Theoretical Characterization of Cyclic Thiolated Copper, Silver, and

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J. Phys. Chem. C 2010, 114, 13571–13576

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Theoretical Characterization of Cyclic Thiolated Copper, Silver, and Gold Clusters Katarzyna A. Kacprzak,† Olga Lopez-Acevedo,† Hannu Ha¨kkinen,†,‡ and Henrik Gro¨nbeck*,§ Department of Physics and Department of Chemistry, Nanoscience Center, P.O. Box 35, UniVersity of JyVa¨skyla¨, FIN-40014 JyVa¨skyla¨, Finland, and Competence Centre for Catalysis and Department of Applied Physics, Chalmers UniVersity of Technology, SE-41296 Go¨teborg, Sweden ReceiVed: May 18, 2010; ReVised Manuscript ReceiVed: July 2, 2010

Density functional theory calculations are used to study structural, electronic, and vibrational properties of cyclic (MeSM)x clusters where MeS is methylthiolate and M is copper, silver, or gold. The clusters show a flexible bond motif where monocyclic rings, catenanes, and helix structures compete in energy. In the investigated series, the copper-sulfur bond is found to be the strongest metal-sulfur bond, followed by gold-sulfur and silver-sulfur. Analysis of the bond character reveals that Cu-S is the most polar bond, whereas Au-S is mainly of covalent type. Vibrational analysis shows characteristic metal-sulfur stretch vibrations for each noble metal. Introduction The high affinity of organothiolate ligands (RS) to metal atoms is commonly utilized to stabilize and functionalize metal surfaces.1 On extended noble metal (Cu, Ag, Au) surfaces, thiolates self-assemble into monolayer films with high order and stability.2 The process is facile and self-assembled monolayers (SAMs) offer a convenient and fast route to tailored surface properties.3-5 The nanoscale analogue to SAMs is thiolateprotected nanoparticles or clusters.6 Such systems can be fabricated with a narrow and controlled size distribution by reduction of metal salt in the presence of thiols7 and may find applications within, for example, molecular electronics8 and biosensing.9 On the molecular scale, gold-thiolate complexes are used to extract metal from ores10 and as therapeutic agents.11 A prerequisite for the stability of SAMs on extended surfaces or protected nanoparticles is the strong RS-metal bond. Due to the buried interface, it is only recently that a consistent picture of the RS adsorption configuration has emerged. On extended surfaces, it is now established that RS drives pronounced reconstruction of all noble metal surfaces.2 Scanning tunneling microscopy (STM) measurements of a monolayer of methyl thiolates (MeS) on Cu(111) shows that the adsorbates occupy 4-fold hollow positions on a pseudo-(100) reconstructed surface12 and low-energy electron diffraction (LEED) measurements of RS adsorption on Ag(111) have revealed a (7 × 7)R 19° surface cell.13 Several models for the structure of RS on Au(111) have been proposed during the past few years:2 (i) adsorption atop Au ad-atoms forming RSAu units,14 (ii) adsorption as RSAuSR complexes,15 and (iii) a combined model that comprise (RSAu)x polymers and RS adsorbed at surface point defects.16 On the basis of density functional theory calculations a surface model with RSAuSR complexes has been predicted17 which is energetically favored over previous suggestions.17,18 Turning to the nanoscale, total structural determinations of two gold clusters have recently been reported, namely Au102(pMBA)44 (where p-MBA is p-mercaptobenzoic acid, SC6H4COOH)19 and Au25(SCH2CH2Ph)18 in anionic20,21 and neutral22 charge states. In both cases, compact metal cores are * To whom correspondence should be addressed. E-mail: [email protected]. † Department of Physics, University of Jyva¨skyla¨. ‡ Department of Chemistry, University of Jyva¨skyla¨. § Chalmers University of Technology.

