Alkyne‐Monofunctionalized Gold Nanoparticles as Massive Molecular Building Blocks

Using a tripodal, thioether-based ligand comprising a remote protected acetylene, we efficiently stabilize small gold nanoparticles (O ≈ 1.2 nm) which are isolated and purified by chromatography. The 1:1 ligand-to-particle ratio is obtained by comparing the particles' dimensions measured by transmission electron microscopy with the weight fraction of the coating ligand determined by thermogravimetric analysis. The single ligand coating of the gold particle guarantees the presence of a single masked alkyne per particle. It can be addressed by wet chemical protocols providing the particles with the properties of “massive molecules”. The “massive molecule” nature of the particles is demonstrated by involving them in wet chemical coupling protocols like oxidative acetylene coupling providing gold nanoparticle dimers (34 % isolated yield) or alkyne-azide “click”-chemistry with a suitable triazide, giving trimeric particle architectures (30 % determined by transmission electron microscopy). The particle stabilization obtained by the coating ligand allows, for the first time, to treat these multi-particle architectures by size exclusion chromatography.

ligand place-exchange reactions on solid support [33,34] or in solution, followed by separation by electrophoresis, [35] on-Au NP surface polymerization of suitable ligands [36] or exchange of the entire ligand shell with carefully tuned thiol mixtures on citratestabilized Au NPs. [37] Recently, the scope of sulfur-containing ligands for Au NPs has been enhanced towards more complex benzylic thioether-based ligands. The reversible coordinative binding of the thioether to the Au NP surface and the flexible adaptive structure of the multidentate coating ligand holds the promise of efficient and size-selective Au NP passivation. Indeed, oligomeric, [38][39][40][41][42][43] tripodal, [44][45][46] dendrimeric, [46][47][48][49][50] and even cage-like [51] thioether-based ligands yielded Au NPs with, in some cases, narrow size-distributions, good yields and remarkable stability features. We reported earlier about the favorable Au NP stabilization features of a family of ligands consisting of a central tetraphenylmethane subunit decorated threefold with multidentate oligomeric chains. [52] Particularly appealing were these ligands with tetraphenylmethane-based benzylic thioether oligomers, as in these cases, a single ligand turned out to be able to stabilize an entire Au NP. Interestingly, as only three of the four phenyl units of the central tetraphenylmethane are decorated with an oligomer, the molecular design leaves the fourth phenyl ring upright-standing on the coated Au NP as ideal site for further chemical functionalization. [53][54][55] Here, we report the successful functionalization of the remaining free phenyl ring by an oligo(phenylene-ethynylene) (OPE) rod exposing a triisopropylsilyl(TIPS)-masked alkyne group. As sketched in Scheme 1, the ligand 1 exposing the OPE rod as a functional group keeps the ability to stabilize an entire Au NP, resulting in monofunctionalized Au NPs which readily undergo chemical reactions in a single position. The combination of a reasonably stable Au NP with a reactive functional group makes these particles addressable by wet chemistry and they can be considered as "massive molecules". The chemical accessibility of the exposed alkyne was demonstrated by standard coupling reactions like oxidative acetylene coupling [39,41,48] resulting in "dumbbell"-type dimers and alkyneazide "click" chemistry [56,57] providing Au NP trimers thanks to a suitable triazide linker. Interestingly, the here reported ligand shell stabilizes the Au NP to an extent enabling to expose these small nano-architectures consisting of interlinked Au NPs to size-exclusion chromatography (SEC), which was not the case for previously reported ligand designs. [39,41,48,49]

