Computational Methods

The VASP-DFT package was used, with a plane-wave basis, kinetic energy cutoff of 400 eV, PAW pseudopotentials,³¹ and the PW91 generalized gradient approximation (GGA) for exchange-correlation .^32,33 In structural optimizations, convergence was achieved for forces <0.001 eV/Å. Calculations were performed for the X-ray determined Ag44(SC6H5)30-4 structure ( hydrogens were added and their positions were relaxed, average d(C-H) = 1.09 Å).

2.3 References and Notes

1. Heiz, U., Landman, U. Nanocatalysis. Springer-Verlag, Berlin, 2007.

2. Anker, J.N. et al., Biosensing with plasmonic nanosensors. Nat. Mater. 2008, 7 442-453.

3. Jin, R. et al., Controlling anisotropic nanoparticle growth through plasmon excitation.

Nature. 2003, 425, 487-490.

4. Maier, S.A. et al., Plasmonics - A route to nanoscale optical devices. Adv. Mat. 2001, 13 1501-1505.

5. Noginov, M.A. et al., Demonstration of a spaser-based nanolaser. Nature. 2009, 460 1110-1113.

6. Atwater, H.A. & Polman, A., Plasmonics for improved photovoltaic devices. Nat.

Mater. 2010, 9, 205-213.

7. Arvizo, R.R. et al., Intrinsic therapeutic applications of noble metal nanoparticles:

past, present and future. Chem. Soc. Rev. 2012, 41 2943-2970.

8. Jadzinsky, P.D., Calero, G., Ackerson, C.J., Bushnell, D.A., & Kornberg, R.D., Structure of a thiol monolayer-protected gold nanoparticle at 1.1 Å resolution.

Science. 2007, 318, 430-433.

9. Heaven, M.W., Dass, A., White, P.S., Holt, K.M., & Murray, R.W., Crystal structure of the gold nanoparticle [N(C8H17)4][Au25(SCH2CH2Ph)18]. J. Am. Chem. Soc. 2008, 130 (12), 3754-3755.

10. Zhu, M., Aikens, C.M., Hollander, F.J., Schatz, G.C., & Jin, R., Correlating the crystal structure of a thiol-protected Au25 cluster and optical properties. J. Am. Chem.

Soc. 2008, 130 (18), 5883–5885.

11. Qian, H., Eckenhoff, W.T., Zhu, Y., Pintauer, T., & Jin, R., Total structure

determination of thiolate-protected Au38 nanoparticles. J. Am. Chem. Soc. 2010, 132, 8280–8281.

12. Zeng, C. et al., Total structure of the golden nanocrystal Au36(SR)24. Angew. Chem.

Int. Ed. 2012, 51 (52), 13114-13118.

13. Williams, R., The Geometrical Foundation of Natural Structure. Dover, New York, 1979.

14. Kratchmer, W., Lamb, L.D., Fostiropoulos, K., & Huffman, D.R., Solid C60: a new form of carbon. Nature. 1990, 318, 354-358.

15. Ebbesen, T.W. & Ajayan, P.M., Large-scale synthesis of carbon nanotubes. Nature.

1992, 358, 220-222.

16. Bethune, D.S. et al., Cobalt-catalysed growth of carbon nano-tubes with single-atomic-layer walls. Nature. 1993, 363 605-607.

17. Murray, C.B., Norris, D.J., & Bawendi, M.G., Synthesis and characterization of nearly monodisperse CdE (E = S, Se, Te) semiconductor nanocrystallites. J. Am.

Chem. Soc. 1993, 113, 8706-8715.

18. Brust, M., Walker, M., Bethell, D., Schiffrin, D.J., & Whyman, R., Synthesis of thiol-derivatised gold nanoparticles in a two-phase liquid-siquid System. J. Chem. Soc., Chem. Commun. 1994, 801-802.

19. Bakr, O.M. et al., Silver nanoparticles with broad multiband linear optical absorption.

Angew. Chem. Int. Ed. 2009, 48, 5921-5926.

