Asymmetric Synthesis of Chiral Bimetallic [Ag 28 Cu 12 (SR) 24 ] 4- Nanoclusters via Ion Pairing

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1 Supporting Information for Asymmetric Synthesis of Chiral Bimetallic [Ag 28 Cu 12 (SR) 24 ] 4- Nanoclusters via Ion Pairing Juanzhu Yan,, Haifeng Su,, Huayan Yang, Chengyi Hu, Sami Malola, Shuichao Lin, Boon K. Teo, *, Hannu Häkkinen, and Nanfeng Zheng *, State Key Laboratory for Physical Chemistry of Solid Surfaces, Collaborative Innovation Center of Chemistry for Energy Materials, and Engineering Research Center for Nano-Preparation Technology of Fujian Province, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen , China. Departments of Physics and Chemistry, Nanoscience Center, University of Jyväskylä, FI Jyväskylä, Finland. Materials Silver nitrate (AgNO 3, 99%), cuprous bromide (CuBr, 99%), 2,4-dichlorobenzenethiol (2,4-SPhCl 2, 97%), tetrabutylammonium bromide ( n Bu 4 NBr, 99%), tetrabutylammonium chloride( n Bu 4 NCl, 99%), tetrabutylammonium tetraphenylborate ( n Bu 4 NBPh 4, 99%), N-benzylcinchonidinium chloride (C 26 H 29 ClN 2 O, 99%), N-benzylcinchoninium chloride (C 26 H 29 ClN 2 O, 99%) were purchased from Alfa Aesar. (Tianjin, China), sodium borohydride (NaBH 4, 98%), triethylamine (C 6 H 15 N, 99.5%), dichloromethane (CH 2 Cl 2, A.R.), and methanol (CH 3 OH, A.R.) were purchased from Sinopharm Chemical Reagent Co. Ltd. (Shanghai, China). The water used in all experiments was ultrapure. All reagents were used as received without further purification. 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). The CD spectra were characterized by Jasco Circular Dichroism Chiroptical Spectrometer. Synthesis of ( n Bu 4 N) 4 [Ag 28 Cu 12 (SR) 24 ] In a typical preparation, 10 mg of AgNO 3 was dissolved in 2 ml of methanol, followed by the addition of 8 mg CuBr in 10 ml of dichloromethane. The mixture was cooled to 0 C in an ice bath, 10 μl of 2,4-dichlorobenzenethiol and 6 mg of tetrabutylammonium bromide were then added. After 20-min stirring, 1 ml of an aqueous solution of NaBH 4 (40 mg/ml) and 50 μl of triethylamine were added quickly to the reaction mixture under vigorous stirring. The reaction mixture was aged S1

