[Ag67(SPhMe2)32(PPh3)8]3+: Synthesis, Total Structure, and Optical Properties of a Large Box-Shaped Silver Nanocluster

: Engineering the surface ligands of metal nanoparticles is critical in designing unique arrangements of metal atoms. Here, we report the synthesis and total structure determination of a large box-shaped Ag 67 nanocluster (NC) protected by a mixed shell of thiolate (2,4-dimethylbenzenethiolate, SPhMe 2 ) and phosphine (triphenylphosphine, PPh 3 ) ligands. Single crystal X-ray diffraction (SCXRD) and electrospray ionization mass spectrometry (ESI-MS) revealed the cluster formula to be [Ag 67 (SPhMe 2 ) 32 (PPh 3 ) 8 ] 3+ . The crystal structure shows an Ag 23 metal core covered by a layer of Ag 44 S 32 P 8 arranged in the shape of a box. The Ag 23 core was formed through an unprecedented centered cuboctahedron i.e., Ag 13 , unlike the common centered Ag 13 icosahedron geometry. Two types of ligand motifs, eight AgS 3 P and eight bridging thiols were found to stabilize the whole cluster. The optical spectrum of this NC displayed highly structured multiple absorption peaks. The electronic structure and optical spectrum of Ag 67 were computed using time-dependent density functional theory (DFT) both for the full cluster [Ag 67 (SPhMe 2 ) 32 (PPh 3 ) 8 ] 3+ and a reduced model [Ag 67 (SH) 32 (PH 3 ) 8 ] 3+ . The lowest metal-to-metal transitions in the range of 500 to 800 nm could be explained by considering the reduced model that shows almost identical electronic states to 32 free electrons in a jellium box. The successful synthesis of the large box-shaped Ag 67 NC facilitated by the combined use of phosphine and thiol paves the way for synthesizing other metal clusters with unprecedented shapes by judicious choice of thiols and phosphines.


INTRODUCTION
Noble metal nanoparticles (NPs), especially ones made of gold and silver, have recently garnered significant attention because of the potential applications of their optical and physicochemical properties, [1][2][3] which led to their use in catalysis, 4,5 bioimaging and biosensing, [6][7][8][9] and light energy conversion 10,11 .Unlike plasmonic NPs (average diameter >2 nm), ultrasmall NPs (typically <2 nm) containing a precise number of atoms completely lose their plasmonic features and display molecular properties 12 due to manifestation of discrete energy levels.Such atomically precise metal NPs, where every single atom influences the overall properties, are often protected by a definite number of ligand molecules.These new functional nanomaterials are called nanoclusters (NCs) or nanomolecules.
Among noble metals, a multitude of gold NCs are reported, including Au25, [13][14][15][16] Au38, 17,18 Au67, 19 Au102, 20,21 and Au144, [22][23][24] and many of them are completely characterized through single crystal X-ray diffraction (SCXRD).In contrast, the exploration of silver NCs is still lagging, mainly due to their instability, resulting in only a few examples of fully characterized silver NCs.][27][28][29][30][31][32][33][34] A key challenge remaining for silver NCs is obtaining highly anisotropic shapes.In gold, NCs of anisotropic shapes were reported through solely thiols 35,36 and co-ligands of phosphines and thiols 37,38 .Such a strategy of co-ligands in sliver has failed to produce analogous results.Zheng and co-workers demonstrated that the use of a phosphine co-ligand, in addition to thiol, could shrink the silver NC size, 39 while the same pure thiol yielded a cluster of increased size 28 under similar reaction conditions.As an example, the ligand, HSPhF2, was shown to produce Ag44 NCs 28 when used alone; however, when used in conjunction with a phosphine co-ligand, a smaller Ag14 NC was found to be the final product 39 .On the other hand, Bakr et al. 33 showed that the use of phosphine does not change the size of the NC, but has a stabilizing effect on the resulting Ag29 NC, wherein phosphine was found to increase the cluster's yield and its stability, and aided in the growth of good-quality single crystals of Ag29 NC for its structure determination.
