J. Phys. Chem. C 2009, 113, 17291–17295
17291
The Smallest Thiolated Gold Superatom Complexes De-en Jiang,*,† Robert L. Whetten,‡ Weidong Luo,§,| and Sheng Dai† Chemical Sciences DiVision, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, School of Chemistry & Biochemistry, Georgia Institute of Technology, Atlanta, Georgia 30332, Department of Physics and Astronomy, Vanderbilt UniVersity, NashVille, Tennessee 37235, and Materials Science and Technology DiVision, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831 ReceiVed: April 19, 2009; ReVised Manuscript ReceiVed: June 5, 2009
The superatom concept of metallic cluster valence is based on the electron-shell model as first proposed to explain the special stability of certain metal-atom clusters generated in the gas phase. It accounts for the magic-number series 2, 8, 18, 34, 58, ... by shell-closing of the superatom orbitals 1S, 1P, 1D, .... Recently, the superatom-complex concept has been introduced to explain the compositions of high-yield gold-cluster compounds, especially Au25(SR)18- and Au102(SR)44 (with -SR being a thiolate group), corresponding to the magic numbers of 8 and 58, respectively. Surprisingly, no thiolated gold cluster accounting for the first closing (electron count 2) has yet been determined. Structure-bonding considerations lead us to propose Au12(SR)9+ as the superior candidate for the smallest thiolated gold superatom. This cluster features an octahedron core covered by three RS(AuSR)2 motifs. It has a unique C3 axis, is chiral, and possesses ideal aurophilic interactions and, therefore, should exist in nature. The folding of thiol-rich biomolecules may help us to realize this complex, which may also be prepared from available phosphine-ligated gold clusters. 1. Introduction Gold-thiolate chemistry has been very beneficial to humankind. Down to the simplest form, gold-thiolate complexes have been used as a drug to treat arthritis.1 Well into the nanoregime, thiolated gold nanoparticles and their assemblies find applications ranging from protein labeling, to drug delivery, to sensing.2 The Brust synthesis3 of thiolated gold nanoparticles first published in 1994 stands as a landmark in nanogold research because it offered a simple wet-chemistry approach to air and thermally stable gold nanoparticles in a large quantity and with narrow size distribution. But such clusters remained far from the dream of “perfect nanometer-scale crystallites, identically replicated in unlimited quantities”.4 That dream came true in 2007 with the total-structure determination of Au102(SR)44,5-7 which clearly shows two structural characteristics of thiolated gold (Au:SR) nanoclusters (with tens to hundreds of gold atoms): a high-symmetry compact metallic core that is complexed Via a unique interfacial structure to both short and long chelating motifs (Figure 1). Walter et al.8 interpreted the electronic structure of ligand-protected gold clusters including Au102(SR)44 by applying the superatomcomplex concept9 to density functional theory (DFT) results. The unique role of the chelating motifs in stabilizing the gold-thiolate interface was demonstrated by DFT-based molecular dynamics simulations.10 Akola et al.11 further advanced this field by correctly predicting the structure for Au25(SR)18-, which yielded to total structure determination by two independent studies.12,13 The structure determinations of Au102(SR)44 and Au25(SR)18-, and the superatom concept underlying their * To whom correspondence should be addressed. E-mail:
[email protected]. Phone: (865)574-5199. Fax: (865) 576-5235. † Chemical Sciences Division, Oak Ridge National Laboratory. ‡ Georgia Institute of Technology. § Vanderbilt University. | Materials Science and Technology Division, Oak Ridge National Laboratory.
Figure 1. Chelating short (RS-Au-SR) and long [RS-(Au-SR)2] motifs. Dashed lines indicate coordination to the cluster core.
