Langmuir 2004, 20, 10379-10381
10379
Synthesis and Characterization of Phosphido-Monolayer-Protected Gold Nanoclusters Diana M. Stefanescu,† David S. Glueck,*,† Rene´e Siegel,‡ and Roderick E. Wasylishen‡ 6128 Burke Laboratory, Department of Chemistry and Center for Nanomaterials Research, Dartmouth College, Hanover, New Hampshire 03755, and Department of Chemistry, University of Alberta, Edmonton, Alberta, Canada T6G 2G2 Received September 21, 2004 Gold-phosphido-monolayer-protected clusters (MPCs) of 1-2-nm diameter, Aux(PR2)y, analogues of the well-known thiolate materials Aux(SR)y, were prepared by NaBH4 reduction of a mixture of HAuCl4‚3H2O and a secondary phosphine PHR2 in tetrahydrofuran/water. In comparison to the Au-thiolate MPCs, fewer of the larger phosphido groups are required to cover the surface, and the Au-P bond is not cleaved as readily in reactions with small molecules as is its Au-S counterpart. 31P NMR spectroscopy provides a direct method to study cluster formation and the interaction of the phosphido ligand with the gold surface.
Introduction
Experimental Section
Nanoscale gold monolayer-protected clusters (MPCs) have been the subject of recent intense research activity.1 The most popular synthesis, reduction of a gold salt in the presence of a thiol, gives MPCs with surface-passivating thiolate (SR-) groups.2 Related clusters have been prepared with several other ligands, such as phosphines,3 amines,4 and thioethers.5 This work, and results with a variety of thiols, showed that fundamental cluster properties, such as size and reactivity, depend on the nature of the surface ligand. To extend these investigations of the surface chemistry of gold, we report here similar syntheses with secondary phosphines to yield MPCs coated with phosphido (PR2-) groups. In contrast to some of the other alternatives, the phosphido group is isoelectronic to thiolate, enabling a direct comparison between these ligands. Moreover, 31P NMR spectroscopy is a convenient method to monitor MPC syntheses and to directly characterize the Au-P interaction in the clusters.
All syntheses and manipulations of Au MPCs (except column chromatography and recrystallization of the eluted clusters) were performed under N2 using standard Schlenk and dry-box techniques. A typical preparation is described below; for additional details of synthesis, characterization, and reactions of the Au-phosphido MPCs, see Supporting Information. Aux(PMes2)y (1). To a solution of 1.181 g (3 mmol) of HAuCl4‚ 3H2O in a mixture of 120 mL of tetrahydrofuran (THF) and 75 mL of water was added 0.811 g of PHMes2 (3 mmol) in 60 mL of a 1:1 THF/water mixture. At this point a very slight darkening of the color of the solution and an increased turbidity could be observed. After stirring for 10 min, a solution of 0.90 g (24 mmol) of NaBH4 in 30 mL of water was added. The color of the solution turned black. After 3 h of stirring, 150 mL of toluene was added and the organic phase was separated from the aqueous one. The organic solvent was evaporated at room temperature to yield a black solid. The solid was suspended in 30 mL of acetonitrile and then vacuum filtered on a fine frit and washed with another 60 mL of acetonitrile. The yield was 658 mg, 68% (based on gold). 1H NMR (C D ): δ 6.89, 6.58, 2.65-1.96 (m), overlap of broad 6 6 and sharp peaks. 31P{1H} NMR (C6D6): δ -16.1, -23.4, -36.1 showing the presence of [Au(PMes2)]n as an impurity.6 TEM: d (nm) ) 1.26 ( 0.42. XPS: Au/P ) 3:1. TGA: 60.96% Au, Au/P ) 2.1:1. Elem anal. (Schwarzkopf, Woodside, NY) found for a similarly prepared sample: C, 24.15; H, 3.46; Au, 58.56; P, 2.89; Cl, < 0.2. To separate [Au(PMes2)]n from Aux(PMes2)y, a sample of 120 mg of as-prepared Aux(PMes2)y was dissolved in 3 mL of toluene and loaded onto a silica column (19 × 400 mm). A total of 500 mL of a 4:1 mixture of hexane and toluene was used to separate the oligomer impurity from the cluster. A clear solution was eluted, yielding a white-yellow solid after solvent evaporation (8 mg of [Au(PMes2)]n, 6.7 wt %, 31P{1H} NMR (C6D6) δ -23.4, -36.1). The clusters remained on the column. The column was washed with 100 mL of ethyl acetate and then with 100 mL of ethanol until the eluted solution turned from black to a faint brown color. The solvent from both solutions was evaporated under vacuum yielding a black solid that was further recrystallized from toluene and acetonitrile. Some black material was still present on the column, but it could not be removed. The recovery was 20 mg (17 wt %). The resulting material was characterized further by solution 1H and 31P NMR, TEM, XPS, TGA, mass spectroscopy, and solid-state 31P NMR.
