Enhanced Surface Ligands Reactivity of Metal Clusters by Bulky Ligands for Controlling Optical and Chiral Properties

: Surface ligands play critical roles in determining the surface properties of metal clusters. However, modulating the properties and controlling the surface structure of clusters through surface-capping agent displacement remain a challenge. In this work, a silver cluster, [Ag 14 (SPh(CF 3 ) 2 ) 12 (PPh 3 ) 4 (DMF) 4 ] ( Ag 14 -DMF , where HSPh(CF 3 ) 2 is 3,5-bis(trifluoromethyl)benzenethiol, PPh 3 is triphenylphosphine and DMF is N,N-Dimethylformamide), with weakly coordinated DMF ligands on the surface silver sites, was synthesized by using a mixed ligands strategy (bulky thiolates, phosphines and small solvents). The as-prepared Ag 14 -DMF is a racemic mixture of chiral molecules. Owing to the unusually high surface reactivity of Ag 14 -DMF , the surface ligands are labile, easily dissociated or exchanged by other ligands. Based on the enhanced surface reactivity, easy modulation on the optical properties of Ag 14 by reversible “on-off” DMF ligation was realized. The mechanism was unraveled and rationalized by density functional theory (DFT) calculations. When chiral amines were introduced to as-prepared products, all eight surface ligands were replaced by amines and the racemic Ag 14 clusters were converted to optically pure homochiral Ag 14 clusters as evidenced by circular dichroism (CD) activity and single-crystal X-ray diffraction (SCXRD). This work not only provides a new insight as to how to modulate the optical properties of metal clusters, but also unveils an efficient way to obtain atomically precise homochiral clusters for specific applications.


Introduction
Metal clusters are deemed as ideal miniatures of organiccapped metal nanoparticles due to their intermediate size regime that bridges the molecules and plasmonic nanoparticles. [1]To date, numerous chemical means have been established for modulating the size, shape and composition of metal nanoclusters for specific applications. [2]Surface-capping agent is one of the most important factors in determining the chemical or physical properties of nanoclusters/nanoparticles.Many studies have revealed that surface ligands on nanoparticles can not only introduce steric effects, but also modulate the surface electronic state of the nanoparticles, which are important for catalysis. [3]For instance, bulky ligands (e.g., thiolates, N-heterocylic carbenes) have been demonstrated to readily promote the surface reactivity of nanoclusters, thereby helping to achieve rapid ligands exchange as well as high catalysis performance. [4]However, modulating the electronic structure of clusters without altering their atomic structures through ligand engineering is still an elusive and unmanageable target.
On the other hand, chiral metal clusters have attracted broad research interest in recent years due to their potential applications, such as asymmetric catalysis, chiral sensing. [5]However, most of the chiral clusters were synthesized in the form of racemic mixtures. [6]Although many efforts (chiral HPLC columns, supramolecular assembly strategy with α-cyclodextrin) have been made to enantioseparate the racemic mixtures, it is still a challenge to produce optically pure clusters through these methods. [7]In order to realize 100% optical purity, chiral ligands are devised and using as protected group for enantioselective synthesis of chiral metal clusters. [8]8a-d, 8f, 9] Thus, devising a general methodology for obtaining chiral clusters with optical purity by commonly used chiral ligands is urgently needed.
Though most of the commonly used thiolate or alkynyl ligands possess strong coordination ability to metals, weakly coordinating ligands (such as CrO4 -, SbF6 -) can also be introduced to metal nanoclusters through rational design. [10]Taking advantage of these weakly coordinated sites, functional or chiral ligands have been introduced on the Ag(I) clusters. [11]It occurs to us that a highly promising way to realize controls of properties of clusters is through surface modification via weakly coordinated sites which possess high reactivities.Inspired by this, we have used a mixed ligands strategy (with bulky thiolates, phosphines and small solvent ligands), to synthesize a chiral silver cluster, [Ag14(SPh(CF3)2)12(PPh3)4(DMF)4] (abbreviated hereafter as Ag14-DMF).Surprisingly, this molecular cluster has unusually high surface reactivities.Furthermore, we show that optical properties and chirality can be modulated by rational design of the solvent conditions and the surface substitution chemistry of these clusters. 10.1002/anie.202101141