Ligand Synthesis
The synthesis of tripodal ligand 1 is shown in Scheme 1. The three benzylic bromides of precursor 2 were substituted with a moderate excess of thiol 3 (1.23 equivalents per bromide), using sodium hydride (NaH) as a base in freshly distilled tetrahydrofuran (THF). After aqueous work-up, the ligand was isolated in 55 % yield by silica plug filtration and automated gel permeation chromatography (GPC, in chloroform).
The synthesis of the central linking unit was performed in six steps with an overall yield of 54 % (Scheme 2). Precursor 4 was obtained via Sandmeyer-type reaction applied to 4-tritylaniline in good 80 % yield. Subsequent treatment with bromine gave the tribromide 5 quantitatively. Profiting from the improved reactivity of the iodine substituent in palladium-catalyzed coupling reactions, [58] the OPE-rod exposing the TIPS-protected alkyne was introduced via Sonogashira cross-coupling reaction performed at room temperature, providing the OPErod 6 exposing three terminal para-bromophenyls. Threefold lithium-halogen exchange was achieved with tert-butyl-lithium in -78°C cold THF, and quenching with N,N-dimethylformamide (DMF) gave the trialdehyde 7 in excellent 88 % yield after aqueous work-up and column chromatography. The three aldehydes were quantitatively reduced to benzylic alcohols in 8 with so-dium borohydride. Finally, in a threefold Appel-reaction, the benzylic alcohols were converted into the benzylic bromides of 2 quantitatively.
The assembly of the teraphenylmethane building block 3 exposing a benzylic thiol was achieved following a previously published protocol. [42] All new compounds were characterized by 1 H-and 13 C-NMR spectroscopy and high-resolution mass spectrometry.

Au NP Synthesis
The Au NPs stabilized by ligand 1 were synthesized following a previously published protocol [38] based on a variation of the Au NP synthesis proposed by Brust et al. [6] In the aqueous phase of a biphasic system, one equivalent of gold salt (HAuCl 4 ) for each sulfur atom in the ligand, i.e. 6 equivalents, was added.
The transfer of the gold salt from the aqueous to the organic phase was achieved by addition of tetra-n-butyl ammonium bromide (TOAB, 2 equivalents with respect to HAuCl 4 , i.e. 12 equivalents) to the organic phase. The ligand was added dissolved in CH 2 Cl 2 . Nucleation of the Au NPs was induced via reduction by aqueous NaBH 4 (8 equivalents with respect to HAuCl 4 , i.e. 48 equivalents). Upon addition of the reducing agent, an immediate color change of the organic phase from bright orange to opaque dark brown was observed. After rigorous stirring for 15 min, the organic phase was separated and concentrated in a N 2 stream to prevent possible thermal aggregation of NPs. Precipitation of the Au NPs from the concentrated organic solution was achieved by addition of ethanol. Repeated centrifugation and subsequent SEC provided pure samples of the Au NPs without remaining traces of ligand molecules or TOAB. The colloidal gold particles stabilized by ligand 1 are labeled as Au-1-TIPS and were isolated in excellent yields above 95 %. Au-1-TIPS was the only detectable form of gold throughout the synthesis and the mass losses were due to the handling during the purification procedures.