stability, offer promising clues to unravel the structures for other Au:SR clusters and to predict new ones. On the basis of the experimental structures for Au102(SR)44 and Au25(SR)18-, Tsukuda et al.14 proposed three simple construction principles for modeling Au:SR clusters: (1) a highsymmetry core; (2) more long motifs for smaller clusters; and (3) each surface atom of the core being bound by terminal thiolates (Figure 1). Although these principles do not consider the electronic structure of the system in whole, they do provide an effective means to generate candidate models for Au:SR clusters whose structures are not yet known. One successful example from the three principles is the prediction of the most stable model for the high-yield Au38(SR)24 compounds. Tsukuda et al.14 first proposed a bi-icosahedral core structure; Zeng and co-workers15 then demonstrated that a face-sharing bi-icosahedral core for Au38(SR)24 (complexed by three short and six long ligands) is indeed significantly lower in energy than all previous models.10,16-18 Coupling Tsukuda’s three principles with the superatom concept that dictates magic numbers of stability for the Au: SR clusters, one can predict Au:SR clusters that are missing from the magic-number series. The magic number corresponding to the major shell-closing electron count for a spherical, squarewell potential goes like 2, 8, 18, (20), 34, (40), 58, 92, ....19 In the structure-known experimental systems, Au25(SR)18- exhibits the eight electron count, while Au102(SR)44 corresponds to 58.8 The Au44(SR)282- cluster20 gives an electron count of 18. Recently, Lopez-Acevedo et al.21 predicted an icosahedral-
10.1021/jp9035937 CCC: $40.75 2009 American Chemical Society Published on Web 07/01/2009
17292
J. Phys. Chem. C, Vol. 113, No. 40, 2009
symmetry structure for the giant Au144(SR)60 cluster which in neutral form is 8 electrons short of the magic number 92. While analysis of high-magic-number clusters is challenging due to high computational cost, one wonders what the clusterstructure would be for the lowest magic number 2. This task should be conceptually and computationally less demanding, given the expectation of a much smaller cluster for this magic number. Surprisingly, there has been no experimental report of small metal-rich Au:SR clusters with less than 15 gold atoms. This is in stark contrast with phosphine-ligated gold clusters where such small clusters are long known.22,23 In this research, we pursue the smallest thiolated gold superatom from several considerations. For the high-symmetry core, we consider the Platonic solids only, assuming that nature prefers order. For the superatom electron-count, we use the rule8 relating the numbers of gold atoms, thiolates, and charge. For the numbers and types of chelate-motifs (short vs long), we apply Tsukuda’s principles.14 The constructed models will be optimized by DFT (section 2). Candidate structures’ electronic structures and optical properties will be interrogated and discussed in section 3. We conclude our findings in section 4. 2. Methods Turbomole V5.10 was used for parallel resolution-of-identity density functional theory (RI-DFT) calculations.24 The nonempirical Tao-Perdew-Staroverov-Scuseria (TPSS)25 form of the meta-generalized gradient approximation (meta-GGA) was used for electron exchange and correlation, because it has been shown26 that the TPSS functional can describe well the aurophilic interactions in gold clusters and gold complexes better than the local density approximation (LDA), GGA, and hybrid functionals. The def2-TZVP orbital and auxiliary basis sets27 were used for all atoms for structural optimization. Effective core potentials which have 19 valence electrons and include scalar relativistic corrections were used for Au.28 The force convergence criterion was set at 1.0 × 10-3 a.u. The timedependent DFT (TDDFT) method was used to compute the excited states.29 Because meta-GGA functionals contain the kinetic energy density, which is not gauge-invariant, the TPSS functional cannot be used in the standard TDDFT method. Instead, we used the closely related Perdew-Burke-Ernzerhof (PBE)30 GGA functional for excited states, based on the TPSSoptimized geometry. From the difference in the HOMO-LUMO gap, we estimate that the PBE functional lowers the excitation energies by 0.05 eV compared to the TPSS functional. In order to simulate temperature and disorder effects, we applied a Gaussian convolution to the computed optical absorption spectrum, with a dispersion of 0.10 eV. Optimized structures were verified to be local minima by normal-mode analysis. 3. Results and Discussion The five Platonic solids include the regular tetrahedron, octahedron, cube, icosahedron, and dodecahedron. The icosahedron forms the core of Au25(SR)18-,11-13 and we exclude it here. The dodecahedron has 20 vertices and so is unlikely to be a core for the magic-number-2 cluster, which is expected to have a much smaller core. The cube has a rather open structure and is unlikely to be stable to serve as a cluster core. Thus, we are left with tetrahedron and octahedron. For such small clusters, the long motifs are preferred to form the protective layer, because their flexible Au-S-Au angle can adapt to the high curvature of the cluster surface.14,18 Two ligands coordinate to the four atoms of the tetrahedron core, whereas three protect the 6-atom octahedron core, yielding formulas of Au8(SR)6 and
Jiang et al.
