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The Complete AgM(DMSA) (M = Ni, Pd, Pt, DMSA = Dimercaptosuccinic Acid) Cluster Series: Optical Properties, Stability, and Structural Characterization Scott R Biltek, Arthur C Reber, Shiv N. Khanna, and Ayusman Sen J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.7b04669 • Publication Date (Web): 28 Jun 2017 Downloaded from http://pubs.acs.org on June 30, 2017

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The Journal of Physical Chemistry

The Complete Ag4M2(DMSA)4 (M = Ni, Pd, Pt, DMSA = Dimercaptosuccinic Acid) Cluster Series: Optical Properties, Stability, and Structural Characterization Scott R. Biltek,1 Arthur C. Reber,2 Shiv N. Khanna,2* Ayusman Sen1* 1

Department of Chemistry, The Pennsylvania State University, University Park, Pennsylvania

16802, United States. 2

Department of Physics, Virginia Commonwealth University, Richmond, Virginia, 23284, United

States. Email: [email protected], [email protected]

Abstract: The cluster series, Ag4M2(DMSA)4 (M = Ni, Pd, Pt), has been synthesized and the optical spectra and stability have been examined as a function of the metal, M. We have also obtained the structure of Ag4Ni2(DMSA)4 using X-ray crystallography, confirming the previously calculated structure. In the optical spectrum, there is a significant blue shift as the substituted metal M progresses down the periodic table. Theoretical calculations suggest that the blue shift is due to the lowering in energy of the d orbitals of the transition metal, M; however the expected metal-metal excitations are optically weak, and the spectra are dominated by metalligand excitations. The Ag4Pd2(DMSA)4 species has exceptionally high stability relative to the previously reported Ni and Pt analogues.

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Introduction Extensive research over the past three decades has shown that atomic clusters exhibit unique size-dependent properties and serve as an intermediate state of matter, in between inorganic complexes and larger nanoparticles.1-7 One promising direction in nanoscience is making materials using clusters as the building blocks.8-10 Since the properties of clusters can be controlled by size and composition, cluster materials offer the unique possibility of making materials with targeted properties. Since naked clusters are highly reactive, stabilizing ligands are often used. Thiol-ligated clusters are particularly important as the strong metal-sulfur bond helps to prevent the reactive surfaces of metal nanoclusters from becoming oxidized or the clusters agglomerating into larger particles.11 Ligands not only protect clusters but can also strongly influence their properties.12-14 The optical properties of ligand-protected noble metal clusters are significantly different from those of larger nanoparticles and are therefore of particular interest. Nanoparticles with sizes above 1.5-2 nm support plasmonic resonances while smaller particles exhibit discrete moleculelike transitions.15-17 Understanding the interplay between the optical excitations related to metalmetal and metal-ligand transitions is critical to understanding these properties in both clusters and larger nanoparticles.18-21 By studying atomically-precise clusters and varying the metal involved in the optical transition, one can study the evolution of optical properties and infer additional details about the origin of the transitions. This requires the systematic synthesis and characterization of a series of similar atomically-precise clusters that exhibit varying optical properties.20


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Bimetallic ligand-protected clusters offer the ability to tune the properties, including optical absorption and reactivity of clusters through composition variations.22-24 Thiolated gold and silver-based nanoclusters ranging in size from 10 to 333 metal atoms have been studied in great detail,25-34 and many of the corresponding bimetallic versions of such gold clusters have been synthesized.35-38 Unfortunately, the synthesis of other transition metal clusters have lagged well behind those of gold leaving a significant knowledge gap. Relatively few bimetallic clusters which do not contain gold have been reported.39-45 We have previously reported the synthesis of bimetallic silver clusters stabilized by the meso-2,3-dimercaptosuccinic acid ligand (DMSA). Our report included both a Ag4Ni2(DMSA)4 cluster and a Ag4Pt2(DMSA)4 cluster,43-44 along with predictions of a possible Ag4Pd2(DMSA)4 cluster. Herein we report the synthesis of the Ag4Pd2(DMSA)4 cluster completing the series of Ag4M2(DMSA)4 (M= Ni, Pd, Pt) clusters. We have also obtained single crystal X-ray diffraction data that confirms the previously predicted structure of the Ag4Ni2(DMSA)4 cluster and helps validate calculations of the other members of the series. Based on the Ag4Ni2(DMSA)4 crystal structure and the other two calculated structures we can reasonably demonstrate that this series is both isoelectronic and isostructural, making any change in properties solely dependent on the identity of the metal M. We have investigated the effect of varying the composition of the bimetallic cluster on the optical properties. We find that, upon going down the periodic table for M, the optical spectrum is blue-shifted, and the HOMO-LUMO gap increases. We find that the primary optical absorptions in the clusters are not due to metal-metal excitations, but rather the metal-ligand excitations, highlighting the importance of the metal-ligand interface in the optical properties of ligand-protected clusters. We expect these trends to be generalizable to other series