Au NP Analysis and Characterization
The Au NPs were characterized by variable temperature UV/ Vis absorption spectroscopy, transmission electron microscopy (TEM), and thermogravimetric analysis (TGA). Finally, the "molecule-like" behavior of Au-1-TIPS was demonstrated by engaging them in wet chemistry coupling protocols producing small nano-architectures like dimers and trimers which could be detected by TEM.
The UV/Vis absorption spectrum of Au-1-TIPS (Figure 1c) displays discrete bands between 300-350 nm, characteristic for the absorbance of the OPE subunit of the ligand 1. Apart from these features, the continuous increase towards shorter wavelength without a surface plasmon band points at Au NPs with diameters below 2 nm. Only with Au NPs of dimensions below 2 nm, the rate of surface scattering exceeds the rate of bulk scattering and the electron-donating character of the coordinating sulfur atoms of the coating ligand increases the surface electron density significantly, reflected in a broadening of the surface plasmon band. [59] It is noteworthy that very similar optical behavior was reported for the parent ligand structure without the exposed functional group. [52] The OPE subunit of 1 acts as an optical label and the very comparable absorption of the OPE subunit of Au-1-TIPS compared with the free ligand 1 not only proves the presence of the ligand on the Au NP, but also supports the hypothetical picture of an exposed and dissolved OPE rod which does not interfere with the Au NP surface. The dimensions of the Au NPs were further analyzed by TEM (Fig-ure 1a-b) and the recorded diameters of 1.21 ± 0.36 nm confirmed the initial claims derived from the UV/Vis experiments. The Au NP size distribution was determined with TEM micrographs comprising more than 6000 NPs, using the threshold and particle analysis tools from ImageJ. [60] TGA of Au-1-TIPS revealed a mass loss of 19.0 % between 200 and 600°C which was attributed to the combustion of the coating organic ligand 1. Assuming a spherical shape, the Au NP diameter of 1.21 nm determined by TEM allowed to determine the Au NP's volume (0.93 nm 3 ). With the density of gold (19.32 g cm -3 ), the mass of an average Au NP was determined to be 1.79 × 10 -20 g, and, by multiplication with Avogadro′s number (N A : 6.022 × 10 23 mol -1 ), the molecular weight of the average uncoated Au NP was determined to be 10792 g mol -1 . The molecular weight of the ligand 1 with the formula C 176 H 188 S 6 Si is 2523.89 g mol -1 . The molecular weight of the proposed particle Au-1-TIPS coated by a single ligand is, with 13316 g mol -1 , the sum of both. The weight contribution of the coating ligand 1 fits, with 18.95 %, perfectly the weight loss recorded during TGA (19.0 %), corroborating the claim that a single ligand 1 is coating and stabilizing an entire Au NP. The same issue is examined in Table 1 displaying that the same number of gold atoms per particle is obtained from the TGA and the TEM analysis.
The Au NPs Au-1-TIPS coated with a single ligand exposing an OPE rod can be regarded as "massive molecules" with a peripheral TIPS-protected alkyne as functional group. To explore the stability of this "massive molecule" and thus also the scope of potential reaction conditions, samples of Au-1-TIPS were dispersed in ortho-xylene and gradually heated while their optical features were monitored by UV/Vis spectroscopy. Applying a heating gradient of 10°C per hour, the UV/Vis spectra of the sample remained unchanged up to 105°C (Figure 1d). Above this temperature, the samples turned red and the emergence of a plasmon band in the UV/Vis was observed. Upon extended exposure to temperatures above 105°C, decoloration of the sample due to precipitation was observed. The thermal decomposition of Au-1-TIPS above 105°C is interpreted as agglomeration to larger particles, either due to too violent collisions for the delicate steric protection provided by coating 1 or due to prior thermal detachment of 1. It is noteworthy that a comparable decomposition temperature was observed for the parent ligand structure (110°C), [52] documenting further the separation between the coating ligand subunit of 1 and the attached OPE rod. Table 1. Ligand-to-gold ratio of Au-1-TIPS. The number of gold atoms of an average particle is calculated from the TGA data (top) and from the dimension  After the determination of the thermal stability of Au-1-TIPS, its potential as precursor in wet-chemical reactions as building block for nano-architectures was explored.

