Transforming Silver Nanoclusters from Racemic to Homochiral via Seeded Crystallization

Chirality has risen as an attractive topic in materials research in recent years, but the attainment of enantiopure materials remains a major challenge. Herein, we obtained homochiral nanoclusters by a recrystallization strategy, without any chiral factors (i.e., chiral ligands, counterions, etc.). Through the rapid flipping of configuration of silver nanoclusters in solution, the initial racemic Ag40 (triclinic) nanoclusters are converted to homochiral (orthorhombic) as revealed by X-ray crystallography. In the seeded crystallization, one homochiral Ag40 crystal is used as a seed to direct the growth of crystals with specific chirality. Furthermore, enantiopure Ag40 nanoclusters can be used as amplifiers for the detection of chiral carboxylic drugs. This work not only provides chiral conversion and amplification strategies to obtain homochiral nanoclusters but also explains the chirality origin of nanoclusters at the molecular level.

I n nature, chirality is a common feature in numerous systems, ranging from small molecules (e.g., tartaric acid and amino acids) to atomic aggregates (e.g., nanoparticles and nanoclusters) and macroscopic objects (e.g., human hands and atmospheric cyclones). In recent years, great progress has been made in the application of chiral nanomaterials in asymmetric catalysis, biorecognition, chiral medicine, and chiroptics. 1−5 However, the origin of chirality generation is still unclear. Further, achieving symmetry breaking and amplification of chiral products are challenging. On the one hand, absolute asymmetric synthesis (AAS) was attained; that is, in the absence of chiral ligands or chiral environments, an enantiomer is spontaneously obtained. 6,7 It is worth noting that external forces are usually required to achieve symmetry breaking, such as stirring, ultrasound, temperature gradients, and so on. 8−11 Without the presence of such factors, the enantiomeric excess (ee) value of the obtained product will be significantly lower, and even racemic results will be obtained. This motivated us to ponder whether the molecule itself can complete the symmetry breaking and give rise to a product with homochirality in the absence of external forces. On the other hand, the seeded crystallization method has been widely used in the synthesis of nanocrystals. 12,13 Furthermore, seed crystals can affect the morphology and properties of crystal products. 12,13 For example, NaClO 3 is achiral in solution, but it can crystallize into chiral crystals, and chiral seed crystals of NaClO 3 can induce the formation of chiral crystal products. 13 Ligand-protected nanoparticles with atomically precise nature, often called nanoclusters (NCs), have been of tremendous interest due to their precise structures and extraordinary properties. 14−32 By introducing chiral factors, such as chiral ligands or chiral counterions, optically active nanoclusters have been synthesized. 33−40 In addition, racemic nanoclusters such as Au 38 were separated into enantiomers through chiral resolution. 41−45 Further studies have shown that it is difficult to make homogold nanoclusters undergo a configuration reversal at room temperature. However, it is interesting that, after doping silver atoms into the kernel of gold nanoclusters, the temperature for configuration reversal can be significantly reduced. 46,47 Theoretically, thiolated silver or heavily silver-doped nanoclusters can only exist in the form of racemates in solution. 47−49 The rapid flip of chirality is achieved by changing the arrangement of the outer metal complex shell. Since the nanocluster itself has a metal core, its chiral construction does not depend on primary nucleation as in the AAS process. In theory, external factors (such as stirring, ultrasound, etc.) are not necessary to achieve symmetry breaking. For the cluster protected by the achiral coligands and crystallized in a noncentrosymmetric space group, if one can build a stable system and obtain only one crystal, then nearly 100% ee value could be obtained. 50 In the seeded crystallization method, the chirality of seed crystals can affect the chirality of final products. The racemic nanoclusters can rapidly flip between chiral structures in solution. If a chiral crystal is used as a seed, it may be feasible for racemic clusters to grow into homochiral crystals with the initial seed; then amplification of chiral crystals will be realized.
