional molecules of stabilizers for metal nanoparticles. Surface modification of metal nanoparticles and the related ligand-exchange reactions are very important in facilitating their application to catalysis and biotechnology.1-4 A number of gold nanoparticles containing functional molecules have been synthesized.1-4 One of the important contributions to this area was the development of a facile method for the preparation of alkanethiol-stabilized gold nanoparticles by Brust and co-workers.2 They used sodium borohydride as the reducing agent, tetraoctylammonium bromide as the phase-transfer agent, and water/toluene for the twophase reaction. In contrast, the synthesis and functionalization of ruthenium nanoparticles has received less attention until now.5 It is very important to develop the facile synthesis of ruthenium nanoparticles because ruthenium nanoparticles and complexes have fascinating
catalytic activity and photoelectroactive properties.5b,6 Although several methods have been developed to prepare ruthenium nanoparticles using stabilizers and ruthenium precursors, there is no report concerning the facile synthesis of water- and organic solvent-soluble amphiphilic ruthenium nanoparticles. In general, alkanethiolstabilized metal nanoparticles are insoluble in water, and catalysis and biological applications require that the clusters readily dissolve in aqueous media and do not aggregate. A more convenient synthesis of functional ruthenium nanoparticles would provide the opportunity to explore the use of these unique building blocks fully. We have developed a new procedure for the preparation of thiol-stabilized ruthenium nanoparticles (OT-Ru, ODTRu, EOT-Ru, and BNT-Ru) with a small core and a narrow
* Corresponding author. E-mail:
[email protected]. (1) (a) Schmid, G. Clusters and Colloids; VCH: Weinheim, Germany, 1994. (b) Feldheim, D. L.; Foss, C. A., Jr. Metal Nanoparticles; Marcel Dekker: New York, 2002. (c) Schmid, G. Nanoparticles; Wiley-VCH: Weinheim, Germany, 2004. (d) Rao, C. N. R.; Muller, A.; Cheetham, A. K. The Chemistry of Nanomaterials; Wiley-VCH: Weinheim, Germany, 2004; Vol. 1. (2) Brust, M.; Walker, M.; Bethell, D.; Schiffrin, D. J.; Whyman, R. J. Chem. Soc., Chem. Commun. 1994, 801. (3) (a) Templeton, A. C.; Wuelfing, W. P.; Murray, R. W. Acc. Chem. Res. 2000, 33, 27. (b) Daniel, M.-C.; Astruc, D. Chem. Rev. 2004, 104, 293. (4) (a) Tamura, M.; Fujihara, H. J. Am. Chem. Soc. 2003, 125, 15742. (b) Tatumi, R.; Fujihara, H. Chem. Commun. 2005, 83. (5) (a) Pan, C.; Pelzer, K.; Philippot, K.; Chaudret, B.; Dassenoy, F.; Lecante, P.; Casanove, M.-J. J. Am. Chem. Soc. 2001, 123, 7584. (b) Spitaleri, A.; Pertici, P.; Scalera, N.; Vitulli, G.; Hoang, M.; Turney, T. W.; Gleria, M. Inorg. Chim. Acta 2003, 352, 61. (c) Viau, G.; Brayner, R.; Poul, L.; Chakroune, N.; Lacaze, E.; Fievet-Vincent, F.; Fievet, F. Chem. Mater. 2003, 15, 486. (d) Yang, J.; Lee, J. Y.; Deivaraj, T. C.; Too, H.-P. J. Colloid Interface Sci. 2004, 271, 308. (e) Chakroune, N.; Viau, G.; Ammar, S.; Poul, L.; Veautier, D.; Chehimi, M. M.; Mangeney, C.; Villain, F.; Fievet, F. Langmuir 2005, 21, 6788.
size distribution from the one-phase reaction of RuCl3 with lithium triethylborohydride as a reducing agent in the presence of several kinds of thiolss1-octanethiol (OT), 1-octadecanethiol (ODT), oligoethyleneoxythiol (EOT), or dithiol, and binaphthyldithiol (BNT). The oligoethyl(6) In contrast to ruthenium nanoparticles, ruthenium complexes are widely used for catalysis and devices because of their unique electrochemical and photochemical properties. See Bruneau, C.; Dixneuf, P. H. Ruthenium Catalysis and Fine Chemistry; Springer: New York, 2004. Hofmeier, H.; Schubert, U. S. Chem. Soc. Rev. 2004, 33, 373 and references therein.