capped by gold-thiolate complexes: For Au102(p-MBA)44, a core with 79 Au atoms is covered by 19 RSAuSR and 2 RS(AuSR)2 units,23 and for Au25(SCH2 CH2Ph)18, a 13 atom core is protected by 6 RS(AuSR)2 units. The possibility of a metal core capped by homoleptic gold-thiolate complexes was first suggested for Au38(RS)2424 and the structural motif observed for Au25(SCH2 CH2Ph)18- was predicted with methyl thiolate (MeS) ligands.25 The recent advances on relevant structural motifs for RS adsorption on extended Au surfaces and nanoparticles shows similarities and, most interestingly, a connection to the structures of homoleptic (RSM)x complexes.26 Moreover, thiolate-metal complexes are known to constitute precursors in the formation of protected nanoparticles and (RSAu)4 has been detected as a dominant fragment in mass-spectrometry of protected gold clusters.27 Metal-thiolate complexes have been the subject of several experimental investigations and it is known that metal(I)-thiolate complexes adopt zigzag structures that develop into rings or strands.11,26 Due to poor solubility, however, the examples of complete structural determinations using X-ray crystallography are relatively few. Ringlike structures have been reported for complexes with four, five, and six gold atoms. Au4[SC(SiMe3)]4 is one such example.28 This compound has a square structure with Au in linear coordination and S atoms in the corners. A similar structure has been determined for Cu4(SSiPh3)429 as well as the larger Ag12(SC6H11)12.30 On the theoretical side, early studies concentrated on small linear compounds.31-35 More recently, cyclic structures have been investigated using approaches based on the density functional theory (DFT).36-38 The connection between homoleptic thiolate complexes and the formation, structure, and fragmentation of protected nanoparticles and clusters makes is important to theoretically characterize (RSM)x systems in better detail. Here we present electronic structure calculations on the basis of DFT for (MeSM)x, with x ) 1-12, where M is Cu, Ag, and Au. As the main focus is the nature of the thiolate-metal interaction, the smallest possible carbon chain is selected, namely, methyl thiolate (R ) MeS). (Previous calculations for Au complexes have shown that the Au-S interaction is only weakly dependent on the ligand structure beyond the first carbon atom.36) The clusters are characterized with respect to structural, electronic, and vibrational properties.

10.1021/jp1045436  2010 American Chemical Society Published on Web 07/23/2010

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Computational Approach DFT39,40 is used in a real space grid implementation41,42 of the projector augmented wave (PAW) method.43 The exchangecorrelation functional is approximated with the spin-polarized Perdew-Burke-Ernzerhof (PBE) formula.44 Previous comparisons show that this functional provides a fair compromise in the description of both the metal phase and the organic ligands.45 The frozen core and projectors are generated with scalar relativistic corrections for Cu, Ag, and Au. Electrons treated in the valence for the different elements are H(1s), C(2s2p), S(3s3p), Cu(3d4s4p), Ag(4d5s5p), and Au(5d6s6p). The grid spacing is set to 0.18 Å in all calculations, which ensure convergence in energy differences. The finite systems are calculated in a monoclinic embedding geometry without periodic images. The structures are relaxed with the quasi-Newton method and regarded as optimized when the largest element of the gradient is smaller than 0.05 eV/Å. All systems are treated in the lowest spin states, namely, singlets for (RSM)x and doublets for the radicals (MeS) and the Cu, Ag, and Au atoms. Vibrational analysis is performed by evaluation of energy gradients for each atom. The harmonic wavenumbers are obtained by means of matrix diagonalization. To investigate the performance of the applied methodology to the organic ligands, the binding energies and bond lengths for MeSH and MeSSMe were calculated. The results are collected and compared to experimental data in Table 1. The agreement between the theoretical results and available experimental data is satisfying. TABLE 1: Comparison of the Theoritical and Experimental Data for Selected Bond Lengths (Å) and Binding Energies (eV) for MeSSMe and MeSH46 MeS-H theory exp.

