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Synthesis and Redox Behavior of Biferrocenyl-Functionalized Ruthenium(II) Terpyridine Gold Clusters Teng-Yuan Dong,* Hao-Wei Shih, and Ling-Shao Chang Department of Chemistry, Center for Nanoscience and Nanotechnology, National Sun Yat-Sen University, Kaohsiung, Taiwan Received April 27, 2004. In Final Form: July 26, 2004 Spectroscopic and electrochemical characterizations of ferrocene- and biferrocene-functionalized terpyridine octanethiolate monolayer-protected clusters were investigated and reported. The electrochemical measurements of Ru2+ coordinated with 4′-ferrocenyl-2,2′:6′,2′′-terpyridine and 4′-biferrocenyl-2,2′:6′,2′′terpyridine complexes were dominated by the Ru2+/Ru3+ redox couple (E1/2 at ∼1.3 V), Fe2+/Fe3+ redox couples (E1/2 from ∼0.6 to ∼0.9 V), and terpy/terpy-/terpy2- redox couples (E1/2 at ca. -1.2 and ca. -1.4 V). The substantial appreciable variations detected in the Ru2+/Ru3+ and Fe2+/Fe3+ oxidation potentials indicate that there is an interaction between the Ru2+ and Fe2+ metal centers. The coordination of the Ru2+ metal center with 4′-ferrocenyl-2,2′:6′,2′′-terpyridine and 4′-biferrocenyl-2,2′:6′,2′′-terpyridine leads to an intense 1[(d(π)Fe)6] f 1[d(π)Fe)5(π*terpyRu)1] transition in the visible region. The 1[(d(π)Fe)6] f 1[d(π) )5(π* Ru 1 Fe terpy ) ] transition observed at ∼510 nm revealed that there was a qualitative electronic coupling between metal centers. The coordination of the Ru2+ transition metal center lowers the energy of the π*terpy orbitals, causing this transition.
Introduction This paper describes a synthetic pathway to the formation of stable biferrocenyl-functionalized terpyridylalkanethiolate monolayer-protected Au clusters (MPCs). It also describes their spectroscopic and electrochemical characterizations. The investigation of alkanethiolate Aubased monolayers includes examples of ferrocenyl alkanethiols (Fc(CH2)nSH, Fc ) (C5H5)Fe(C5H4)),1-5 carbonylfunctionalized ferrocenoyl alkanethiols (FcCO(CH2)nSH and FcCO2(CH2)nSH),4-12 and carbonyl-functionalized biferrocenoyl alkanethiols (BifcCO(CH2)nSH, Bifc ) (C5H5)Fe(C10H10)Fe(C5H5)).13-15 Very recently, we have * To whom correspondence should be addressed. Tel: +886-75252000 ext 3937. Fax: +886-7-5253908. E-mail: dty@ mail.nsysu.edu.tw. (1) Collard, D. M.; Fox, M. A. Langmuir 1991, 7, 1192-1197. (2) Uosaki, K.; Sato, Y.; Kita, H. Langmuir 1991, 7, 1510-1514. (3) Ingram, R. S.; Hostetlar, M. J.; Murray, R. W. J. Am. Chem. Soc. 1997, 119, 9175-9178. (4) Chidsey, C. E. D.; Bertozzi, C. R.; Putvinski, T. M.; Mujsce, A. M. J. Am. Chem. Soc. 1990, 112, 4301-4306. (5) (a) Ingram, R. S.; Hostetler, M. J.; Murray, R. W.; Schaaf, T. G.; Khoury, J. T.; Whetten, R. L.; Bigioni, T. P.; Guthrie, D. K.; First, P. N. J. Am. Chem. Soc. 1997, 119, 9279-9280. (b) Chen, S.; Ingram, R. S.; Hostetler, M. J.; Pietron, J. J.; Murray, R. W.; Schaaff, T. G.; Khoury, J. T.; Alvarez, M. M.; Whetten, R. L. Science 1998, 280, 2098-2101. (6) Viana, A. S.; Abrantes, L. M.; Jin, G.; Floate, S.; Nichols, R. J.; Kalaji, M. Phys. Chem. Chem. Phys. 2001, 3, 3411-3419. (7) Zhang, L.; Godinez, L. A.; Lu, T.; Gokel, G. W.; Kaifer, A. E. Angew. Chem., Int. Ed. Engl. 1995, 34, 235-237. (8) Green, S. J.; Stokes, J. J.; Hostetler, M. J.; Pietron, J. J.; Murray, R. W. J. Phys. Chem. B 1997, 101, 2663-2668. (9) Terrill, R. H.; Postlethwaite, T. A.; Chen, C.