10.1021/la052332t CCC: $30.25 © 2005 American Chemical Society Published on Web 11/17/2005
12094
Langmuir, Vol. 21, No. 26, 2005 Scheme 1. Reagents and Conditionsa
a (i) SOCl2, C6H5N, THF, reflux, 4 h; (ii) MeCOSK, MeCN, reflux, 4 h; (iii) K2CO3, MeOH, room temp., 1 h.
Scheme 2. Reagents and Conditionsa
a (i) n-BuLi, -60 °C, 2 h; (ii) Br-(CH2)10-Br, room temp., 17 h; (iii) MeCOSK, MeCN, reflux, 2 h; (iv) K2CO3, MeOH-benzene, room temp., 1 h.
eneoxythiol-stabilized ruthenium nanoparticles were found to be soluble and stable in water and organic solvents. This letter reports the facile procedure for the preparation of water- and organic solvent-soluble ruthenium nanoparticles and redox-active ruthenium nanoparticles by a ligand-exchange reaction. To our knowledge, this is the first example of the preparation of amphiphilic ruthenium nanoparticles. Stable dispersions of metal nanoparticles in aqueous media are important to many applications. The synthetic sequence of thiols (EOT and PTZ) is described in Schemes 1 and 2. Binaphthyldithiol (BNT) was synthesized according to a procedure of Lucchi et al.7 The convenient one-phase reaction conditions produce small ruthenium nanoparticles that can be easily prepared from commercially available materials. As a typical run, the preparation of ruthenium nanoparticles (ODT-Ru) using 1-octadecanethiol as a stabilizer is as follows. A solution of RuCl3 (103 mg, 0.5 mmol) and 1-octadecanethiol (286 mg, 1.0 mmol) in 10 mL of anhydrous tetrahydrofuran was stirred for 1 h at room temperature under an Ar atmosphere. To this vigorously stirred solution was added lithium triethylborohydride (1.0 M, 10 mL) at 0 °C, and the resulting solution was stirred for 2 h at room temperature. To the reaction mixture was added anhydrous EtOH (40 mL) at 0 °C. After stirring, the organic phase was evaporated to 5 mL in vacuo and mixed with EtOH (300 mL). The resulting precipitate was collected by filtration and washed serially with EtOH. The nanoparticles were redissolved in THF for purification and precipitated with EtOH, and then the particles were isolated by filtration. These processes were repeated until no free thiol remained, as detected by TLC and 1H and 13C NMR spectroscopy. Other thiol-stabilized ruthenium nanoparticles (OT-Ru, BNT-Ru, and EOT-Ru) were prepared using 1-octanethiol (OT), binaphthyldithiol (BNT), and oligoethyleneoxythiol (EOT) as stabilizers of ruthenium nanoparticles. In the case of the purification of oligoethyleneoxythiol-stabilized ruthenium nanoparticles (EOT-Ru), EOT-Ru was redissolved in CH2Cl2 for purification and precipitated with diethyl ether, and then the particles were isolated by filtration. (7) Fabbri, D.; Delogu, G.; Lucchi, O. De. J. Org. Chem. 1993, 58, 1748.
Letters
The ruthenium nanoparticles (OT-Ru, ODT-Ru, and BNT-Ru) were found to be soluble in organic solvents such as n-hexane, benzene, toluene, CH2Cl2, CHCl3, THF, and diethyl ether. Significantly, the oligoether-functionalized ruthenium nanoparticles (EOT-Ru) were soluble in water and organic solvents such as benzene, toluene, CH2Cl2, CHCl3, THF, ethyl acetate, MeOH, acetone, and CH3CN. The ruthenium nanoparticles were quite stable in the solid state. Lyophilized particles could be readily redissolved in the solvents to form a clear solution. The proton signals in the 1H NMR spectra of the ruthenium nanoparticles, though significantly broadened, appeared at positions that are almost identical to those of free thiol.8 The core size and size distribution of the ruthenium nanoparticles were examined by transmission electron microscopy (TEM). The TEM picture of Ru nanoparticles showed narrowly dispersed nanoparticles: 2.2 ( 0.4 nm for OT-Ru (Figure 1a), 1.6 ( 0.3 nm for ODT-Ru (Figure 1b), 2.0 ( 0.4 nm for EOT-Ru (Figure 1c), and 2.0 ( 0.2 nm in diameter for BNT-Ru (Figure 1d). Thus, dispersed ruthenium nanoparticles were easily obtained by the onephase reduction of RuCl3 with lithium triethylborohydride in THF. In contrast to our results, it was reported that the reaction of a Ru