Small symmetry-breaking triggering large chiroptical responses of Ag70 nanoclusters

The origins of the chiroptical activities of inorganic nanostructures have perplexed scientists, and deracemization of high-nuclearity metal nanoclusters (NCs) remains challenging. Here, we report a single-crystal structure of Rac-Ag70 that contains enantiomeric pairs of 70-nuclearity silver clusters with 20 free valence electrons (Ag70), and each of these clusters is a doubly truncated tetrahedron with pseudo-T symmetry. A deracemization method using a chiral metal precursor not only stabilizes Ag70 in solution but also enables monitoring of the gradual enlargement of the electronic circular dichroism (CD) responses and anisotropy factor gabs. The chiral crystals of R/S-Ag70 in space group P21 containing a pseudo-T-symmetric enantiomeric NC show significant kernel-based and shell-based CD responses. The small symmetry breaking of Td symmetry arising from local distortion of Ag−S motifs and rotation of the apical Ag3 trigons results in large chiroptical responses. This work opens an avenue to construct chiral medium/large-sized NCs and nanoparticles, which are promising for asymmetric catalysis, nonlinear optics, chiral sensing, and biomedicine. Having control over the chirality of metal nanoclusters is challenging. Here, the authors report the deracemization of silver nanoclusters and monitor the chiroptical responses.

Herein, we report the preparation and structural characterization, including X-ray crystallography, of {NH 2 (CH 3 ) 2 } 2 {Ag 70 S 4 (S i Pr) 24 (CF 3 COO) 20 (DMF) 3 }·4DMF (Rac-Ag 70 ). It crystallizes in the P 3c1 space group (No. 165) and contains enantiomeric pairs of 70-nuclearity silver clusters, each of which is a pseudo-T-symmetric doubly truncated tetrahedron ( Fig. 1), lacking a C 3 axis (and thus strictly speaking should be called C 1 symmetry). By using a small Ag(I) cluster as a stabilizer, we obtain regularly T d -symmetric Ag 70 (Fig. 1a) in the structure of achiral cocrystal Ag 70 ·Ag 12 . By controlling the chiral metal precursor, we monitor a gradual increase in CD responses and g abs in solution, demonstrating that the chiral carboxylate enters the coordination layer step-by-step and causes a progressive deracemization of the Ag 70 racemates. Interestingly, the process can be reversed. Moreover, the chiral crystals of R/S-Ag 70 in space group P2 1 only contain pseudo-T-symmetric enantiomeric nanoclusters similar to but with more deviation from those in Rac-Ag 70 (Fig. 1b). The CD responses of crystalline R/S-Ag 70 are consistent with those in solution, which originate from kernel-based and shell-based electronic transitions, as evidenced by theoretical calculations. Structural analysis reveals that the small symmetry breaking of T d symmetry arises from local distortion of Ag-S motifs and rotation of the apical Ag 3 trigons, resulting in large chiroptical responses.
This work opens an avenue to construct chiral medium-sized or large-sized nanoclusters and nanoparticles.

Results
Synthesis and characterization. The synthesis of Rac-Ag 70 was achieved by an acid reduction method. {Ag(S i Pr)} n and CF 3 COOAg together with CF 3 COOH, an indispensable acid, reacted in a mixed solvent containing N,N′-dimethylformamide (DMF) and isopropanol ( i PrOH) under solvothermal conditions at 80°C for 24 h. The reaction solution changed from colourless to black-red, indicating that slow reduction (Ag I to Ag 0 ) had occurred during the solvothermal process. Subsequently, black crystals of Rac-Ag 70 were deposited after the filtrate evaporated in the dark for 1 week (Supplementary Fig. 1).
According to previous reports, Ag + ions can be reduced to Ag 0 atoms by heating DMF solution in a neutral or alkaline environment [24][25][26][27][28][29] . However, the silver clusters prepared under these conditions bear a maximum of two free electrons [24][25][26][27] . Here, we changed the environment from basic to acidic (CF 3 COOH) for the preparation of Rac-Ag 70 nanoclusters (NCs) under solvothermal conditions. At high temperature, the addition of more CF 3 COOH may slow down the DMF reduction and the nucleation of Ag nanoparticles, and the S 2− anions slowly generated in situ simultaneously capture an appropriate aggregation state, resulting in a high-nuclearity structure with a distinct Ag core. Thus, a reduction method was established for the synthesis of Ag NCs possessing multiple free electrons.