MeS-SMe

dS-H

Eb

dS-S

Eb

1.35 1.34

3.84 3.79

2.05 2.03

2.90 2.83

Results and Discussion Structure and Stability of Cyclic (MeSM)x Clusters. Here, we explore homoleptic (MeSM)x structures based on cyclic motifs. For each cluster size, a large number of structural configurations were relaxed to local minima. The lowest energy structures for (MeSM)x (x ) 2-12) are shown in Figure 1. As the gold systems have been reported previously,36 low-energy isomers are reported only for Cu and Ag. In cases where the two metals adopt the same configuration, the Cu version is depicted. In all cases, the methyl thiolates are bonded in bridge configurations, building up closed rings or crowns. The smallest clusters (n e 4) are planar, whereas a zigzag motif is developed for the larger sizes. The total energy is not sensitive to the orientation of the methyl groups and for all sizes several isomers exist within 0.05 eV. The dimer is the only system where the methyl groups are oriented in a cis-configuration. This structure is favored over the trans-isomer by 0.02, 0.03, and 0.04 eV for Cu, Ag, and Au, respectively. For the trimer, the cis-isomer is higher in energy than that reported in Figure 1 by 0.04, 0.02, and 0.02 eV for Cu, Ag, and Au, respectively. (MeSM)4 in the transconformation is preferred by 0.17 (Cu), 0.08 (Ag), and 0.08 (Au) eV over the corresponding cis-geometry. The pentamer is the first size with a bent structure. This structural motif develops and becomes clearer for the larger sizes. For all odd systems,

Figure 1. Optimized ring structures for (MeSM)x. Atomic color codes: brown, Cu; gray, Ag; yellow, S; black, C; and white, H.

TABLE 2: Structural Parameters for the Low Energy Isomers of Cyclic (MeSM)x Ringsa (MeSCu)2 (MeSCu)3 (MeSCu)4 (MeSCu)5 (MeSCu)6 (MeSCu)7 (MeSCu)8 (MeSCu)9 (MeSCu)10 (MeSCu)11 (MeSCu)12 (MeSAg)2 (MeSAg)3 (MeSAg)4 (MeSAg)5 (MeSAg)6 (MeSAg)7 (MeSAg)8 (MeSAg)9 (MeSAg)10 (MeSAg)11 (MeSAg)12 (MeSAu)2 (MeSAu)3 (MeSAu)4 (MeSAu)5 (MeSAu)6 (MeSAu)7 (MeSAu)8 (MeSAu)9 (MeSAu)10 (MeSAu)11 (MeSAu)12

dM-S

dS-C

dM-M

RM-S-M

RS-M-S

2.25 2.19 2.17 2.17 2.17 2.17 2.17 2.17 2.17 2.16 2.17 2.52 2.42 2.39 2.39 2.39 2.39 2.39 2.39 2.39 2.39 2.39 2.48 2.37 2.34 2.33 2.33 2.33 2.33 2.33 2.33 2.33 2.33

1.84 1.84 1.84 1.84 1.84 1.84 1.84 1.84 1.84 1.84 1.84 1.84 1.84 1.84 1.84 1.84 1.84 1.84 1.84 1.84 1.84 1.84 1.84 1.84 1.84 1.84 1.84 1.84 1.84 1.84 1.84 1.84 1.84

2.34 2.58 2.77 2.94 2.78 2.88 2.71 2.78 2.76 2.92 2.78 2.73 2.94 3.26 3.40 3.21 3.33 3.23 3.38 3.36 3.41 3.40 2.73 3.00 3.32 3.44 3.36 3.38 3.33 3.44 3.36 3.44 3.40

62 72 79 86 80 84 77 80 79 85 80 65 75 86 91 84 89 85 90 90 91 91 67 79 90 95 92 93 91 95 92 95 94

117 166 173 170 171 172 174 174 172 171 175 114 165 178 175 175 175 174 177 176 176 177 113 161 177 178 178 177 178 178 177 176 177

a

dM-S, dM-M, dS-C are the mean M-S, M-M, and S-C bond distances (Å), respectively. RM-S-M and RS-M-S are the M-S-M and S-M-S angles, respectively.

