NHC-Stabilized Au10 Nanoclusters and Their Conversion to Au25 Nanoclusters

Herein, we describe the synthesis of a toroidal Au10 cluster stabilized by N-heterocyclic carbene and halide ligands via reduction of the corresponding NHC–Au–X complexes (X = Cl, Br, I). The significant effect of the halide ligands on the formation, stability, and further conversions of these clusters is presented. While solutions of the chloride derivatives of Au10 show no change even upon heating, the bromide derivative readily undergoes conversion to form a biicosahedral Au25 cluster at room temperature. For the iodide derivative, the formation of a significant amount of Au25 was observed even upon the reduction of NHC–Au–I. The isolated bromide derivative of the Au25 cluster displays a relatively high (ca. 15%) photoluminescence quantum yield, attributed to the high rigidity of the cluster, which is enforced by multiple CH−π interactions within the molecular structure. Density functional theory computations are used to characterize the electronic structure and optical absorption of the Au10 cluster. 13C-Labeling is employed to assist with characterization of the products and to observe their conversions by NMR spectroscopy.

Electronic density of states in the gas-phase model of [3b] + by using the PBE (top) and GLLB-SC (bottom) xc functional. 73 Table S10. Bader charges for gold atoms and ligands in the gas-phase model of [3b] + calculated using the GLLB-SC functional. 73 Figure S76.
Visualisation 1 General information

Instruments and sample preparation
1 H, 13 C{ 1 H} and all 2-D NMR spectra were recorded on Bruker AV400, AV600, AV700 or JEOL ECA-II 600 MHz spectrometers and referenced externally to SiMe4. All 1 H NMR spectra were referenced to CDCl3 at 7.26 ppm, CD2Cl2 at 5.32 ppm or CD3OD at 3.31 ppm; all 13 C NMR spectra were referenced to CDCl3 at 77.16 ppm, CD2Cl2 at 53.84 ppm or CD3OD at 49.00. All spectra were recorded at room temperature unless otherwise noted. All NMR data was processed and displayed using MestReNova software. In cases of residual solvent contamination of NMR solvents, these contaminants are noted on each spectrum. Other impurities are noted where assignment was possible. Elemental analyses were performed on a Thermo Scientific Flash 2000 CHNS-O analyzer. UV-vis absorbance spectra were recorded using an Agilent Cary 60 or Cary 3500 Multizone UV-vis spectrophotometers, or Shimadzu UV-1800 spectrometer using 1 cm 2 quartz cuvettes. UVvis absorbance spectra were recorded at ambient temperature unless otherwise specified. Monitoring reaction progress over time in situ (kinetics experiment) at controlled temperature was performed on Agilent Cary 3500 Multizone UV-vis spectrophotometer using Cary UV Workstation software. Time intervals for recurrent measurements of reaction mixture and temperature of the cuvette compartment are specified for each experiment of this kind. Alternatively, for ex situ measurements, small aliquots were taken from the reaction mixture at given reaction times, diluted with corresponding solvent at room temperature to obtain a 3.0 mL of solution, and their UV−vis absorbance spectra recorded. Excitation, emission, and fluorescence excitation-emission matrix (EEM) data were obtained using the Horiba Duetta Fluorescence and Absorbance Spectrometer, and inner-filter effects and Rayleigh scattering were corrected using EzSpecTM software. Samples for photoluminescence studies (sufficiently dilute to diminish inner-filter effects) were prepared in 1 cm 2 pathlength quartz fluorescence cuvettes with graded seal tube, sealed with rubber septa, and deoxygenated by purging with argon.