In this work, we demonstrate that the use of a phosphine coligand with controlled synthetic parameters plays an important role in determining the shape and size of the NC, where we observed -in contrast to previous reports-an increase of the NC size that results in a unique anisotropic shape.Particularly, we synthesized and fully characterized a novel NC of [Ag67(SPhMe2)32(PPh3)8] 3+ using a one-pot synthetic strategy, in which both thiol and phosphine co-ligands were judiciously chosen to produce this large stable NC of a box-like shape.While the syntheses of Ag NCs with similar or larger sizes have been reported, 40,41 they were plagued by instability issues precluding their structure elucidation and precise molecular formula determination (e.g.restricting the estimation of NCs' formulae to matrix-assisted laser desorption ionization mass spectrometry).Thus the [Ag67(SPhMe2)32(PPh3)8] 3+ cluster is particularly significant, as it represents the largest known non-superatom Ag NC to-date; its synthesis opens new avenues of tailoring and elucidating the properties of Ag NCs.Unlike most NCs, theoretical modeling of the Ag67 structure reveals a free electron count of n = 32 and a lack of an electronically closed shell as a superatom would, which is a result of the cluster's non-spherical shape.Furthermore, the electronic structure is identifiable using a reduced atomistic model [Ag67(SH)32(PH3)8] 3+ , which shows delocalized metal-core states akin to 32 electrons in a box-shaped jellium.We demonstrate that our reduced atomistic model yields a good description of the lowest of transitions in the linear optical absorption spectrum.

Synthesis, purification, and crystallization
Synthesis of [Ag67(SPhMe2)32(PPh3)8] 3+ NCs involves the reaction of sliver nitrate with HSPhMe2 in a MeOH/DCM solvent mixture (see the experimental section for details and Scheme 1 in the Supporting Information (SI)).Later on, triphenylphosphine (PPh3) was introduced into the reaction mixture under vigorous stirring to form Ag-thiolate-phosphine complexes.Subsequently, these complexes were reduced using NaBH4 to produce Ag NCs.After completion of the reaction, the NC product was washed with MeOH.The UV-vis absorption spectrum of the as-prepared or raw product showed a novel spectrum (Figure S1A) with optical features completely different from those of Ag25 and its doped clusters, 26,42,43 despite the use of same thiol.The photoluminescence (PL) spectrum of the same raw product showed a weak emission at ~820 nm (Figure S2).Interestingly, electrospray ionization mass spectrometry (ESI-MS) of this raw product, and also the singly-washed sample, revealed both [Ag25(SPhMe2)18] - and [Ag67(SPhMe2)32(PPh3)8] 3+ NCs in negative and positive-ion modes, respectively (Figure S3A and B).However, we found that the room-temperature aging of the singly-washed raw product (i.e., the product after the first MeOH-wash) changes color from reddish brown to greenish brown (insets of Figure S1).This greenish brown product was extracted with MeOH, cleaned and redissolved in DCM, and referred to as the 'purified' or the 'final' product (inset of Figure 1).The UV-vis spectrum of this solution consists of strong and well-defined multiple absorption bands in the visible range (Figure 1), which is strikingly different from that of the as-prepared cluster product (Figure S1A).The convergence and sharpening of the features in the UV-vis spectrum of the final product are indicative of an improvement in the monodispersity of the NCs.The ESI-MS of the purified product showed the absence of Ag25 (Figure S3C), while only a predominant peak at m/z ~4571.87corresponding to [Ag67(SPhMe2)32(PPh3)8] 3+ cluster was observed (Figure 2).The absence of Ag25 after aging is attributed to its decomposition or conversion to Ag67 as Ag25 is unstable under ambient conditions for more than a day.The sole charge-state 3+ of Ag67 