Single-crystal structure of Rac-Ag 70 . Based on the SCXRD structural analysis (detailed below), 70-nuclearity Ag NCs of Rac-Ag 70 were determined to be −2 valence-state anionic clusters with   Table 1). The detailed silver coordination modes and local structural features are discussed in the Supplementary Information .
From the innermost core outward ( Supplementary Fig. 13), the 70-Ag atom NC, denoted Ag 4 @Ag 12 @Ag 12 @Ag 6 @Ag 24 @Ag 12 , features a tetrahedron of four Ag atoms (an idealized Platonic solid), a truncated tetrahedron of twelve Ag atoms (an idealized Archimedean solid), a truncated tetrahedron of 12 Ag atoms (an Archimedean solid), an octahedron of six Ag atoms (an idealized Platonic solid), a truncated octahedron of 24 Ag atoms (an Archimedean solid), and a doubly truncated tetrahedron of twelve Ag atoms, with each of the vertices occupied by a Ag atom. This 70-Ag atom aggregate possesses tetrahedral topology 12,21,22,[33][34][35] , which incorporates S 2− -passivated FCCbased Ag 16 inner core (Ag−μ 3 -S bond lengths: 2.469(5)−2.489(6) Å; Supplementary Fig. 8). This kernel may provide a valuable clue for a deep insight into the nucleation and evolution of FCC-based Ag NCs and silver bulk materials. From single ion to three-layer FCC close-packing (Ag + →Ag 6 →Ag 16 , Supplementary Fig. 14), the regular aggregation states of small Ag species captured by four S 2− anion-templates may represent the early-growth stage of Ag bulk or Ag nanoparticles with FCC close-packing. Based on the ideal tetrahedron growth pattern, the next larger member of this family should have a Ag 31 S 4 core with FCC four-layer closepacking ( Supplementary Fig. 14).
The 24 μ 4 -S i Pr − ligands are evenly distributed and anchored on the surface of the cluster with Ag−S bond lengths in the range of 2.432(8)−2.616(5) Å. These organic components can be divided into two groups according to tetrahedral arrangement characteristics ( Supplementary Fig. 15). The 20 CF 3 COO − ligands, which are located on the rectangular or triangular faces of the double truncated tetrahedron protect the Ag 70 in μ 2 -η 1 ,η 1 or μ 1 −η 1 ,η 1 coordination mode (Ag−O bond lengths: 2.29(2) −2.44(4) Å, Fig. 2e and Supplementary Fig. 13a). Three DMF molecules are located at a Ag 3 vertex of polyhedra (Ag−O bond lengths: 2.35(2) Å). In addition, it was found that there are multiple C − H···F hydrogen bonds between CF 3 COO − and -CH 3 groups of S i Pr − and DMF ligands ( Supplementary Fig. 16), suggesting the strong interactions between the clusters.
Careful analysis of the geometric structure of an individual cluster in Rac-Ag 70 indicated that it lacks a mirror plane (σ) and inversion (i). Meanwhile, breakage of T d symmetry occurred due to distortion of the surface Ag−S motifs 3 , although the rotation angle along the pseudo-C 3 axes is slight ( Supplementary Fig. 17). Notably, the inner kernel (Ag 4 (core) and Ag 12 S 4 (1st shell), Supplementary Fig. 18) presents achiral structural characteristics due to the existence of the three mirror planes. This slight change based on surface motifs could be an important cause of large chiroptical responses of inorganic nanoparticles, which will be further verified by homochiral single crystals. As a result, left-and right-handed enantiomers coexist in the unit cell of Rac-Ag 70 ( Fig. 2e and Supplementary Fig. 17).  Fig. 25). The solution of Rac-Ag 70 shows broad emission peaks centred at 1300 nm in the NIR-II region ( Supplementary Fig. 27), which may arise from core-based transitions with low energy 14,36 .