Mass spectrometry
Electrospray ionization mass spectra (ESI-MS) of small molecules were recorded at Queen's University using a Thermo Fisher Orbitrap Velos Pro mass spectrometer with a heated-electrospray ionization probe. ESI-MS of clusters were recorded L. J. Matthias, C. G. Overberger, J. Org. Chem., 1978, 43, 1978 S6 at The University of Tokyo using a JEOL JMS-T100LC time-of-flight mass spectrometer. Acetonitrile-dichloromethane (1:1 v/v) dispersions of samples (ca. 0.1 mg/mL) were directly infused at a flow rate of 3.0 mL/h by a syringe pump. Typical parameters used for the measurements were as follows: spray voltage 2.5 kV; ring electrode 45 V; orifice 1 voltage 40 V; orifice 2 voltage 0 V; desolvation temperature 120 °C; cone temperature 80 °C. The spectra were calibrated with respect to [Cs(CsI)n] + clusters generated from a 99% aqueous methanolic solution of CsI (0.5 mg/mL) under the same parameters, as an external reference. Additional ESI-MS of clusters was recorded at Nagoya University on a Bruker Compact QTOF mass spectrometer. Acetonitrile-dicholoromethane (1:1 v/v) dispersions of samples (ca. 0.1 mg/mL) were infused at a flow rate of 180 μL/h by a syringe pump. Parameters were as follows: end plate offset 500 V, capillary 3.6 kV, nebulizer 0.3 Bar, dry gas 3.0 L/min and dry temp 200 °C. The instrument was first calibrated with Agilent Technologies ESI Tuning Mix under the same parameters as an external reference. Theoretical isotope patterns were calculated using enviPat Web 2.4.

X-ray crystallography
Crystals of appropriate quality for X-ray diffraction studies were covered with a thin layer of hydrocarbon oil (Paratone-N). A suitable crystal was then selected, attached to a glass fibre or a microsampler, and quickly placed in a low-temperature stream of nitrogen. The data was corrected for Lorentz polarization, and absorption correction was performed analytically using CrysAlisPro software. The initial phases were determined by the direct method using SHELXS-2018, and the structures were refined using SHELXL-2018. All non-hydrogen atoms were refined anisotropically and hydrogen atoms were treated as riding models. All aromatic rings were treated using rigid constraints to minimize the number of refinement parameters. Since the cluster had a large solvent-accessible void in the lattice (ca. 33%: 26000 Å 3 ), solvent molecules and one of the counter anions could not be modeled due to the significant disorder. These disordered electron densities were treated by the SQUEEZE program on the PLATON platform. 6, 7 Moreover, the large void made the molecular packing loose. This weakened the reflections at higher resolution. Therefore, in order to maintain a reasonable ligand structure, the bond lengths and angles of the moiety were restrained using DFIX and DANG commands.

Photoluminescence quantum yield
Relative photoluminescence quantum yield (ΦF) was determined by the comparative method. 8 According to the method, the fluorescence quantum yield of a sample is related to that of a standard by the following expression: where F and Fstd are the areas under the fluorescence emission curves of the sample and the standard, respectively. A and Astd are the absorbances of the samples and standard at the excitation wavelengths, respectively. n and nstd are the refractive indices of solvents used for the sample and standard, respectively. Zinc phthalocyanine (ZnPc, Acros Organics) solution in freshly distilled pyridine was employed as the standard (ΦF(std) = 0.30). 9 The refractive indexes of the solvents were taken

S7
to be npyridine = 1.5095 and nDCM = 1.4242 (both at 20 °C). 10 All solutions were filtered using 0.2 μm PTFE syringe filters, diluted to desired A, placed into 1 cm 2 pathlength quartz fluorescence cuvettes with graded seal tube, sealed with rubber septa, and deoxygenated by purging with argon. In several independent measurements, sample and standard solutions were prepared to have equal absorbances at their respective excitation wavelengths after purging. UV-vis absorbance spectra were recorded on an Agilent Cary60 spectrophotometer with the corresponding pure solvents for background correction. Fluorescence emission spectra were recorded for the same solutions on a Horiba Duetta Fluorescence and Absorbance Spectrometer with EzSpecTM software. Both the sample and standard were excited at the same relevant wavelength (indicated for each particular dataset). The spectra were corrected for wavelength-dependent instrument sensitivity. The areas under the fluorescence emission curves were calculated numerically using Origin Pro 2016 software.