the structure has to incorporate a “structural defect”, whereas the even number complexes have all metal atoms in a plane. Selected structural parameters for the lowest energy isomers of the monocyclic rings are reported in Table 2. The bond lengths show a rapid convergence with cluster size. The metal-sulfur bond lengths converge to 2.17, 2.39, and 2.33 Å, for Cu, Ag, and Au, respectively. The differences can, in part, be related to differences in atomic radii, which are 1.35 (Cu), 1.60 (Ag), and 1.35 (Au) Å. The structural defects for the odd

Cyclic Thiolated Cu, Ag, and Au Clusters

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TABLE 3: Fragmentation Energy (eV) to MeSM Units Per Unit (MeSM)2 (MeSM)3 (MeSM)4 (MeSM)5 (MeSM)6 (MeSM)7 (MeSM)8 (MeSM)9 (MeSM)10 (MeSM)11 (MeSM)12

Cu

Ag

Au

1.18 2.05 2.15 2.12 2.13 2.13 2.16 2.15 2.15 2.13 2.15

0.87 1.62 1.73 1.72 1.73 1.73 1.73 1.72 1.73 1.72 1.72

0.56 1.76 1.99 2.00 2.01 2.00 2.01 2.00 2.01 2.00 2.00

systems are visible in the metal-metal distances, which have an odd/even pattern. The metal-sulfur-metal angle is close to 90° for the larger Ag and Au rings. For Cu, on the other hand, this angle is smaller than 90° and does not show a clear convergence. For all three elements the metal is close to linearly coordinated with a sulfur-metal-sulfur angle of ∼175°. The calculated structures compare well with available experimental data on complexes with other ligands. The structure of Cu4(SSiPh3)4 has been solved29 using X-ray crystallography and it adopts a square structure with Cu in close to linear coordination and S atoms in the corners. The mean Cu-S distance and S-Cu-S angle were reported to be 2.16 Å and 170°, respectively. The structure of Ag12(SC6H11)12 been measured30 and the average intracycle Ag-S distance was determined to be 2.45 Å. The S-Ag-S angles were reported to be ∼170°. In ref 28, among various compounds, (AuSC(SiMe3))4 was prepared, crystallized, and characterized with X-ray crystallography. The mean Au-S distance, Au-S-Au angle, and S-Au-S angle were reported to be 2.30 Å, 91.9°, and 177.5°, respectively. A cyclic hexamer (AuSC15H23)6 was prepared and the crystal structurally characterized in ref 47. For this compound, the average Au-S and S-C distances were reported to be 2.29 and 1.80 Å, respectively, the Au-S-Au angle to be 102°, and the S-Au-S angle to be 175°. The calculated values for Cu and Au are in fair agreement with the experiments, and the slight expansion of the S-Au distance as compared to the experiments can be attributed to the approximation used for the exchangecorrelation as elaborated on in ref 45. For Ag, the difference is somewhat too large and it is possible that intercyclic interactions are of importance in the experimental situation.30 The stability of the (MeSM)x low energy isomers is reported in Table 3. The fragmentation energy (Ef) per MeSM unit is calculated according to Ef ) (xE[MeSM] - E[(MeSM)x]/x. Similar to the structural properties, the energetic stability converges rapidly. Already for the tetramer, the fragmentation can be regarded as converged. The stabilities show small but clear odd-even alternations. The even clusters are somewhat more stable than the odd. This is related to the possibility that even clusters adopt a more regular zigzag pattern with linearly coordinated metal atoms and S-M-S angles close to 90°. In fact, also the standard deviation of the S-M-S and M-S-M angles for the low-energy isomers obey an odd-even pattern (not shown) with a smaller deviation for the even clusters as compared to the adjacent odd clusters. The cohesion of the cyclic clusters should be related to other relevant bond strengths in the system: the MeS-M bond strength and the M-M cohesion. The strengths of the MeS-M bond for the MeSM monomer are calculated to be 2.80, 2.16, and 2.55 eV for M ) Cu, Ag, and Au, respectively. In fact, those values are close to the metal-sulfur bonds in the rings; open

Figure 2. Optimized catenane structure for (MeSCu)10, (MeSAg)10, (MeSCu)11, (MeSAg)11, (MeSCu)12, and (MeSAg)12. See Figure 1 for atomic color codes.