* To whom correspondence should be addressed. glueck@ dartmouth.edu. † Dartmouth College. ‡ University of Alberta. (1) (a) Daniel, M.-C.; Astruc, D. Chem. Rev. 2004, 104, 293-346. (b) Templeton, A. C.; Wuelfing, W. P.; Murray, R. W. Acc. Chem. Res. 2000, 33, 27-36. (2) Brust, M.; Walker, M.; Bethell, D.; Schiffrin, D. J.; Whyman, R. J. Chem. Soc., Chem. Commun. 1994, 801-802. (3) (a) Tamura, M.; Fujihara, H. J. Am. Chem. Soc. 2003, 125, 1574215743. (b) Schmid, G.; Corain, B. Eur. J. Inorg. Chem. 2003, 30813098. (c) Weare, W. W.; Reed, S. M.; Warner, M. G.; Hutchison, J. E. J. Am. Chem. Soc. 2000, 122, 12890-12891. (4) (a) Brown, L. O.; Hutchison, J. E. J. Am. Chem. Soc. 1999, 121, 882-883. (b) Leff, D. V.; Brandt, L.; Heath, J. R. Langmuir 1995, 12, 4723-4730. (5) (a) Shelley, E. J.; Ryan, D.; Johnson, S. R.; Couillard, M.; Fitzmaurice, D.; Nellist, P. D.; Chen, Y.; Palmer, R. E.; Preece, J. A. Langmuir 2002, 18, 1791-1795. (b) Li, X.-M.; de Jong, M. R.; Inoue, K.; Shinkai, S.; Huskens, J.; Reinhoudt, D. N. J. Mater. Chem. 2001, 11, 1919-1923.
(6) (a) Stefanescu, D. M.; Yuen, H. F.; Glueck, D. S.; Golen, J. A.; Rheingold, A. L. Angew. Chem., Int. Ed. 2003, 42, 1046-1048. (b) Stefanescu, D. M.; Yuen, H. F.; Glueck, D. S.; Golen, J. A.; Zakharov, L. N.; Incarvito, C. D.; Rheingold, A. L. Inorg. Chem. 2003, 42, 88918901.
10.1021/la0476478 CCC: $27.50 © 2004 American Chemical Society Published on Web 10/21/2004
10380
Langmuir, Vol. 20, No. 24, 2004
Letters
Scheme 1
Table 1. Analytical Data for Phosphido-Monolayer-Protected Gold Nanoclusters 1-4 PR2
% Au (TGA)
Au/P (TGA)
Au/P (XPS)
dTEM (nm)a
average formula
PMes2b PPhMesb PCy2c P(t-Bu)2c
66.19 67.38 75.1 83.62
2.7:1 2.4:1 3.0:1 3.8:1
2.8:1 2.2:1 4.5:1 4.4:1
1.14 1.07 1.67 1.96
Au38(PMes2)14 Au38(PPhMes)16 Au146(PCy2)38 Au314(P(t-Bu)2)67
Average core diameter. b After column chromatography. c Assynthesized. Chromatography allowed estimation of the amount of oligomer impurity, but recovery of cluster 3 was too low for further characterization and 4 could not be completely separated from the oligomer. a
1H
NMR (C6D6): δ 6.6 (broad, Ar), 3.2-1.6 (very broad, Me). NMR (C6D6): no peak was observed. IR (KBr, cm-1): 3022, 2911, 2856, 1728, 1600, 1550, 1456, 1400, 1378, 1289, 1239, 1161, 1083, 1028, 850, 694, 611, 556, 450. TEM: d (nm) ) 1.14 ( 0.34. XPS: Au/P ) 2.8:1. TGA: 66.19% Au, Au/P ) 2.7:1. MALDI TOF-MS (HABA): maximum around m/z 11 405. The m/z value calculated for Au38(PMes2)14 is 11 256. The solid-state 31P NMR spectrum of the material after chromatography was similar to that of the as-synthesized material, with an additional sharp peak at 0.8 ppm (Supporting Information, Figures S1 and S2). Although no oligomers were observed by solution 31P NMR in a sample of chromatographed material, a peak at -17 ppm assigned to an oligomer was observed in the solid state. Presumably the small amount of material and its low solubility precluded observation of this peak in solution. 31P{1H}
Results and Discussion Treatment of an aqueous solution of HAuCl4 with 1 equiv of a secondary phosphine PHR2, followed by reduction with excess aqueous NaBH4, gave dark solutions from which toluene-soluble MPCs formulated as Aux(PR2)y (14) could be extracted and isolated as black powders (Scheme 1).7 31 P and 1H NMR spectra of these crude materials showed that they were contaminated with the previously described oligomers [Au(PR2)]n.6 Column chromatography on silica removed most of the oligomers from 1-3, but some decomposition occurred and much of the cluster sample could not be eluted from the column. After further recrystallization from toluene/acetonitrile, only