Computational methods
All the properties of [3b] + were calculated using density-functional theory (DFT) as in the implementation of the codepackage GPAW 11 (Grid-based projector-augmented wave method) using a real-space grid. All calculations included scalarrelativistic correlations. The Perdew-Burke-Ernzerhof (PBE) 12 and GLLB-SC 13 were used as exchange-correlationfunctionals. At first, the experimental crystal structure was relaxed with PBE-functional using 0.2 Å grid spacing and 0.05 eV/Å convergence criterion for the maximum forces acting on atoms in the cluster. The electronic structure and HOMO-LUMO gap were analysed for the relaxed structure with both PBE and GLLB-SC functionals. Related to the electronic structure, the Kohn-Sham wavefunctions were projected to spherical harmonic functions centered at the center of mass of the cluster inside a spherical region of 3.5 Å radius to reveal the superatomic nature. 14 Localization of the wavefunctions to Au-, Br-, C-and N-atoms was also investigated in a similar fashion but using the position of each atom as a central point in analysis. Cutoff radii in the latter analysis were selected to be 1.4 Å, 1.2 Å, 0.7 Å and 0.7 Å for Au, Br, C and N respectively. Due to the more realistic HOMO-LUMO gap, all further analyses were conducted only with the GLLB-SC functional. For these analyses grid spacing was set to 0.25 Å. Optical absorbance spectra were calculated using linear-response timedependent density functional theory lr-TDDFT, 15 for which the kernel was calculated with PBE whereas GLLBSC was used to describe the wave functions. Eventually, dipole transition contribution maps (DTCM) were probed to check the strongest electron-hole transitions of the individual absorbance peaks by using time-dependent density functional perturbation theory TD-DFPT. 16 DTCM reveals the increasing and decreasing contributions to the induced dipole moment at the selected excitation energy. In this study, DTCM is represented by taking summation over the contributions from three independent directions of the electric field. Directions were determined based on the principal axis of the moments of inertia of the cluster. Local atomic charges were calculated from all electron density using Bader charge analysis. 17 2 Synthetic procedures and purification protocols

Synthesis of 1,3-di(2,4,6-trimethylbenzyl)benzimidazolium chloride (1a)
Benzimidazole (0.591 g, 5.0 mmol), ground potassium hydroxide (0.421 g, 7.5 mmol) and acetonitrile (10 mL) were combined in a 50 mL pressure tube charged with a stir bar. The mixture was stirred and heated for 30 min, then 2,4,6trimethylbenzyl chloride (1.771 g, 10.50 mmol) was added in small portions. The pressure tube was sealed, and the reaction mixture was stirred and heated at 85 °C for 24 hours, then cooled to room temperature, and volatiles were removed by air flow resulting in a white to pale yellow coloured solid residue. Distilled water (8 mL) and DCM (10 mL) were added to the crude residue, and the pressure tube was sealed and shaken/stirred until all the solid had dissolved and the aqueous (turbid and colourless) and organic (pale yellow-orange) layers had separated. The mixture was transferred into a separation funnel, and the organic layer was separated. The aqueous layer was washed with additional portions of DCM (2 x 10 mL), and the organic layers were separated and combined, dried with anhydrous sodium sulfate and gravity-filtered. The volume of liquid was reduced in vacuo in a rotary evaporator until crystallization of the product occurred, and the yellowish mixture was layered with an excess of pentane and left to crystallize for 1-2 days at -4 °C. White crystals were collected by vacuum filtration, washed with pentane and dried in vacuo to yield the title compound as a white powder (1.887 g, 90 % yield

Synthesis of 1,3-di(2,4,6-trimethylbenzyl)benzimidazolium bromide (1b)
2,4,6-trimethylbenzyl bromide (2.240 g, 10.50 mmol), benzimidazole (0.591g, 5.0 mmol) and potassium carbonate (2.200 g, 15.90 mmol) were added to a 150 mL pressure tube with acetonitrile (50 mL). The mixture was heated at 85 °C for 20 hours, and then cooled to room temperature. After cooling, distilled water (50 mL) was added, and this mixture was decanted into a flask charged with DCM (100 mL). The organic layer was separated, and the aqueous layer washed with DCM (3 x 30 mL). The organic fractions were combined, dried with anhydrous sodium sulfate, and collected by gravity filtration. The volatiles were removed in vacuo, yielding an off-white crude solid, which was redissolved in DCM (10 mL 2.3 Synthesis of 1,3-di(2,4,6-trimethylbenzyl)-[2-13 C]-benzimidazolium bromide (1b*) 13 C-labeled benzimidazolium salt 1b* was synthesized in a similar manner as the corresponding non-labeled compound 1b using [2-13 C]benzimidazole; NMR spectroscopic data in CDCl3 was identical to the non-labeled compound 1b except for increased intensity of the carbene carbon signal as expected. Data collected in CD3OD is presented here for comparison. Note that the NCHN proton was not observed in CD3OD due to facile exchange with the solvent on the NMR timescale, and difficulties in shimming to this solvent led to broadening of the 1 H NMR spectrum.