(MeSM)4 structures are 2.61 (Cu), 2.18 (Ag), and 2.49 (Au) eV higher in energy than the closed rings reported in Figure 1. The RS-M bond strengths can be compared to the bond strengths in the metal dimers, which are computed to be 2.24, 1.74, and 2.27 eV for Cu2, Ag2, and Au2, respectively. Thus, the metal-sulfur bond is stronger than the dimer metal-metal bond for all systems. This is consistent with the bond motif recently revealed for gold nanoparticles19,20 and the pronounced reconstruction of extended metal surfaces observed for all coinage metals.2 Structure and Stability of Alternative (MeSM)x Structures. In addition to structures based on a monocyclic motif, catenane and helix structures are investigated. Catenane structures (two cyclic molecules that interpenetrate each other) have been experimentally observed for Au with other ligands.48,49 In ref 48, (AuSC6H4-p-CMe3)10 and (AuSC6H4-o-CMe3)12 were prepared and structurally characterized. It was found that the two pentamers or hexamers are interlocked with one Au atom of one ring situated close to the center of the other ring. A catenane structure with chiral properties for the undecamer with RS ) [2,3,4,6-tetra-O-acetyl-β-1-D-thioglucopyranosato-S] has also been characterized with penetrating penta- and hexamers.49 Helix structures are known to be relevant for gold thiomalate,50 which is used as an antiarthritic drug.11 Figure 2 shows examples of catenane structure for copper and silver. This kind of structures were explored for (MeSM)x with x ) 8-12. However, structures with (MeSM)4 rings turned out to correspond to high-energy isomers, and interlocked rings are relevant only for those sized larger than the nonamer. For (MeSM)10, the catenane structure is preferred with respect to one monocyclic ring by 0.33 (Cu), 0.46 (Ag), and 0.17 (Au) eV, respectively. The preference is enhanced with ring size and for (MeSM)12 the corresponding values are 0.63 (Cu), 0.73 (Ag), and 0.27 (Au) eV. The decamer consists of two pentamer rings that are close to planar. Thus, the ring-ring interaction counteracts some of the bent structure present for the single pentamer geometry. This can be attributed to steric interactions. The structures has two M-S distances. The M-S distance that involve the metal atom at the center of the ring is 2.23 (2.44) Å for Cu (Ag), whereas the other Cu-S (Ag-S) distance is 2.18 (2.40) Å. For the corresponding Au complex, these distances are calculated to be 2.40 and 2.33 Å, respectively. The result for Au is in good agreement with the result in ref 48, where the two different distances were measured to be 2.34 and 2.28 Å. For the undecamer, two different ring sizes are interlocked. Just as for the decamer, the steric interaction between the rings counteracts the zigzag structure and the rings turn more planar

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Figure 3. Optimized helix structures for (MeSCu)8, (MeSCu)9, and (MeSAg)10. See Figure 1 for atomic color codes.