20. Harkness, K.M. et al., Ag44(SR)304−: a silver–thiolate superatom complex. Nanoscale.

2012, 4, 4269-4274.

21. Shichibu, Y. et al., Extremely High stability of glutathione-Protected Au25 clusters against core etching. Small. 2007, 3 835-839.

22. Cathcart, N. & Kitaev, V., Silver nanoclusters: single-stage scaleable synthesis of mono-disperse species and their chiro-optical properties. J. Phys. Chem. C. 2010, 114 16010-16017.

23. Dharmaratne, A.C., Krick, T., & Dass, A., Nanocluster size evolution studied by mass spectrometry in room temperature Au25(SR)18 synthesis. J. Am. Chem. Soc. 2009, 131 13604-13605.

24. Jana, N.R., Gearheart, L., & Murphy, C.J., Seeding growth for size control of 5−40 nm diameter gold nanoparticles. Langmuir. 2001, 17 6782-6786.

25. Charaborty, I. et al., The superstable 25-kDa monolayer protected silver nanoparticle:

measurements and interpretation as an icosahedral Ag152(SCH2CH2Ph)60 cluster.

Nano Lett. 2012, 12, 5861-5866.

26. Dance, I.G., The structural chemistry of metal thiolate complexes. Polyhedron. 1986, 5, 1037-1104.

27. Herron, N., Calabrese, J.C., Farneth, W.E., & Wang, Y., Crystal structure and optical properties of Cd32S14(SC6H5)36•DMF4, a sluster with a 15 Å CdS core. Science. 1993, 259, 1426-1428.

28. Yoon, B. et al., Size-dependent structural evolution and chemical reactivity of gold clusters. Chem. Phys. Chem. 2007, 8, 157-161.

29. Knight, W.D. et al., Electronic shell structure and abundances of sodium clusters.

Phys.Rev. Lett. 1984, 52 (5), 2141-2143.

30. Yannouleas, C., Landman, U. Shell-correction and orbital-free density-functional methods for finite systems. In recent progress in orbital-free density functional theory, Wesolowski, T.A., Wang, Y.A., eds., World Scientific: Singapore, 2013 p.

203-250.

31. Kresse, G., Joubert, D., From ultrasoft pseudopotentials to the projector augmented-wave method. Phys. Rev. B. 1999, 59 1758-1775.

32. Perdew, J.P., Unified theory of exchange and correlation beyond the Local density approximation. In electronic structure of solids, P. Ziesche, P., Eschrig, H., ed., Akademie Verlag, Berlin, 1991, p. 11-20.

33. Perdew, J.P. et al., Atoms, molecules, solids, and surfaces: Applications of the

generalized gradient approximation for exchange and correlation. Phys. Rev. B. 1992, 46, 6671-6687.; Erratum: Phys. Rev. B. 1993, 48, 4978-4978.

Chapter 3

M4Ag44p-MBA30 Molecular Nanoparticles

3.1 Introduction

Michael Faraday first produced colloidal gold nanoparticles in the mid 1800s.

Remarkably, they are still stable today.¹ Their stability could be attributed partly to the nobility of gold and partly to the charge carried by the particles, which suppressed particle-particle collisions and subsequent aggregation. In general, however, metal nanoparticles do not share the same longevity of the Faraday gold colloids. Large colloidal silver nanoparticles may benefit from the same charge stabilization mechanism common to colloids, but like most metals they are susceptible to chemical oxidation and therefore have a much shorter shelf life than colloidal gold nanoparticles.

Among small ligand-passivated nanoparticles, the so-called “magic-number” gold^2,3 and silver nanoparticles⁴ are expected to be exceptionally stable since they are molecules (i.e. discrete formulae and structures) and not random combinations of metal atoms and ligands⁵. This field has its origin in gas phase “magic-number” clusters whose stability was attributed to molecule-like closed electronic shells.⁶ Molecular nanoparticles with noble metal cores are still far less stable than their colloidal counterparts, however, in part

due to their ability to convert to other sizes and structures in addition to metal-thiolates.^7,8 Since molecular silver nanoparticles are also susceptible to chemical oxidation, they are also less stable than molecular gold nanoparticles.