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Results and Discussion
Synthesis and Characterization of Ag14-DMF.The Ag14-DMF cluster was synthesized by reducing a mixture of Ag compound, PPh3 and (CF3)2PhSH with NaBH4 in a mixture of three different solvents of DMF, dichloromethane (DCM) and water (see experimental section for more details).Yellow single crystals were grown by diffusion with hexane after ten days.(Figure S1) Singlecrystal X-ray diffraction (SCXRD) revealed that Ag14-DMF is a racemate which crystallized in C2/c space group and the unit cell comprises a pair (Z=2) of enantiomers (Table S1 and Figure S2).No counter ions were found in the unit cell, which means this cluster belongs to two electron superatomic clusters (14-12=2).The structure of Ag14-DMF is similar to the previously reported two electron superatomic cluster, [Ag14(SC6H3F2)12(PPh3)8] (HSC6H3F2 = 3,4-difluoro-benzenethiol), [12] except that four of the phosphine ligands were replaced by DMF solvent molecules (Figure S3).As shown in Figure 1a and b, the 14 silver atoms of the Ag14-DMF form a twisted fcc structure.Each edge of the twisted cube is coordinated by a thiolate ligand.Four PPh3 and four DMF ligands coordinate to the eight corners of the twisted cube.Color legend: green sphere, Ag; yellow sphere, S; pink sphere, P; red sphere, O; blue sphere, N; sky blue sphere, F; grey sphere, C. All hydrogen atoms are omitted for clarity. [15]detailed analysis of the structures revealed that, due to the steric hindrance of the bulky thiolate ligands (the percent buried volume (%Vbur) of HSPh(CF3)2 and HSC6H3F2 have been calculated by SambVca (Salerno molecular buried volume calculation) software [13] , the result for HSPh(CF3)2 was 15.6%, and for HSC6H3F2 was 12.0%.), it is virtually impossible for the cluster to accommodate eight PPh3 ligands as in the case of [Ag14(SC6H3F2)12(PPh3)8] cluster (Figure S3).Instead, only four PPh3 ligands are observed on each Ag14-DMF.The four PPh3 ligands are arranged in a tetrahedral array (with an idealized Td point-group symmetry).The remaining four vertices of the cube are occupied by four DMF molecules.Despite this arrangement, there remains severe steric hindrance between the twelve bulky thiolate ligands and the four PPh3 ligands.As a result, the four silver atoms coordinated with PPh3 are farther from the octahedral Ag6 4+ core with an average (nonbonding) Ag-Ag distance of 3.88 Å (Figure S4a).In contrast, the four silver atoms coordinated with the small DMF molecules are closer to the Ag6 4+ core, with an average bond length of 3.05 Å (Figure S4b and Table S2).Both of these Ag-Ag distances are significantly longer than the average bond length of 2.85 Å within the octahedral Ag6 4+ core.
Interestingly, the crystals of Ag14-DMF exhibits distinctly different colors when dissolved in different solvents.As shown in Figure 2a, pink, orange and light yellow colored solutions were obtained when crystals of Ag14-DMF were dissolved in diethyl ether, methanol and DMF solution, respectively.The UV-vis spectra of Ag14-DMF in commonly used solvents are shown in Figure 2b and S5.When the clusters were dissolved in diethyl ether, DCM, toluene and ethanol solution, two strong bands at 450 and 527 nm and a shoulder peak at 375 nm were observed.In methanol, a strong peak at 380 nm and a shoulder peak at 520 nm were observed.However, only one strong band at around 375 nm appeared in the UV-vis spectra of the clusters in DMF, DMSO and pyridine (Py).Moreover, the Ag14-DMF in different solvents also showed different fluorescent spectra at room temperature.While Ag14-DMF in diethyl ether showed a red emission around 640 nm, two emission peaks at 570 and 620 nm were observed for the clusters dissolved in methanol.Yellow emission with peak at around 560 nm was observed for the DMF solution of Ag14-DMF (Figure S6).Hence, the common solvents studied here can be qualitatively classified into three categories based on their different relative coordination abilities: (a) no (or very weak) coordination ability such as diethyl ether, DCM, toluene and ethanol; (b) weak coordination capability such as methanol; and (c) relatively strong coordination power such as DMF, DMSO and pyridine.Based on this analysis, we reason that the different colors exhibited by Ag14-DMF in different solvents may be due to the weak bonding between DMF ligands and surface Ag atoms.When dissolved in solvents with low coordination abilities (such as diethyl ether and methanol), DMF molecules would dissociate from the cluster (as in ether or DCM) or replaced by the solvent molecules (as in methanol), thereby altering the optical properties of the Ag14 cluster and causing the color change from yellow to orange (in methanol) or to red (in ether or DCM).When yellow crystals of Ag14-DMF were dissolved in a solvent with a higher coordination power (such as DMSO, pyridine), the DMF molecules should be replaced, leading to a small changes in the optical spectrum and the resulting solution remains yellow in color.S3, S4 and S5) The 1 H NMR spectra of the crystals of Ag14-MeOH, Ag14-DMSO and Ag14-Py are shown in Figure S7, S8 and S9, respectively.As shown in Figure S10, the structures of Ag14-MeOH, Ag14-DMSO and Ag14-Py are similar with Ag14-DMF except that the four DMF molecules were replaced by four CH3OH, DMSO or pyridine molecules, respectively.This observation implies that the four DMF ligands are only weakly bonded on the Ag14 cluster.Interestingly, the color of the crystals of Ag14-DMF changed from yellow to dark red gradually when it was exposed to air, and it turned back to yellow again when DMF was added (Figure S11).This process is completely reversible.As shown in the 1 H NMR spectra of the crystals of Ag14-DMF before and after air exposure (Figure S12), the peaks of DMF (8.02, 2.97, 2.97 ppm) almost disappeared after exposure to air, implying that the DMF molecules were readily dissociated.The solid state UV-vis spectrum of the red crystals remained the absorption peaks at the wavelengths similar to those for the ether solution of Ag14-DMF (Figure S13), suggesting that DMF molecules were also dissociated from Ag14-DMF when it was dissolved in diethyl ether.All these observations indicate the DMF molecules in Ag14-DMF are labile.It occurred to us that, given the weakly protected surface Ag sites, the Ag14 cluster should possess high surface reactivity.Modulating the optical properties of Ag14 cluster.The high surface reactivity of the DMF-Ag sites in Ag14-DMF provides an opportunity to modulate the optical properties of Ag14 cluster through external stimuli.For instance, when DMF was added to diethyl ether solution of Ag14-DMF, the color of the solution changed from pink to yellow rapidly.Due to the significantly different absorption profiles of the pink and light yellow solutions, the conversion process can be monitored by absorption and emission spectroscopy.As shown in Figure 3a, upon adding DMF, the color of the solution was changed from pink to light yellow gradually.The variation of the UV-vis spectra and fluorescent spectra are shown in Figure 3b and Figure S14, respectively.With the increase of DMF, the peak at 375 nm increased while the peaks at 527 and 450 nm disappeared concomitantly in the UVvis spectra.In their fluorescent spectra, the red emission at 640 nm changed to yellow emission at 550 nm gradually.Moreover, this process is totally reversible.When DMF extracted from the diethyl ether solution by layering with water, the color of the solution was changed from light yellow to pink immediately, which can be recycled many times without intensity loss in the UV-vis spectra (Figure 3c).Other coordinative solvents can also be used to modulate the optical properties of Ag14.According to the change of UV-vis spectra after adding the same amount of MeOH, DMSO, DMF and pyridine molecules to the diethyl ether solution of the crystals of Ag14-DMF, the bond strength between different solvent molecules and Ag14 can be thus defined in the order of MeOH <DMF ~ DMSO <Py (Figure S15).These results suggest that it is possible to modulate the optical properties of a cluster based on the coordinating power of the solvent due to the high surface reactivity of the Ag14 cluster.
DFT calculations.In order to understand the reason why stimulation with DMF can result in the dramatic change in optical properties of Ag14, we performed DFT calculations for the clusters in different conformations.We used the GPAW software with realspace grid (details are given in Computational Methods section).Structure relaxation confirmed that the experimental structures are stable local energy minimum structures.During the relaxation with the PBE-functional, the metal core expanded slightly but the overall structure remained otherwise intact.The differences in the coordination ability and strength of the weakly bound ligands (MeOH, DMF, DMSO, Py) were estimated with a full ligand exchange reaction, e.g., Ag14-DMF + 4Py > Ag14-Py + 4DMF, in which the incoming and the detached ligands were treated as isolated molecules.By comparing energetics between all cluster pairs could be ordered from the strongest to the weakest binder as follows: 1) DMSO, 2) DMF, 3) Py and 4) MeOH.A list of all calculated energies is given in Table S6.The difference to the experimental result may be explained by the solvation energies which are missing in our calculations and would be difficult to calculate reliably with DFT.
The most important unrevealed observation relates to the change of optical spectrum when stimulating the Ag14-DMF sample with DMF.Based on the hypothesis that the detachment of DMF creates a spectrum with three peaks whereas the intact Ag14-DMF cluster shows only one main peak.To explain the experimental observations we formed a model structure by removing all weakly bound DMF-ligands from the surface and relaxed the system to the local minimum configuration.Calculated spectra in Figure S16 show two distinguished peaks at 471 nm and 335 nm for the intact cluster with DMF and three for the cluster without DMF-molecules at 473 nm, 384 nm and 331 nm.Surprisingly, the positions of the peaks are in good agreement with the experimentally measured spectrum for the latter, but the original Ag14-DMF cluster has clear discrepancy at low energy spectrum.Note that the measured spectral series in Figure 3b show also a spectrum with two features, but at the higher 10.1002/anie.202101141