Figure 2. (a) Schematic construction of the tetrahedron-core cluster Au8(SR)6; the second arrow indicates the DFT-geometry-optimization (G.O.) process. (b) Schematic construction of the octahedron-core cluster Au12(SR)9. Au, green; S, blue; R, not shown.
Au12(SR)9, respectively (see the left column of Figure 2). Now we need to determine the charge for the two clusters. For an arbitrary cluster Aux(SR)Lq, the electron count is n ) x - L q.8 For n ) 2, q ) x - L - 2. So the tetrahedron-core cluster should be neutral while the octahedron-core cluster should be positively (+1) charged. Figure 2a shows how the tetrahedron-based cluster Au8(SR)6 is constructed; namely, the two long motifs wrap around two faces of the tetrahedron and connect two vertices. The dihedral angle of the two faces (70.5°) causes a large strain in the long motif, because the preferred Au-S-Au angle for the long motif is 100 to 120°, as shown in the experimental structures of Au25(SR)18- (refs 12 and 13) and Au102(SR)44 (ref 5) and from computationally scanning the Au-S-Au angle for the longmotif anion in the gas phase (see Supporting Information Figure S1 online). Consequently, the initial structure underwent a dramatic change after DFT optimization, leading to an optimized structure shown in Figure 2a. Interestingly enough, the optimized structure still features a tetrahedron core but now the two long motifs simply cap opposing edges of the tetrahedron with a 90° Au-S-Au angle. This structure has a HOMO-LUMO gap of 3.23 eV. Can this cluster be the smallest superatom complex? We think probably not. Although the electronic structure indicates great stability and Tsukuda’s three principles are fulfilled, the structure is quite open and exposes the core to further chemical attacks, indicating that the cluster would be very labile. However, the optimized structure of Au8(SR)6 does demonstrate that these ligands can chelate the edges of a small polyhedron. We further considered capping the two opposite edges of the tetrahedron with the short chelating motifs, to generate Au6(SR)4 (see Supporting Information Figure S2 online). The neutral state of this cluster also has a shell-closing electron count of 2. In the optimized structure, the tetrahedral core is well maintained with a slightly bended S-Au-S bond (∼160°) in the short motif. The cluster has a HOMO-LUMO gap of 2.40 eV. We think that this cluster is likely to be as labile as Au8(SR)6, due to the half naked core. We now turn our attention to the octahedron core. Figure 2b shows that each of the three long motifs wraps around two neighboring faces and connects two opposing vertices. Here the dihedral angle between two faces is 109.4°, which can comfortably accommodate the long motif. Hence, DFT optimization changes the initial structure only slightly. The optimized structure for Au12(SCH3)9+ is shown in Figure 3. Within 0.125-Å tolerance, the optimized octahedron core retains Oh symmetry, but within 0.06-Å tolerance, the symmetry lowers to D3d, and within 0.004-Å tolerance, it lowers to D3. Six of the original
The Smallest Thiolated Gold Superatom Complexes
J. Phys. Chem. C, Vol. 113, No. 40, 2009 17293
Figure 3. DFT-optimized structure of Au12(SCH3)9+, viewed along the C3 axis: (a) ball-and-stick model; (b) space-filling model; (c) Au12S9 framework with Au-Au distances labeled (in Å). Au, green; S, blue; C, red; H, black.
Figure 4. The Au12S9 framework (a) of Au12(SR)9+ is related to the trefoil knot (c) by viewing the connections in the framework as in part b. Au, green; S, blue; C and H, not shown.
