Thiol-Stabilized Atomically Precise, Superatomic Silver Nanoparticles for Catalyzing Cycloisomerization of Alkynyl Amines

Both the electronic and surface structures of metal nanomaterials play critical roles in determining their chemical properties. However, the non-molecular nature of conventional nanoparticles makes it extremely challenging to understand the molecular mechanism behind many of their unique electronic and surface properties. In this work, we report the synthesis, molecular and electronic structures of an atomically precise nanoparticle, [Ag 206 L 72 ] q (L = thiolate, halide; q = charge). With a four-shell Ag 7 @Ag 32 @Ag 77 @Ag 90 Ino-decahedral structure having a nearly perfect D 5h symmetry, the metal core of the nanoparticle is co-stabilized by 68 thiolate and 4 halide ligands. Both electrochemistry and plasmonic absorption reveal the metallic nature of the nanoparticles, which is explained by density functional theory calculations. Electronically, the nanoparticle can be considered as a superatom, just short of a major electron shell closing of 138 electrons ( q = -4). More importantly, many of ligands capping on the nanoparticle are labile due to their low-coordination modes, leading to high surface reactivity for catalysing the synthesis of indoles from 2-ethynylaniline derivatives. The results exemplify the power of the atomic-precision nanocluster approach to catalysis in probing reaction mechanisms and in revealing the interplay of heterogeneous reactivities, electronic and surface structural dynamics, thereby providing ways for optimization.

NaBH4 (40 mg/mL) and 50 μl of triethylamine were added quickly to the reaction mixture under vigorous stirring.The reaction mixture was aged for 12 h at 0 °C.The aqueous phase was then removed.The organic phase was washed several times with water and evaporated for further analysis.Dark single crystals suitable for X-ray diffraction study were grown by a double-layer of hexane/CH2Cl2 solution of crude product at 4 °C for two weeks.The yield of 1 was ~25 % (based on Ag).Compound 2a was prepared according to the known procedures.S1-S3

Characterizations
The UV-vis spectra were measured by Shimadzu UV-2550 Spectrophotometer with dichloromethane as solvent.Mass spectra were recorded on an Agilent Technologies ESI-TOF-MS (6224).Thin-layer chromatography (TLC) was carried out with silicycle pre-coated silica gel plates. 1 H NMR and 13 C NMR spectra were recorded at room temperature on a Bruker AV-400 spectrometer and a Bruker AV-500 spectrometer in chloroform-d3 with TMS (0.0 ppm) and chloroform (77.0 ppm) signal as an internal reference.Raman spectra were acquired using a confocal Raman system (Xplora, Horiba) using a 532 nm laser as the excitation light with the power around 0.15 mW.TEM spectra were recorded on a TECNAI F-30 transmission electron microscope operating at 300 kV.

Electrochemistry
Electrochemical measurements of nanoparticle were performed with an electrochemical workstation (CHI 760e) using a glass carbon working electrode (diameter 0.1 mm), a Pt counter electrode, and a SCE-reference electrode in 0.1 M Bu4NPF6/CH2Cl2.Nanoparticle solutions were degassed and blanketed with a high-purity N2 atmosphere during measurement.Ferrocene (Fc 0/+ ) was used as an internal reference for the SCE-reference electrode.The Fc 0/+ couple was found to be 0.554 V versus SCE in 0.1 M Bu4NPF6/CH2Cl2.
All potentials in this article are reported with respect to Fc 0/+ .Voltammograms of the nanoparticle solutions were acquired at 0 °C using an ice bath.

Catalyst Preparation
Preparation of (AgSR)n polymer. 1 mmol cyclohexanethiol was added into 10 ml silver hexafluoroantimonate CH2Cl2 dispersion (0.1 M) under vigorous stirring.The suspension was aged at r.t. for 3 h.The resulting white precipitate, (AgSR)n polymer, was collected via centrifugation, washed with CH2Cl2 for five times and then dried in vacuum.

Catalytic Experiments
Cyclization Reaction of N-(2-ethynylphenyl)-4-methylbenzenesulfonamide (2a) was carried out in a vial.For each reaction, a mixture of reactant, catalyst and solvent was placed into reactor.Then reaction mixture was vigorously stirred at the designed temperature for required time.After cooling the room temperature, the supernatant was collected via centrifugation, and purified by flash chromatography or preparative chromatography on a silica gel.The yields of 1-tosyl-1H-indole (2b) were calculated with reference to the obtained product.The product identification was carried out using 1 H and 13 C NMR.

Recycling procedure for 2a
The reused 5 mol% catalyst (1 wt% Ag206/TiO2), 2a (0.1 M), chlorobenzene (2 ml) and a magnetic stir bar were charged.The vial was sealed with a septum and placed into an oil bath, which was preheated to 80 °C.The reaction mixture was held at this temperature for 2 h.The reaction mixture was worked up and purified to afford 2b.The 1 wt% Ag206/TiO2 was washed several times with hexane, then dried in 80 °C, and used again in the next round of recycling.

DFT calculations
All the atomistic and jellium DFT computations were done by using the real-space code package GPAW.S6 The experimental crystal structure of Ag206(SR)68F2Cl2 (SR = cyclohexanethiol) was used as the starting point, and total energies of [Ag206(SR)68F2Cl2] q with 4- q  5+ were evaluated via single-point calculations, i.e., ignoring the structural relaxation.For a cluster of this size, this is a reasonable approximation.The electronelectron interactions were described by the PBE-functional.The real-space grid spacing was 0.25 Å. Auxiliary DFT calculations were done within the jellium description without explicit atoms, but smearing the positive background charge uniformly in a polyhedral shape taken from the outermost Ag90 layer in the core of (1).The real-space grid spacing in these calculations was set to 0.4 Å.The electronic structure of both the atomistic [Ag206(SR)68F2Cl2] q and jellium model of the core with q = 4-(corresponding to 138 free electrons) was analyzed via projection of the Kohn-Sham orbitals to spherical harmonics as described previously.S7

X-ray single-crystal analysis
The diffraction data of the single crystals grown from the solutions of 1 were collected on an Agilent Technologies SuperNova system X-ray single-crystal diffractometer with Mo (λ =0.71073 Å) and Cu Kα radiation (λ = 1.54184Å) at 100 K.The data were processed using CrysAlisPro.S8 The structure was solved and refined using Full-matrix least-squares based on F2 using ShelXT S9 and ShelXL S10 in Olex2 S11 and Shelxle.S12 Detailed crystal data and structure refinements for both compounds are given in Supplementary Table S1.
Supplementary Table S1 Ag206/TiO2 and Ag206/C.5 ml monodisperse CH2Cl2 solution of Ag206 nanoparticle (2 mg/mL) was slowly dropped into 20 ml TiO2 or C CH2Cl2 dispersion (1g TiO2 or C) under vigorous stirring.The suspension was aged at r.t. for 6 h.The solid was collected via centrifugation, washed with CH2Cl2 for five times and then dried in vacuum.Preparation of 1 wt% Ag44/TiO2 and Ag374/TiO2.Other monodisperse Ag44 and Ag374 nanoparticles were prepared according to the protocol demonstrated in refs.22 and 23 (main text).The obtained nanoparticles were loaded on TiO2 as same as the preparation of 1 wt% Ag206/TiO2. S4