In this work, we devise a crystallization approach for achieving homochiral recrystallization and seeded crystallization. We first synthesized racemic Ag 40 nanoclusters. Then, we developed an automatic crystallization system by using a robot in order to quickly screen the crystallization conditions. After optimizing the conditions, the probability of obtaining one crystal in the crystallization vial is close to 10%. After testing more than 40 crystals, we found that 2 of the crystals are in the orthorhombic system (P2 1 2 1 2 1 , flack = 0.006), which means that an enantiomeric excess close to 100% has been achieved, and the chirality of the resulting crystal is random in our current work without introducing any chiral factors and external forces. Further, based on the principle of rapid structure flipping of silver nanoclusters in solution, using one homochiral crystal obtained by recrystallization as the seed allows rapid construction of crystals with conspecific chirality in a racemic solution. The chirality of the final product crystals is consistent with the chirality of the initial seed. It should be noted that the state of the resulting products is many crystals and not a single one. In addition, Ag 40 nanoclusters can be used as chiral amplifiers to determine the ee values of chiral carboxylic acids. This work not only provides new insight into the chiral origin of nanomaterials but also provides new insight for chiral separation and chiral crystal amplification.
Racemic Ag 40 was first synthesized, and crystals were obtained ( Figure S1a). Details of the synthesis are provided in the Supporting Information. X-ray crystallography revealed that racemic Ag 40 NCs were crystallized in a triclinic P1̅ space group, which is achiral. The crystal structure of Ag 40 can be viewed as a kernel-shell structure, which contains an icosahedral Ag 13 kernel and an Ag 27 complex shell ( Figure  S2). Notably, the Ag 27 complex shell consists of three Ag 7 chains, every two Ag 7 chains are connected by an Ag 2 (CH 3 COO)(TBBM) 4 "button", and the chiral structure is determined by the direction of the chain rotation (Figures 1  and S3). The structure of Ag 40 obtained in this work is not the same as the Ag 40 previously reported; 51 however, it is similar to the doped AuAg 39 nanocluster. 51 The UV−vis absorption spectrum of Ag 40 in CH 2 Cl 2 (DCM) exhibits three characteristic peaks at 460, 500, and 610 nm ( Figure S4a), the corresponding circular dichroism (CD) spectrum shows that Ag 40 is racemic in DCM solution ( Figure S4b). ESI-MS was performed; however, no meaningful signal of Ag 40 was obtained in either the positive or negative ion mode ( Figure  S5). After the addition of cesium ions, the situation did not improve ( Figure S5c). This may be due to the Ag 40 nanocluster being neutral.
The ratio between TBBM and acetic ligands were confirmed by 1 H NMR in Figure S6. Silver or silver-doped nanoclusters undergo rapid racemization in solution. 47 We rationalize that it should be possible for a rapidly racemizing nanocluster to produce a monochirality crystal during the crystallization. There are two processes involved: (i) synthesizing racemic silver nanoclusters; here, the newly obtained Ag 40 nanocluster is chosen as a model in testing our chiral crystallization approach; (ii) screening the crystallization conditions to obtain one crystal in the solution.
In order to achieve rapid screening, we designed a set of high-throughput automated crystallization system (Figures 2  and S10). As such, a grain of homochiral Ag 40 crystal was acquired and divided into five pieces ( Figure 3). All pieces were homochiral and right-handed unraveled by SC-XRD.
Furthermore, the homochiral Ag 40 was found to crystallize in an orthorhombic P2 1 2 1 2 1 space group. There are three important variables to be concerned with in this model: (1) Concentration of the DCM solution. To obtain Ag 40 nanoclusters in one-and fine-crystal form, the concentration of Ag 40 in the DCM solution is very crucial. When the solution is highly concentrated, it gives rise to the primary form of racemic crystals. Here, we control the concentrations to 3, 4, 5, 6, 7, 8, 9, and 10 mg/mL, respectively. (2) Ratio and volume of the diffusion layer. In this experiment, the ratio of the twosolvent diffusion layer is chosen as V DCM :V CHd 3 CN = 1:1, 1:2, and 1:3, and the volume of the diffusion layer is 0.5 or an equal multiple of the Ag 40 DCM solution that is injected. Too much diffusion layer can make the crystal growth difficult. (3) Recrystallization temperature. To explore the temperature gradient influence, temperature gradients of 10, 20, and 30°C are controlled to recrystallize Ag 40 crystals. With the help of the automatic crystallization system, 144 vials of crystallization can be completed uniformly each time (Figures 2g,h and S10). All the crystallization results are shown in Table S1.