Furthermore, ESI-MS was used to investigate the composition, charge state, and solution behaviour of Rac-Ag 70 in detail . Crystals of Rac-Ag 70 were dissolved in EtOH and measured in negative-ion mode with varied declustering potential and collision energy, showing three main grouped peaks in the mass-to-charge ratio (m/z) ranges of 3500-4000 (−3 charge state), 4200-4600 (−2 charge state), and 5500-6100 (−2 charge state). As shown in Supplementary Fig. 29, the peaks (Rac-Ag 70 : 2 h − 2 l) corresponding to the complete cluster skeleton (Ag 70 S 4 ) are easy to find, confirming that the entire cluster molecule can stably exist in solution. The correlations between the species mainly involve the same Ag 70 S 4 skeleton and the exchange of molecules outside the shell (S i Pr − and CF 3 COO − ), in which the weakly bound CF 3 COO − molecules are easily removed from the surface of the cluster under the prevailing ionization conditions. When a collision energy is applied to the system (from 0 to −15 V), fragment peaks 1a − 1j appear, and their intensity gradually increases; these peaks may be ascribed to some large fragments (Ag ( Table 2), suggesting that Rac-Ag 70 is unstable in solution, consistent with the results of timedependent UV-Vis (Fig. 3b). Meanwhile, the presence of peaks 2a − 2c (Ag 68 S 4 ), and 2d − 2 g (Ag 69 S 4 ) indicates that the dissociation of Rac-Ag 70 starts with the loss of CF 3 COOAg small molecules step-by-step. The appearance of the peak corresponding to the Ag 12 cluster ([Ag 12 (S i Pr) 6 (CF 3 COO) 7 ] − , Supplementary  Fig. 29d) indicates that this stable small cluster formed immediately after Rac-Ag 70 decomposition.
Inspired by the relatively strong interactions between the anionic cluster and the small molecules (CF 3 COOAg), as revealed by ESI − MS (2 m − 2t, Supplementary Fig. 29c and Supplementary Table 2), we proposed that CF 3 COOAg could stabilize the Rac-Ag 70 cluster and that dissociation-coordination equilibrium could occur between the anionic NC and CF 3 COOAg. This speculation was confirmed by time-dependent UV-Vis absorption spectra and ESI-MS experiments. When 250 equivalents of CF 3 COOAg were added to the solution of Rac-Ag 70 , Ag 70 remained stable for dozens of days (Fig. 3c-d). As depicted in Supplementary Fig. 29h-i, supramolecular assembly was carried out in a solution of Rac-Ag 70 after the introduction of 250 eq. of CF 3 COOAg to form a more stable hybrid product, Ag 70 ·(1-4) CF 3 COOAg (Supplementary Table 3), which completely inhibited the removal of CF 3 COO − and the dissociation of clusters. When C 2 F 5 COOAg was used, we obtained the same stability trend of Rac-Ag 70 in solution ( Supplementary Fig. 30 and Supplementary  Tables 4-5). This direct and sufficient evidence suggested that stable and rapid dissociation-coordination equilibrium occurred between the anionic NC and the small molecules (RCOOAg) in the solution. Based on the above experimental data, a reasonable stability mechanism in solution is proposed. For the solution of Rac-Ag 70 , NCs decompose and form small molecules (such as CF 3 COOAg) and fragments (such as Ag 12 clusters) until equilibrium is reached. Note that when the concentration of Rac-Ag 70 is too low, the NCs will be completely destroyed. If there is a large number of small molecules in the Rac-Ag 70 solution, then the decomposition of the NCs will be inhibited, and some stable products (Ag 70 ·(1-4)CF 3 COOAg) will be formed. Note that excessive RCOOAg (more than 500 eq.) is detrimental to these NCs.