Synthesis of 1,3-di(2,4,6-trimethylbenzyl)benzimidazolium-ylidene gold chloride (2a)
The title compound was prepared using modifications to established synthetic procedures for the preparation of NHC gold complexes published by Nolan et al.i Me2SAuCl (0.147 g, 0.5 mmol), 1a (0.210 g, 0.5 mmol), and potassium carbonate (0.207 g, 1.5 mmol) were combined with acetone (30 mL) in a 100 mL round-bottom flask fitted with a condenser, and charged with a stir bar. The solution was stirred and refluxed under an air atmosphere for 3-4 hours. After cooling, the pale yellow solution was filtered by gravity filtration, and volatiles were removed in vacuo. The resulting off-white or pale yellow solid was dissolved in DCM (3 mL) and passed through glass wool and a short silica gel plug in a Pasteur pipette or small column to give a colourless filtrate. The column with the yellow by-products adsorbed to the silica was washed with additional portions of DCM (2 x 3 mL), and filtrates were combined. The solvent from the filtrate was removed in vacuo to yield the title gold complex 2a as a white powder (0.273 g, 89 %

Isolation of [Au10( MesCH2 Bimy)6Cl3]Cl ([3a]Cl) from reduction of 2a
A round bottom flask (100 mL) was charged with a stir bar and 2a (0.0738 g, 0.12 mmol), and DCM (40 mL) was added to dissolve the gold complex. To the stirring solution of gold complex was added a thoroughly sonicated solution of NaBH4 (0.00454 g, 0.12 mmol) in absolute EtOH (12 mL
[3a]Cl (0.015 g, 0.0034 mmol) was combined with an excess of NH4PF6 (0.120 g, 0.736 mmol), 10 mL of MeOH were added, and the mixture was stirred overnight at room temperature. Suspended orange solid was allowed to settle down, and the liquid was decanted. Orange precipitate was dissolved in 2 mL of DCM, and the solution was run through a small column with PSQ60AB silica (~1x5 cm column size), collecting the coloured band separately and pumping that to dryness in vacuo.

Isolation of [Au10( MesCH2 Bimy)6Br3]Br ([3b]Br) from reduction of 2b
A round bottom flask (50 mL) was charged with a stir bar and 2b (0.0405 g, 0.0614 mmol), and DCM (20 mL) was added to dissolve the gold complex. To the stirring solution of gold complex was added a thoroughly sonicated solution of NaBH4 (0.00230 g, 0.0608 mmol) in absolute EtOH (6 mL). The mixture was stirred 18-20 h at room temperature. The red-brown coloured reaction mixture was evaporated to dryness in vacuo. According to UV-vis absorbance spectroscopy, NMR and ESI-MS (see the corresponding figures below), at this stage crude reaction mixture contains gold clusters along with large amount of 2b and [( MesCH2 Bimy)2Au]Br gold complexes. Initial separation of the clusters was performed by extraction and precipitation, leveraging higher solubility of clusters in Et2O compared with gold complexes. Et2O (~4 mL) was added to heterogeneous red-brown and beige dry residue, and the mixture was sonicated 5-10 min until a uniform brown suspension was formed. Then the mixture was filtered through a Celite plug, removing a beige or pale brown precipitate and collecting red-brown filtrate. The precipitate was collected separately by washing the Celite plug with DCM and pumping the filtrate to dryness to yield a pale brown coloured residue, identified by UV-vis absorbance and NMR spectroscopy, and ESI-MS as a mixture of majority [( MesCH2 Bimy)2Au]Br byproduct and 2b (0.0250 g). The Et2O filtrate was dried in vacuo to yield the crude cluster mixture as a red-brown residue (0.0121 g). The crude cluster mixture was purified by silica gel column chromatography (see Figure S1; PSQ60AB silica; 12:1 DCM/MeOH eluent; ~3x10 cm column size). Three coloured bands were collected separately from the column and evaporated to dryness in vacuo as follows: 1) an initial red coloured band, yielding a red residue (0.0057 g) containing mostly the starting complex 2b and unknown byproducts; 2) a second brown band at the front of the main orange coloured cluster band, which yields a brown residue (0.0041 g) containing a mixture of cluster species, including some [3b]Br; and 3) an orange band, which yields the title cluster [3b]Br as an orange residue (0.0077 g, 27 % based on Au). While the cluster obtained from column purification is typically fairly pure, trace contamination of 2b and other impurities can remain. The purity can be improved by precipitation of the cluster from an Et2O or DCM solution (~1-2 mg/mL) with pentane or hexane (~4x v/v). Careful removal of the supernatant solution, followed by washing the precipitate with Et2O-pentane (1:4 v/v; 1 x 0.5 mL) and pentane (2 x 1 mL) and drying in vacuo typically affords the cluster with only trace impurities by NMR spectroscopy. For the above example, precipitation was carried out with DCM-hexane, yielding pure cluster [3b]Br as a bright orange solid (0.0045 g, 16 % based on Au  (dd, 3 JHH = 6.0 Hz, 4 JHH = 3.0 Hz, 8H, ArHBimy), 6.43 (br s, 16H, ArHMes), 6.01 (br m, 16H, NCH2Ar), 5.72 (broad s, 8H, NCH2Ar), 2.22 (s, 48H, ArCH3), 2.17 (s, 24H, ArCH3), 2.12 (s, 12H, ArCH3), 2.05 (s, 24H, ArCH3). 13 C{ 1 H} NMR (CD2Cl2, 176 MHz): δ 214. 9,200.7 (NCN),138.6,138.4,137.7,137.6,134.5,133.6,129.9,129.9,127.4,124.4,124.4,112.8,112.7 (ArC),50.6,49.7 (NCH2Ar),21.9,21.7,21.1,21.0 (ArCH3)