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of clusters, helping to guide future synthesis of bimetallic clusters tailored to desired applications. Experimental Procedures Chemicals: All reagents were available commercially and used as purchased without further purification. Silver nitrate (99.9999%), nickel nitrate hexahydrate, meso-2,3-dimercaptosuccinic acid (~98%), palladium acetate, silver acetate (99.9%), hexachloroplatinic acid (≥ 37.5% Pt basis), acetonitrile and methanol (Chromasolv >99.9%) were purchased from Sigma-Aldrich. Potassium tetrachloropalladate (99.9%) was purchased from Strem chemicals. Sodium borohydride (≥ 99%) was purchased from Fluka. Ethanol (200 proof) was purchased from Koptec. Bio-gel P-4 size exclusion chromatography media (fractionation range 800-4000 MW, 45-90 µm bead size) was purchased from Bio-Rad.

Cluster Synthesis: Nanoclusters were synthesized by a two-step process; the formation of metal thiolate precursor complexes and then the reduction of those complexes into nanoclusters. Ag4Ni2(DMSA)4 and Ag4Pt2(DMSA)4 clusters were prepared as previously reported.44-45 The Ag4Pd2(DMSA)4 clusters were prepared in a similar manner as follows. In a three-neck round-bottom flask under a nitrogen blanket, silver acetate (85.2 mg) and palladium acetate (20.2 mg) were dissolved in 60 mL ethanol and cooled to 0°C. Dimercaptosuccinic acid (108 mg) was then added and allowed to react under slow stirring for 4 h. Sodium borohydride (26.1 mg) was then added under vigorous


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stirring and allowed to react for 48 hours. This crude reaction mixture was then centrifuged out of the ethanol suspension and washed repeatedly with ethanol to remove any excess salts and reactants. The solid was then dried under nitrogen flow and extracted with a small amount of water to separate clusters from insoluble side products. Finally, the clusters were precipitated from solution via addition of ethanol and again dried under nitrogen flow. Cluster Isolation via Size Exclusion Chromatography: The mixture of obtained clusters was purified by size exclusion chromatography using a column of polyacrylamide beads (800-4000 MW range). Concentrated solutions of clusters (~50100 mg/ml) were prepared and loaded onto the column and run with 150 mM sodium chloride to provide the required ionic strength. Clusters separated into bands which were collected separately and precipitated with ethanol. Purity of the fraction of interest was determined via UV-vis spectroscopy and fractions were rerun for additional purification if required. Cluster Isolation via Spin Filtration: A 100 mg/ml solution of clusters was run through a 3k MWCO (Molecular Weight Cut Off) spin filter at 10000 RPM for 30 minutes. The filters were washed with DI water and the filtrate was run through the filter a second time. Clusters were then precipitated with ethanol and dried under nitrogen. Product Characterization: Clusters were primarily characterized by electrospray ionization mass spectrometry, UVvisible spectroscopy, and single crystal X-ray crystallography. ESI Mass spectrometric analysis was performed on a Waters LCT Premier time-of-flight (TOF) mass spectrometer. Clusters were 5 ACS Paragon Plus Environment