Figure 3-1: M4Ag44(p-MBA)30 product. M4Ag44(p-MBA)30 nanoparticles shown in   solid and in solution forms. The dice represent the four Platonic solids that can be found in the structure. (photo credit: Daniel Miller)

It was recently discovered⁹ that M4Ag44(p-MBA)30 molecular nanoparticles are even more stable than molecular gold nanoparticles. This surprising stability has been attributed to a closed electronic shell with a large HOMO-LUMO gap, an ultrastable 32-silver-atom excavated-dodecahedral core, coordinating solvent molecules, and protective Ag2S5 capping structures that have been named “mounts”, a term taken from the decorative arts. M4Ag44(p-MBA)30 belongs to a family of Ag44L30 compounds,^10-14 where L is a phenyl-containing thiolate ligand, which suggests that the phenyl rings may play a key role in the stability of the cluster.

The exceptional stability of M4Ag44(p-MBA)30 has several important consequences.

First, M4Ag44(p-MBA)30 has been shown to be inert under reaction conditions therefore the product contains only a single species with no need for separations. This is presumably a consequence of the product being much more energetically favoured than any other competing size. Second, the selectivity of the reaction for M4Ag44(p-MBA)30

results in quantitative yields. Third, due to the aforementioned points the reaction is easily scaled to efficiently produce very large quantities of pure M4Ag44(p-MBA)30

product, as shown in Figure 3-1.

The synthesis and properties of M4Ag44(p-MBA)30 molecular nanoparticles have been studied in detail and are the subject of this report. Important issues such as the role of the phenyl group and the coordinating solvent are also addressed along with general stability and decay of the particles.

3.2  Experimental    

3.2.1  Chemicals  

  Sodium   borohydride,   ethanol,   methanol,   toluene,   N,N-­‐dimethylformamide   (DMF),   dimethyl sulfoxide (DMSO), citric acid, acetic acid, sodium chloride, sodium hydroxide, and cesium hydroxide were purchased from Fisher. Silver nitrate, 4-mercaptobenzoic acid (p-MBA), and ammonium acetate were purchased from Sigma-Aldrich, cetyl trimethylammonium bromide (CTAB) was purchased from Alfa Aesar. All reagents were used as received, without further purification. De-ionized water (18.2 MΩ cm) was used.

3.2.2 Synthesis

Briefly, 0.75 mmol of AgNO3 and 1 mmol of p-MBA were added to 33 ml of 7:4 water-ethanol solvent and stirred for about 5 min followed by ~2 min of ultrasonication.

These reagents reacted to form a cloudy light yellow precipitate, which was the Ag-thiolate precursor. The pH was then adjusted to 12 using 50% w/v aqueous CsOH. The precursor dissolved when the pH was above 9, forming a clear, light yellow solution.

Next, 7.5 mmol of NaBH4 reducing agent in 9 ml or water was added dropwise over a period of 30 min and was left to stir for 1 hour. Once the reaction was complete, the dark red solution was centrifuged for 5 min and then separated from the insoluble byproducts by decanting. The product was then precipitated using DMF and then separated from the supernatant by centrifugation and decanting. It is important not to dry this raw product.

The raw product was an alkali metal salt of the final product since some of the carboxylates on the p-MBA ligands had sodium or cesium counterions in place of their protons. This material was protonated in DMF using an acid (e.g. acetic acid, citric acid, ascorbic acid, sulfuric acid) to displace the alkali metal cations. To protonate, pure DMF was added to the precipitated particles, which did not initially solubilize. Glacial acetic acid was then added dropwise to the solution until the precipitate dissolved into the DMF, forming a deep red solution. Protonation was repeated 3 times, precipitating with toluene each time. Once fully protonated, the final product contained only 4 alkali metal cations, which served as the counterions to the -4 charge on the metal core.