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extremum of DMF addition the lowest energy peak at 527 nm finally vanishes.At this point the nature of the peaks is important to be clarified, for which time dependent perturbation theory was used.The analysis was done with so called dipole transition contribution maps visualizing the strengthening and screening excitations to the peaks in Kohn-Sham basis.Figure 4 and Figure S17 show clearly that in both studied clusters the lowest energy peaks are from HOMO to LUMO type of transitions.These states are localized to the metal core and correspond to 1S to 1P type of superatom transitions based on superatom electron state symmetries shown in Figure 5.Both of the states are well separated from the ligand states.At higher energies the ligands states start to contribute, the highest energy peak being solely from ligand-to-ligand type of transitions.States that are localized to DMF ligands are not involved in the contributions as they are localized further away from the Fermi energy and therefore is an underlying indirect effect to the structure that changes the lower energy spectrum under stimulation with DMF.Obviously, that change correlates with the changes in the superatom states.For comparison we calculated the spectrum of the analogous, [Ag14(SC6H3F2)12(PPh3)8]. [12]All-phosphine cluster with the same level of theory which is shown in Figure S18.Calculations give only one clear peak in the spectrum at 415 nm.The reason can be seen from the electronic structure.For the all-phosphine cluster the HOMO-LUMO gap is 0.3 eV larger (2.65 eV) compared to the same of Ag14-DMF cluster (2.35 eV).The HOMO-LUMO gap of Ag14 cluster without DMF was 2.31 eV.In addition the superatom 1S and 1P states are accompanied with the ligand states and not separated as shown in Figure 5c.As a consequence, the optical spectrum gets more monotonous regardless of the formation of one main peak which is still mostly caused by 1S to 1P type of transitions as visualized in Figure S19.