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dissolved in deionized H2O at concentration of 0.4 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 set to scan from 100-3000 m/z in negative ion mode, using electrospray ionization (ESI). The drying gas temperature was set to 120°C, the source temperature was set to 100°C and the capillary voltage was 2800V. UV-visible spectroscopy was performed on ThermoFisher Evolution 220 UV-VIS Spectrophotometer using solutions of clusters in water. ESI-MS allowed for identification of the cluster composition, UVvisible spectroscopy allowed for determination of cluster purity based on the sharpness of the peaks and single crystal X-ray crystallography allowed for structural determination of the Ag4Ni2(DMSA)4 cluster. Stability Measurements by UV-Visble Spectroscopy: Solutions of clusters were prepared in deoxygenated water. This solution was diluted to a maximum absorbance of approximately 1.0. The solution was then divided into two cuvettes, one left often to air with stirring and one carefully sealed against oxygen. Measurements were taken every 24 h with the contents of the open air flask being diluted back to its original volume to offset any evaporation. Crystallization Experiments: Attempts to grow crystals of the clusters were carried out as follows. Solutions of clusters in water (10-30 mg/ml) were loaded into cut NMR tube vials or 2 mL sample vials. The caps of these vials were punctured with needles of various gauges (18 to 30) to control the rate of vapor diffusion. These capped vials were then placed into large 20 mL scintillation vials containing ~3 mL of ethanol or acetonitrile. These vials were purged with nitrogen gas and sealed with their 6 ACS Paragon Plus Environment

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caps and a parafilm wrap. The vials were left undisturbed as the organic vapor diffused into the aqueous solution causing gradual precipitation. This process was carried out at several temperatures (0°C, 20°C, 40°C). The best crystals were obtained from a solution combining 10 mg/ml cluster with 5 mg/ml sodium acetate at room temperature. Details of data collection and refinement are given in Table S1. Theoretical Methods: The electronic structure calculations were performed within a density functional framework using the Amsterdam Density Functional (ADF) set of codes.46 The PBE gradient-corrected density-functional47 was used with orbitals represented by a linear combination of Slater-type orbitals (STO) located at the atomic sites. All structures were allowed full variational freedom and fully optimized without constraints or symmetry. A large number of structures with multiple core structures were optimized, and the lowest energy structures are reported here. Relativistic effects were taken into account using the Zeroth Order Regular Approximation (ZORA)48-49 while employing a TZ2P basis. The optical spectrum was calculated using Time Dependent Density Functional Theory (TD-DFT) with 600 excitations. Results and Discussion: We have successfully synthesized a series of clusters of the formula Ag4M2(DMSA)4 (M= Ni, Pd, Pt). These clusters have all been synthesized and isolated by size exclusion chromatography, and, based on DFT calculations,44 are isoelectronic and have similar geometric structures. Each of these clusters was synthesized by a similar synthetic process modified for the individual system.50 In order to synthesize the silver palladium cluster, compatible ethanol soluble salts of both palladium and silver had to be found. While the most common silver feedstock is silver 7 ACS Paragon Plus Environment


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nitrate, we found that when using silver nitrate ethanol was capable of reducing the palladium salt before our desired reaction could be carried out. Literature review yielded an example of a similar reaction between palladium nitrate and ethanol being used to generate palladium nanoparticles with ethanol serving as the reducing agent.51 In order to bypass this problem, we used silver acetate and palladium acetate as our feedstock salts. Metal ion ratios were modified to generate the purest product, with the best result at 15% palladium and 85% silver. As with the silver nickel and silver platinum reactions, absolute control over size and composition was not entirely possible and the final product does not consist purely of the desired clusters, requiring additional purification. A good separation of the silver palladium product could not be achieved by gel electrophoresis (Figure S1) and so size exclusion chromatography was employed. Gel electrophoresis was also unsuccessful for separating the silver platinum product. The crude product resolved into bands which were monitored by UV-visible as they eluted. The cluster band was run through the column again to ensure complete purification. This band was then precipitated by addition of ethanol and dried under N2 flow. Because the side products isolated from the silver palladium reaction were not observed to contain actual clusters, but instead consisted of larger particles, we also experimented with low molecular weight cut off spin filtration as a means of purification. This proved to be highly effective, with results comparable to those of size exclusion chromatography (Figure S2). This provides additional confirmation of the absence of cluster side products which would easily pass through the spin filter instead indicating that the side products are large nanoparticles. A dilute solution of cluster solid was then prepared and identified by electrospray ionization mass spectroscopy (Figure 1A). A broad series of peaks was present in the spectrum, but each can be identified as originating from the expected Ag4Pd2(DMSA)4 8 ACS Paragon Plus Environment