Figure 5. Nondegenerate HOMO and doubly degenerate LUMOs of Au12(SCH3)9+. The cluster is viewed at the same angle as in Figure 3. Isovalues are at 0.025 a.u.
facets are now “wrapped” by ligands, while the 3-fold axis contains the centers of the two unwrapped faces, which provide ample space for the terminal methyl groups, as shown in the space-filling representation (Figure 3b). Each R- (methyl-) group can adopt two positions relative to the S-Au-S bond, and therefore, many close-energy isomers exist from permutation of methyl positions. For the isomer shown in Figure 3, the whole cluster has C3 symmetry with a tolerance of 0.07 Å and is therefore chiral. Considering the coordination of the ligands to the core, one perceives three planes bisecting the octahedron, configured as in the common three-blade propeller or fan (see Figure 3c). Topologically, the Au12S9 framework can be related to the familiar trefoil knot, which is also chiral (Figure 4). Figure 3c shows Au-Au distances in Au12(SCH3)9+, and one can see that all the formally Au(I) ions of the ligands interact strongly with the Au6 core, manifested by the short distances (ranging from 2.85 to 3.05 Å). This aurophilic interaction due to the strong relativistic effect in gold is well documented for gold compounds.31-34 These attractions contribute to the overall stability of the structure, even as larger R-groups provide ample steric protection.
Figure 6. LUMO + 1 of Au12(SCH3)9+. The left figure is viewed at the same angle as in Figures 3 and 5. The right figure shows the back side.
The electronic structure of the Au12(SCH3)9+ cluster features a 1.70-eV gap between the unique HOMO and the two degenerate LUMOs, as well as a low-lying LUMO+1, all depicted in Figures 5 and 6. These diffuse frontier orbitals can be viewed within the electron-shell model as the 1S level and three 1Px,y,z levels. The small splitting among the LUMOs, consistent with a modest symmetry lowering, is attributed to the squeezing of
17294
J. Phys. Chem. C, Vol. 113, No. 40, 2009
Figure 7. Computed optical absorption spectrum for Au12(SCH3)9+. Vertical lines give computed energies for transitions, and their heights represent oscillator strengths.
the octahedron core along the C3 axis. (The face-to-face distance along the C3- or z-axis, namely, between the two unwrapped faces, is 2.24 Å, in contrast with 2.34 Å for the other three face-to-face pairs.) As a result, the Pz orbital along the C3 axis is pushed higher in energy (by 0.5 eV) relative to the case of the (Px,Py) orbitals. Both HOMO and LUMO + 1 clearly reflect the C3 symmetry of the cluster, while the sum of the electron density for the two degenerate LUMO orbitals also has a C3 symmetry (see Supporting Information Figure S3 online). From both the geometric and electronic views, we consider this Au12(SCH3)9+ cluster an excellent candidate for the smallest Au:SR superatom featuring a magic number of 2 for the shellclosing electron count. We now examine the redox property of this Au12(SCH3)9+ cluster. Here we computed the adiabatic (structurally unrelaxed) energy for this cluster to lose or gain electrons. We found that it needs 8.92 eV to become a dication, indicating that the Au12(SCH3)9+ cluster is difficult to oxidize. On the other hand, the cluster gains 4.39 eV energy by acquiring an electron, indicating that Au12(SCH3)9+ can be easily reduced and may be a good oxidizing agent. Further acquiring an electron gains energy only slightly by 1.46 eV, to generate Au12(SCH3)9-. The conversion between the positive and the neutral states of the Au12(SCH3)9 superatom may also display reversible switching of magnetism, as the Au25(SR)18 superatom has shown.35 We next examine the optical properties of the Au12(SCH3)9+ cluster. The computed absorption spectrum is shown in Figure 7. Compared with Au25(SR)18-,12,13,36,37 Au38(SR)24,14,37-39 and larger cluster homologues,20,40-42 Au12(SCH3)9+ shows more molecular-like adsorption features of well separated absorption peaks. The first peak, at 1.8 eV, corresponds to the HOMOto-LUMO transition. The smaller peak at ∼2.25 eV is accounted for dominantly by the transition from HOMO to LUMO + 1. The other major peaks (for example, at 2.6, 3.0, and 3.6 eV) all have multiple contributions (largest contribution less than 30%). We expect that Au12(SR)9+ will also show interesting