than in the monocyclic case. The distances between the metal atoms in the periphery of pentamer and the center atom of the pentamer are (with small variations ∼0.1 Å) 2.65, 3.15, and 3.13 Å, for Cu, Ag, and Au, respectively. For the hexamer, however, the distances between metal atom, inside ring, and metal atoms on the periphery vary substantially: 1.40, 1.06, and 0.55 Å for Cu, Ag, and Au, respectively. This could simply be related to the larger ring size and reduced steric constraints. In ref 48, (AuSC6H4-o-CMe3)12 was measured to consist of slightly buckled hexamers and Au-S separations in the range 2.29-2.32 Å, with no appreciable difference for the Au-S distances for Au-atoms making the ring-ring contact. The calculations yield similar results for the Au dodecamer, all Au-S distances are close to 2.34 Å. A similar result is obtained for Ag (2.40 Å), whereas a difference in Cu-S is calculated; the short distance is 2.19 Å and the long distance is 2.22 Å. The differences between the metals can be attributed to the different sizes of the rings. The longest S-S distance in the monocyclic (MeSM)6 rings are 7.4, 8.1, and 8.0 Å for Cu, Ag, and Au, respectively. Helix structures are experimentally known to be relevant for gold thiolate complexes.50 On the basis of DFT calculations within a linear combination of atomic orbitals (LCAO) approach, it was recently suggested that helical structures for (MeSAu)x are slightly energetically favored for x ) 6-9.38 Here, we have investigated helical structures for sizes larger than the octamer. Some examples are reported in Figure 3. Contrary to the finding of Shao et al.,38 we do not find helical structures to be energetically preferred for gold-thiolate complexes. For (MeSAu)8, for example, the helix is calculated to be 0.1 eV higher in energy than the monocyclic ring. To investigate possible reasons for the difference we repeated the calculations with the same implementation51,52 of the Kohn-Sham equations that was used in ref 38. The result of Shao et al. for the octamer was reproduced only if the real-space radial cutoff value for evaluation of matrix elements was decreased to 5.0 Å. For copper and silver, cyclic rings are found to be energetically preferred over helical structures with the exception of the octamer, for which the helix is somewhat lower in energy, 0.25 (0.21) eV, for (MeSCu)8 ((MeSAg)8). Electronic Properties. The electronic density of states (EDOS) for the low-energy isomers of the monocyclic ring structures are reported in Figure 4. The EDOS has been obtained by a 0.1-eV Gaussian broadening of the one electron Kohn-Sham energy levels. To facilitate comparisons, each spectrum has been normalized by the number of MeSAu units in the cluster. The EDOS reveals that these systems are “molecular” in the sense that all sizes have large HOMO-LUMO (HL) gaps.53 The smallest gaps are calculated for the dimers: 2.06 eV (Cu), 2.13 eV (Ag), and 1.50 (Au) eV. The largest HL gaps for Ag and Au are calculated for the tetramers: 3.82 and 3.97 eV for Ag and Au, respectively. For Cu, the largest HL separation is found for the hexamer (3.17 eV).

Figure 4. Electronic density of states (EDOS) for cyclic (MeSM)x with x ) 4, 7, 9, 11. The total density of state is marked by a gray line. The shaded areas correspond to the projected density of state (PDOS) associated with metal atoms. Blue, green, and red color denote d, p, and s contributions, respectively. The one-electron Kohn-Sham energies have been broadened with a 0.1 eV Gaussian and are reported with respect to the HOMO level. The DOS have been normalized by division of the number of (MeSM) units in each cluster.

The shaded part of the spectra in Figure 4 is the density of states projected onto the metal atoms (PDOS). The metal d-states are located mainly 1.1, 2.1, and 1.75 eV below the HOMO level, for Cu, Ag, and Au, respectively. Although states close to the HOMO level are of s-character, Cu and Au also show some weights on d. In all cases, these states are hybridized with sulfur p-states. The EDOS for catenane structures together with the corresponding monocyclic rings are reported in Figure 5. The HL gaps are significantly reduced for the catenanes. The HOMO and HOMO-1 of the interlocked rings are bonding and antibonding combinations of s-derived Au states. As both bonding and antibonding states are occupied, the net chemical interaction of the ring-ring contact is nonbonding. However, the two (MeSAu)5 rings are bonded by 0.17 eV. This interaction is of physical nature and it should be realized that DFT with exchange-correlation functionals that only incorporate semilocal contributions (as the one used here) do not describe this kind of bonding (aurophilic) in an accurate manner. In order to study the difference in bond character for the three metals, a Bader analysis54,55 was performed on the relaxed structures. The charges on the different systems did not change appreciably within the series of sizes and conformations. The metal atoms were found to be positively charged by 0.34 (Cu), 0.27 (Ag), and 0.09 (Au) electrons, respectively. Thus, the largest charge separation is obtained for copper, whereas the smallest is calculated for gold. This is consistent with the difference in ionization potentials (IP) for the different atoms. The IP for Cu