-H.; Poon, C.-D.; Terzis, A.; Chen, A.; Hutchison, J. E.; Clark, M. R.; Wignall, G.; Londono, J. D.; Superfine, R.; Falvo, M.; Johnson, C. S.; Samulski, E. T.; Murray, R. W. J. Am. Chem. Soc. 1995, 117, 12537-12548. (10) Hostetler, M. J.; Green, S. J.; Stokes, J. J.; Murray, R. W. J. Am. Chem. Soc. 1996, 118, 4212-4213. (11) Hostetler, M. J.; Wingate, J.; Zhong, C.-J.; Harris, J. E.; Vachet, R. W.; Clark, M. R.; Londono, J. D.; Green, S. J.; Stokes, J. J.; Wignall, G. W.; Glish, G. L.; Porter, M. D.; Evans, N. D.; Murray, R. W. Langmuir 1998, 14, 17-30. (12) Green, S. J.; Pietron, J. J.; Stokes, J. J.; Hostetler, M. J.; Vu, H.; Wuelfing, W. P.; Murray, R. W. Langmuir 1998, 14, 5612-5619. (13) Kubo, K.; Kondow, H.; Nishihara, H. Electrochemistry 1999, 12, 1129-1131.
reported biferrocenyl alkanethiol (Bifc(CH2)nSH) Au-based monolayers.16 The pioneering studies of electroactive self-assembled monolayers (SAMs) of ferrocene-terminated alkanethiols coadsorbed with unsubstituted n-alkanethiols on evaporated gold films were reported by Chidsey.4,17 Monolayers containing low concentrations (mole fraction (χFc) e 0.25) of alkanethiols linked to ferrocene by a polar ester group (FcCO2(CH2)nSH) coadsorbed with CH3(CH2)nSH (n ) 7, 9, 11) showed thermodynamically ideal surface electrochemistry. At low mole fractions, Chidsey suggested that the ferrocene groups were homogeneous and noninteracting. At higher mole fractions of the electroactive thiol in the adsorption solution, the resulting cyclic voltammograms broadened, developed an asymmetry, and finally developed an additional set of peaks as the amount of ferrocene was increased. Chidsey suggested that the breakdown of the thermodynamically ideal behavior of this system is probably due to a combination of interactions between ferrocene sites and the inhomogeneity of those sites at higher surface contractions. The use of ferrocene-terminated thiols (Fc(CH2)11SH) in which the ferrocene is attached directly to the polymethylene chain without a polar ester group led to broadened electrochemical features, even at the lowest fraction (χFc ) 0.05) examined. This highly nonideal behavior at low surface coverage suggested a strong interaction between the ferrocene groups, which was perhaps due to aggregation. Following the report by Brust et al. on the synthesis of gold clusters stabilized by monolayers of alkanethiolate ligands,18,19 Murray et al. launched an investigation of (14) Horikoshi, T.; Itoh, M.; Kurihara, M.; Kubo, K.; Nishihara, H. J. Electroanal. Chem. 1999, 473, 113-116. (15) Men, Y.; Kubo, K.; Kurihara, M.; Nishihara, H. Phys. Chem. Chem. Phys. 2001, 3, 3427-3430. (16) Dong, T.-Y.; Chang, L. S.; Tseng, I. M.; Huang, S. J. Langmuir 2004, 20, 4471-4479 . (17) Chidsey, C. E. D. Science 1991, 251, 919-922. (18) Brust, M.; Walker, M.; Bethell, D.; Schiffrin, D. J.; Whyman, R. J. Chem. Soc., Chem. Commun. 1994, 801-802.