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(Figure S3). Additional peaks stem from differing charge states, ion exchange with the ligand carboxylic acid protons, and in some cases minor fragmentation. A close up on an individual peak from a series shows another distribution of peaks based on the isotopic distribution of the cluster. This distribution serves as a fingerprint for a particular cluster and aids greatly in a positive identification. The expected distribution of a specific compound in a particular charge state can be calculated from the isotope abundances for all the elements involved and compared with the experimental distribution to confirm the cluster identity (Figure 1B). The geometric structures of all three clusters in the Ag4M2(DMSA)4 series have been previously calculated.44 It is valuable to confirm these structures through single crystal X-ray crystallography to validate the method and to eliminate all uncertainty. Unfortunately, this requires the growth of high quality crystals which is often challenging for thiolated nanoclusters. After significant tailoring of crystallization conditions, we have successfully grown high quality crystals of Ag4Ni2(DMSA)4, allowing for the complete determination of the cluster structure (Figure 2 and S4). This structure is consistent with the structure previously calculated using DFT. Although the ligand orientation is slightly different, the vital core structure is essentially identical. The cluster contains an octahedral core composed of a plane of four silver atoms with the two nickel atoms centered on opposing sides. Bond angles and lengths within the structure indicate significant metal-metal bonding interactions between the silver atoms themselves and between the silver and nickel atoms. The average Ag-Ag bond length is 2.99 Å. This value can be compared to both the Ag-Ag bond length in bulk silver (2.89 Å) and Ag-Ag bond lengths observed for another published silver thiol cluster crystal structure Ag44(SPh)304- (2.83 Å internal) (3.18 Å external).29 The Ag-Ag bond length is slightly longer than those in both bulk silver metal and in the metallic core of the larger Ag44 cluster due to the impact of the thiol 9 ACS Paragon Plus Environment


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bonding. This effect has also been seen in both gold clusters and the Ag44 cluster which show a significantly longer M-M bond lengths for the metal atoms participating in thiol bonding.52-53 The Ag-Ni bond length within the Ag4Ni2(DMSA)4 cluster is 3.17 Å. This is slightly longer than the Ag-Ag bond (2.99 Å), the covalent radius of Ag and Ni predicts a bond distance of 2.69 Å, while the Van der Waals radii suggests that a nonbonding distance would correspond to 3.35 Å, Thus the Ag-Ni distance is intermediate between a covalent and a nonbonding interaction. If we assume no Ag-Ni bonding, we would expect the nickel and the four surrounding sulfur atoms to be within in a plane, giving rise to 180° bond angles between nickel and the trans sulfur atoms. Within the Ag4Ni2(DMSA)4 cluster, the nickel is pulled slightly downwards towards the silver atoms and out of the sulfur plane by 0.05 Å, resulting in bond angles averaging 178.12°, which is a rather small shift, but it may suggest a weak interaction between the Ag and the Ni. The core is predominately stabilized by metal-sulfur bonding; however, it is possible that there is also a stabilization effect from a silver-oxygen interaction. An oxygen on one carboxylic group of each ligand is particularly close to one of each of the four silver atoms (2.542 Å). The square plane of silver atoms is slightly twisted out of alignment from the two sulfur planes in the direction of this interaction. For comparison, we have included the theoretical bond distances and angles in Table 1. We find that both the calculated Ag-Ag bond distances and the calculated Ag-Ni bond distances are generally longer than experiment. In order to confirm similar structures in Ag4Pd2(DMSA)4 and Ag4Pt2(DMSA)4, efforts to grow crystals of these clusters are ongoing but while crystals have been obtained, they are not yet suitable for high quality data collection. Based, however, on the similarity of the DFT calculated structures, and the now proven accuracy of the computed Ag4Ni2(DMSA)4 structure, it is reasonable to assume that that the our structures