Cyclic Thiolated Cu, Ag, and Au Clusters

Figure 5. Electronic density of states (EDOS) for cyclic (MeSM)5 and (MeSM)6 and catenane conformations (MeSM)10 and (MeSM)12. The Kohn-Sham orbitals for HOMO and HOMO-1 are shown for (MeSAu)10. The isosurfaces are shown at a value of (0.1 1/Å3/2.

Figure 6. Slices through the charge density differences for MeSCu, MeSAg, and MeSAu. Blue corresponds to charge gain and red to charge depletion. Isosurfaces are shown in the range [-0.1, 0.1] (e/Å3).

and Ag are similar (the experimental values are 7.7 and 7.6 eV, respectively), whereas Au has a considerably higher IP (9.2 eV). It should be noted that although the formal charge on the metal atoms is +1, the calculated values are clearly smaller. The difference in bonding for the three metals was also investigated by evaluation of the charge density difference for the corresponding monomers (MeSM). The results are shown in Figure 6. The density differences were calculated as ∆F ) F(MeSM) - F(M) - F(MeS) in the frozen positions of MeSM. The most pronounced charge rearrangement is found for MeSCu. Charge is depleted from the metal site and polarized toward the thiolate. A similar pattern is obtained for MeSAu. In this case, however, there is a clearer sign of a covalent bond type with both charge depletion and accumulation along the Au-S bond. The charge rearrangement for MeSAg is less pronounced, which is consistent with a longer bond distance and weaker bond strength. Vibrational Analysis of (MeSM)4. Vibrational spectroscopy is often used for characterization of molecules and molecular fragments, and has, for example, been utilized to investigate benzene thiolate covered gold nanoparticles (AuNP).56 In Figure 7, the results for a vibrational analysis of the stable tetramers (trans-configuration) are reported. The highest wavenumbers in

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Figure 7. Vibrational spectra for (MeSCu)4, (MeSAg)4, and (MeSAu)4.

the system (∼3000 cm-1) are owing to C-H stretch vibrations. The vibrational modes at 1450, 1310, and 960 cm-1 are also related to the methyl group and correspond to the scissors, umbrella, and waving modes, respectively. The S-C stretch vibrations appear at 670 cm-1. These modes are not IR active, which instead is the case for the M-S stretch vibrations, which appear at ∼350 cm-1. For Cu, Ag, and Au, the strongest mode is at 380, 310, and 320 cm-1, respectively. The modes at lower wavenumbers are Me-S wiggles (∼150 cm-1) and the softest wavenumbers are modes of the metal-S framework. Nonstochiometric Fragments. Gold-thiolate complexes have recently been shown to be of importance as protective units in ligand-stabilized Au nanoparticles (AuNP).19,21,23,25 The complexes are in these cases of nonstochiometric type, i.e., Aux(RS)x+1, where x is 1 or 2. The Au102(RS)44 particle is composed of a 79 atoms core protected by 19 Au(RS)2 and 2 Au2(RS)3, whereas Au25(RS)18 consists of a 13 atom core covered by 6 Au2(RS)3 units. In addition, a structural suggestion for a 29 kDa system, with a tentative assignment Au144(RS)60, includes an Au114 core covered by 30 Au(RS)2 units.57 Here, the structural and electronic properties of Au(MeS)2 and Au2(MeS)3 are investigated. The relaxed structures in neutral and anionic form are presented in Figure 8. For Au(MeS)2, the MeS-Au bond length is 2.27 Å in the neutral charge state. Au(MeS)2 is a radical with one unpaired electron and a small HOMO-LUMO gap of 0.02 eV owing to the exchange splitting. Negatively charged Au(MeS)2 is a closed shell system with a large HOMO-LUMO separation of 2.73 eV, similar to the stochiometric (MeSAu)x systems (Figure 3). The MeS-Au is enlarged by 0.05 Å upon charging. In fact, the relaxed value of 2.32 Å is similar to the mean value of 2.34 Å calculated for Au144(MeS)60 within the same DFT implementation.57 This is a structural signature of the localization of one conduction electron from the metal core per Aux(RS)x+1 unit in the AuNP. The capacity to localize electrons is furthermore indicted by a large electron affinity (EA). The adiabatic EA is calculated to be 2.6 eV. The highest weights of the HOMO level is found at the S-atoms, which corresponds to the anchoring points of Au(MeS)2 on AuNP or Au(111).18