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DFT Molecular Dynamics simulations.We performed molecular dynamics (MD) simulations for Ag14 clusters with and without DMF, using Langevin dynamics at room temperature in total for 7.5 ps and 8.6 ps, respectively.The structural evolution is shown in the SI-videos.No major deformations were seen in the structures by qualitative inspection.To reveal the effects of smaller deformations to the optical spectra, 8 snapshot structures were taken after thermalization (at 3.0 ps) approximately at each 300 steps.The spectra show rich variation in shape including everything between one and three distinguished peaks, see Figure S20.Interestingly, for the intact cluster with DMF ligands there are more spectra with single peak and cluster without DMF has more spectra with three peaks and lower energy features.The average spectra for the ensemble in Figure 6 confirm this qualitatively inspected difference.The result is close to the measured spectra and prominently different from the calculated result got for the static, crystal structure-based clusters.It can be further confirmed that during MD-simulation the HOMO-LUMO gap is on average larger for Ag14 cluster with DMF (2.00 eV) than without DMF (1.89 eV).The gap is lowered in both systems as compared to static structures but the relative difference in the gap is getting larger.Because the initial static Ag14-DMF structure for MD-simulation was not supporting the experimental observation, it can be expected that the same qualitative behavior even with larger time scales and increased sampling.Obviously, the time scale used here is long enough to capture the required structure dynamics and deformations behind the observation.Nevertheless, considerable increase in the time scale and in the sampling would require significantly more computational resources.
Conversion of the racemic mixtures of Ag14-DMF to homochiral Ag14 clusters.Based on the above observations, it occurred to us that chiral amines may be introduced to form optically pure chiral Ag14 clusters.As is well known, chiral amines are often used to modify nanoparticles for chirality-related applications. [14]However, it is often difficult to incorporate amine ligands on clusters due to the weak coordination ability of amines in comparison with thiolate, alkynyl, or phosphine ligands.The situation is different here.The high surface reactivity of Ag14-DMF induced by bulky thiolate ligands described above provides an excellent opportunity to achieve this goal.In this work, R-1phenylpropylamine (R-1-PPA) and S-1-phenylpropylamine (S-1-PPA), a pair of commercially available chiral amines, were chosen.These two enantiomeric amines exhibit two peaks at 243 and 291 nm, as mirror images, in the CD spectra (Figure S21).Here they were used to post-modify the ligand shell of Ag14-DMF to form the corresponding enantiomeric pair of [Ag14(SPh(CF3)2)12(R-1-PPA)8] and [Ag14(SPh(CF3)2)12(S-1-PPA)8] (abbre.as Ag14-R-1-PPA and Ag14-S-1-PPA, respectively.Synthetic details can be found in the Experimental Section).To the best of our knowledge, the present work is the first time that chiral amines were incorporated into a silver cluster to realize chirality.
SCXRD revealed that the homochiral Ag14 clusters Ag14-R-1-PPA and Ag14-S-1-PPA crystallized in chiral space group (C2).(cf.Table S7 and S8) The high purity of the 1 H NMR spectra of the crystals of Ag14-S-1-PPA indicates the homogeneity of the samples (Figure S22).As shown in Figure S23, the resulting enantiomeric clusters exhibit perfect mirror symmetry.The overall structures of Ag14-R/S-1-PPA are similar to Ag14-DMF, except the DMF and PPh3 ligands in the outer shell of Ag14-DMF were substituted by eight chiral amine ligands.Detailed analysis of the molecular structures of Ag14-DMF revealed that, initially, the silver framework of this cluster conforms to an idealized Oh symmetry and is achiral.However, due to severe steric hindrance between the twelve bulky thiolate ligands and the eight chiral amines (or four PPh3 and four DMF), the thiolate ligands of Ag14 clusters are heavily twisted, lowering the point-group symmetry to C2 symmetry (Figure 7a, b) and the cluster becomes chiral.For Ag14-DMF, the twisting directions are equally probable, thereby producing a racemate.For Ag14-R-1-PPA and Ag14-S-1-PPA, the chiral amine ligands induce the formation of optically pure chiral clusters.The degree of twisting of the sulfur, silver and nitrogen atoms can be measured as twist angles with reference to the Ag6 core (viewed along the C2 symmetry axis) (Figure S24) and the results are shown in Table S9.Color legend: green sphere, Ag; yellow sphere, S; pink sphere, P; blue sphere, N; red sphere, O. [15] 10.1002/anie.202101141