10.1021/la0489458 CCC: $27.50 © 2004 American Chemical Society Published on Web 09/04/2004
Ruthenium(II) Terpyridine Gold Clusters
their synthesis to find the core size and the chemical, electrochemical, structural, and physical properties in ferrocenyl alkanethiolate Au clusters.8-12 The voltammetric results showed that ferrocenyl alkanethiolate Au clusters not only exhibited ferrocene electrochemistry but also exhibited a double layer charging phenomenon analogous to that of a metal electrode/solution interface. The cyclic voltammetry (CV) results revealed that the physisorption of clusters acted as tiny electrodes on the working electrode. Recently, the design of new bis-2,2′:6′,2′′-terpyridine (terpy-terpy) ligands created by connecting two terpyridine (terpy) moieties via a rigid spacer attached to their 4′-positions has received a surge in interest, owing to their great utility in the fields of molecular electronics and artificial photosynthesis.20-24 The terpy-terpy ligands that have been prepared and used further to make dinuclear transition metal complexes are illustrated in Chart 1.25-38 Numerous studies on transition metal 2,2′:6′,2′′-terpyridine (terpy) complexes exhibiting luminescence from metal-to-ligand charge-transfer (MLCT) excited states have been reported. The complexes formed between terpy and Ru2+ are normally weakly luminescent at room temperature, but attaching a functionalized group at the 4′-position provides the desired emission. It has been shown that if the Ru2+ terpy complex substituted an alkynylene group at the 4′-position of the terpy moiety, it could possess a room-temperature luminescence lifetime (τp ) 565 ns) 1000-fold longer than that of the Ru(terpy)22+ complex (τp ) 0.56 ns) in deoxygenated CH3CN at 25 °C.31 (19) Brust, M.; Fink, J.; Bethell, D.; Schiffrin, D. J.; Kiely, C. J. J. Chem. Soc., Chem. Commun. 1995, 1655-1656. (20) Balzani, V.; Scandola, F. Supramolecular Photochemistry; Ellis Horwood; Chichester, 1991. (21) Sauvage, J.-P.; Collin, J.-P.; Chambron, J.-C.; Guillerez, S.; Coudret, C.; Balzani, V.; Barigolletti, F.; Cloa, L. De; Flamigni, L. Chem. Rev. 1994, 94, 993-1019. (22) Collin, J.-P.; Gavin˜a, P.; Heitz, V.; Sauvage, J.-P. Eur. J. Inorg. Chem. 1998, 1-14. (23) Harriman, A.; Ziessel, R. Coord. Chem. Rev. 1998, 171, 331339. (24) Barigelletti, F.; Flamigni, L. Chem. Soc. Rev. 2000, 29, 1-12. (25) Indelli, M. T.; Scandola, F.; Collin, J.-P.; Sauvage, J.-P.; Sour, A. Inorg. Chem. 1996, 35, 303-312. (26) Patoux, C.; Launay, J.-P.; Beley, M.; Chodorowski-Kimmes, S.; Collin, J.-P.; James, S.; Sauvage, J.-P. J. Am. Chem. Soc. 1998, 120, 3717-3725. (27) Beley, M.; Chodorowski-Kimmes, S.; Collin, J.-P.; Laine´, P.; Launay, J.-P.; Sauvage, J.-P. Angew. Chem., Int. Ed. Engl. 1994, 33, 1775-1778. (28) Schu¨tte, M.; Kurth, D. G.; Linford, M. R.; Co¨lfen, H.; Mo¨hwald, H. Angew. Chem., Int. Ed. Engl. 1998, 37, 2891-2893. (29) Hammarstro¨m, L.; Barigelletti, F.; Flamigni, L.; Armaroli, N.; Sour, A.; Collin, J.-P.; Sauvage, J.-P. J. Am. Chem. Soc. 1996, 118, 11972-11973. (30) Grosshenny, V.; Harriman, A.; Gisselbrecht, J.-P.; Ziessel, R. J. Am. Chem. Soc. 1996, 118, 10315-10316. (31) Benniston, A. C.; Grosshenny, V.; Harriman, A.; Ziessel, R. Angew. Chem., Int. Ed. Engl. 1994, 33, 1884-1885. (32) Grosshenny, V.; Harriman, A.; Ziessel, R. Angew. Chem., Int. Ed. Engl. 1995, 34, 2705-2708. (33) Grosshenny, V.; Harriman, A.; Ziessel, R. Angew. Chem., Int. Ed. Engl. 1995, 34, 1100-1102. (34) El-ghayoury, A.; Harriman, A.; Khatyr, A.; Ziessel, R. Angew. Chem., Int. Ed. Engl. 2000, 39, 185-189. (35) Harriman, A.; Mayeux, A.; Nicola, A. De; Ziesel, R. Phys. Chem. Chem. Phys. 2002, 4, 2229-2235. (36) Hissler, M.; El-ghayoury, A.; Harriman, A.; Ziessel, R. Angew. Chem., Int. Ed. Engl. 1998, 37, 1717-1720. (37) Harriman, A.; Khatyr, A.; Ziessel, R.; Benniston, A. C. Angew. Chem., Int. Ed. Engl. 2000, 39, 4287-4290. (38) Barigelletti, F.; Flamigni, L.; Calogero, G.; Hammarstro¨m, L.; Sauvage, J.-P.; Collin, J.-P. J. Chem. Soc., Chem. Commun. 1998, 23332334.