Isolation and crystallization of [3b][OTf] from [3b]Br
For crystallization purposes, [3b]Br was anion exchanged to the triflate (OTf) salt. Crystals suitable for X-ray crystallography were grown from a saturated solution of [3b] [OTf] in DCM layered with n-hexane.

Reduction of 2c and evidence for the formation of [Au10( MesCH2 Bimy)6I3]I and [Au25( MesCH2 Bimy)10I7]I2
A round bottom flask (50 mL) was charged with a stir bar and 2c (0.03022 g, 0.04278 mmol), and DCM (10 mL) was added to dissolve the gold complex. The flask was then covered with aluminum foil to avoid excess light exposure. To the stirring solution of gold complex was added a thoroughly sonicated solution of NaBH4 (0.00194 g, 0.0513 mmol) in absolute EtOH (3 mL). Samples of the reaction mixture (0.1 mL) were removed via syringe at 10 minutes, 2 hours and 19 hours and dried in vacuo before analysis by UV-vis and NMR spectroscopy as well as ESI-MS. The remaining reaction mixture at 19 hours was evaporated to dryness in vacuo, then the residue was extracted with Et2O (4 x 1 mL), and the supernatant solution filtered through a Celite plug and evaporated to dryness yielding a red-brown residue (0.0079 g). The precipitate from Et2O was collected separately from the Celite plug via washing with DCM, and pumping the filtrate to dryness yielding purplebrown residue (0.0192 g). The crude Et2O supernatant residue was purified by silica gel column chromatography (PSQ60AB silica; 12:1 DCM/MeOH eluent; ~2x5 cm column size). Two initial coloured bands were collected separately from the column and evaporated to dryness in vacuo as follows: 1) an initial red coloured band, yielding a red-brown residue (0.0053 g) containing a complex mixture of unknown species as well as gold complexes and some [3c]I, and 2) a second brown band, which yielded a brown residue (0.0017 g) containing a mixture of mostly gold complexes and [3c]I (as identified by ESI-MS). A third brown band moved slowly at the top of the column, and was eluted by adding NH4PF6 (~5-10 mg) into the remaining eluent (12:1 DCM/MeOH) and eluting all brown coloured fractions with MeOH. The fractions were evaporated to dryness and the residue redissolved in DCM and filtered through Celite to remove excess salts, followed by drying in vacuo yielding a brown residue, which was washed with pentane (3 x 1 mL) and dried in vacuo (0.0020 g).