small amounts of MPCs 1 and 2 were recovered. For 4, less of the oligomer could be removed by chromatography.8 The MPCs were characterized by a range of techniques (Table 1). Transmission electron microscopy (TEM) showed that the clusters contained gold cores of 1-2-nm diameter, depending on the phosphine used, with modest polydispersity (ca. 30-40%, Figure 1). These results are consistent with UV-vis spectra (in CH2Cl2), which lacked the gold surface plasmon resonance band at 520 nm typically observed for larger clusters (diameter > 2 nm).9 The TEM results for as-prepared and chromatographed MPCs did not differ significantly. (7) For analogous single-phase syntheses of Au-thiolate MPCs, see: (a) Brust, M.; Fink, J.; Bethell, D.; Schiffrin, D. J.; Kiely, C. J. Chem. Soc., Chem. Commun. 1995, 1655-1656. (b) Yee, C. K.; Jordan, R.; Ulman, A.; White, H.; King, A.; Rafailovich, M.; Sokolov, J. Langmuir 1999, 15, 3846-3491. (8) Weight percentage of oligomer impurity: 7% (for 1), 4% (for 2), ∼15% (for 3), and ∼20% (for 4). (9) Alvarez, M. M.; Khoury, J. T.; Schaaff, T. G.; Shafigullin, M. N.; Vezmar, I.; Whetten, R. L. J. Phys. Chem. B 1997, 101, 3706-3712.
Figure 1. TEM micrograph and size distribution for Aux(PMes2)y.
The elemental composition was established by a combination of elemental analysis, X-ray photoelectron spectroscopy (XPS), and thermogravimetric analysis (TGA). Microanalysis on selected samples showed that no chlorine was present, in agreement with XPS results, ruling out formulas like Aux(PHR2)yClz, as observed in MPCs with tertiary phosphine ligands.3c,10 XPS also provided Au/P ratios consistent with TGA measurements. The number of gold core atoms can be estimated from the average diameter of the clusters, yielding the average formulas in Table 1.11 In comparison to Au-thiolate MPCs, the phosphido materials are similar in size, but fewer of the larger PR2 groups are required to cover the surface.12 In NMR studies of Au-SR MPCs, resonances of nuclei close to the gold core were broadened and difficult to observe.13 Consistent with these results, the PR2 signals in the solution 1H NMR spectra of 1-4 were broad and no peaks due to the MPCs could be seen in the 31P NMR spectra.14 For comparison, the solution 77Se NMR spectrum of octaselenolate-capped Au nanoparticles showed a very broad peak (line width about 34 000 Hz).15 Although few solid-state 31P NMR studies of P-containing MPCs have been reported, this technique is likely to be generally useful in the characterization of these materials.16 Here, it revealed broad (full width at halfmaximum, fwhm, ranges from ca. 4000-18 000 Hz) peaks for materials 1-4 (Table 2), as well as the oligomer impurities and, in some cases,17 the phosphine oxide PHR2(O).11b Spectra of 1 before and after chromatography showed that peaks assigned to the cluster survived, while most of the oligomer impurity was removed. The largest (10) Schmid, G.; Pfeil, R.; Boese, R.; Bandermann, F.; Meyer, S.; Calis, G. H. M.; van der Velden, J. W. A. Chem. Ber. 1981, 114, 36343642. (11) (a) Schaaff, T. G.; Shafigullin, M. N.; Khoury, J. T.; Vezmar, I.; Whetten, R. L.; Cullen, W. G.; First, P. N.; Gutie´rrez-Wing, C.; Ascensio, J.; Jose-Yacama´n, M. J. J. Phys. Chem. B 1997, 101, 7885-7891. (b) Formulas were calculated for the idealized composition Aux(PR2)y. XPS also showed the presence of O, presumably from the phosphine oxide (observed by NMR, see below) and/or water or residual solvent. We cannot rule out the possibility that some of the Au-PR2 groups are also oxidized. (12) Hostetler, M. J.; Wingate, J. E.; Zhong, C.