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As shown in Figure 8a, the UV-vis spectrum of Ag14-R-1-PPA exhibit one strong band at 370 nm, which is slightly blue-shifted compared to the Ag14-DMF in DMF (380 nm).Calculated optical absorption spectrum in Figure 8b shows two lower intensity features in range 375-500 nm and a steep increase below it, associated with a high energy feature at 325 nm.The circular dichroism (CD) spectra of the crystals of Ag14-R-1-PPA and Ag14-S-1-PPA in diethyl ether solution give perfect mirror images from 310 to 440 nm and two clear signals (minima or maxima: 340 and 375 nm) are observed.(Figure 8c) The calculated CD spectrum of Ag14-R-1-PPA is in good agreement with the measured spectrum showing similar minimum and maximum signals at the studied wavelength range (Figures 8c and d).Positions of the calculated signals are seen at 325 nm and 360 nm.Apparently the mentioned lowering of the structural symmetry connects to the changes in the optical spectra similarly as in the non-chiral systems.But here the structural changes are driven by the assembly of chiral ligands on the surface of cluster and the structure is probably more rigid.
Interestingly, when the crystals of Ag14-S-1-PPA were dissolved in 5.0 × 10 -2 mmol/mL R-1-PPA diethyl ether solution, it will convert into Ag14-R-1-PPA.These conversions have been confirmed by SCXRD and CD spectral evidence (Figure S25).These observations confirm the high surface reactivities of the Ag14 clusters that provide an excellent opportunity for reversible dynamic control of their physical and chemical properties.

