Isolation and Structural Characterization of a Silver–Platinum

Article pubs.acs.org/JPCA

Isolation and Structural Characterization of a Silver−Platinum Nanocluster, Ag4Pt2(DMSA)4 Scott R. Biltek,† Ayusman Sen,*,† Anthony F. Pedicini,‡ Arthur C. Reber,‡ and Shiv N. Khanna*,‡ †

Department of Chemistry, The Pennsylvania State University, University Park, Pennsylvania 16802, United States Department of Physics, Virginia Commonwealth University, Richmond, Virginia 23284, United States

‡

S Supporting Information *

ABSTRACT: We report the synthesis, isolation, and characterization of the ligandprotected bimetallic cluster, Ag4Pt2(DMSA)4 (DMSA = meso-2,3,-dimercaptosuccinic acid). The procedure is similar to the one employed for the synthesis of Ag4Ni2(DMSA)4. Theoretical studies suggest that the Pt and Ni atoms have square planar configurations. Because the crystal field splitting of 5d orbitals is typically larger than that for 3d orbitals, the Pt-based cluster has an optical spectrum that is significantly blue-shifted as compared to the Ni-based cluster.

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INTRODUCTION Molecular scale metal nanoclusters of precise composition are of great importance for their unique electronic and catalytic properties.1−4 Of particular interest are the monolayer protected clusters (MPCs) where a small metal core is passivated by thiolate ligands. The strength of the metal thiolate bond and the ready availability of thiols make thiolates an ideal class of ligands for forming stable clusters.5 The majority of the reported work on MPCs has focused on monometallic gold and silver clusters with a large number of them isolated and identified by mass spectrometry6−9 and complete structural determination achieved for a small subset.10−15 More recently, the focus has shifted to bimetallic clusters.16,17 The incorporation of a second metal into a MPC offers many advantages including enhanced optical properties,18 improved stability,19 altered reactivity,20,21 and the tuning of existing properties through compositional variations.22 Theoretical studies also suggest interesting and valuable properties in bimetallic clusters that have yet to be synthesized.22−24 Research on silver based bimetallic MPCs lags behind that on gold clusters. Several clusters including Au6 Ag 7 SR 10 ,25 Au 2 4 PtSR 1 8 , 2 1 Au 2 4 Pd SR 1 8 , 1 9 Au 3 6 Pd 2 SR 2 4 , 2 6 and Au12Ag32SR3017 have been isolated and characterized. However, only a few nongold bimetallic clusters incorporating silver have been observed,27,28 and almost none have been isolated. To fully realize the potential of MPCs, a wider variety of bimetallic clusters need to be synthesized, isolated, and characterized. In this paper we address two key questions: (a) What happens when a known synthetic route to a bimetallic cluster is repeated with a different congener? Does an isostructural and isoelectronic cluster form? (b) If a metal in a bimetallic cluster © XXXX American Chemical Society

is replaced by its congener, how do the electronic and optical properties of the cluster change? We had previously reported the isolation of the dimercaptosuccinic acid (DMSA)-protected cluster, Ag4Ni2(DMSA)4.29 Herein, we report the synthesis, isolation, and theoretical investigation of Ag4Pt2(DMSA)4. To obtain Ag4Pt2(DMSA)4, we modified the synthesis of Ag4Ni2(DMSA)4 by replacing the Ni source with a source of its congener Pt and found that it produced a bimetallic MPC with the same stoichiometry as the original synthesis. Theoretical investigations determined the structure of the Ag−Pt bimetallic MPC and found it to be isostructural and isoelectronic with the Ag4Ni2(DMSA)4 cluster. The optical properties of the Pt containing MPC are different; because the crystal field splitting of 5d electrons is typically much larger than 3d electrons, the HOMO−LUMO gap of the Pt containing MPC is larger than that of the Ni containing MPC. Because of this, the Pt based MPC has a substantial blue shift in the optical spectrum, suggesting a strategy for tuning the optical spectrum of clusters. Theoretical investigations into the yet to be synthesized Ag4Pd2(DMSA)4 cluster confirm this by revealing a HOMO− LUMO gap and optical spectrum that is intermediate in energy between the Ni and Pt based MPCs.