2 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-red single crystals suitable for X-ray diffraction study were grown by a double-layer of toluene/ch 2 Cl 2 solution of crude product at 4 C for two weeks. Synthesis of [Ag 28 Cu 12 (SR) 24 ] 4- -BCNC, [Ag 28 Cu 12 (SR) 24 ] 4- -BCDC Same as (1) with the exception of the use of 10 mg of N-benzylcinchoninium chloride (BCNC) or N-benzylcinchonidinium chloride (BCDC), in place of 6 mg of tetrabutylammonium bromide. Synthesis of Na 4 [Ag 28 Cu 12 (SR) 24 ] or H 4 [Ag 28 Cu 12 (SR) 24 ] without any couterion Same as (1) with the omission of tetrabutylammonium bromide. Single-Crystal X-ray Structure Analysis The diffraction data of ( n Bu 4 N) 4 [Ag 28 Cu 12 (SR) 24 ] were collected on an Agilent Technologies SuperNova single-crystal X-ray diffractometer with Mo Kα radiation (λ = Å) at 100 K. The data were processed using CrysAlis Pro. S1 The structure was solved and refined using Full-matrix least-squares based on F 2 with program SHELXS-97 and SHELXL-97 S2 within Olex2. S3 Single Crystal Analysis of ( n Bu 4 N) 4 [Ag 28 Cu 12 (SR) 24 ] Dark-red block crystal of dimensions 0.15 mm 0.1 mm 0.1 mm, space group P 2 1 /n, a = (12) Å, b = (2) Å, c = (3) Å, α = 90, β = (6), γ = 90, V = (3) Å 3, Z = 4, Mo Kα, T =100 K, 2θ max = reflections were measured, of which were unique with R int = , Final R 1 = 7.28%, wr 2 = for 3034 parameters and reflections with I > 2σ(I). Computational Details The computations were done by adopting the similar method reported in our previous work. S4 We used density-functional theory (DFT) calculations with projector augmented waves (PAW) as implemented in the real-space grid code-package GPAW. S5,S6 Structure of the [Ag 28 Cu 12 (SR) 24 ] 4- cluster was relaxed using PBE-functional, S Å grid spacing and 0.05 ev/å criterion for residual forces acting on atoms. Total charge of the clusters was -4. Superatom shell structure was analyzed by projecting the wave functions to spherical harmonics with respect to the center of mass of the cluster as described in our previous work. S4 The optical absorption spectra for the optimized structures were calculated by using the linear response time-dependent DFT (LR-TDDFT) as implemented in GPAW, S8 using 0.25 Å grid spacing and the PBE functional. The CD spectra were calculated using PBE-functional for the structures of [Ag 28 Cu 12 (SR) 24 ] 4- cluster and its Ag 28 core. S2

3 Reference S1. CrysAlis Pro Version (2011). Agilent Technologies Inc. Santa Clara, CA, USA. S2. Sheldrick, G. M. A short history of SHELX. Acta Cryst. 2008, A64, S3. Dolomanov et al. (2009). OLEX2: a complete structure solution, refinement and analysis program. J. Appl. Cryst. 2009, 42, S4. Yang, H. Y.; Wang, Y.; Huang, H. Q.; Gell, L.; Lehtovaara, L.; Malola, S.; Häkkinen, H.; Zheng, N. F. All-thiol-stabilized Ag 44 and Au 12 Ag 32 nanoparticles with single-crystal structures. Nat. Commun. 2013, 4, S5. Mortensen, J.; Hansen, L.; Jacobsen, K. Real-Space Grid Implementation of the Projector Augmented Wave Method. Phys. Rev. B 2005, 71, S6. Enkovaara, J.; Rostgaard, C.; Mortensen, J. J.; Chen, J.; Dulak, M.; Ferrighi, L.; Gavnholt, J.; Glinsvad, C.; Haikola, V.; Hansen, H. A.; et al. Electronic Structure Calculations with GPAW: A Real-Space Implementation of the Projector Augmented-Wave Method. J. Phys.: Condens. Matter 2010, 22, S7. Perdew, J.P; Burke, K.; Ernzerhof, M. Generalized Gradient Approximation Made Simple. Phys. Rev. Lett. 1996, 77, S8. Walter, M.; Häkkinen, H.; Lehtovaara, L.; Puska, M.; Enkovaara, J.; Rostgaard, C.; Mortensen, J. J. Time-Dependent Density-Functional Theory in the Projector Augmented-Wave Method. J. Chem. Phys. 2008, 128, S3

4 Figure S1. TEM image of [Ag 28 Cu 12 (SR) 24 ] 4- statistics of particle distribution. nanoparticles. Inset plot showed S4

5 Figure S2. The packing diagram of [Ag 28 Cu 12 (SR) 24 ] 4- clusters with n Bu 4 N + counterions. Color codes: orange, Ag; blue, Cu; yellow, S; light green sphere, Cl; gray, C. All hydrogen and chlorine atoms are omitted for clarity. Figure S3. Detailed structure analysis of the [Ag 28 Cu 12 (SR) 24 ] 4- cluster. (a) An inner 1 tetrahedral Ag 4 core capped on the four faces with four 2 triangular Ag 6 facets. Dashed lines represent Ag Ag interactions between neighboring Ag 6 facets. (b) Three pairwise interactions between the six thiolates of Cu 3 (SR) 6 motifs. Color codes: green, turquoise, pink red and orange, Ag; blue, Cu; yellow, S; gray, C. All hydrogen and chlorine atoms are omitted for clarity. S5