Isolation and crystallization of [Au25( MesCH2 Bimy)10Br8][B(C6F5)4] ([5b][B(C6F5)4]) from conversion of [3b]Br in EtOH at room temperature
Cluster [3b]Br (0.0120 g, 0.00262 mmol) was dissolved in EtOH (6 mL) and stirred at room temperature for 7 days. Following this, the sample was filtered through a 0.22 µm PTFE syringe filter, and the filtrate was dialyzed against ~800 mL of 95% EtOH for two days. Next, the EtOH solution was transferred to a 20 mL vial and the solvent removed in vacuo yielding a dark red solid. DCM (5 mL) and K[B(C6F5)4] (5.0 mg, 6.96 µmol) were added to the solid, and the mixture was stirred for 24 hours before being filtered to remove excess K[B(C6F5)4]. The resultant solid was dissolved in fluorobenzene (1 mL), layered with n-hexane (3 mL), and allowed to sit at room temperature for 48 hours to afford dark brown-black crystals of [5b][B(C6F5)4] (0.0020 mg, 15 % based on Au). ESI-MS for [C270H300Au25Br8N20] + : Calculated -9388.9; could not be observed experimentally, likely due to facile loss of one bromide ligand on the core during ionization. To a solution of 2b (0.0424 g, 0.0643 mmol) in DCM (20 mL) was added a sonicated suspension of NaBH4 (0.00246 g, 0.0650 mmol) in EtOH (6 mL), and the mixture was stirred at room temperature for 24 hours. The mixture was evaporated to dryness in vacuo, then the crude residue was extracted with Et2O (4 x 2 mL). The supernatant solutions were filtered through Celite and evaporated to dryness yielding crude [3b]Br as an orange-brown residue (0.0140 g). This mixture was dissolved in MeOH (5 mL) and transferred to a screw top glass reaction tube (50 mL) charged with a stir bar. The tube was capped with a Teflon lined screw cap and the mixture was heated in a 60 °C oil bath for 5 days after which the colour had changed from orange-brown to dark green-brown with some purple-black gold nanoparticle precipitate and gold(0) plating on the walls of the tube. The mixture was cooled and excess NH4PF6 (~50 mg) was added directly to the solution, and it was stirred a few minutes. The solvent was removed in vacuo and the residue redissolved in DCM and filtered through a Celite plug, removing some purple-black precipitate and excess salts. The resultant dark green-brown crude residue was purified by column chromatography (PSQ60AB silica; 12:1 DCM/MeOH eluent; ~3x10 cm column size). The initial colourless fraction, followed by pale orange fraction, were collected separately and evaporated to dryness yielding pale orange residues (0.0022 g and 0.0051 g, respectively), which contained mostly the starting complex 2b and by-product [( MesCH2 Bimy)2Au]Br (as judged by NMR spectroscopy; see below). Next, a dark olive-green band containing the title cluster was eluted. The fraction was evaporated to dryness in vacuo, and the residue was washed with pentane (2 x 1 mL) and dried in vacuo yielding cluster [4b][PF6]2 as a dark olive-green coloured residue (0.0025 g, 10 % based on Au). Typically, the cluster after column contains small impurities (gold complexes or other unidentified by-products) therefore the cluster was purified by precipitation as follows. The column residue was partially dissolved in Et2O (1-2 mL), then DCM was added dropwise to until the cluster just dissolved. The cluster was then precipitated with the addition of pentane (~2x volume). The precipitate was allowed to settle, and the supernatant solution was carefully removed via pipet. The precipitate was washed with pentane (2 x 1 mL) and dried in vacuo yielding pure cluster [4b][PF6]2 as a dark olive-green coloured powder (0.0019 g, 8 % based on Au). UV-vis absorbance and NMR spectra tracking the purification of this cluster are shown in Figure S9- δ 206.7 (NCN), 139.1,138.3,137.7,136.8,136.3,133.9,133.1,130.5,130.0,129.5,127.8,126.9,124.8,123.9,113.1,113.0 (ArC),52.3,51.4 (NCH2Ar),21.6,21.0,20.9,20.6,19.7,18.9 (ArCH3)   Spectroscopic data are consistent with the non-labeled cluster except for higher intensity of the carbene carbon signal as expected.