Interestingly, in the crystallization conditions of 30°C, concentration of 4 mg/mL, and volume of the diffusion layer (V DCM :V CHd 3 CN = 1:1) was half of the DCM solution, homochiral Ag 40 crystal was successfully acquired (Figure  2i,j). X-ray diffraction identified orthorhombic Ag 40 crystals. This remarkable discovery indicates that the crystal system of Ag 40 crystals is converted from the originally triclinic system (racemic) to the orthorhombic system (homochiral) after recrystallization. Further, Ag 40 nanoclusters show excellent stability in the solution and solid state, which indicates that the clusters are not decomposed during the recrystallization ( Figure S11). Under this set of crystallization conditions, more than 40 crystals from different vials were tested, and two of them were found to be homochiral (one crystal is lefthanded and the other is right-handed) and orthorhombic (P2 1 2 1 2 1 ), therefore the probability of racemic Ag 40 nanoclusters being converted to one homochiral Ag 40 crystal is 5% (Figure 2d,e).
Based on the principle that silver nanoclusters flip rapidly in solution, we have obtained homochiral Ag 40 crystals. Meanwhile, this principle also provides the possibility for the amplification of chiral crystals. We used one homochiral Ag 40 crystal as seed to make the racemic Ag 40 clusters in solution convert into homochiral crystals by the seeded crystallization (Figure 2e,f). Simply, we first identified one homochiral Ag 40 crystal (i.e., left-handed Ag 40 , abbrev. L-Ag 40 ) by SC-XRD. Then, it was placed in a saturated DCM solution of Ag 40 (racemic) and CH 3 CN was added; this L-Ag 40 crystal remained in the crystalline state. After about 1 week, many   (Figures 2j and S1). Expectedly, during the seeded crystallization process, racemic Ag 40 clusters in solution rapidly flip, nucleate, and grow into homochiral crystals in the presence of one chiral crystal. We also note that the chirality of the crystals via seeded growth is consistent with the chirality of the initial seed. It is interesting that the Ag 40 nanoclusters which are crystallized in P2 1 2 1 2 1 space group show optical activity in solid state but no activity in solution. This is due to the rapid racemization of the crystal upon redissolution. The CD spectra and corresponding UV−vis spectra of Racemic-Ag 40 (Rac-Ag 40 ), L-Ag 40 , and right-handed Ag 40 (R-Ag 40 ) crystals in solid state were collected at 250−800 nm (Figures 4a,b and S12a). The CD spectra of L-Ag 40 and R-Ag 40 are in mirror image of each other, confirming that they are pure enantiomers. As shown in Figure 4b A) were calculated over the spectral range; g max is about 5.9 × 10 −4 at 261 nm (Figure 4c). It is worth noting that, using homochiral crystals (i.e., L-Ag 40 ) as seeds, the obtained crystals have CD spectra similar to that of the original crystal (Figure 4d). Notably, the CD value of L-Ag 40 seeded crystals decreased by approximately 10%, compared to the initial seed. Additionally, in order to better analyze the chirality of the samples, the corresponding UV−vis spectra and gfactors of L-Ag 40 crystal and L-Ag 40 seeded crystals in solid state were compared (Figures 4a, S12b, and S13). Notably, the g-factors of L-Ag 40 seeded crystals decreased by about 16%. The g max of L-Ag 40 seeded crystals is about 4.5 × 10 −3 at 260 nm. The possible reason might be that not all crystals were nucleated and grown with the added chiral crystal.
There are 12 Ag 40 molecules around one central Ag 40 molecule in racemic or homochiral Ag 40 crystals ( Figures  S14−S27). The density of the two crystals is calculated. After deducting the influence of the solvent, the density of the racemic Ag 40 crystals is 1.6 g/cm 3 and the homochiral is 1.9 g/ cm 3 , explaining that homochiral Ag 40 crystals has a higher packing density. For the intermolecular weak interactions, the C−H···π and π−π interactions play a vital role in the packing of Ag 40 crystals. 52 In racemic Ag 40 crystals, only Ag 40 molecules that are located in the b-axis direction have the C−H···π interaction with the central Ag 40 molecule, as shown in Figure S28. The average distance for the C−H···π interaction is 3.073 Å for the left or right Ag 40 molecule. In plane 1 of the b-axis direction of homochiral Ag 40 , the average distance of the eight C−H···π interactions is 3.466 Å ( Figure  S29). As shown in Figure S30, the average distance for the C− H···π interaction is 3.560 Å in plane 2 of the c-axis direction of homochiral Ag 40 . The π···π interactions are also discovered, and the average spacing is 5.023 Å. In homochiral Ag 40 crystals, the 12 C−H···π interactions and four π···π interactions allow a central molecule to form a closed-ring structure with eight surrounding molecules in planes 1 and 2, which results in restrictions of the motions of the whole nanocluster. The connectivity via C−H···π and π···π interactions continues all the way along the a-axis of the unit cell, leading to the formation of the needle-like single crystal along the [100] direction. By contrast, C−H···π interactions exist only in a single plane along the [010] direction, which makes a less compact packing in racemic Ag 40 crystals. Under the recrystallization, it is highly likely to obtain the racemic Ag 40 crystal of the P1̅ space group. With a difference in the kinetic barrier (Ea, high for homochiral crystals), such crystals are not easy to form. This also shows that the acquisition of chiral crystals is a thermodynamic process in this case: higher temperatures favor the denser phase, which happens to be the chiral phase.