Langmuir, Vol. 20, No. 21, 2004 9341 Chart 1
The preparation of 4′-ferrocenyl-2,2′:6′,2′′-terpyridine (fcterpy) was reported,39,40 and cyclic voltammetric measurements of its Ru2+ metal complexes ([Ru(terpy)(fcterpy)]2+ and [Ru(fcterpy)2]2+) indicated that the Ru2+/3+ and ferrocenium/ferrocene redox coupled at ∼1.3 and ∼0.6 V versus Ag/AgCl, respectively.41 Due to the fact that electronic coupling between the Ru2+ and ferrocenyl center is small, attachment of a ferrocene moiety to the 4′-position of terpy has minimal influence on the electrochemical properties in comparison with its monomeric components.41 In addition to the πterpy-π*terpy absorptions (240280 nm) and the dπRu-π*terpy MLCT absorption (∼480 nm), the [Ru(terpy)(fcterpy)]2+ and the [Ru(fcterpy)2]2+ complexes exhibited an unusual 1[(d(π)fc)6] f 1[(d(π)fc)5(π*terpyRu)1] MLCT absorption at ∼510 nm.41 Very recently, our interest in the design of molecular electronic wire has focused on how to arrange the redoxactive subunits. It is hoped that it will be possible to control transfer or store information along the wire. We have prepared a series of molecular wires derived from terpyridines bridged by a ferrocenyl spacer.42 Due to the Ru 1 1[(d(π) )6] to 1[d(π) )5(π* Fe Fe terpy ) ] transition, the binuclear biferrocene appears to be promising bridge that could ensure fast and quantitative transfer of energy within the array. We recognized that the biferrocene ligand usually employed in mixed-valence chemistry was not readily susceptible to structural variations and would be a very good system for systematic work. The spectroscopic measurements for mixed-valence biferrocenium showed that electron transfer in this system was quite easy.43 (39) Farlow, B.; Nile, T. A.; Walsh, J. L.; Mcphail, A. T. Polyhedron 1993, 12, 2891-2894. (40) Constable, E. C.; Edwards, A. J.; Martinez-Ma´n˜ez, R.; Raithby, P. R.; Cargill Thompson, A. M. W. J. Chem. Soc., Dalton Trans. 1994, 645-650. (41) Hutchison, K.; Morris, J. C.; Nile, T. A.; Walsh, J. L.; Thompson, J. R.; Petersen, J. D.; Schoonover, J. R. Inorg. Chem. 1999, 38, 25162523. (42) Dong, T.-Y.; Lin, M.; Chiang, Y. N. M. Inorg. Chem. Commun. 2004, 7, 687-690. (43) Dong, T.-Y.; Chang, L. S.; Lee, G. H.; Peng, S. M. Organometallics 2002, 21, 4192-4200 and references therein.