Au-1-TIPS as Precursor in Coupling Reactions I: Dimerization
The analytical data of Au-1-TIPS displayed a Au NP coated by a single ligand and, consequently, also only one OPE rod per Au NP. Furthermore, the comparison of the UV/Vis spectra of Au-1-TIPS and the free ligand 1 suggested an entirely dissolved and thus wet-chemically addressable OPE subunit exposed at the Au NP surface. The OPE rod exposes a TIPS-protected terminal alkyne as masked but reactive functional group which can be involved e.g. in various coupling protocols. To demonstrate the engagement of Au-1-TIPS as "massive molecule" in coupling chemistry, the TIPS-masked alkyne was deprotected profiting from the fluoride ions of tetra-n-butylammoniumfluoride (TBAF) in CH 2 Cl 2 (Scheme 3). After aqueous work-up, the deprotected Au NPs were exposed to typical Glaser-Hay oxidative acetylene coupling conditions. [61,62] They were re-dissolved in CH 2 Cl 2 and Cu (I) ions were added together with N,N,N′,N′-tetramethylethane-1,2-diamine (TMEDA) as ligand dissolving the copper ion. The reaction was vigorously stirred at ambient air to guarantee the presence of dissolved oxygen. To avoid aggregation of the coated gold particles, the reaction was kept for 4 hours at room temperature. After aqueous work-up and subsequent repetitive separation by SEC using CH 2 Cl 2 and toluene as eluent, Au NP dimers [(Au-1) 2 ] were isolated in 34 % yield. It is noteworthy that these dimeric Au NP architectures based on the ligand 1 were the first larger systems resisting decomposition on the SEC column, documenting further the superior stability of Au NP coated by the ligand 1. All our earlier ligand designs based on benzylic thioethers survived SEC conditions as monomers, but the resulting interlinked NP architectures disintegrated when exposed to the shear forces of SEC. [39,41,48] The yield of (Au-1) 2 was also determined by analyzing TEM micrographs of the crude reaction mixture. The reaction mixture was diluted to an extent avoiding the agglomeration of Au NPs on the TEM grid. 53 micrographs (each 340 nm × 260 nm) comprising a total of 2077 Au NPs were analyzed, covering 4.69 μm 2 of the TEM grid. Pairs of Au NPs with a spacing up to 3.2 nm were considered as (Au-1) 2 . With 826 of 2077 NPs being part of a dimer corresponding to a yield of 39.8 %, the TEM analysis confirmed the yield isolated by SEC.
Additional structural features of (Au-1) 2 were extracted from TEM micrographs (Figure 2a). The size-distribution of the Au NPs after the coupling reaction (Figure 2b) resembles, with 1.18 ± 0.29 nm, within the limits of the method, the one recorded for the parent sample Au-1-TIPS. This suggests that, during the entire deprotection-homocoupling protocol, the Au NPs remain stable and protected by the coating ligand, corroborating the "massive molecule" behavior of Au-1-TIPS in the reaction sequence.
TEM micrographs of the isolated, "dumbbell-like" Au NP dimers ( Figure 2a) enabled to analyze the inter-NP distances in more details. Interestingly, the distribution of the NP spacing clearly displayed two maxima (Figure 2c). The majority of about 80 % of the recorded Au-NP distances was, with 2.62 ± 0.21 nm, in the dimensions of the diethynyl-linker expected as product of the oxidative acetylene coupling, as the distance for the fully stretched linker estimated by simple MM2 simulation was 2.8 nm (distance between both tertiary methane carbon atoms of the rod-terminating tetraphenylmethane subunits, see Scheme 3a). In principle, any shorter distance might be attributed to the projection of the Au NPs of a "dumbbell" dimer not lying flat on the TEM grid. For such an explanation, one would, however, rather expect a continuous tailing to shorter distances and not a discrete maximum. Thus, in similarity to "dumbbell" architectures based on dendrimer-coated Au-NPs, [48] the shorter inter-NP distance of 1.64 ± 0.20 nm (recorded for about 20 %) most likely points at direct coordinative contacts between an Au NP and the terminal acetylene exposed by the OPE rod of the deprotected Au-1. [63] Such a coordination between alkyne and Au NP requires access to the Au NP surface, showing that the ligand 1 is not covering the entire surface of the stabilized Au NP in Au-1. Interestingly, trimeric structures e.g. consisting of "dumbbell"-type dimers formed by oxidative acetylene coupling with one of its terminal Au NPs engaged in a coupling with the terminal acetylene of a third deprotected Au-1 were, with only 4 observed cases, too rare to exclude random arrangements on the TEM grid. The Au NP dimers with short spacing are of comparable stability to the longer ones and do also not decompose during the SEC process. Also, we were not able to distinguish both structures during the SEC experiment, which was not surprising, considering the working hypothesis that both structures exclusively differ in the interlinking of two Au-1-type subunits.