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EXPERIMENTAL SECTION Synthesis. Ag−Pt clusters were synthesized by methods similar to those reported earlier for Ag4Ni2DMSA4.29 In brief, Special Issue: A. W. Castleman, Jr. Festschrift Received: January 31, 2014 Revised: April 17, 2014

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exposure and minimize sample damage. Image analysis was performed with Gatan Digitial Micrograph software. EDX data were recorded using ES-Vision software. The electronic structure calculations on various cluster configurations were carried out using a gradient-corrected density-functional formalism30 with orbitals represented by a linear combination of Slater-type orbitals (STO) located at the atomic sites. Actual calculations were carried out using the Amsterdam Density Functional (ADF) set of codes.31 All structures were allowed full variational freedom and fully optimized without constraint or symmetry. The functional proposed by Perdew, Burke, and Ernzerhof (PBE) was used to account for exchange and correlation. Relativistic effects were taken into account using the zeroth-order regular approximation32−34 while employing a TZ2P all-electron basis. The optical spectrum was calculated using time dependent density functional theory (TD-DFT) with 300 excitations.

an ethanolic solution of silver nitrate (86.6 mg in 60 mL ethanol) was prepared in a 100 mL three-neck round-bottom flask. To this solution was added 75 mg of chloroplatinic acid hexahydrate. The solution was then chilled to 0 °C in an ice bath and flushed with nitrogen. Meso-dimercaptosuccinic acid (108 mg) was then added to the solution under slow stirring (∼60 rpm). The temperature and gentle nitrogen flow were maintained, and the mixture was allowed to react for 4 h; the color changed from light yellow to a dark orange, indicating the formation of thiolate precursors. Sodium borohydride (75 mg) was then added, and stirring was increased to high speed (∼1200 rpm). The solution was then allowed to react overnight. During this time, the temperature was allowed to rise to ambient levels, and the color of the solution darkened significantly to a black/brown color. The crude reaction mixture was then washed with ethanol to remove excess salts and ligand. The solid was dried under nitrogen and extracted with water. The clusters were then precipitated with ethanol and again dried under nitrogen. Polyacrylamide Gel Electrophoresis. The native polyacrylamide gel electrophoresis (PAGE) of the cluster utilized a Bio-Rad Mini PROTEAN Tetra Cell with 1.5 mm gels. Approximately 75 mg of the as prepared clusters were dissolved in 1 mL of water containing 5% glycerol. The cluster solution was loaded equally onto the four gels and run at 100 V for 2.5 h with a 192 mM glycine and 25 mM tris(hydroxymethylamine) running buffer. The bands were cut out and extracted with cold H2O overnight under nitrogen. The clusters were precipitated by the addition of ethanol and dried under nitrogen. Numerous gel concentrations and conditions were utilized, but the best results were found with the following conditions: 25% T, 3.3% C, pH 8.8 resolving gel; 5% T, 3.3% C, pH 6.8 stacking gel. Size Exclusion Chromatography. A 1 cm diameter column was prepared with 3.5 g of Bio-Rad Biogel P4̅ size exclusion chromatography media (fractionation range 800− 4000 MW, 45−90 μm bead size). A 150 mM solution of either sodium chloride or sodium nitrate was used as the running buffer to maintain adequate ionic strength. The beads were hydrated and the column packed according to manufacturer instructions. A 100 mg/mL solution of clusters in buffer was prepared, filtered through a 0.22 μM filter, and run down the column at a low flow rate (∼5 mL/hour). Fractions were collected and precipitated by the addition of ethanol. The set of fractions corresponding to each band were then redissolved, combined, and run through the column a second time with only the primary band being collected to increase final sample purity. Characterization Instruments. UV−vis spectroscopy was carried out on an Agilent 8453 UV−vis spectrophotometer. Electrospray ionization mass spectrometric analysis was performed on a Waters LCT Premier time-of-flight (TOF) mass spectrometer. Operation of the mass spectrometer was performed using MassLynx software Version 4.0. Clusters were dissolved in deionized H2O at concentration of 1.2 mg/mL and diluted 50% with methanol. Samples were introduced into the mass spectrometer using direct infusion via a syringe pump built into the instrument. The mass spectrometer was run in negative ion mode, using electrospray ionization (ESI). Transmission electron microscopy was performed on a JEOL 2010F field-emission TEM with STEM capability operated at 200 kV. Samples were drop cast on copper 200 mesh lacey carbon grids. Long-term beam exposure led to particle aggregation, so images were collected quickly to limit beam