The final product dissolved readily into DMF without the aid of an acid. Further, it was possible to form toluene solutions of the final product, after complete protonation, although with lower concentrations than with DMF. More concentrated toluene solutions

could be made using small amounts of DMF cosolvent. To improve shelf life of the final product, samples were stored as damp sticky solids rather than completely dry and free-flowing powders since a coordinating solvent is needed to protect the nanoparticles.

3.2.3 Electrospray Ionization Mass Spectrometry (ESI-MS)

A Synapt HDMS quadrupole-time-of-flight ion mobility mass spectrometer (Waters Corp.) was used for the collection of all mass spectrometry data. (Anal Chem 2012, 84, p5304-5308) The instrument was equipped with a nanospray source with negative ionization and operated in V-mode using continuous-flow fused silica emitters (made in house). Instrumental parameters were: capillary voltage, 2.0−4.0 kV; sampling cone, 15 V; extraction cone, 4.0 V; cone gas flow rate, 0 L/h; trap collision energy, 0.5 V; transfer collision energy, 1.0 V; source temperature, 40 °C; desolvation temperature, 120 °C.

External calibration was performed in the range 100 ≤ m/z ≤ 4000 using cesium iodide.

Approximately 600 scans were averaged and processed using Masslynx 4.1 software (Waters Corp.). Samples were protonated using citric acid and dissolved in DMF, with a concentration of approximately 0.5 mg/mL.

3.2.4 Optical measurements

Solutions were prepared in neat DMF or DMSO solvents. Optical absorption spectra were recorded on a Perkin Elmer Lambda 950 spectrophotometer. The molar absorptivity was measured by preparing a solution with a known mass of damp solid and then determining the silver metal fraction of that mass by thermal gravimetric analysis. This was performed with an SDT Q600 thermogravimetric analyzer from TA Instruments.

Thermograms were obtained with ~22 mg of sample in a Pt pan under a nitrogen atmosphere by ramping the temperature at 2 °C/min from room temperature to 1000 °C.

For the spectral evolution during the synthesis, a dilute reaction mixture (1/8 conc.) was made and the entire amount of NaBH4 was added in ~30 s. As the reaction proceeded, 0.5 ml aliquots were taken, centrifuged, and placed in a 1 mm path length cuvette to record the absorption spectra.

3.2.5 NMR

Fully protonated M4Ag44(p-MBA)30 nanoparticles were cleaned of residual acetic acid for use in NMR experiments. To clean the nanoparticles, they were redissolved in neat DMF, precipitated with toluene, and then washed with toluene to remove the residual DMF. The clean precipitate was dried under flowing nitrogen for approximately six hours to remove the toluene. A DMF-d7 solution was prepared from the dried nanoparticles with a concentration of 5 mg/mL. The 1H-NMR spectra were collected on a Bruker Avance III 600 MHz spectrometer.

3.2.6 Stability measurements

A DMSO solution of protonated M4Ag44(p-MBA)30 nanoparticles was prepared with a concentration of 2.0 mg/ml. A 10 ml aliquot was stored in a 15 ml sample vial on the lab bench top. For each measurement, a 0.5 ml aliquot was pipetted out, and centrifuged at 5000 rcf for 5 min to remove precipitates before recording the absorption spectra in a 1 mm path length cuvette.

For oxygen-free stability experiments, a stock solution was prepared in DMSO such that the absorbance at 490 nm was 1.5 for a 1 mm path length cuvette. To this

solution, 0.5 mM NaCl and 0.25 mM ammonium acetate were added. The solution was then purged with N2 gas. The solution was split into 12 equal parts (1.5 ml each), placed in capped vials, and was purged again with N2 gas before storage. Spectra for each time point were recorded using freshly unsealed solutions, which were centrifuged at 5000 rcf for 1 min before recording the absorption spectra in a 1 mm path length cuvette.