6 Figure S4. Lambert Beer plot for [Ag 28 Cu 12 (SR) 24 ] 4- at 465 nm in CH 2 Cl 2 solvent (0.5 cm quartz cell; A = absorbance; C = concentration; = molar extinction coefficient; R 2 = linearly dependent coefficient ). S6

7 Figure S5. Analysis of the electronic density of states of [Ag 28 Cu 12 (SR) 24 ] 4- cluster in terms of the superatom orbitals. The HOMO-LUMO gap is centered at zero. Note that for the Ag 28 Cu 12 cluster the 2S superatom state is below the 1D manifold, while for the other clusters the 2S state is empty (it forms the LUMO state). Figure S6. The chemical structures of BCNC and BCDC. S7

8 Figure S7 The experimental CD spectra of BCNC and BCDC in CH 2 Cl 2 solution. Figure S8 Time tracking of ESI spectra of the samples in air at 0 C during chiral separation of racemic mixture of [Ag 28 Cu 12 (SR) 24 ] 4- by introducing BCDC. The main MS peaks from left to right were assigned as follows: [Ag 28 Cu 12 (SR) 24 ] 4- ; [Ag 28 Cu 12 (SR) 24 (BCNC) n ] 4- (n=1, 2); [Ag 28 Cu 12 (SR) 24 (BCN)(BCNC) m ] 3- (m=0, 1, 2, 3). (1 is abbreviated here as (AgCu) 40 ). S8

9 Figure S9. (a) The plot of increment of CD signal against decrement of UV absorption at 465 nm during the 3 h enantioseparation process of 1 (1 is abbreviated here as (AgCu) 40 ). There is a nice linear correlation between the enhancement of the CD signal and the weakening of the UV-vis absorption. At the same wavelength, both R- and L- enantiomers should have their anisotropy factors with the same intensity but opposite signs. The observed linear correlation indicated that the R-enantiomer was dramatically degraded with the introduction of BCNC while the L-enantiomer was well-stabilized. (b) The corresponding profile of anisotropy factors by using the increased CD signal and reduced UV-vis absorption at 3 h. The anisotropy factor g= A/A= [mdeg]/(32980 A). In the calculation, the degradation of only L R-enantiomer was assumed. The increased CD signals were due to the increased concentration of L-enantiomer by 2.20 M. S9

10 Figure S10. Time tracking of UV vis spectra of solid-state ( n Bu 4 N) 4 [Ag 28 Cu 12 (SR) 24 ] in air at room temperature to evaluate the stability of cluster. The solid-state ( n Bu 4 N) 4 [Ag 28 Cu 12 (SR) 24 ] at different times was dissolved in CH 2 Cl 2 for UV vis measurement. Figure S11. Time tracking of UV vis spectra of ( n Bu 4 N) 4 [Ag 28 Cu 12 (SR) 24 ] DMF solution in air at room temperature to monitor the stability of cluster. S10

11 Figure S12. Variations of the absorption maximum of 1 at 465 nm as a function of time in CH 2 Cl 2 solutions containing different salts n Bu 4 N + X - (X= I, Br, Cl, BPh 4 and F) CH 2 Cl 2 environment at 0 C in air or under N 2. S11

12 Abs. / a.u. (AgCu) BCDC 1.2 (AgCu) BCNC Wavelength / nm Figure S13. The UV-vis spectra of 1 BCNC and 1 BCDC corresponding to the samples used for the CD spectra shown in Figure 4a. Based on the absorption spectra, the concentrations of 1 BCNC and 1 BCDC were estimated to be and 8.53 M, respectively, although the samples had similar CD intensity. The comparison results clearly suggested that the selectivity of the asymmetric synthesis was not 100% and highly depended on the chiral cations applied in the synthesis. More work is still required to determine the selectivity quantitatively. The non-prefect selectivity might be attributed to the weak stereochemical interaction between the chiral clusters and chiral cations. S12