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RESULTS AND DISCUSSION Synthesis of the nanoclusters was carried out by a pathway similar to our prior synthesis29 of Ag4Ni2(DMSA)4 and an earlier synthesis of Ag7(DMSA)4.35 First, a low temperature reaction of a mixture of silver and platinum salts with dimercaptosuccinic acid thiol ligand was carried out for the controlled formation of thiolate precursor complexes as indicated by a change in color from pale yellow to a darker orange. This solution was then reduced with sodium borohydride leading to formation of nanoclusters, as indicated by another change in color to a black/brown solution. This product had a broad, featureless UV−visible spectrum, indicative of significant impurity within the sample. This necessitated the separation of clusters into distinct monodisperse samples to meaningfully study their properties. Polyacrylamide gel electrophoresis (PAGE) has a strong history of success for the separation of water-soluble thiolated nanoclusters36,37 and was, therefore, applied to our mixture. Unfortunately, despite some minor separation, the bands were poorly resolved and the compounds were only recovered in poor yields. To address this problem, we turned to size exclusion chromatography (SEC), which has recently seen considerable success in separating organic soluble clusters.9,26,38 Compared with PAGE, much cleaner separation of the desired clusters from additional products was achieved with far better yields (Figure 1). The mixture separated into two bands. The first of which eluted very rapidly, consistent with larger particles outside of the exclusion range of the gel; the second of which eluted more slowly, consistent with a small nanocluster. Additional purity was obtained by concentrating and recycling bands through the column. The sharpening of the UV peak in the cluster fraction relative to the crude mixture through successive column cycles provided strong evidence of the purity of the fractions eluted from the column (Figure 2). Once isolated, the clusters could be identified and characterized. Electrospray ionization mass spectrometry (ESI-MS) was applied to both fractions obtained from SEC. In the first fraction, there was very little signal, indicating that the major component of the fraction was not ionizing. This, along with the speed at which the band moved through the column, suggested the presence of larger nanoparticles rather than nanoclusters. Transmission electron microscopy (TEM) showed the presence of a broad size range of nanoparticles and associated energy-dispersive X-ray spectroscopy (EDX) analysis confirmed the presence of silver, platinum, and sulfur (Figure B

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The ground state geometry of Ag4Pt2(DMSA)4 is shown in Figure 5, and the next three lowest energy isomers are shown in Figure 6. In the ground state structure of Ag4Pt2(DMSA)4, Ag4 and the two Pt atoms together form a distorted octahedron, with the sulfur atoms of the DMSA each binding to one Pt atom and one Ag atom forming a rectangular cuboid. The cluster has a HOMO−LUMO gap of 2.43 eV. The structure is stabilized by a charge transfer from the Ag atoms to the Pt(DMSA)2 fragments. Each Pt atom is surrounded by 4 sulfur atoms in a square planar configuration, and charge transfer from the Ag atom results in the Pt atom having a 5d8 electronic configuration. This can be understood through crystal field theory, in which d8 metals have a tendency to form square planar complexes.40 The alternate isomers of Ag4Pt2(DMSA)4 also have the Pt in a square planar configuration surrounded by sulfur atoms. The Ag4Pt2S8 core structure is the same in the three lowest energy structures, so the exact arrangement of the ligands is difficult to confirm using only theory. We find that the most stable structure is the staggered configuration with the carboxylic acid groups pointing toward the center of the cluster. We next compare the structure of the Ag4Pt2(DMSA)4 cluster with their congener replacements, Ag4Ni2(DMSA)4 and Ag4Pd2(DMSA)4. First of all, the structure of the Ag4X2S8 core (X = Ni, Pd, Pt) is identical in all clusters in Figure 7. The geometries of the lowest energy structures for these clusters are shown in Figures S3−S5 (Supporting Information). We have used the staggered configuration for all three clusters. Although the staggered structure is most stable for Ag4Pt2(DMSA)4, the eclipsed structure is more stable in the Ag4Ni2(DMSA)4 cluster because of hydrogen bonding between the carboxylic acid groups. In solution, this hydrogen bonding will be much less important because the hydrogen bonding may also occur between the cluster and solvent. The optical spectra of the staggered cluster in Ag4Ni2(DMSA)4 better matches the experimental spectra, so we have used this structure. We have plotted the HOMO and LUMO of the clusters in Figure 7, and in all cases the HOMO is the ndxz orbital on the Ni, Pd, and Pt atom, and the LUMO is an antibonding orbital between the S ligand and the ndx2−y2 orbital, confirming that these clusters are isoelectronic. The HOMO− LUMO gap of the Ag4Pt2(DMSA)4 cluster is 2.43 eV, markedly larger than that for Ag4Pd2(DMSA)4 at 2.14 eV, and Ag4Ni2(DMSA)4 at 1.55 eV. This is because 5d orbitals undergo larger crystal field splittings than 4d orbitals, which typically have larger crystal field splittings than 3d orbitals. This suggests that the optical spectrum in bimetallic clusters stabilized through crystal field splitting may be tuned through congener replacement. To demonstrate the effect of congener replacement on the optical spectrum, the calculated UV−vis spectra of Ag4Ni2(DMSA)4, Ag4Pd2(DMSA)4, and Ag4Pt2(DMSA)4 are shown in Figure 8. The molecular orbital diagram, and the excitations with the largest oscillator strengths are shown in Figures S6−S8 (Supporting Information). Ag4Ni2(DMSA)4 has a calculated absorption peak centered at 590 nm, and a shoulder that absorbs 400−500 nm light, resulting in the observed green coloration. This is consistent with the measured UV−vis spectra of Ag4Ni2(DMSA)4.29 Ag4Pd2(DMSA)4 has a similar UV−vis spectra; however, the peaks are blue-shifted with lowest energy absorption centered at 550 nm, and a shoulder that rises up beginning at 450 nm. Finally, Ag4Pt2(DMSA)4 undergoes an additional blue shift with the lowest energy absorption found around 475 nm, and the