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Rotello’s research group has recently synthesized44 nanoparticles bearing terpyridine ligands and studied their self-assembly using a variety of transition metals. The present paper describes the first step in the preparation of 4′-ferrocenyl-2,2′:6′,2′′-terpyridine and 4′-biferrocenyl-2,2′:6′,2′′-terpyridine ruthenium(II) terpyridyloctanethiolate complexes (1 and 2 in Scheme 1) and studies of the self-assembly of 1 and 2 chemisorbed on Au nanoparticles. Expansion of this study to the synthesis of molecular wire assembled on a gold surface is underway and will be reported in due course. Experimental Section General Information. The preparations involving airsensitive materials were carried out by using standard Schlenk techniques under an inert atmosphere of N2. Chromatography was performed on neutral Al2O3 (act. III). Dried CH2Cl2 and THF were distilled from P2O5 and Na, respectively. Samples of Ru(DMSO)4Cl2,45 dibromobiferrocene,46 and 4′-ferrocenyl-2,2′: 6′,2′′-terpyridine40 were prepared according to the literature procedure. All other chemicals used were reagent grade and used as received. Ferrocene- and biferrocene-functionalized terpyridine octanethiolate monolayer-protected clusters (MPCs 3 and 4) were prepared as illustrated in Scheme 1. Octanethiolated Au MPCs (AuC8) were prepared as described by Murray.11 Synthesis of Biferrocenecarboxaldehyde. At -78 °C, 4.7 mL of n-BuLi (1.6 M in hexane, 7.2 mmol) was added under nitrogen to the dried THF (30 mL) solution of dibromobiferrocene (1.80 g, 3.41 mmol). The resulting solution was stirred at -78 °C for 1 h. DMF (0.25 mL) was then added, and the solution was stirred at -78 °C for 1 h. Water was then added, and the mixture was extracted with CH2Cl2. The combined extracts were dried over MgSO4 and concentrated by means of a rotatory evaporator. The residue was then chromatographed on Al2O3 (act. III). Elution with hexane/CH2Cl2 (1/1) gave the desired compound with a yield of 28%. Anal. Calcd for C21H18Fe2O‚H2O: C, 60.62; H, 4.84. Found: C, 61.34; H, 4.61. 1H NMR (CDCl3): d 3.96 (s, 5H, Cp), 4.20 (dd, J ) 2; 2 Hz, 2H, Cp), 4.27 (dd, J ) 2; 1.5 Hz, 2H, Cp), 4.33 (dd, J ) 2.5; 5.0 Hz, 2H, Cp), 4.39 (dd, J ) 3.0; 3.0 Hz, 2H, Cp), 4.43 (dd, J ) 2.0; 2.0 Hz, 2H, Cp), 4.58 (dd, J ) 3.0; 1.5 Hz, 2H, Cp), 9.74 (s, 1H, CHO). Mass spectrum (EI): M+ at m/z 398. Synthesis of 4′-Biferrocenyl-2,2′:6′,2′′-terpyridine (bifcterpy). The biferrocenyl-terpyridine was prepared in a fashion similar to that of the fcterpy.40 Anal. Calcd for C35H27Fe2N3‚H2O: C, 67.85; H, 4.68; N, 6.79. Found: C, 68.53; H, 4.57; N, 6.54. 1H NMR (acetone-d6): d 3.83 (dd, J ) 1.5; 1.0 Hz, 2H, Cp), 3.84 (s, 5H, Cp), 4.10 (dd, J ) 2.0; 2.0 Hz, 2H, Cp), 4.13 (dd, J ) 1.5; 2.0 Hz, 2H, Cp), 4.24 (dd, J ) 2.0; 2.0 Hz, 2H, Cp), 4.32 (dd, J ) 1.5; 2.0 Hz, 2H, Cp), 4.83 (dd, J ) 2.0; 1.5 Hz, 2H, Cp), 7.35 (ddd, J ) 4.5; 2.5; 5.0 Hz, 2H, py), 7.88 (dt, J ) 15.5; 14.0 Hz, 2H, py), 8.28 (s, 2H, py), 8.63 (d, J ) 7.0 Hz, 2H, py), 8.75 (d, J ) 4.5 Hz, 2H, py). Mass spectrum (EI): M+ at m/z 601. Mp: 205-206 °C. Synthesis of Dichloro(DMSO)(4′-ferrocenyl-2,2′:6′,2′′terpyridine)Ru(II). An ethanol solution (10 mL) of Ru(DMSO)4Cl2 (0.140 g, 0.29 mmol) and fcterpy (0.126 g, 0.3 mmol) was refluxed for 15 h under N2. After cooling to room temperature, the precipitate was collected by filtration and the crude product (44) Norsten, B. T.; Frankamp, B. L.; Rotello, V. M. Nano Lett. 2002, 2, 1345-1348. (45) Evans, I. P.; Spencer, A.; Wilkinson, G. J. Chem. Soc., Dalton. Trans. 1973, 204-209. (46) Lai, L. L.; Dong, T.-Y. J. Chem. Soc., Chem. Commun. 1994, 2347-2349.