luminescent properties, as many Au:SR clusters do.42-44 To relate to the previous experiments, we note that gelfractionated Au:SG (GSH ) glutathione) clusters include a small component which has been assigned the stoichiometric formulas (AuSG)10, (AuSG)11, and (AuSG)12,42 which are conventionally understood as nonmetallic oligomeric ring structures. But to our knowledge, a formula of Au12(SR)9 has not been proposed based on mass spectroscopic detection of intact cluster compounds. Our work here calls for a closer examination at this size region. Given the predicted stability for Au12(SR)9+, one question arises as to how one would go about realizing it. One idea is to use thiol-rich biomolecules (such as a peptide or protein with
Jiang et al. many cysteine residues) when reducing AuCl4-, such that in folding one encapsulates the small clusters formed. A recent example of using proteins to make gold nanoclusters45 indicates that this idea is indeed a potential direction to pursue the smallest thiolated gold clusters. Another idea is to exploit the fact that many small-cored phosphine-ligated gold clusters have been made.22,23 One can use these clusters as precursors to make small-cored thiolated gold clusters by ligand exchange. Encouragingly, Tsukuda and co-workers46 successfully made Au25(SG)18 by reacting Au11(PPh3)8Cl3 with GSH (glutathione). This approach could be combined with the use of Cys-rich peptides. In particular, it seems plausible that clusters such as Au12(SR)9+ may be made by using smaller phosphine-ligated gold clusters such as [Au4(PR3)4]2+ (ref 47) and [Au6(PPh3)6]2+.48 The former features a tetrahedron Au4 core47 and also has a shell-closing electroncount of 2 according to the electron-shell model.8 Our models for Au8(SR)6 (Figure 2a) and Au6(SR)4 (Supporting Information Figure S2 online) are close analogues of [Au4(PR3)4]2+. But unlike [Au4(PR3)4]2+, which is charged and has a compact ligand layer, our clusters are neutral and quite naked. Another example of phosphine-liganded Au clusters is [C@Au6(PPh3)6]2+, which has a carbon-centered octahedron core.49 This cluster has an electron-count of 8 according to the electron-shell model and, therefore, is stable. Here the center atom C is necessary for maintaining the octahedron shape. Without it, the core prefers other geometries such as an edge-sharing bitetrahedral structure.23,48 Our model for Au12(SR)9+, however, has an electron-count of 2 and is noncentered. Because of Au12(SR)9+’s stable electron count, it is in fact the first example of a hexanuclear gold cluster featuring a noncentered octahedron core. 4. Summary and Conclusions We have proposed three thiolated gold clusters that feature a shell-closing electron count of 2, thereby corresponding to the smallest member in the magic-number series. Two of the three clusters feature a tetrahedron core whose two opposite edges are capped by two short and long motifs, respectively. The half naked core of the two clusters indicates that they would be quite labile without really bulky thiolate groups. The third cluster has a formula of Au12(SR)9+, which comprises an octahedron core and three long motifs. We consider this cluster an excellent candidate for the smallest thiolated gold superatom. The cluster frame (Au12S9) has a D3 symmetry and shows ideal aurophilic interactions between ligands and the core. The octahedron core is slightly squeezed (by ∼0.1 Å) along the C3 axis. As a result, the three lowest unoccupied orbitals split into doubly degenerate LUMOs and nondegenerate LUMO + 1, which affects the optical absorption. The computed spectrum shows well-separated adsorption peaks, with the first peak located at 1.8 eV, corresponding to the HOMO-to-LUMO transition. The success stories of Au102(SR)44 and Au25(SR)18- suggest that additional homologous clusters may be subjected to total structure determination, which normally proceeds most efficiently for smaller compounds. Here, we have suggested Au12(SR)9+ as an especially suitable target for such efforts. Added Note. After this work had been completed, we learned of the experimental isolation of stable Au12(SR)9 complexes (HSR ) N-acetylcysteine) as the smallest metal-rich thiolated gold complex.50 Further work will be required to determine whether this new cluster compound has the predicted structure and superatom character.