The corresponding ESI-MS spectrum (Supplementary Fig. 35 and Supplementary Tables 6-7) shows the grouped peaks assigned to {Ag 70 S 4 (S i Pr) 24 (TFL/CF 3 COO) 21 } 3− ·(0-3)TFLAg and {Ag 70 S 4 (-S i Pr) 24 (TFL/CF 3 COO) 20 } 2− ·(0-4)TFLAg, suggesting that the increase of Cotton effects is positively associated with gradual ligand exchange (CF 3 COO − →TFL − ). In addition, the corresponding UV-Vis absorption does not change within a certain period of time ( Supplementary Fig. 31d), indicating that the main metal skeleton of the cluster remains unchanged, and dynamic equilibrium and long-term stability of the system are achieved. Interestingly, in the range of n(R-/S-TFLAg):n(Rac-Ag 70 ) = 0-15, the Cotton effects are almost linearly enhanced, while beyond the ratio of 15, these effects are basically unchanged (Fig. 4c and Supplementary Fig. 32c). The amplification of chiral signals may stem from the fact that the achiral ligands (CF 3 COO − ) are continuously replaced by chiral ligands (TFL − ) on the cluster surface, and the symmetry breaking of the whole structure is gradually amplified. A reversible decrease in Cotton effects is recovered by using CF 3 COOAg titration (Fig. 4b) Cocrystals of Ag 70 ·Ag 12 evidencing the proposed stability mechanism. Aiming to further verify the above-proposed stability mechanism in solution, we attempted to prepare single crystals that contained an anionic Ag 70 cluster and a small Ag(I) 12 cluster ({Ag 12 (S i Pr) 6 (CF 3 COO) 6 }) as a stabilizer, which was named Ag 70 ·Ag 12 (Supplementary Figs. 41-57) [37][38][39][40][41][42][43] . The appearance of Ag(I) 12 cluster could be attributed to the remainder of the unreduced silver(I) fragments in the EtOH-DMF system. The smaller Ag(I) 12 cluster possesses a slightly distorted cuboctahedral metal framework (Ag···Ag distances: 3.06(1)-3.09(2) Å, Cocrystallization with Ag(I) 12 leads to a more regular stacking of Ag 70 (Supplementary Figs. 50-51); thus, Ag 70 ·Ag 12 crystallizes in a higher-symmetry space group (Fd 3m, number 227) compared to Rac-Ag 70 (P 3c1, number 165). More interestingly, the 70-nuclearity Ag NC in Ag 70 ·Ag 12 exhibits nearly perfect T d symmetry ( Supplementary Fig. 47b), and its polyhedral skeleton is similar to that in Rac-Ag 70 , except that the slight symmetry breaking leads to different Ag···Ag distances (Supplementary  Figs. 48-49 and Supplementary Table 1). Therefore, the small Ag(I) 12 cluster not only stabilizes Ag 70 but also changes the crystal packing, which probably weakens the intercluster strain in  Table 10) similar to those of Rac-Ag 70 . The Ag(I) 12 cluster truly stabilizes Ag 70 in solution: Ag 70 ·Ag 12 remains stable in EtOH for more than 7 days, as revealed by time-dependent UV-Vis absorption spectra ( Supplementary  Fig. 56); Ag 70 ·Ag 12 is also more thermally stable than Rac-Ag 70 in solution (Supplementary Fig. 57).
Single crystal of chiral Ag 70 verifying the proposed deracemization mechanism. We subsequently attempted to prepare single crystals of homochiral Ag 70 . This is considerably difficult because homochiral high-nuclearity NCs remain an open challenge 9,13,16 . After continuous attempts, we obtained enantiomeric R/S-Ag 70 crystals ( Supplementary Fig. 58) in chiral space group P2 1 in the presence of R/S-TFLAg. The main composition of R/S-Ag 70 was determined as {Ag 70 S 4 (S i Pr) 24 Fig. 59). Although the peripheral ligands of R-Ag 70 could not be positioned, its Ag−S skeleton was well resolved (see the Methods for the refinement of the structure of R-Ag 70 ). Structural analysis indicates that the pseudo-T-symmetric metal skeleton of R-Ag 70 (Fig. 5a-b) is more severely distorted than that of Rac-Ag 70 (Supplementary Figs. 60-63 and  Supplementary Table 1). Therefore, the absence of mirror planes in the Ag 70 cluster molecule leads to the obvious shell-based and metal kernel-based Cotton effects of R-Ag 70 and S-Ag 70 crystals ( Fig. 5c and Supplementary Fig. 64). The CD spectra of R-Ag 70 and S-Ag 70 are consistent with those of the above deracemized Rac-Ag 70 solution with R/S-TFLAg, verifying the small symmetry breaking related to the considerable Cotton effects. Combining the single-crystal structure analysis, and the nearly identical CD signals of solution and solid state, we propose that the incoming TFL − (chiral organic ligand) induces the structural deformation, which plays a critical role in chiroptical response and the deracemization process. In addition, TFLAg (chiral complex), which was used to trigger the reaction in our experiment, is found to be the concomitants of the silver cluster in ESI-MS (Supplementary  Figs. 35-37 and Supplementary Tables 6-9), and also plays a role in stabilizing the clusters (Supplementary Fig. 31d and Supplementary Fig. 40).