Dong et al. was washed with ether several times. The crude product was used without any further purification. The yield was ∼72%. 1H NMR (DMSO-d6): d 3.86 (dd, 2H, Cp), 3.89 (s, 5H, Cp), 4.38 (dd, 2H, Cp), 4.54 (dd, 2H, Cp), 4.69 (dd, 4H, Cp), 5.33 (dd, 2H, Cp), 7.78 (dd, 2H, Py), 8.17 (dd, 2H, Py), 8.61 (s, 2H, Py), 8.75 (dd, 2H, Py), 9.01 (dd, 2H, Py). Mass spectrum (ESI-MS): M+ at m/z 850. Synthesis of Dichloro(DMSO)(4′-biferrocenyl-2,2′:6′,2′′terpyridine)Ru(II). An ethanol solution (10 mL) of Ru(DMSO)4Cl2 (0.140 g, 0.29 mmol) and bifcterpy (0.180 g, 0.3 mmol) was refluxed for 15 h under N2. After cooling to room temperature, the precipitate was collected by filtration and the crude product was washed with ether several times. The crude product was used without any further purification. Synthesis of 8-(2,2′:6′,2′′-Terpyridin-4′-yloxy)octane-1thiol. Functionalized terpy thiol was prepared through a similar synthetic route described in the literature with some slight modifications.44 Briefly, the 1,8-diiodooctane was converted to 8-(2,2′:6′,2′′-terpyridin-4′yloxy)-1-iodooctane by treatment with 4′-hydroxy-tepyridine and K2CO3 in acetone, and the resulting solution was then refluxed for 24 h. The halide group was then replaced by treatment with NaSH in THF and refluxed for 8 h under N2 to create the desired terpy thiol. Synthesis of 1. The ferrocene thiol derivative 1 (illustrated in Scheme 1) was prepared by reacting terpy thiol (21.5 mg, 0.022 mmol) and fcterpy[Ru(DMSO)Cl2] (29.5 mg, 0.044 mmol) in ethanol (50 mL) and refluxing the resulting solution for 48 h under N2. After cooling to room temperature, the volume of the solution was reduced by one-half using a rotary evaporator. A saturated aqueous solution of NH4PF6 (2 mL) was then added to this dark-colored solution to produce a violet precipitate. The violet precipitate was washed several times with ethanol and purified by chromatography. Elution with acetone gave the desired compound with a yield of 32%. Anal. Calcd for C48H46F12FeN6OP2RuS‚H2O: C, 47.23; H, 3.93; N, 6.89; S, 2.62. Found C, 47.34; H, 3.92; N, 6.90; S, 2.58. 1H NMR (acetone-d6): d 1.49 (m, 6H, CH2), 1.69 (m, 2H, CH2), 1.76 (m, 2H, CH2), 2.05 (m, 2H, CH2), 2.78 (m, 2H, CH2), 3.76 (s,1H, SH), 4.33 (s, 5H, Cp), 4.63 (t, J ) 13.5 Hz, 2H, Cp), 4.78 (t, J ) 4.0 Hz, 2H, Cp), 5.51 (dd, J ) 1.5; 2.5 Hz, 2H, CH2), 7.32 (dd, J ) 7.5; 5.0 Hz, 4H, py), 7.75 (t, J ) 11.5 Hz, 4H, py), 8.04 (dd, J ) 8.0; 13.0 Hz, 4H, py), 8.70 (s, 2H, py), 8.83 (d, J ) 8.5 Hz, 2H, py), 8.95 (d, J ) 8.0 Hz, 2H, py), 9.12 (s, 2H, py). Mass spectrum (ESI-MS): M+ at m/z 1201. Synthesis of 2. The biferrocene thiol derivative 2 (illustrated in Scheme 1) was prepared in a similar way to the ferrocene thiol derivative 1. In an N2 atmosphere, an ethanol solution (20 mL) of terpy thiol (0.12 g, 30 mmol) and bifcterpy[Ru(DMSO)Cl2] (0.24 g, 28 mmol) was refluxed for 24 h. The volume of the solution was reduced by one-half using a rotary evaporator after cooling to room temperature. A saturated aqueous solution of NH4PF6 (2 mL) was added then to this dark-colored solution to produce a violet precipitate. The violet precipitate was washed several times with ethanol and purified by chromatography. Elution with pure acetone gave the desired compound with a yield of 35%. Anal. Calcd for C58H53F12Fe2N6OP2RuS‚H2O: C, 49.62; H, 3.92; N, 5.99; S, 2.28. Found: C, 49.97; H, 3.94; N, 6.24; S, 2.58. 