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Figure 8. Structures for neutral (top) and anionic (bottom) Au(MeS)2 and Au2(MeS)3. The Kohn-Sham orbitals for the HOMO levels are shown for the charged systems. The isosurfaces are shown at a value of (0.1 1/Å3/2.

The properties of Au2(MeS)3 are similar to those of Au(MeS)2. The MeS-Au distances are for the neutral case calculated to be 2.28 and 2.36 Å for the central and terminal bonds, respectively. The corresponding distances for the anionic case are 2.31 and 2.34 Å, respectively. HOMO-LUMO separation for Au2(MeS)3- is calculated to be 3.03 eV and the adiabatic EA is 3.0 eV. The HOMO level is localized on the terminal S. The first state with central sulfur character is found 0.91 eV below the HOMO level. Vibrational analysis of the anionic systems shows clear similarities with the stochiometric systems. In particular, strong IR active bands are found near 300 cm-1 owing to Au-S stretch vibrations. For Au(MeS)2-, the strongest mode is located to be at 318 cm-1, whereas for Au2(MeS)3-, the most pronounced mode is at 335 cm-1. Conclusions We have used electronic structure calculations on the basis of the density functional theory to investigate completely thiolated copper silver and gold clusters in the range from the monomer to the dodecamer. The clusters show a flexible bond motif, where monocyclic rings, catenanes, and helix structures all are close in energy. In the investigated series, the copper-sulfur bond is found to be the strongest metal-sulfur bond, followed by gold-sulfur and silver-sulfur bonds. The high affinity of the metals toward thiolates is one reason for the pronounced reconstruction of noble metal surfaces upon RS adsorption and, in the case of gold, the formation of gold-thiolate complexes. Analysis of bond character reveals that Cu-S is the most polar bond, whereas Au-S is mainly of covalent type. Vibrational analysis shows characteristic metal-sulfur stretch vibrations for each noble metal. Acknowledgment. Support from the Swedish Research Council and the Academy of Finland is gratefully acknowledged. CPU time has been provided by CSC (Espoo), NSC (Jyva¨skyla¨), and C3SE (Go¨teborg). References and Notes (1) Bernard, A.; Renault, J.; Michel, B.; Bosshard, H.; Delamarche, E. AdV. Mater. 2000, 12, 1067. (2) Woodruff, D. P. Phys. Chem. Chem. Phys. 2008, 10, 7211. (3) Ulman, A. Chem. ReV. 1996, 96, 1533. (4) Smith, R. K.; Lewis, P. A.; Weiss, P. S. Prog. Surf. Sci. 2004, 75, 1. (5) Love, J. C.; Estroff, L. A.; Kriebel, J. K.; Nuzzo, R. G.; Whitesides, G. M. Chem. ReV. 2005, 105, 1103. (6) Daniel, M. C.; Astruc, D. Chem. ReV. 2004, 104, 293. (7) Brust, M.; Walker, M.; Bethell, D.; Schiffrin, D. J.; Whyman, R. J. J. Chem. Soc. Chem. Commun. 1994, 801.

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