-J.; Harris, J. E.; Vachet, R. W.; Clark, M. R.; Londono, J. D.; Green, S. J.; Stokes, J. J.; Wignall, G. D.; Glish, G. L.; Porter, M. D.; Evans, N. D.; Murray, R. W. Langmuir 1998, 14, 17-30. (13) Kohlmann, O.; Steinmetz, W. E.; Mao, X.-A.; Wuelfing, W. P.; Templeton, A. C.; Murray, R. W.; Johnson, C. S., Jr. J. Phys. Chem. B 2001, 105, 8801-8809. (14) For solution 31P NMR studies of Au-PPh3 nanoparticles, see: Petroski, J.; Chou, M. H.; Creutz, C. Inorg. Chem. 2004, 43, 15971599. For solution 15N NMR studies of Au-amine nanoparticles, see: Thomas, K. G.; Zajicek, J.; Kamat, P. V. Langmuir 2002, 18, 37223727. (15) Zelakiewicz, B. S.; Yonezawa, T.; Tong, Y. J. Am. Chem. Soc. 2004, 126, 8112-8113. (16) (a) Kolbert, A. C.; de Groot, H. J. M.; van der Putten, D.; Brom, H. B.; de Jongh, L. J.; Schmid, G.; Krautscheid, H.; Fenske, D. Z. Phys. D., At., Mol. Clusters (Suppl.) 1993, 26, S24-26. (b) van Staveren, M. P. J.; Brom, H. B.; de Jongh, L. J.; Schmid, G. Z. Phys. D.: At., Mol. Clusters 1989, 12, 451-452.
Letters
Langmuir, Vol. 20, No. 24, 2004 10381
Table 2. Solid-State
31P
Scheme 3
NMR Data for Au-Phosphido MPCs
material
δ (cluster)a (fwhm)
δ (oligomer impurity)b (pure oligomer)
Aux(PMes2)y (1) Aux(PPh(Mes))y (2) Aux(PCy2)y (3) Aux(P(t-Bu)2)y (4)
109 (4000)c 113 (4840)c 125 (4860) 215 (18 000)c
-17, -34 (-17, -34) -20 (shoulder) (-29) 60, 41 (64, 50, 44) 116 (117, 113, 95, 81)
a Chemical shifts in ppm, peak widths in Hz. b Signals in the MPC samples assigned to the oligomers match those observed for pure oligomer samples. c Besides the main peaks assigned to the clusters, additional signals were observed: for 1, δ 27 (sharp); for 2, δ 13, 3, overlapping a spinning sideband; for 4, δ 52 (broad), 1 (sharp).
Scheme 2
cluster, 4, displayed the broadest peak, with a chemical shift significantly different from those of 1-3. For comparison, the solution 13C NMR chemical shift of 13C1labeled Au-octanethiolate MPCs depended on particle size.18 Here, the 31P NMR chemical shift might also depend on the PR2 groups. Treatment of aqueous HAuCl4 with a thiol RSH causes reduction of gold and formation of the thiolate oligomer [Au(SR)]n.19 Subsequent reduction with NaBH4 yields MPCs. Monitoring the Au-phosphido MPC synthesis by 31 P NMR spectroscopy revealed a similar reduction process (Scheme 2). Treatment of aqueous HAuCl4 with PHR2 in THF gave the known compounds Au(PHR2)(Cl) and PR2Cl; conversion of PMes2Cl to the phosphine oxide PHMes2(O) was observed in the presence of water.20 After borohydride reduction, extraction into toluene, and isolation of the MPCs, washing with acetonitrile removed PHR2 and PHR2(O), leaving clusters and oligomers [Au(PR2)]n behind. Although they were observed in the synthesis, independent treatment of [Au(PMes2)]n or Au(PHMes2)(Cl) with NaBH4 did not give the same cluster product.3c Treatment of Au-thiolate MPCs with iodine gives the disulfides RSSR and is commonly used to characterize the ligands on a MPC surface.21 In contrast, treatment of 4 with I2 yielded the iodophosphine complex Au(P(t-Bu)2I)(I) (5) as the only soluble product (32% yield), along with a black insoluble byproduct (Scheme 3). Because reaction of I2 with [Au(P(t-Bu)2)]n is known to give 5,6b we cannot (17) Phosphine oxides [(a) Lindner, E.; Frey, G. Chem. Ber. 1980, 113, 2769-2778; (b) Fagan, P. J.; Li, G. WO 0021663, 2000; (c) Li, G. Y. Angew. Chem., Int. Ed. 2001, 40, 1513-1516] were observed in samples of 1 and 2 but not in 4. Overlap of the expected 31P NMR signal for the phosphine oxide [(d) Li, G. Y. WO 0200574, 2002] with the peaks due to the oligomer in 3 prevented detection of PHCy2(O) by this technique. (18) Zelakiewicz, B. S.; de Dios, A. C.; Tong, Y. J. Am. Chem. Soc. 2003, 125, 18-19. (19) Shon, Y.