Au-1-TIPS as Precursor in Coupling Reactions II: Trimerization
The lack of trimers observed in the homocoupling protocol discussed above raised the question if there could be an intrinsic limitation disqualifying trimerization. As alternative approach to couple three Au NPs to a trimeric nano-architecture, alkyneazide "click"-chemistry has already been applied successfully for dendrimer-stabilized Au NPs. [49] Thus, a sample of Au-1-TIPS was deprotected and the liberated alkynes were exposed to a "click" protocol with the triazide 1,3,5-tris[p-(azidomethyl)phenyl]benzene (9 in Scheme 3) as threefold central coupling unit. To the deprotected Au-1 dissolved in CH 2 Cl 2 , the triazide 9 was added and, while vigorously stirring, aqueous solutions of copper sulfate and sodium ascorbate as reducing agent were added (Scheme 3b). To avoid aggregation of the coated gold particles, the reaction was kept for 4 hours at room tempera- ture. After extraction with CH 2 Cl 2 , the composition of the crude reaction mixture was analyzed by TEM. 42 micrographs (each 340 × 260 nm) comprising a total of 1587 Au NPs were analyzed, covering 3.71 μm 2 of the TEM grid. Groups of 3 NPs with a maximal separation smaller than 4.5 nm were considered as the trimeric (Au-1) 3 9. The slightly larger accepted radius compared with the homocoupled (Au-1) 2 is due to both the larger dimensions and the increased flexibility of the particle-interlinking organic structure. From the 1587 NPs, 477 NPs or 30.1 % were engaged in a (Au-1) 3 9 structure, 324 NPs or 20.4 % were observed as dimers of Au NPs, and 786 NPs or 49.5 % were recorded as monomers. Whether the dimers are formed due to only twofold "click"-chemistry-type coupling to 9, or to homocoupled (Au-1) 2 , cannot be distinguished in the TEM micrographs.
Also for the "click"-chemistry protocol, a very comparable size-distribution (Figure 3b, 1.28 ± 0.30 nm) of the Au NPs after the coupling reaction was observed, pointing at the integrity of Au-1 during the applied reaction conditions. In analogy to the homocoupled (Au-1) 2 , also (Au-1) 3 9 survived SEC conditions. However, while fractions comprising (Au-1) 3 9 and the di-meric Au NP architectures could be separated from the monomeric Au NPs, the method failed to isolate pure (Au-1) 3 9 fractions.
TEM micrographs of (Au-1) 3 9 (Figure 3a) enabled the analyses of the interparticle distances (Figure 3c). The rather broad and unspecific distance distribution reflects the increased structural variety in the arrangement of the three Au NP in (Au-1) 3 9, due to the flexibility of the interlinking scaffold comprising triazole subunits attached at benzylic positions.
While the spatial grouping of Au NPs by TEM analyses of the reaction mixtures support for both coupling reactions the formation of the desired oligo-NP nano-architectures, the method does not comprise any further chemical information. As example, an interesting chemical issue was the rather moderate yield of the desired oligo-NP objects with 39.8 % for (Au-1) 2 and 30.1 % for (Au-1) 3 9 respectively. Whether these moderate yields reflect the lack of reactivity of the "massive molecule" Au-1 in the coupling reaction or, in the step before, an incomplete deprotection of Au-1-TIPS cannot be distinguished by the here reported studies.