Isolation and crystallization of [Au25( MesCH2 Bimy)10Br7][B(C6F5)4]2 ([4b][B(C6F5)4]2) from [4b][PF6]2
Cluster [4b][PF6]2 (0.0028 g, 2.9x10 -4 mmol) was dissolved in MeOH (3 mL) and a solution of excess Li[B(C6F5)4][OEt2] (0.00269 g, 0.00309 mmol) in MeOH (2 mL) was added. The mixture was stirred at room temperature for 2 hours, then evaporated to dryness in vacuo, redissolved in DCM, filtered through Celite, and evaporated to dryness yielding a dark olive-green residue (0.0035 g). The residue was purified by column chromatography (PSQ60AB silica; 12:1 DCM/MeOH eluent; ~1x5 cm column size). The dark green fractions were collected and evaporated to dryness yielding a dark green residue of [4b][B(C6F5)4]2 (0.0015 g). The residue was redissolved in Et2O (~1 mL) and hexane (~1 mL) was added until the solution became cloudy, then Et2O was added dropwise to just dissolved the cluster and the solution was syringe filtered and left in a capped vial overnight at room temperature. Overnight, dark green-black cube/block shaped crystals had formed. A portion of these crystals and mother liquor was carefully removed via pipet and analyzed by X-ray crystallography; however, the quality of the data was not suitable for structural analysis. A new batch of crystals suitable for X-ray crystallography was regrown from this material by layering n-hexane onto a DCM solution of the cluster (ca. 5 mg/mL) at leaving the mixture -4 °C for several days. The remaining crystals were carefully washed with pentane (3 x 1 mL) with a few drops of Et2O added, then pentane only (3 x 1 mL), followed by drying in vacuo yielding a dark green-black blocky crystalline solid (0.0001 g). S22 3 Monitoring reaction progress
In situ monitoring of the reduction of (A) 2a (X = Cl), (B) 2b (X = Br) and (C) 2c (X = I) at 2.31 mM in DCM/EtOH (10:3) with NaBH4 by UV-vis absorbance spectroscopy. Spectra were automatically recorded every 30 min for 2.5 days. Note that the noise observed in the spectra recorded at 0 h is related to solution inhomogeneity due to H2 evolution at early reaction stages.  Figure S12.
In situ monitoring of the reduction of (A) 2a (X = Cl), (B) 2b (X = Br) and (C) 2c (X = I) at 2.31 mM in DCM/EtOH (10:3) with NaBH4 by UV-vis absorbance spectroscopy (continued). The same data as on Figure S11 are grouped in two time periods: initial (left), characterized by growth of intermediate gold species ($), and main (right), when formation of Au10 (#) and Au25 (&) clusters was observed.
The following trends can be observed. Intermediate gold species were detected in all reactions (X = Cl, Br, and I), and the initial period was the shortest for X = Cl and the longest for X = I. Au10 cluster forms for all halides. Au25 is detected in significant amounts for X = I, whereas the amount of Au25 formed with X = Br over the same time was small. Au25 clusters were not observed when X = Cl. Note that ESI-MS reveals several peaks of unidentified gold nanoclusters larger than Au10 (e.g., at m/z 2792.2 or 2514.8), which are present in reaction mixture at early stages only. Such species potentially may be associated with "intermediate gold species" observed in UV-vis absorbance spectra.                 Z is the number of formula units per unit cell. Comparison of Z with the multiplicity of the general position na of the space group will then require that the asymmetric unit is Z/na times the formula unit. The asymmetric unit is the minimum group of atoms whose positions, together with those generated by the symmetry operations of the space group generate the complete contents of the unit cell.       For [Au25( iPr Bimy)10Br7] 2+ nanoclusters, 21 calculations were made using deposited data CCDC1916156.

Mass spectra
The following general trends can be observed: average radial Au-Au bonds are shorter than average peripheral; Au-Au bonds within icosahedron are shorter than those between the two icosahedra; average Au-Br terminal bonds are shorter than Au-Br bridging ones.   Figure S66.

UV-vis absorbance and fluorescence spectra
UV-vis absorbance spectra of 2a, 2b, and 2c in EtOH.  Note that the weak emission of [3b]PF6 and low stability of this cluster under irradiation complicate obtaining its excitation spectra.   Optimized structure of the gas-phase model of [3b] + using the PBE exchange-correlational functional. Table S9.
Experimental and computed (PBE) atom-atom bond lengths in the crystal structure and in the gas-phase model of [3b] + .
Atom indexing is shown in Figure S58. Figure S75.

S73
Electronic density of states in the gas-phase model of [3b] + by using the PBE (top) and GLLB-SC (bottom) xc functional.
Color coding shows the projection of the orbitals into spherical symmetry about the c.o.m of the cluster. The projection is done in a sphere of radius 3.5 Å. The grey area labeled as "out" denotes that part of the electron density that is either outside this radius (in the ligands) or cannot be spanned by the angular momentum components up to J symmetry.  Bottom left panel shows the occupied electron states and the vertical panel on the right shows the empty states. Red/blue areas in the map denote constructive/destructive contributions of the electron-hole transitions to the total transition dipole. Along the dashed line, the hole-electron orbital energy difference matches the peak energy. The occupied and empty electron states are decomposed by the elemental weights as shown in the color coding.