Furthermore, the Ag 40 nanocluster could react with chiral 2chloropropionic acid/ibuprofen/naproxen to result in an optically pure enantiomer. 1 H NMR and CD spectra showed that the CH 3 COO − ligand on the Ag 40 surface could be rapidly replaced by these chiral acids (Figures S31−S50). The UV−vis spectra of Ag 40 did not change with ligand exchange, indicating that the structure of Ag 40 was retained (Figures S46a and  S50b). The CD spectra of Ag 40 (R-2-chloropropionic acid) and Ag 40 (S-2-chloropropionic acid) showed symmetrical peaks at 241, 275, 337, 423, 462, 507, and 623 nm, respectively ( Figure S46b). The anisotropy factors were calculated, and g max is about 1.1 × 10 −3 at 343 nm ( Figure S46c), which is comparable to that of chiral silver nanoclusters. 53,54 When using Ag 40 to detect chiral compounds such as ibuprofen or naproxen, the maximum CD signal can be obtained when the molar ratio of the chiral compound is around 7 times higher than the cluster ( Figure S47c,d). The 1 H NMR results showed an average of ∼7 ibuprofen or naproxen per cluster ( Figures  S39, S40, S43, and S44). For chiral 2-chloropropionic acid, the CD signals approached the maximum at 1:12 ( Figure S47a). The 1 H NMR results showed an average of ∼6 "2chloropropionic acid" per cluster ( Figures S34 and S36). These results indicate that chiral inversion can be achieved in the Ag 40 clusters if approximately half of the carboxyl groups are replaced by chiral acids. In order to prove that excessive chiral acids can cause the chiral inversion of Ag 40 nanoclusters, the ligand exchange between the homochiral Ag 40 (chiral acids) and the chiral acids with the opposite chirality and the corresponding CD spectra confirm that the opposite chirality can be induced ( Figure S48). 48 Meanwhile, the enantiomerdependent CD intensity in the R/S-2-chloropropionic acid ligand-exchange process also linearly correlates with the ee values, and the detection range was from 0 to 100% ee ( Figure  5a,b). Notably, R/S-2-chloropropionic acid in CH 2 Cl 2 exhibited only one CD signal at 232 nm ( Figure S49), which is close to the far-ultraviolet region and requires high sensitivity for the instrument. For R/S-2-chloropropionic acid, the Ag 40 cluster can effectively extend the detection range from deep ultraviolet to visible light ( Figure S47a). Meanwhile, the Ag 40 nanocluster can be used as chiral amplifiers for chiral carboxyl drugs such as R/S-ibuprofen and R/S-naproxen ( Figures S50  and S51).
In summary, the Ag 40 nanoclusters are analyzed by SC-XRD, UV−vis, 1 H NMR, XPS, TGA, ESI-MS, and CD. Chiral symmetry breaking, absolute asymmetric synthesis, and the amplification of chiral crystals in metal nanoclusters from racemic Ag 40 nanoclusters to homochiral ones are accomplished with the help of the crystallization model. In addition, Ag 40 nanoclusters can be used as chiral amplifiers and provide guidance for cluster-based chiral sensors. Our work demonstrates the possibility of homochiral construction without the influence of chiral factors (including ligands, environment, etc.), nor the external forces. Further, the seeded crystallization method proved that the effect of chiral crystal seed on the chirality construction of final products and amplification of chiral crystals were achieved. The results provide important implications for the transformation of chirality between nanoclusters, chiral separation, the amplification of chiral crystals, chiral detection, and the origin of chirality generation.