Conclusion
In summary, a new ligand for the efficient and controlled synthesis of monofunctionalized Au NPs combining the massive nature of the particles with the wet chemical addressability of a molecule and thus behaving as "massive molecules" with superior stability properties is reported. The ligand design is based on a central tetraphenylmethane subunit unifying three oligomeric benzyl sulfide chains forming the Au NP-stabilizing subunit and exposing a masked alkyne on an OPE rod attached to the fourth phenyl ring of the central unit. Au NPs with narrow size distribution around 1.2 nm coated by a single ligand are obtained efficiently and display very good stability features, withstanding decomposition up to 105°C in dispersion. The "massive molecule"-like behavior is displayed by deprotecting the terminal alkyne of the coated Au NP and engaging it in coupling protocols providing interlinked dimeric and trimeric Au NP systems as displayed by TEM analyses. With the here reported coating ligand, the particles of these nano-architectures are stable enough to resist the shear forces during purification by SEC.
We are currently investigating the catalytic activity of these ligand-coated Au NPs and explore their scope as labels in proteins and even larger biological systems.

Materials
All commercially available starting materials were of reagent grade and used as received, unless stated differently. Absolute THF was purchased from Acros, stored over 4 Å molecular sieves, pre-dried with CaH, handled under argon and freshly distilled from sodium before each use. Pure CH 2 Cl 2 was purchased from J. T. Baker. Pure toluene was purchased from Acros. Pure DMF was purchased from Acros Organics. tert-butylmethyl ether (TBME), c-hexane, ethyl acetate (EtOAc), and CH 2 Cl 2 from Biosolve were used for purification and were of technical grade. Column chromatography was carried out on SiliaFlash P60 (particle size 40-63 μm) from SiliCycle. SEC for the purification of Au NPs mono-, di-and trimers was performed manually using Bio-Rad Bio-Beads S-X1 (operating range 600-14 000 g mol -1 ) with CH 2 Cl 2 or toluene as eluent. Deuterated solvents were purchased from Cambridge Isotope Laboratories.

EurJIC
European Journal of Inorganic Chemistry stants (J) are given in Hertz (Hz), the multiplicities are denoted as: s (singlet), d (duplet), t (triplet), m (multiplet), and br (broad). Highresolution mass spectra (HRMS) were measured as HR-ESI-ToF-MS with a Maxis 4G instrument from Bruker or a Bruker UltraFlex II -MALDI-ToF-MS. For purification of the ligands, a (automated) Shimadzu Prominence System was used with SDV preparative columns from Polymer Standards Service (two Showdex columns in series, 20 mm × 600 mm each, exclusion limit: 30 000 g mol -1 ) with chloroform as eluent. UV/Vis measurements were recorded on a Jasco V-770 spectrophotometer using 117.100F-QS cuvettes from Hellma Analytics (10 mm light path). TEM was performed on a Philips CM100 TEM at 80 kV using copper grids (Cu-400HD) from Pacific Grid Tech. TGA was measured on a Mettler Toledo TGA/SDTA851 e with a heating rate of 10°C min -1 .
p-Iodophenyltriphenylmethane (4): [64] To a degassed 250 mL two-neck flask equipped with rubber septum and thermometer, BF 3 OEt 2 (3.67 mL, 29.0 mmol, 2 equiv.) was added and cooled to -10°C. In a separate flask, 4-tritylaniline (5.01 g, 14.5 mmol, 1 equiv.) was dissolved in 100 mL of degassed CH 2 Cl 2 and was added dropwise via transfer cannula. The mixture was stirred at -10°C for 45 minutes after which time tBuNO 2 (3.00 mL, 25.0 mmol, 1.75 equiv.) dissolved in 50 mL of degassed CH 2 Cl 2 was added dropwise via transfer cannula. The mixture was stirred at -10°C during 45 minutes. I 2 (4.78 g, 18.9 mmol, 1.3 equiv.) and KI (3.61 g, 21.8 mmol, 1.5 equiv.) were added to the mixture which was thereafter stirred vigorously during 15 h and allowed to slowly warm up to r.t. Excess halogen was quenched upon addition of saturated aqueous Na 2 S 2 O 3 . After phase separation, the aqueous phase was extracted twice more with CH 2 Cl 2 and dried with Na 2 SO 4 . After evaporation of the volatile in vacuo, compound 4 was purified by column chromatography (c-hexane) as a pale solid (5.2 g, 11.6 mmol, 80 %). 1  p-Iodophenyltri(p-bromophenyl)methane (5): [65] In a 250 mL flask, 4 (4.91 g, 11.0 mmol, 1 equiv.) was dissolved in Br 2 (20 mL, 385 mmol, 35 equiv.) and stirred at r.t. for 30 minutes. The reaction was diluted with CH 2 Cl 2 and crushed ice was added. The mixture was further cooled with an ice bath and saturated aqueous Na 2 S 2 O 3 was slowly added until all the excess bromine was quenched. The mixture was extracted twice with CH 2 Cl 2 , dried with Na 2 SO 4 , and the volatile evaporated in vacuo. The crude was subjected to silica plug filtration (CH 2 Cl 2 ) and recrystallized from hot c-hexane to yield 5 as a pale solid (7.51 g, 11.0 mmol, quant.). 1