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for Ag4Pd2(DMSA)4 and Ag4Pt2(DMSA)4 are also accurate, and that the series is likely to be isoelectronic and isostructural. We investigated two major properties of the cluster series. First, we looked at the electronic structures of the compounds. Many of the key applications for clusters are based on their electronic structures, and as such it is highly desirable to be able to tune these properties for any given cluster. The previous calculations on this series suggested a shift in the HOMO-LUMO gap of the clusters as the metal M moved down the periodic table.44 We are now able to experimentally confirm this prediction. The electronic structure of each cluster can be determined by simple UV-vis spectroscopy. When the spectrum of the Ag4Pd2(DMSA)4 cluster was obtained and compared with those from Ag4Ni2(DMSA)4 and Ag4Pt2(DMSA)4, a clear trend can be observed (Figure 3). The main peak of each spectrum shifts to higher energy on moving from nickel to palladium to platinum. This is further demonstrated in the HOMO-LUMO gaps and the structures of the clusters, as shown in Figure 4. The stability of these clusters can be understood in that the M(DMSA)2 square planar complex is electron deficient by two electrons. The four Ni-S bonds each reduce the effective valence of Ni by one, so the Ni atom has 6 valence electrons in the isolated Ni(DMSA)2 complex. In a square planar complex, the Ni atom is electronically stable in the 3d8 configuration, so it needs 8 valence electrons and thus the Ni becomes stable after gaining two electrons from Ag. This means that the 2:1 ratio of Ag electron donors and M(DMSA)2 species results in a closed electronic shell. Within this 2:1 ratio, 2M and 4Ag results in a stable octahedral structure with the maximum number of thiol bonds. Thus the Ag4M2(DMSA)4 cluster is both electronically and geometrically stable. To understand the changes in the optical spectra of the Ag4M2(DMSA)4 clusters, for M=Ni, Pd, and Pt, we also calculated the optical spectra of the clusters and compared them to the 11 ACS Paragon Plus Environment


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experimental results. The HOMO-LUMO gaps of the clusters are in the range of 1.44 eV for the Ni substituted cluster up to 2.47 eV for the Pt substituted cluster. The absorption peaks observed in the experimental spectra are significantly higher in energy; about 3.76 eV for Ni, 4.28 eV for Pd, and about 5.06 eV for Pt. These energies are significantly higher than the expected HOMOLUMO gap of any metal cluster. We note that there is a weak absorption in the Ni spectra at 2.07 eV, which is much closer to the HOMO-LUMO gap, and a weak Pt absorption at around 3.75 eV. In Figure 5, we have shown the calculated optical spectrum of the three clusters using TD-DFT by calculating the lowest 600 excitations for each cluster. For Ag4Ni2(DMSA)4 we find a weak absorption at 599 nm, labeled “a” in the figure that corresponds to a transition between 3d (3d→3d) states of the nickel atom. This absorption is observed in experiment, and the clusters have a faint greenish color, consistent with absorption in the red-orange color range. There is a stronger pair of absorptions, labeled “b” at 410 nm and 438 nm, and these peaks correspond to an excitation from the higher energy portion of the 3d orbitals mostly localized on the nickel to an excited state on the DMSA ligand. There is an even stronger metal-ligand absorption peak at 351 nm that corresponds to electron in a deeper 3d orbital of nickel being excited to a state mostly located on the ligand, labeled c, and the experiment shows a strong absorption at 340 nm. The strongest absorption peaks are from the occupied 3d orbitals of nickel to the ligand. Perhaps the 3d-3d transitions on the nickel that correspond to the HOMO-LUMO gap have weak oscillator strengths because selection rules generally forbid d-d optical transitions. The calculated optical spectrum of Ag4Pd2(DMSA)4 results in a similar series of peaks, a weak 4d-4d transition at 555 nm, a stronger shoulder for a 4d-ligand transition at 402 nm, and an even stronger absorption at 347 nm. Ag4Pt2(DMSA)4 also reveals a similar pattern with a weak absorption at 472 nm, a 12 ACS Paragon Plus Environment