NCs was confirmed from a characteristic peak separation of m/z 0.33.The molecular formula of [Ag67(SPhMe2)32(PPh3)8] 3+ NCs was validated by matching the experimental isotopic pattern with their simulated spectrum (inset of Figure 2), consistent with SCXRD (vide infra).It is worth noting that the number of phosphine (PPh3) ligands in the mass spectrum of Ag67 NCs is sensitive to instrumental parameters (see the SI for details).Under soft conditions, the peak for [Ag67(SPhMe2)32(PPh3)8] 3+ was predominant, while at moderately harsh conditions, cluster peaks with sequentially-lost PPh3 were more noticeable (Figure S3).Consequently, a series of nine peaks of [Ag67(SPhMe2)32(PPh3)x] 3+ could be observed, corresponding to x = 0-8, where the difference (m/z ~87) between every two adjacent peaks was one PPh3 (Figure S3).The loss of PPh3 ligands during the electrospray ionization is likely due to the weak bonding of the phosphine ligands to the Ag67 NCs, as it was also noted in other Ag and doped clusters.To further confirm the purity and monodispersity of the final Ag67 NCs, analytical ultracentrifugation (AUC) was performed.The AUC results show that Ag67 displays a high degree of monodispersity in DCM as seen by the 3D representation of sedimentation (s) and diffusion (D) coefficient distributions (Figure S4).Also, the near vertical plot of the van Holde-Weichet (vHW) analysis is a distinguishing feature of monodisperse systems (Figure S5).Polydisperse systems usually show a slanting non-vertical vHW profile of the sedimentation coefficient distributions.Ag67 has a standard sedimentation coefficient of 3.0S (or 3.0 x 10 -13 s), within the range observed for other silver NCs. 30,31 31  and 1 H nuclear magnetic resonance (NMR) spectroscopy was used to probe the chemical composition and the ligand environments of Ag67 NCs.The 1 H NMR (Figure S6) shows different peaks at the aromatic and aliphatic regions.The broad peaks in the aromatic region between 6.8-7.3 ppm are associated with the aromatic protons of SPhMe2 ligands, while the three peaks between 7.3-7.6ppm are corresponding to the aromatic protons of PPh3.In agreement with published reports, 45 the spectra show different peaks between 0.75-2.5 ppm related to the methyl group (CH3) of SPhMe2 (Figure S6B).Different chemical shifts of CH3 confirmed different chemical environments of various methyl groups within the [Ag67(SPhMe2)32(PPh3)8] 3+ NC. Figure S7 shows the 31 P NMR peaks of the clusters in comparison to 31 P NMR peak of free PPh3.A broad peak of the cluster at 8.1 ppm is due to the phosphine bound to the cluster, while a sharp peak at 27.4 ppm could be due to free triphenylphosphine oxide (PPh3O), probably resulted from the oxidation of free PPh3 (if anything left in the solution during washing steps).
Clearly, the aging, or size-focusing step, is essential to concentrate the final product of Ag67 as it allows the separation and precipitation of unstable species (Ag25) spontaneously.The selective formation of Ag67 as the final product over Ag25 or other size clusters may be due to its thermodynamic and geometric stability (vide infra), which was gained by the unique combination of Ag, PPh3 and HSPhMe2.The final Ag67 NCs was found to be compatible with MeOH and 1-propanol in addition to DCM, as its UV-vis features were appeared consistently in these solvents (Figure S8A).Furthermore, the Ag67 was found to be stable at 5 °C in DCM for at least five days (Figure S8B).However, unlike Ag25, 26 the purified Ag67 NCs were found not to luminesce (Figure S2).The emission peak of the as-prepared product was most likely due to the presence of Ag25.This fact was further confirmed by PL measurements of a mixture of pure Ag25 and Ag67, where the emission peak appeared at the position of as-prepared Ag67 product and also pure Ag25 (Figure S2).The yield of the final Ag67 NCs was ~25% (based on Ag atom content).