p-[p-(Triisopropylsilylethynyl)phenylethynyl]phenyltris[p-(formyl)phenyl]methane (7):
In a dry, degassed 250 mL flask, 6 (2.10 g, 2.51 mmol, 1 equiv.) was dissolved in 50 mL of freshly distilled THF and cooled to -78°C. tBuLi (1.7 M in pentane, 20 mL, 34.1 mmol, 13.5 equiv.) was added to the mixture and stirred for 2 hours. DMF (5 mL, 65.3 mmol, 26 equiv.) was added to the mixture which was warmed up to r.t. over 2 hours and was thereafter stirred for 2 more hours. The reaction was cautiously quenched by addition of water, the organic phase separated, and the aqueous phase extracted twice with CH 2 Cl 2 . The combined organic phase was dried with Na 2 SO 4 , and the solvent evaporated. The purification was afforded by column chromatography (c-hexane 4:1 EtOAc) to yield 7 as a yellowish solid (1.51 g, 2.20 mmol, 88 %). 1

p-[p-(Triisopropylsilylethynyl)phenylethynyl]phenyltri(phydroxymethylphenyl)methane (8):
In a 100 mL flask, 7 (1.51 mg, 2.20 mmol, 1 equiv.) was dissolved in 20 mL of MeOH and 20 mL of THF and cooled to 0°C. NaBH 4 (749 mg, 19.8 mmol, 9 equiv.) was added to the solution portionwise over 1 hour. The mixture was allowed to stir for 1 more hour until the reaction was quenched upon careful addition of aqueous HCl (10 %). The mixture was diluted with CH 2 Cl 2 and transferred to a separation funnel. The aqueous phase was extracted three times with CH 2 Cl 2 , the combined organic phase dried with Na 2 SO 4 and the solvent removed in vacuo. The crude product was filtered through a silica plug (c-

EurJIC
European Journal of Inorganic Chemistry mixture was allowed to stir at room temperature for 15 hours after which time, the reaction was quenched upon addition of minimum amounts of water. The mixture was dried with Na 2 SO 4 , and the solvents evaporated to dryness. The crude was filtered through a silica plug (CH 2 Cl 2 ) and subjected to automated GPC (chloroform) to give an orange oil (44 mg, 0.018 mmol, 55 %). 1