complex, Ru(1,5-cyclooctadiene ) COD)(1,3,5-cyclooctatriene ) COT), in THF in the presence of an alkanethiol with 3 bar of dihydrogen at 193 K for 15 h led to Ru nanoparticles that were found to be strongly agglomerated.5a Such a preparative method of ruthenium nanoparticles from the reduction of the ruthenium precursor, Ru(COD)(COT), with 3 bar of dihydrogen seems to be inconvenient. However, the X-ray photoelectron spectroscopy (XPS) spectrum of the ruthenium nanoparticles showed Ru 3d5/2 binding energy of 280.1 eV for OTRu and ODT-Ru, 280.3 eV for BNT-Ru, and 280.2 eV for EOT-Ru corresponding to the Ru0 state. A number of ligand-exchange reaction of alkanethiolstabilized gold nanoparticles with thiols have been described,3a but very few ruthenium nanoparicles are known.5a In addition, no clear-cut example of redox-active ruthenium nanoparticles has been hitherto known. The incorporation of photo-redox-active molecules into ruthenium nanoparticles using a ligand-exchange reaction can yield new nanocomposites having unique electrochemical and photochemical properties. New redox-active ruthenium nanoparticles were synthesized by a ligand-exchange reaction of octadecanethiol-stabilized ruthenium nanoparticles (ODT-Ru) with a phenothiazinethiol9 (PTZ as shown in Scheme 2). A solution of a thiol, PTZ (20 mg), and ODT-Ru nanoparticles (60 mg) in toluene (5 mL) was stirred for 24 h at room temperature. After the solution was stirred, the organic phase was evaporated in vacuo and mixed with EtOH (200 mL). The resulting precipitate was collected by filtration and washed serially with EtOH. The phenothiazine-functionalized ruthenium nanoparticles (ODT-PTZ-Ru) obtained were very stable and soluble in benzene, toluene, CH2Cl2, CHCl3, and THF. The core size of ODT-PTZ-Ru is 1.9 ( 0.4 nm. This finding shows that the ligand-exchange reaction of thiolstabilized ruthenium nanoparticles with the functional thiol can be proceed without aggregation of the nanoparticles. The XPS spectrum of ODT-PTZ-Ru showed the Ru 3d5/2 binding energy at 280.1 eV. The electrochemical properties of ODT-PTZ-Ru were studied by cyclic voltammetry. The cyclic voltammogram of ODT-PTZ-Ru was (8) The broadening effects of 1H NMR signals in alkanethiolatestabilized gold nanoparticles have been discussed.3a (9) Phenothiazine derivatives are known as good electron donors and photoelectroactive compounds. See Bosch, E.; Kochi, J. K. J. Chem. Soc., Perkin Trans. 1 1995, 1057 and references therein.
Letters
Langmuir, Vol. 21, No. 26, 2005 12095
Figure 1. TEM micrographs and histograms of size distribution for (a) OT-Ru, (b) ODT-Ru, (c) EOT-Ru, and (d) BNT-Ru.
measured in 0.1 M Bu4NPF6-CH2Cl2 at a glassy carbon electrode and a Ag/0.1 M AgNO3 reference electrode (scan rate: 100 mV s-1). The half-wave potential (E1/2) and the cathodic-anodic peak separation (∆Ep) value for the phenothiazine redox process were +0.51 V and 61 mV, respectively. These results indicate that the ligandexchange reaction of the thiol-stabilized ruthenium nanoparticles with the redox-active thiol led to the formation of redox-active ruthenium nanoparticles. In summary, we have successfully prepared a series of small water- and organic solvent-soluble ruthenium nanoparticles with a narrow dispersity from the one-phase reduction of RuCl3 with lithium triethylborohydride in the presence of alkane monothiols and aromatic dithiol as stabilizers. The functionalization of alkanethiol-stabilized
ruthenium nanoparticles was accomplished by the ligandexchange reaction using a functional thiol. These first examples of amphiphilic dispersed ruthenium nanoparticles and their functionalization open interesting perspectives in the field of reaction and catalysis and in electronic applications. Further studies of the amphiphilic functional ruthenium nanoparticles are in progress. Acknowledgment. This work was supported in part by a Grant-in-Aid for Scientific Research on Priority Areas (no. 17036067 “Chemistry of Coordination Space”) from the Ministry of Education, Science, Sports and Culture, Japan. LA052332T