3.3 Results and Discussion

3.3.1 Synthesis

The strategy for synthesizing M4Ag44(p-MBA)30 nanoparticles involves the reduction of a silver-thiolate precursor in semi-aqueous solution in the presence of a coordinating solvent, which helps protect the clusters. The silver-thiolate precursor forms upon combination of silver ions and p-MBA, with p-MBA in excess, precipitating at neutral pH. Increasing the pH with CsOH solubilizes the silver thiolate to create a more homogeneous starting material, although this is not required to produce M4Ag44 (p-MBA)30 nanoparticles. It is required, however, to raise the pH to ~12 when using water/alcohol solvents in order to deprotonate the alcohol. Note that if the reaction is carried out in an alcoholic solution at a pH that is much lower than 12, reduction with NaBH4 will be vigorous (e.g. vigorous bubbling is observed), which could result in the formation of a black precipitate rather than the red cluster product. If the pH is higher than 12, however, the reaction will require a larger amount of NaBH4. While other alkali metal hydroxides can also be used, CsOH was found to produce a higher yield. Both Na and Cs ions are present during the reaction, however, therefore the formula of the fully

protonated final product is written as M4Ag44(p-MBA)30 to represent a mixture of alkali counterions. When the reaction is done in solutions containing only Na ions, the final product is Na4Ag44(p-MBA)30.

The identity of the precursor remains to be shown, however the two most likely possibilities are a silver-thiolate polymer and a silver-thiolate dimer complex, analogous to silver benzoate.[ref] The solid precursor dissolves at a pH of 9, which is consistent with the dimer complex since it correlates with the deprotonation of the thiol group (pKa 6.8) rather than the carboxylate (pKa 5.8).¹⁵ Further, when the carboxylate is replaced with an amine (4-mercaptoaniline) to ensure the formation of the polymer, the precursor does not dissolve as the pH is raised to 12. This indicates that the thiol is intact in the precursor and that the carboxylate complexes with the silver ions.

Figure 3-2: M4Ag44(p-MBA)30 absorbance. Molar absorptivity of M4Ag44(p-MBA)30

nanoparticles in DMF solution. Inset shows the absorbance as a function of energy.

Whether the silver thiolate is a solid or a solution, its reduction produces the same deep red solutions of M4Ag44(p-MBA)30 nanoparticles, as shown in Figure 3-1. The color

and characteristic optical absorption spectrum (see Figure 3-2) can be used to evaluate the success of the reaction since the absorbance provides a measure of the yield and the highly-structured spectra are indicative of the purity of the product, in that it contains only a single species. The M4Ag44(SR)30 product is unique in this sense, as no processing or size sorting is required to get a precisely single-sized molecular product.

The M4Ag44(p-MBA)30 nanoparticles are synthesized in the presence of alkali metal ions therefore the raw product is the conjugate base of the final product, where some of the carboxylate groups have alkali metal counterions. Alkali metals in excess of the four stoichiometric counterions have been identified by elemental analysis, but the increase in pH upon dissolution in semi-aqueous solvents also demonstrates the basic nature of the raw product. The conjugate base nanoparticles dissolve readily in polar solvents and are not soluble in nonpolar solvents. The alkali metal counterions on the carboxylates can be easily displaced by protons using acids to produce the fully protonated final product. Once protonated, the clusters dissolve readily in a variety of polar aprotic solvents such as DMF, DMSO, and pyridine as well as toluene when a small amount of coordinating solvent is added. The fully protonated nanoparticle contains four stoichiometric cations to balance the -4 charge of the metal core. This was shown by elemental analysis and by high-resolution mass spectrometry of the final product, wherein all protons were precisely accounted for.⁹

Figure 3-3: ESI-MS of M4Ag44(p-MBA)30. (A) Raw product, containing excess alkali   metal counterions. (B) Fully protonated final product.