Gold Nanoparticle Formation and Purification
Au NP syntheses were carried out on a 30-50 μmol scale with respect to the Au equivalents. Tetrachloroauric acid (6 equiv., where 6 is the number of sulfur atoms in the used ligand) was dissolved in 2.0 mL of deionized water. A solution of TOAB (12 equiv.) dissolved in 2.0 mL of CH 2 Cl 2 was added and the two-phase mixture was stirred for 15 minutes after which time, the aqueous phase had turned colorless and the bright orange color of the organic phase indicated complete phase transfer of the gold. The ligand (1 equiv.), dissolved in 2.5 mL of CH 2 Cl 2 , was added to the reaction mixture which was stirred for 15 minutes, allowing the thioether moieties and the gold to pre-organize their conformation. A freshly prepared solution of sodium borohydride (48 equiv.) in 2.0 mL of water was then added quickly to the reaction mixture, reducing the gold species and thereby causing an immediate color change to dark brown. After 15 min stirring, the resulting opaque organic phase was separated from the aqueous phase via Pasteur pipette and the aqueous phase was washed three times with CH 2 Cl 2 and separated in the same manner. The combined organic fractions were concentrated to a volume of approximately 0.5 mL in a Falcon tube by constant bubbling of protection gas to avoid possible thermal decomposition. 35 mL of ethanol was added in order to precipitate the particles which were then centrifuged three times at during 25 minutes at 5°C and 4000 rpm, discarding the supernatant after every centrifugation step. After redispersion in CH 2 Cl 2 , the Au NPs were separated from excess ligand via size-exclusion chromatography, and stored in the freezer dispersed in CH 2 Cl 2 at a concentration of 1 mg mL -1 .

Gold Nanoparticle Dimerization
The formation of Au NP dimers was done on a 2 mg scale with respect to Au-1. The acetylene-functionalized Au NPs (Au-1-TIPS) were dispersed in CH 2 Cl 2 (200 μL) and TBAF (1 M in THF, 50 μL) was added. The mixture was left stirring for 1 hour, quenched with water, extracted with dichloromethane and dried with Na 2 SO 4 . After filtration, the solvent was evaporated and the deprotected particles were redispersed in CH 2 Cl 2 (200 μL). TMEDA (50 μL) and copper(I) chloride (2 mg) were added. The dimerization reaction was left stirring for 4 hours and then quenched with a saturated aqueous solution of ammonium chloride, extracted with CH 2 Cl 2 and dried with Na 2 SO 4 . After filtration, the solution was concentrated and subjected to size-exclusion chromatography (once in CH 2 Cl 2 and twice in toluene). The obtained fractions were investigated by TEM on carbon-coated copper S5 grids in order to identify pure dimer fractions. 0.67 mg of dimers were obtained (34 %; with respect to Au-1-TIPS containing 54.7 Au atoms in average). Highly diluted solutions were used for deposition on the grids to avoid accidental proximity of non-linked NPs. Interparticle distances were measured manually from the TEM micrographs.

Gold Nanoparticle Trimerization
The Au NP "click" reactions were carried out with 3 mg of Au NPs (MW 13357 g/mol [one ligand 1 with 55 Au atoms]). The acetylenemonofunctionalized Au NPs were dispersed in dichloromethane (200 μL) and TBAF (1 M in THF, 50 μL) was added. The mixture was left stirring for 1 hour and quenched with water, extracted with dichloromethane and dried with Na 2 SO 4 . After filtration, the solution was concentrated by vacuum at 30°C. The dry, deprotected Au NPs were dissolved in 50 μL dichloromethane. Corresponding to the amount of azide moieties, 0.33 equiv. 1,3,5-tris[p-(azidomethyl)phenyl]benzene was dissolved in 50μL CH 2 Cl 2 and added to the solution. While intensely stirring, CuSO 4 (20 mol-% in respect to Au-1-TIPS dissolved in 20 μL water) and sodium ascorbate (30 mol-% in respect to Au-1-TIPS dissolved in 20 μL water) were added. After 4 h reaction time, the mixture was extracted with dichloromethane and dried with Na 2 SO 4 and dried in vacuo to yield a mixture of Au NP mono-, di-and trimers. The crude was subjected to four cycles of SEC with toluene as an eluent to remove unreacted monomers. Di-and trimers were not separated. Highly diluted solutions were used for deposition on the grids to avoid accidental proximity of non-linked NPs. Interparticle distances and di-and trimerization yields were measured manually from the TEM micrographs.