The Smallest Thiolated Gold Superatom Complexes Acknowledgment. This work was supported by the Office of Basic Energy Sciences, U.S. Department of Energy, under Contract No. DE-AC05-00OR22725 with UT-Battelle, LLC. Supporting Information Available: Potential energy surface of the long-motif anion in the gas phase [RS(AuSR)2]- along the Au-S-Au angle, schematic construction of the tetrahedroncored cluster Au6(SR)4, and the sum of the electron densities of the two degenerate LUMO orbitals of Au12(SCH3)9+. This material is available free of charge via the Internet at http:// pubs.acs.org. References and Notes (1) Shaw, C. F. Chem. ReV. 1999, 99, 2589. (2) Daniel, M. C.; Astruc, D. Chem. ReV. 2004, 104, 293. (3) Brust, M.; Walker, M.; Bethell, D.; Schiffrin, D. J.; Whyman, R. J. Chem. Soc., Chem. Commun. 1994, 801. (4) Whetten, R. L.; Khoury, J. T.; Alvarez, M. M.; Murthy, S.; Vezmar, I.; Wang, Z. L.; Stephens, P. W.; Cleveland, C. L.; Luedtke, W. D.; Landman, U. AdV. Mater. 1996, 8, 428. (5) Jadzinsky, P. D.; Calero, G.; Ackerson, C. J.; Bushnell, D. A.; Kornberg, R. D. Science 2007, 318, 430. (6) Whetten, R. L.; Price, R. C. Science 2007, 318, 407. (7) Ball, P. Nat. Mater. 2007, 6, 927. (8) Walter, M.; Akola, J.; Lopez-Acevedo, O.; Jadzinsky, P. D.; Calero, G.; Ackerson, C. J.; Whetten, R. L.; Go¨nbeck, H.; Ha¨kkinen, H. Proc. Natl. Acad. Sci. U. S. A. 2008, 105, 9157. (9) Khanna, S. N.; Jena, P. Phys. ReV. Lett. 1992, 69, 1664. (10) Jiang, D. E.; Tiago, M. L.; Luo, W. D.; Dai, S. J. Am. Chem. Soc. 2008, 130, 2777. (11) Akola, J.; Walter, M.; Whetten, R. L.; Ha¨kkinen, H.; Gro¨nbeck, H. J. Am. Chem. Soc. 2008, 130, 3756. (12) Heaven, M. W.; Dass, A.; White, P. S.; Holt, K. M.; Murray, R. W. J. Am. Chem. Soc. 2008, 130, 3754. (13) Zhu, M.; Aikens, C. M.; Hollander, F. J.; Schatz, G. C.; Jin, R. J. Am. Chem. Soc. 2008, 130, 5883. (14) Chaki, N. K.; Negishi, Y.; Tsunoyama, H.; Shichibu, Y.; Tsukuda, T. J. Am. Chem. Soc. 2008, 130, 8608. (15) Pei, Y.; Gao, Y.; Zeng, X. C. J. Am. Chem. Soc. 2008, 130, 7830. (16) Garzon, I. L.; Rovira, C.; Michaelian, K.; Beltran, M. R.; Ordejon, P.; Junquera, J.; Sanchez-Portal, D.; Artacho, E.; Soler, J. M. Phys. ReV. Lett. 2000, 85, 5250. (17) Ha¨kkinen, H.; Walter, M.; Gro¨nbeck, H. J. Phys. Chem. B 2006, 110, 9927. (18) Jiang, D. E.; Luo, W.; Tiago, M. L.; Dai, S. J. Phys. Chem. C 2008, 112, 13905. (19) de Heer, W. A. ReV. Mod. Phys. 1993, 65, 611. (20) Price, R. C.; Whetten, R. L. J. Am. Chem. Soc. 2005, 127, 13750. (21) Lopez-Acevedo, O.; Akola, J.; Whetten, R. L.; Gro¨nbeck, H.; Ha¨kkinen, H. J. Phys. Chem. C 2009, 113, 5035.