X-ray Crystal Structure
Single crystal X-ray diffraction (SCXRD) measurements were applied to single crystals that were grown at 4 °C by layering hexane onto a DCM solution of freshly purified Ag67 NCs.The crystal data analysis shows that the overall composition of NCs is Ag67(SPhMe2)32(PPh3)8, which is consistent with the ESI-MS formula.The Ag67 NC crystallizes into an orthorhombic crystal system (space group Ccc2), see the SI for details of the crystal data and the refinement.
The total structure of Ag67 is shown in Figure S9.The packing structure clearly reveals the location of two [BPh4] -ions per cluster, while it was difficult to localize the third counterion, which is most probably disordered in the cavities.However, the sole 3+ charge-state is evident from ESI-MS and DFT (vide infra).The total structure of Ag67 without hydrogens and counterions is shown in Figure 3A.For clarity, all the carbon atoms are omitted in Figure 3B, which shows a box-shape of the Ag67S32P8.The dissection of Ag67S32P8 was conducted based on the connectivity of metal atoms to ligands (both thiols and phosphines).The metal structure that is not connected to ligands is shown in Figure 3C.It consists of 23 Ag atoms in a box-shape.The remaining Ag44(SPhMe2)32(PPh3)8 framework (Figure 3D) acts as a closed metal-ligand protecting layer that encapsulates the Ag23 core.
The formation of the Ag23 core is illustrated in Figure S10.Ag23 consists of a centered Ag13 cuboctahedrona core shape seldom observed in thiol-and phosphine-protected silver NCs (Figure S10A).Usually in Ag NCs, centered 26,33 and non-centered icosahedra 28,29 are the archetypal core-shapes.Two opposite square faces of Ag13 are shared (Figure S10B) with two Ag8 crowns to form an Ag21 unit as in Figure S10C.Finally, the two open crown positions are closed by two silver atoms to form the Ag23 structure as in Figure S10D (the slight rotation of the structure of Figure S10D gives the Figure S10E).This crowning of the Ag13 cuboctahedron leads to the growth of Ag67 cluster in a box-shape.The average Ag-Ag distance from the central atom to cuboctahedron atoms and between cuboctahedron atoms of Ag13 are 2.860 and 2.860 Å, respectively, comparable to the Ag-Ag bond length 2.88 Å in bulk silver, indicating the strong interaction among the Ag13 core atoms.Notably, the Agcuboctahedron-Agcrown and Agcuboctahedron-Agcrown-center distances are 2.806 and 2.910 Å, respectively.The large bond distance of Agcuboctahedron-Agcrown-center and the resultant relatively less interactions among these Ag atoms inclines the growth of cluster towards a boxshape.The formation of the Ag44(SPhMe2)32(PPh3)8 metal-ligand encapsulating layer starts from a closed Ag36 cuboid-like box (Figure S11A).The structure of Ag36 has eight planar hexagons, ten planar squares and four non-planar hexagons.The Ag-Ag bond distance range in this Ag36 metal layer is 2.846-3.048Å, with an average of 2.9195 Å.Note that four non-planar hexagons are located at the central part of Ag36.Out of six Ag atoms of a non-planar hexagon, four Ag atoms are in one plane, away from Ag23 core, while the remaining two Ag atoms are in a different plane, towards the Ag23 core.Each sulfur atom (at the central part of the Ag36) of a thiol connects to three Ag atoms of a non-planar hexagon (Figure S11B).In total, two bridging thiols 46,47 are present per every non-planar hexagon.Therefore, four non-planar hexagons have eight bridging sulfurs to form the structure of Ag36S8 (Figure S11B).In addition to this simple bridging thiol motif-type, there is another type of ligand motif, AgS3P.As mentioned earlier that there are eight planar hexagons.Each sulfur of the AgS3P motif bridges the two adjacent Ag atoms of a planar hexagon.In other words, one AgS3P motif is completely bound to single planar hexagon.Therefore, eight AgS3P motifs cover the eight planar hexagons to form a metalligand layer structure of Ag44S32P8 (Figure S11C).Encapsulation of Ag23 core by Ag44S32P8 layer gives the total structure of Ag67S32P8 (Figure 3B).After encapsulation of Ag23 core in Ag44S32P8 layer, all the four crown Ag atoms (top Ag atoms) of a crown of Ag23 core are positioned at the centers of four planar hexagons of Ag36.The crown-closing Ag atom of the same crown is connected to a planar square face at one extreme of the Ag36 cuboid-like box.Similarly, the remainder five Ag atoms of other bottom crown are positioned at the other extreme of the Ag36 box and the Ag13 cuboctahedron is positioned at the center of Ag36 box.