-S.; Mazzitelli, C.; Murray, R. W. Langmuir 2001, 17, 7735-7741. (20) (a) For Au(PHR2)(Cl) complexes, see: Schmidbaur, H.; Weidenhiller, G.; Aly, A. A. M.; Steigelmann, O.; Muller, G. Z. Naturforsch., B 1989, 44, 1503-1508. Schmidbaur, H.; Aly, A. A. M. Z. Naturforsch., B 1979, 34, 23-26. (b) For dimesitylphosphine derivatives, see: Bartlett, R. A.; Olmstead, M. M.; Power, P. P.; Sigel, G. A. Inorg. Chem. 1987, 26, 1941-1946 and ref 17a. (c) We have not yet been able to identify all the intermediates formed in these syntheses. See Supporting Information for details of these experiments and their dependence on the PR2 group. (21) Templeton, A. C.; Hostetler, M. J.; Kraft, C. T.; Murray, R. W. J. Am. Chem. Soc. 1998, 120, 1906-1911.
tell if it results solely from the impurity present in the MPC sample or also via etching of [Au(PR2)] units from the cluster. Consistent with the latter possibility, treatment of chromatographed 1 and 2 with I2 also gave products formulated as Au(PR2I)(I) (6,7), but these rapidly decomposed in solution. Place-exchange reactions of thiolate MPCs with thiols have been widely explored,1 and thiols displace amines and phosphines from gold MPCs.4a,22 However, dodecanethiol did not react with 3 or 4. In contrast, similar reactions of 1 and 2 quickly yielded the secondary phosphine PHR2, along with some phosphine oxide PHR2(O). The origin of the phosphine oxide is not clear. Some is formed in the synthesis and remains as an impurity in MPC samples.11b Deprotonation of a phosphine oxide or oxidation of a phosphido ligand might give a surface PR2(O)- group17c which could yield PHR2(O) on place exchange with the thiol. Phosphine oxide might also result from redox chemistry in the reaction with thiol.23 Oxidation of the MPCs does not appear to occur in the solid state. After samples of 1 and 2 were aged in the air for 1 month, treatment with dodecanethiol gave phosphine and phosphine oxide in a ratio similar to that observed for samples stored under N2. Conclusion We report a single-phase synthesis of gold-phosphido MPCs Aux(PR2)y, analogues of the well-known thiolate materials Aux(SR)y. The syntheses, physical properties, and sizes of the two classes of clusters are similar, but there are some significant differences: (i) fewer of the larger phosphido groups are required to cover the surface; (ii) 31P NMR spectroscopy provides a direct method to study cluster formation and to characterize the interaction of the capping ligand with the gold surface; and (iii) the Au-P bond is not cleaved as readily in reactions with small molecules as is its Au-S counterpart. Further study of the dependence of the physical properties of Au-phosphido MPCs on the substituents and the cluster size will provide fundamental information on these and related materials. Acknowledgment. We thank Charles Daghlian and Ursula Gibson for assistance with TEM and XPS and the National Science Foundation, the American Chemical Society Petroleum Research Fund, and NSERC (Canada) for support. Supporting Information Available: Additional experimental details and characterization data. This material is available free of charge via the Internet at http://pubs.acs.org. LA0476478 (22) (a) Woehrle, G. H.; Warner, M. H.; Hutchison, J. E. J. Phys. Chem. B 2002, 106, 9979-9981. (b) Warner, M. G.; Reed, S. M.; Hutchison, J. E. Chem. Mater. 2000, 12, 3316-3320. (c) Brown, L. O.; Hutchison, J. E. J. Am. Chem. Soc. 1997, 119, 12384-12385. (23) Song, Y.; Huang, T.; Murray, R. W. J. Am. Chem. Soc. 2003, 125, 11694-11701.