Figure 1. Comparison of the separation achieved by size exclusion chromatography (SEC) (left), and polyacrylamide gel electrophoresis (PAGE) (right) is shown. SEC resulted in significantly improved separation while being able to handle larger amounts of material with greater recovery.

Figure 2. UV−visible spectra of fractions obtained with successive cycles of size exclusion chromatography (SEC). A qualitative increase in purity can be seen by the sharpening the band at 250 nm. Note that these samples were not controlled for concentration and, therefore, absolute absorbance is not meaningful.

3). To further analyze the nature of the particles, STEM mode EDX was performed on individual particles, showing the presence of both larger pure silver particles and smaller silver− platinum particles (Figure S1, Supporting Information). In the second fraction, on the other hand, ESI-MS resulted in a strong signal, allowing the identification of the primary component of the cluster-containing band (Figure 4). The large series of peaks beginning at 770 m/z correspond to the −2 charge state of Ag4Pt2(DMSA)4 and its subsequent sodium exchange peaks. Some additional smaller peaks may be attributed to fragments of or other ion adducts of Ag4Pt2DMSA4. A close look at the isotope distribution of the first major peak shows an excellent match with the expected distribution and peak locations of Ag4Pt2(DMSA)4, providing confirmation of the composition.39 Additional detailed peak identifications are shown in Figure S2 (Supporting Information). Once the composition of the cluster was determined via mass spectrometry, the structure of the cluster could then be modeled computationally. C

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Figure 3. (A) TEM image of the isolated large particle fraction from SEC. Lattice fringes consistent with larger crystalline nanoparticles can be seen throughout. Fringes measuring both 2.4 and 2.0 Å were observed, consistent with the (111) and (200) planes of silver. A wide variety of particle sizes were present at different locations within the sample from around ∼3 to ∼10 nm. These large sizes are consistent with the rapid rate at which the band traveled through the SEC column. (B) Wide area EDX elemental analysis confirming the presence of both silver and platinum within the sample.

Figure 5. Ground state structure of Ag4Pt2(DMSA)4.

peak at 475 nm is too weak to observe experimentally. The absorption at 240 nm seen in the experimental spectrum is too high in energy to be diagnostic and is likely to be found on the DMSA ligand. The lack of agreement between theory and experiment may be caused by significant spin−orbit splitting of the 5d states on Pt. To test this hypothesis, we calculated the absorption spectra of Ag4Pt2(DMSA)4 by including spin−orbit coupling as a perturbation. The results are shown in Figure S9 (Supporting Information). The inclusion of the spin−orbit coupling slightly red-shifted the absorption spectra but did not improve the agreement with experiment. The method used in this calculation41 for incorporating the spin−orbit coupling does not couple the filled and virtual orbitals and only includes spin−orbit as a perturbation. We suspect that a fully selfconsistent treatment of the spin−orbit effects is needed to accurately account for the observed shifts. Such calculations are, however, beyond the scope of the present work. We note that the dramatic blue shift in the absorption spectra in going from Ni to Pt is observed in both experiment and theory. To understand the mechanism by which the cluster forms, we investigated the binding energy of DMSA to Ni, Pd, Pt, and Ag. We found that the binding energy of the first DMSA to Ag is 2.79 eV, 5.27 eV for Ni, 4.17 eV for Pd, and 5.11 eV for Pt. The Pd−S bond is weaker than the Ni−S and Pt−S bond in