Alternatively, the Ag44S32P8 metal-ligand layer can also be viewed as follows.There are two Ag16 units (Figure S12A), each look like a bowl.Each bowl has four planar hexagons and a planar square as the base.These two bowls are joined (from open side) by four one-dimensional S-Ag-S (average Ag-S distance = 2.619 Å) motifs (Figure S12B) to form Ag36S8 (Figure S12C).This Ag36S8 is exactly the same as in Figure S11B.Subsequently, eight three-dimensional AgS3P motifs cover eight planar hexagons to produce Ag44S32P8 (Figure S12E).This Ag44S32P8 scaffolding accommodates the Ag23 core as described before to form the total structure of Ag67S32P8 (Figure S12F,G).

Theoretical analysis of the electronic structure
The electronic structure of the [Ag67(SPhMe2)32(PPh3)8] 3+ cluster was analyzed by using the experimental crystal structure and also by building a reduced model cluster [Ag67(SH)32(PH3)8] 3+ .In that model, positions of Ag, S, and P atoms were fixed to experimental crystal coordinates (as shown in Figure 3B) and S-H and P-H bonds were relaxed to equilibrium.The electronic structure was solved by using the density functional theory (DFT) implemented in the real-space code GPAW. 48The electronic exchange-correlation interaction was described by the PBE functional. 49e found that the [Ag67(SPhMe2)32(PPh3)8] 3+ cluster has an appreciable energy gap, about 0.36 eV, between the highest occupied (HOMO) and lowest unoccupied (LUMO) molecular orbitals, rendering the cluster electronically stable (the reduced model cluster has a similar gap of 0.43 eV).Visual inspection of several of the silver-based Kohn-Sham molecular orbitals of the [Ag67(SH)32(PH3)8] 3+ model in the region of the HOMO-LUMO gap showed remarkable similarity to box-like quantum states in the jellium model, where each state could be labeled with (nx, ny, nz) triplets giving the number of nodes in x-, y-, and z-directions, respectively.By employing the widely-used rule 50 to count the "free electrons" (Ag(5s) electrons that are not needed for stabilization of the ligand layer) in the metal core, one gets 67-32-3 = 32 free electrons.In fact, state symmetries and their energy order in [Ag67(SH)32(PH3)8] 3+ were identical to states of 32 electrons in a jellium box, where the background charge was set to +12 to mimic the approximate total of Bader charges of all silver atoms in the [Ag67(SH)32(PH3)8] 3+ model cluster (the details of this analysis is a subject of a follow-up theoretical paper).We calculated the optical absorption spectrum of both the [Ag67(SPhMe2)32(PPh3)8] 3+ cluster and the [Ag67(SH)32(PH3)8] 3+ model cluster by using DFT in the linear-response formalism as implemented in GPAW. 51The PBE functional was used for calculating the oscillator matrix.Figure 4 shows that both calculations reproduce the shape of the lower-energy part of the calculated spectrum very well with the experimental data, albeit a slight blue-shift of about 87 nm for [Ag67(SH)32(PH3)8] 3+ and 95 nm red-shift of the full cluster [Ag67(SPhMe2)32(PPh3)8] 3+ .This red-shift could be expected to arise from the chosen PBE functional.The observed blue-shift for [Ag67(SH)32(PH3)8] 3+ model is caused by the lack of the true ligand layer around the metal core, thus resulting in a slightly modified (compressed) electron density in the core that shifts all the transitions to blue.Analysis of the lowest metal-to-metal transitions for the [Ag67(SH)32(PH3)8] 3+ model showed that they arise from contributions from Kohn-Sham states where the total number of nodes (nx + ny + nz) changed by 1, thus confirming the dipole selection rule in a quantum box.