1H NMR (acetone-d6): δ 1.49 (m, 6H, CH2), 1.68 (m, 2H, CH2), 1.76 (m, 2H, CH2), 2.09 (s, 1H, SH), 2.10 (m, 2H, CH2), 2.92 (m, 2H, CH2), 3.83 (t, J ) 3.5 Hz, 2H, Cp), 3.89 (s, 5H, Cp), 4.38 (t, J ) 3.5 Hz, 2H, Cp), 4.54 (t, J ) 2.0 Hz, 2H, Cp), 4.55 (m, 2H, CH2), 4.68 (m, 4H, Cp), 5.34 (t, J ) 3.5 Hz, 2H, Cp), 7.29 (m, 2H, py), 7.40 (m, 2H, py), 7.63 (d, J ) 0.5 Hz, 2H, py), 7.76 (m, 2H, py), 8.05 (m, 4H, py), 8.54 (m, 1H, py), 8.76 (m, 3H, py), 8.80 (d, J ) 8.5 Hz, 2H, py), 9.05 (d, J ) 8.0 Hz, 2H, py). Mass spectrum (ESI-MS): M+ at m/z 1385. Au MPCs. Octanethiol-modified Au MPCs have been prepared by the method outlined in the literature published by Murray.11 Briefly, a toluene solution containing a 2:1 mole ratio of octanethiol to HAuCl4 was reduced using NaBH4 at room temperature. Following the steps of concentrating the organic layer, pouring into ethanol, standing overnight at -4 °C, centrifuging, and rinsing with ethanol gave the desired Au MPCs. The Au MPCs had average core diameters of approximately 2.2 ( 0.7 nm. The ∼2 nm sized particles were used to induce size and shape evolution with heating treatment.47 These particles were then dissolved in a small amount of toluene together with N(C8H17)4Br and C8H17SH. The resulting solution was heated in a silicon oil bath
Ruthenium(II) Terpyridine Gold Clusters
Langmuir, Vol. 20, No. 21, 2004 9343 Scheme 1
at 138 °C. When the color of this solution changed from brown to dark red, the solution was heated further at 110 °C for an additional 5 h. The change in color also included a size evolution. The solvent was removed in a rotary evaporator (under 50 °C) followed by washing several times with ethanol and acetone. The Au MPCs created by using the heating treatment had average core diameters of approximately 4.5 nm (measured by transmission electron microscopy (TEM)). Synthesis of Au MPCs 3 and 4. A mixture containing octanethiol-modified Au clusters (50 mg) and derivative 1 or 2 (47) Zhong, C. J.; Maye, M. M.; Zheng, W.; Leibowitz, F. L.; Ly, N. K. Langmuir 2000, 16, 490-497.
(10 mg) in a 5 mL toluene/CH2Cl2 (3:7) solution was stirred at room temperature for 48 h. The solvent was removed under a vacuum. The resulting product was cleaned by washing with hexane and acetone, and it was then suspended in acetone and isolated with a centrifuge. This procedure was repeated several times in order to obtain purified modified Au MPCs. Electrochemical Measurements. Cyclic voltammetry was performed using a BAS 100W system. The reference electrode was Ag/AgCl in a saturated KCl solution. The CV measurements were carried out in a standard three-compartment cell under N2 at 25 °C equipped with a Pt counter electrode, glassy carbon working electrode, and Ag/AgCl reference electrode in a CH2Cl2/ CH3CN (3:7) solution containing a 0.1 M (n-C4H9)4NPF6 elec-
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Dong et al. Table 1. CV Data of 1-4 and Related Compounds with a Scan Rate of 100 mV s-1 Ru center compda
E1/2 (V)b
∆Ep (mV)c
fc bifc
Fe center E1/2 (V)b 0.46 0.28 0.59 0.64
fctpy bifctpy 1
1.28
121
2
1.33
119
3
1.32
90
4
1.37
77
∆E1/2 (V)d 0.31
terpy center
∆Ep (mV)c 70 70 75 73
0.45 0.88 0.62
0.43
70 79 62
0.48 0.86 0.68
0.38
66 67 42
0.58 0.89
0.32
42 23
E1/2 (V)b
∆Ep (mV)c