Figure 4. Electrospray ionization mass spectrum of the isolated nanocluster fraction from SEC. Top: broad view from 500 to 1500 m/ z. The large series of peaks beginning at 770 m/z correspond to the −2 charge state of Ag4Pt2(DMSA)4 and its subsequent sodium exchange peaks. Bottom: zoomed in view of first major peak (red) overlaid with simulated isotope distribution for [Ag4Pt2(DMSA)4 − 2H]2− (blue).

second peak at 410 nm. In the experimental UV−vis spectra of Ag4Pt2(DMSA)4 we find that the observed absorption peaks are significantly more blue-shifted than those calculated (Figure 2 versus Figure 8). Our interpretation of the spectra is that the peak at 425 nm in the theoretical spectrum corresponds to the 350 nm shoulder in the experimental spectrum, and that the D

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Figure 6. Three subsequent higher geometry isomers of Ag4Pt2(DMSA)4 and their respective energy difference vs the ground state: (A) isomer 1; (B) isomer 2; (C) isomer 3.

Figure 7. HOMO−LUMO atomic orbital clouds of (A) Ag4Ni2(DMSA)4, (B) Ag4Pd2(DMSA)4, and (C) Ag4Pt2(DMSA)4 (HOMO in red-blue, LUMO in orange-light blue).

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CONCLUSIONS We have reported the synthesis, characterization, and theoretical structural determination of a novel bimetallic Ag− Pt cluster, Ag4Pt2DMSA4. The cluster is water-soluble and highly stable. We found that by taking a synthetic procedure for a MPC bimetallic cluster and replacing Ni with a congener Pt results in the formation of a cluster with same structure as the original bimetallic cluster. The optical absorption spectra of Pt is blue-shifted from that of Ni because the crystal field splitting of 5d orbitals is markedly larger than 4d and 3d orbitals. This offers a strategy for tuning the optical spectrum in bimetallic clusters. In addition to its fundamental importance, the new cluster may also have significant catalytic applications due to the presence of highly strained and accessible platinum atoms.

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ASSOCIATED CONTENT

S Supporting Information *

Figure 8. Calculated UV−vis absorption spectrum for Ag 4 Ni 2 (DMSA) 4 (top), Ag 4 Pd 2 (DMSA) 4 (middle), and Ag4Pt2(DMSA)4 (bottom), respectively.

STEM-EDX analysis of larger particle fraction, additional electrospray ionization mass spectrometry peak identifications, additional geometries of the lowest energy structures for clusters, molecular orbital diagrams with the excitations with the largest oscillator strengths, and calculated UV−vis spectrum. This material is available free of charge via the Internet at http://pubs.acs.org.

X(DMSA)1,2 and X−SH because the Pd atom forms weaker πbonds with the S atom. This demonstrates that the binding of DMSA to the Pt/Pd/Ni atom is much larger than that to Ag. Furthermore, the binding energy of the second DMSA to form X(DMSA)2 is 2.71 eV for Ag, 3.22 eV for Ni, 3.68 eV for Pd, and 4.20 eV for Pt. We suspect that this cluster forms first through the formation of the square planar Pt(DMSA)2 fragment. This complex is electron deficient by two electrons, so it is stabilized by two silver atoms per Pt transferring an electron to the Pt complex. The 2 Ag:1 Pt ratio is completed in Ag4Pt2(DMSA)4. This structure also forms the optimal number of metal−sulfur bonds, which further explains the cluster stability.

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AUTHOR INFORMATION

Corresponding Authors

*A. Sen: e-mail, [email protected]. *S. N. Khanna: e-mail, [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The work was supported by the U.S. Department of Energy, Office of Basic Energy Sciences. The theoretical portions of this E

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work (AFP, ACR, and SNK) were supported by the U.S. Department of Energy (DOE) through grant DE-FG0211ER16213.

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dx.doi.org/10.1021/jp501124q | J. Phys. Chem. A XXXX, XXX, XXX−XXX