J. Phys. Chem. C, Vol. 113, No. 40, 2009 17295 (22) Jones, P. G. Gold Bull. 1983, 16, 114. (23) Mingos, D. M. P. Gold Bull. 1984, 17, 5. (24) Ahlrichs, R.; Bar, M.; Haser, M.; Horn, H.; Kolmel, C. Chem. Phys. Lett. 1989, 162, 165. (25) Tao, J. M.; Perdew, J. P.; Staroverov, V. N.; Scuseria, G. E. Phys. ReV. Lett. 2003, 91, 146401. (26) Johansson, M. P.; Lechtken, A.; Schooss, D.; Kappes, M. M.; Furche, F. Phys. ReV. A 2008, 77, 053202. (27) Weigend, F.; Haser, M.; Patzelt, H.; Ahlrichs, R. Chem. Phys. Lett. 1998, 294, 143. (28) Andrae, D.; Haussermann, U.; Dolg, M.; Stoll, H.; Preuss, H. Theor. Chim. Acta 1990, 77, 123. (29) Furche, F.; Rappoport, D. Density functional methods for excited states: equilibrium structure and electronic spectra. In Computational Photochemistry; Olivucci, M., Ed.; Elsevier: Amsterdam, 2005; p 93. (30) Perdew, J. P.; Burke, K.; Ernzerhof, M. Phys. ReV. Lett. 1996, 77, 3865. (31) Pyykko¨, P. Angew. Chem., Int. Ed. 2004, 43, 4412. (32) Pyykko¨, P. Inorg. Chim. Acta 2005, 358, 4113. (33) Pyykko¨, P. Chem. Soc. ReV. 2008, 37, 1967. (34) Schmidbaur, H.; Schier, A. Chem. Soc. ReV. 2008, 37, 1931. (35) Zhu, M.; Aikens, C. M.; Hendrich, M. P.; Gupta, R.; Qian, H.; Schatz, G. C.; Jin, R. J. Am. Chem. Soc. 2009, 131, 2490. (36) Negishi, Y.; Chaki, N. K.; Shichibu, Y.; Whetten, R. L.; Tsukuda, T. J. Am. Chem. Soc. 2007, 129, 11322. (37) Schaaff, T. G.; Whetten, R. L. J. Phys. Chem. B 2000, 104, 2630. (38) Schaaff, T. G.; Shafigullin, M. N.; Khoury, J. T.; Vezmar, I.; Whetten, R. L.; Cullen, W. G.; First, P. N.; GutierrezWing, C.; Ascensio, J.; JoseYacaman, M. J. J. Phys. Chem. B 1997, 101, 7885. (39) Tsunoyama, H.; Nickut, P.; Negishi, Y.; Al-Shamery, K.; Matsumoto, Y.; Tsukuda, T. J. Phys. Chem. C 2007, 111, 4153. (40) Alvarez, M. M.; Khoury, J. T.; Schaaff, T. G.; Shafigullin, M. N.; Vezmar, I.; Whetten, R. L. J. Phys. Chem. B 1997, 101, 3706. (41) Schaaff, T. G.; Knight, G.; Shafigullin, M. N.; Borkman, R. F.; Whetten, R. L. J. Phys. Chem. B 1998, 102, 10643. (42) Negishi, Y.; Nobusada, K.; Tsukuda, T. J. Am. Chem. Soc. 2005, 127, 5261. (43) Bigioni, T. P.; Whetten, R. L.; Dag, O. J. Phys. Chem. B 2000, 104, 6983. (44) Wang, G. L.; Guo, R.; Kalyuzhny, G.; Choi, J. P.; Murray, R. W. J. Phys. Chem. B 2006, 110, 20282. (45) Xie, J.; Zheng, Y.; Ying, J. Y. J. Am. Chem. Soc. 2009, 131, 888. (46) Shichibu, Y.; Negishi, Y.; Tsukuda, T.; Teranishi, T. J. Am. Chem. Soc. 2005, 127, 13464. (47) Zeller, E.; Beruda, H.; Schmidbaur, H. Inorg. Chem. 1993, 32, 3203. (48) Briant, C. E.; Hall, K. P.; Mingos, D. M. P.; Wheeler, A. C. J. Chem. Soc., Dalton Trans. 1986, 687. (49) Scherbaum, F.; Grohmann, A.; Huber, B.; Kruger, C.; Schmidbaur, H. Angew. Chem., Int. Ed. 1988, 27, 1544. (50) Zhang, Y.; Shuang, S.; Dong, C.; Lo, C. K.; Paau, M. C.; Choi, M. M. F. Anal. Chem. 2009, 81, 1676.
JP9035937