Protonation also improves the stability of the -4 ion in the gas phase. Figure 3-3(a) shows the ESI mass spectrum of Ag44(p-MBA)30−4 in DMF for the raw product. In this case, the parent peak at m/z 2335 can be easily detected however its intensity is 5 and 6 times lower than the daughter peaks Ag43(p-MBA)28−3 at m/z 2975 and Ag44(p-MBA)30−3

at m/z 3115, respectively. Note that the complementary Ag43(p-MBA)28−3 and AgL2−

fragments were previously shown to result from the spontaneous fragmentation of desolvated Ag44(p-MBA)30−4 ions in the gas phase⁹.

Figure 3-3(b) shows that the ESI-MS is significantly improved after protonation with citric acid (3 times). The relative abundance of the Ag44(p-MBA)30−4 parent species dramatically increased, such that it became the highest intensity nanocluster peak. The increase in the ratio of Ag44(p-MBA)30−4 relative to the Ag43(p-MBA)28−3 fragment indicates that protonation had a stabilizing effect on Ag44(p-MBA)30−4. The overall reduction in fragmentation also indicates that protonation significantly improves the gas

phase stability of Ag44(p-MBA)30−4 ions.

The -4 charge on the metal core is required to complete the filling of the 1D orbitals such that the nanoparticle has a closed electronic shell.⁹ The distribution of charge can be most accurately visualized as the orbital shapes previously calculated.⁹

3.3.2 Coordinating solvent

The role of the coordinating solvent was studied by synthesizing the nanoparticles in semi-aqueous solutions with a variety of coordinating cosolvents, using a 7:4 ratio of water to cosolvent. The yields of these reactions corresponded to the coordinating ability of the solvent, in the order pyridine > DMSO > DMF > EtO- > acetate > acetone. All reactions produced the same M4Ag44(p-MBA)30 product regardless of the coordinating solvents used. The spectra of the reaction products differed only as a result of environmental effects. Although using neat coordinating solvents would seem to be desirable, semi-aqueous solvents are generally used due to solubility requirements of the reagents as well as the poor activity of non-aqueous sodium borohydride.

Although empirical evidence clearly demonstrates the importance of coordinating solvents, the details of their role in ensuring the stability of this silver molecule remain unclear. The crystal structure of Na4Ag44(p-MBA)30 can begin to provide insight into the role of the coordinating solvent molecules. Normally ligand shells on noble metal nanoparticles are considered to be well-solvated and isotropic coatings that saturate and completely protect the metal cores. This is not the case for Na4Ag44(p-MBA)30. Instead, ligand bundling has been observed,^16,17 which leads to an uneven protective coating around the Ag nanoparticle in the solid state, as shown in Figure 3-4.

Figure 3-4: M4Ag44(p-MBA)30 crystal structure. Space-filling view of the structure down a 3-fold axis. Ligand bundling creates gaps that leave the Ag atoms in the mounts (blue) exposed to chemical attack. Color scheme: grey – C; orange – O; yellow – S; blue – Ag in mounts; green – Ag in decahedral outer core shell.

Since the molecular structure shown in Figure 3-4 was determined by x-ray diffraction from the solid-state material, it is not necessarily representative of the solution-phase structure. For example, the observed ligand bundling could be a consequence of intramolecular hydrogen-bonds within the crystal,¹⁷ in which case the bundling would not be present in solution and the ligands would be expected to be solvated and isotropic. If bundling were a consequence of pi-interactions between the ligands, however, then bundling could be present in solution. This would have implications with regard to the

Since the molecular structure shown in Figure 3-4 was determined by x-ray diffraction from the solid-state material, it is not necessarily representative of the solution-phase structure. For example, the observed ligand bundling could be a consequence of intramolecular hydrogen-bonds within the crystal,¹⁷ in which case the bundling would not be present in solution and the ligands would be expected to be solvated and isotropic. If bundling were a consequence of pi-interactions between the ligands, however, then bundling could be present in solution. This would have implications with regard to the