CONCLUSION
In conclusion, we report the synthesis and total structure of a new silver NC of [Ag67(SPhMe2)32(PPh3)8] 3+ using SCXRD, DFT, ESI-MS, and NMR.The combined use of thiol and phosphine ligands in our study demonstrates that silver clusters can be made in stable non-spherical box-like shapes, in sharp contrast to previously reported Ag44 and related "superatoms" that have an approximate spherical core.The optical absorption spectrum of Ag67 NC shows distinct optical features, and the theoretical calculations reveal that the lowest optical transitions can be understood by invoking delocalized electronic states in a quantum box.This achievement will push the limit of synthesizing large silver NCs with high-stability and previously unknown shapes providing a new degree of freedom in tailoring their properties.

Synthesis and purification of [Ag67(SPhMe2)32(PPh3)8] 3+
NCs.In a glass vial, 40 mg of AgNO3 was completely dissolved in 5 mL of MeOH by sonication.Then, 10 mL of DCM that containing 40 µL of HSPhMe2 was added to the reaction vial.The reaction mixture was vigorously stirred inside a fume hood at room temperature (25 °C) that produced a turbid yellow mixture.After 20 min. of stirring, 200 mg of PPh3 dissolved in 2 mL of DCM was immediately added to the reaction solution.The turbid yellow mixture turned to a clear colorless solution.After that, 500 µL of an aqueous solution of NaBH4 (42 mg, 1.11 mmol of NaBH4 dissolved in 2 mL H2O) was added drop-wise to the solution producing a dark solution, which was allowed to react at 1200 rpm for 2 h.
The dark as-prepared solution was first centrifuged at 8000 rpm for 5 min.to get rid of any insoluble byproducts.The solution was then evaporated to decrease the solution volume to 10 mL, and then MeOH was added to precipitate the NCs followed by centrifugation (8000 rpm/ 5 min.).The obtained precipitate was dissolved in 5 mL of DCM, while the supernatant was discarded.After first wash, i.e., the sample now in DCM was aged overnight at 25 °C under ambient conditions.During the aging time, the sample color slowly started changing from reddish brown to greenish brown/olive.In the next day, the sample was dried and the precipitate was extracted with MeOH, and then centrifuged (8000 rpm for 5 min.)to eliminate insoluble species.The solution of NCs in MeOH was again aged at 25 °C for few hours such that during the time of aging, the sample showed orange/pink junk precipitates which can be removed by centrifugation (8000 rpm for 5 min.).Using a rotary evaporator, the sample was dried.The purified NCs were dissolved in ~2 mL of DCM.The final product of Ag67 NCs solution was allowed to react with NaBPh4 (which had an equal molar of AgNO3) to further stabilize the clusters, [Ag67(SPhMe2)32(PPh3)8] 3+ .This stabilized final product was stored in a tightly capped vial inside the fridge (4 °C) to obtain single crystals (see below).

Crystallization.
A very concentrated DCM solution (~8-10 mg/mL) of Ag67 NCs was taken from the original purified solution into a small clean NMR tube after filtration using a syringe filter (0.22 µm of pore size).Hexane was layered on the top of DCM solution of NCs and then

Figure 1 .
Figure 1.UV-vis absorption spectrum of the purified Ag67 NCs in DCM.Inset is a photograph of the purified final product of Ag67 NCs.
Figure 2. Positive mode ESI-MS of purified [Ag67(SPhMe2)32(PPh3)8] 3+ NC in DCM.The inset shows a matching of isotope patterns between the experimental and simulated spectra.