Conclusion
In conclusion, a silver cluster with weakly coordinated DMF ligands on the surface was synthesized through a large and small steric ligands mixed strategy.Through its high surface reactivities, we achieved modulating the optical properties of Ag14 cluster by removal or addition the DMF ligands.DFT calculations revealed that DMF ligands "on-off" on the silver sites changes the electron structure of Ag14 cluster, which further influence the optical properties.The best agreement to experiments is seen for dynamical ensemble of cluster structures which indicates the sensibility of the optical response to small deformations in structural symmetry of the cluster.Furthermore, chiral amine ligands (R/S-1-PPA) are successfully used to convert the racemic mixtures of Ag14-DMF to homochiral Ag14 clusters.This is not only the first report of enantioselective conversion of racemic mixtures to homochiral clusters exhibiting 100% optical purity, but also opens the door for synthesis of atomic precise chiral clusters using chiral amine ligands.We believe the strategy for modulating the optical properties and generating chirality of metal clusters reported in this paper are useful in clusters design and chiral applications.

Figure 2 .
Figure 2. Photographs (a) and UV-vis spectra (b) of the crystals of Ag14-DMF dissolved in diethyl ether, methanol and DMF solutions.

Figure 3 .
Figure 3. a) Photographs of the diethyl ether solution of Ag14-DMF when adding different amount of DMF.b) UV-vis spectra variation of the diethyl ether solution of Ag14-DMF when adding different amount of DMF.c) UV-vis spectra of Ag14-DMF in diethyl ether after add-remove DMF in different cycles.

Figure 4 .
Figure 4. Dipole transition contribution map of Ag14-DMF cluster: a) 471 nm; b) 335 nm.Contour plot top left corner shows the strengthening (in red) and screening (in blue) contributions to the transition dipole moment of the peak labeled in lower right corner optical spectrum.Lower left and top right panels shows the occupied and unoccupied Kohn-Sham states projected to different parts (DMF, thiolates=SR, phosphines=PPh3 and silver=Ag) of the structure.

Figure 5 .
Figure 5. Electronic structure of Ag14 clusters with DMF in (a) and without DMF in (b) as a projection to spherical harmonics functions that denote the superatom state symmetries as labeled in figure.The same analysis is shown for [Ag14(SC6H3F2)12(PPh3)8] cluster in (c) and for the chiral [Ag14(SPh(CF3)2)12(R-1-PPA)8] cluster in (d).

Figure 6 .
Figure 6.Ensemble-averaged optical absorption spectra of Ag14 cluster with and without DMF from DFT MD simulations.

Figure 8 .
Figure 8. a) Experimental UV-vis spectrum of Ag14-R-1-PPA.b) Calculated UVvis spectrum of Ag14-R-1-PPA.c) Experimental Circular dichroism (CD) spectra of the enantiomers of Ag14-R/S-1-PPA.d) Calculated CD spectrum of Ag14-R-1-PPA.Due to the dynamic nature of amine-Ag sites in solution, the crystals of Ag14-R/S-1-PPA are dissolved in 2.0 × 10 -2 mmol/mL R/S-1-PPA diethyl ether solution for optical spectra measurement.In panel (b) and (d) the most important peaks are labeled with A and B.