Discussion
In summary, we prepared a medium-sized silver NC racemate (Rac-Ag 70 ) through acid reduction synthesis, in which each NC featured the largest doubly truncated tetrahedron with pseudo-T symmetry. The dissociation-coordination mechanism between ligands and the silver framework enables stabilization of Ag 70 by cocrystallization with Ag(I) 12 clusters, deracemization of Rac-Ag 70 in solution, and achievement of enantiomeric crystals of R/ S-Ag 70 through chiral metal precursors. SCXRD analysis revealed that small symmetry breaking from T d symmetry is responsible for the large chiroptical response of chiral clusters. This work provides not only an insight into the stabilization mechanism of high-nuclearity metal clusters and symmetry breaking related to the chiroptical response but also a significant case for the exploration of growth of truncated tetrahedron-shaped noblemetal NCs. Synthesis of R/S-Ag 70 . Chiral R/S-Ag 70 was synthesized by a ligand-exchange method from Ag 70 ·Ag 12 NCs. Herein, the synthetic method for {Ag 70 S 4 (-S i Pr) 24 (CF 3 COO) 20−n (R-TFL) n } 2− (R-Ag 70 , n = 8 based on 19 F-NMR (Supplementary Fig. 59)) is used as an example. Three milligrams of Ag 70 ·Ag 12 were dissolved in 2 mL DMF. One milligram of R-TFLAg was added to the above solution under stirring and stirred for 2 min. The black-red solution was filtered and evaporated in the dark for 1 week. The black block crystals of R-Ag 70 were isolated and washed with dichloride/n-hexane (yield: 70 %).

Methods
Crystallographic data collection and refinement of the structure. SCXRD measurements of Rac-Ag 70 , Ag 70 ·Ag 12 and R-Ag 70 were performed at 200 K on a Rigaku XtaLAB Pro diffractometer with Cu-Kα radiation (λ = 1.54184 Å). Data collection and reduction were performed using the program CrysAlisPro 44,45 . All structures were solved with direct methods (SHELXS) 46 and refined by full-matrix least squares on F 2 using OLEX2 47 , which utilizes the SHELXL-2018/3 module 48 . All non-hydrogen atoms were refined anisotropically, and hydrogen atoms were placed in calculated positions with idealized geometries and assigned fixed isotropic displacement parameters. Appropriate restraints and/or constraints were applied to the geometry, and the atomic displacement parameters of the atoms in the clusters were determined. The absence of {NH 2 (CH 3 ) 2 } + , CF 3 COO − , and DMF molecules in the SCXRD data of Ag 70 ·Ag 12 could be caused by weak diffraction, high symmetry, and a highly disordered state in the lattice.
Due to the extremely large unit cell and high disorder on the ligands, it is not very easy to solve the structure of the whole cluster for R-Ag 70 , particularly for the organic ligands. Even, to be honest the space group cannot be confidently confirmed from the X-ray diffraction data at 100 K. According to the results from XPREP, other than the suggested "A" lattice by default, primitive lattice should be the more suitable one since there are in total 15139 exceptions of I > 3σ and the mean intensity is 10.2 with the man I/σ of 1.9. The exceptions for other lattices show similar total reflection number, and only slightly high mean intensity (21.4-27.7) and mean I/σ (2.4-2.6). The high statistic mean |E*E−1| value (1.019 in this case) normally points to the centrosymmetry. However, a large number of reflection exceptions tend to suggest the absence of any glide planes (a, n, c) in the structure. In detail, comparing with that only 30 weak reflections (I mean = 1.1, I/σ = 0.5) violate the 2 1 axis, 1835 reflections where 86 have intensity stronger than 3σ don't support the existence of c glide. The exceptions for a and n symmetries show similar total number as that for c (1835 for a and 1843 for n) ( Supplementary  Fig. 65). Therefore, the possible space group might be P2 1 . The confusing part is that much more strong (I/3σ) reflections violate the a and c plane, which suggests the P2 1 /c as an alternative. Considering the ligand that we used to synthesize the cluster is chiral with analytical pure (97%) and the relatively high yield (70%) of the single-crystal product, the correct space group of R-Ag 70 should be P2 1 . Finally, we solved the structure in P2 1 space group. Due to the bad disorder on the organic ligands, only the Ag 70 S 28 core can be assigned and freely refined with anisotropic displacement parameters. With this incomplete model, a Flack parameter of 0.42 (6) was obtained, indicating the twinning. However, the apparently high Flack parameter is also possibly caused by the lack of the organic part of the structure, particularly the chiral ligand.
Detailed information about the X-ray crystal data, intensity collection procedure, and refinement results for Rac-Ag 70 , Ag 70 ·Ag 12 and R-Ag 70 are summarized in Supplementary Table 11.
Quantum chemical calculations. The ground state of the metal cluster for Ag 70 was optimized by the semiempirical tight binding method (GFN-xTB) with the GBSA model for methanol 49 , and all excitations up to 6 eV were calculated with the simplified Tamm-Dancoff approach (sTDA) [50][51][52][53] . The molecular orbitals were extracted from Molden format by Multiwfn 54 and then rendered and virtualized by the VMD program 55 .