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second absorption at 412 nm and a major peak at 302 nm. We believe that the measured optical spectra is dominated by this peak “c”, which shifts from 351 nm to 347 nm and to 302 nm as we progress from Ni to Pd to Pt. We note that calculated excitation energies are lower than the experimental results, and that this is a well-known issue with TD-DFT. The shifts seen in experiment are larger than those in the theory, especially between Ni and Pd. One possibility is that the Ni peak c is wider than the Pd peak and has a lower energy peak at 373 nm, while the Pd has a sharper peak at 347 nm. Thus the Ni peak absorption is found at lower energy than the Pd peak. The reason for the changes in the optical spectrum of the Ag4M2(DMSA)4 series is due to the lowering of the 3d/4d/5d orbital energy due to larger crystal field splitting. The HOMO and LUMO are predominantly made up of the Ni/Pd/Pt 3d/4d/5d orbitals, so the HOMO-LUMO gaps depend on the crystal field splitting of the atom in a square planar complex. The most noticeable shift across the three clusters is that the HOMO LUMO gap increases from Ni to Pd to Pt due to this increase in crystal field splitting. This also effects the energy of the metal-ligand excitations as the crystal field splitting lowers the relative position of the filled 3d/4d/5d orbitals of the Ni/Pd/Pt atoms. For this reason, the metal-ligand excitation that dominates the optical spectrum moves to higher energy as we move down the periodic table. In addition to changes in the electronic structures we also considered the changes in stability of the cluster with each metal. The stability of each cluster was determined by time resolved UVVis spectroscopy (Figure 6). Stability of the cluster solutions was monitored both exposed to oxygen and in sealed cuvettes flushed with nitrogen. Under both conditions the silver-palladium cluster was by far the most stable, followed by the silver-platinum cluster, with the silver-nickel cluster exhibiting the worst stability. Solutions of Ag4Pd2(DMSA)4 are exceptionally stable, showing little to no change in the first 5 days in solution even with significant oxygen exposure



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and remaining relatively stable for over a year in solution when protected from nitrogen (Figure S5). Ag4Ni2(DMSA)4 and Ag4Pt2(DMSA)4 on the other hand decompose significantly faster when exposed to oxygen, and are not indefinitely stable even when sealed under nitrogen. The nature of this difference in stability is easy to rationalize when comparing Ag4Pd2(DMSA)4 and Ag4Ni2(DMSA)4 considering the relative ease at which nickel oxidizes, and the larger HOMOLUMO gap of Ag4Pd2(DMSA)4 will reduce reactivity with O2,54 but these explanations do not hold up for the Ag4Pt2(DMSA)4 cluster. We also note that the binding of the DMSA is weaker to Pd than to Ni or Pt, so the binding strength of the ligand does not play role in the increased stability of Ag4Pd2(DMSA)4. The reason for the markedly increased stability with palladium relative to that of platinum requires additional study.

Conclusion In summary, we have accomplished the synthesis of the Ag4Pd2(DMSA)4 cluster, completing the series: Ag4M2(DMSA)4 (M= Ni, Pd, Pt). We have confirmed the structure of Ag4Ni2(DMSA)4 using single crystal X-ray crystallography. A blue-shift in the optical spectrum and increase in the HOMO-LUMO gap with the change in the metal M has been experimentally observed. An analysis of the excitation spectra reveals that the metal-metal excitations have quite small oscillator strengths, and that the optical absorption spectra are dominated by metal-ligand excitations. The blue shift may be understood because by replacing Ni with Pd and Pt, the d band shifts towards lower energies so that the metal-ligand transition requires more energy as we move down the periodic table. Ag4Pd2(DMSA)4 is found to be the most stable cluster in the 14 ACS Paragon Plus Environment

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series. We believe these trends will be generalizable to other series of clusters, and highlight the importance of metal-ligand interactions in understanding the physical properties of ligandprotected clusters and nanoparticles. ASSOCIATED CONTENT Supporting Information. CIF file, crystal and structure refinement data, additional stability data, additional mass spectroscopy data, and additional separation data. This material is available free of charge via the Internet at http://pubs.acs.org

AUTHOR INFORMATION Corresponding Authors *[email protected], [email protected] Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. ACKNOWLEDGMENT The experimental work was supported by U.S. Department of Energy, Office of Basic Energy Sciences. The theoretical work (ACR, SNK) was supported by the U. S. Department of Energy (DE-SC0006420). The authors gratefully acknowledge Dr. Hemant Yennawar for helpful discussions regarding X-ray crystallography. ABBREVIATIONS HOMO, Highest Occupied Molecular Orbital; LUMO, Lowest Unoccupied Molecular Orbital;



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FIGURES

Figure 1. A: Full ESI-MS spectrum of Ag4Pd2(DMSA)4 sample. Multiple peaks arise from ion exchange and differences in charge states. B: A close up of the [Ag4Pd2(DMSA)4 -2H]2- peak showing the experimental isotope distribution (red) overlaid with the expected isotope distribution (dashed blue).



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Figure 2. Structure of the Ag4Ni2(DMSA)4 cluster as determined by single crystal X-ray crystallography with core atoms labeled. The metal atoms form an octahedron with the nickel atoms at opposite vertices consistent with the previously predicted structure. (Ag – dark blue, Nigreen, S – yellow, O – Red, C – Gray, H – White). Image prepared with Olex2 software.


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Figure 3. UV-visible spectra of the cluster series Ag4M2(DMSA)4 (M= Ni, Pd, Pt). A shift in the major peak can be seen with the change in the identity of M.



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Figure 4. Calculated Structure of Ag4Ni2(DMSA)4, Ag4Pd2(DMSA)4, and Ag4Pt2(DMSA)4.


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Figure 5. Calculated Density of States, and Optical Absorption Spectra for Ag4Ni2(DMSA)4, Ag4Pd2(DMSA)4, and Ag4Pt2(DMSA)4. The major absorption peaks in each spectra are marked by a, b, and c in the absorption spectra graph with the major initial orbital from excitation shown in red, and the final orbital from the excitation is shown in blue. The energy of the major orbitals are shown on the DOS.



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Figure 6. A series of time resolved UV-visible spectra for the cluster series Ag4M2(DMSA)4 (M= Ni, Pd, Pt) both sealed under nitrogen gas and open to atmosphere.


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TABLES Table 1. Key bond lengths, and angles for Ag4Ni2(DMSA)4 crystal structure and theory. (See Figure 2 for atom designations)

Experiment Bond length (Å) Ag1-Ag2 2.942 Ag1-Ag4 2.98 Ag3-Ag2 3.016 Ag3-Ag4 3.005 Ni1-Ag1 3.155 Ni1-Ag2 3.19 Ni1-Ag3 3.153 Ni1-Ag4 3.198 Ni2-Ag1 3.111 Ni2-Ag2 3.239 Ni2-Ag3 3.119 Ni2-Ag4 3.169 Bond angle (°) S2-Ni1-S8 177.32 S3-Ni1-S6 178.967 S1-Ni2-S7 179.509 S4-Ni2-S5 176.675

Theory Bond length (Å) Ag1-Ag2 3.03 Ag1-Ag4 3.06 Ag3-Ag2 3.08 Ag3-Ag4 3.06 Ni1-Ag1 3.17 Ni1-Ag2 3.21 Ni1-Ag3 3.26 Ni1-Ag4 3.16 Ni2-Ag1 3.15 Ni2-Ag2 3.27 Ni2-Ag3 3.16 Ni2-Ag4 3.26 Bond angle (°) S2-Ni1-S8 172.1 S3-Ni1-S6 178.3 S1-Ni2-S7 172.4 S4-Ni2-S5 178.6



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