Introduction

Over the last decades, the chemistry of atomically and structurally precise Au and Ag nanoclusters (NCs) and their alloys have gained a broad attention in modern science owing to their potential applications in catalysis, optoelectronics, electrochemical studies, chemical sensing, biomedicine and chiral cluster syntheses1,2,3,4,5,6,7,8,9. Size focusing synthesis in combination with atomic precision studied by X-ray crystallography sets apart these ultra-small sub-nanometer size NCs from their colloidal analogs for achieving aforementioned properties. To date, hundreds of atomically defined ligand-protected coinage metal NCs and their alloys have been synthesized. In this regard, the more common protecting ligands employed to isolate different NCs are thiols, phosphines, alkynes, hydrides or their combinations1,10,11,12,13,14,15,16. In contrast, structurally precise Au and Ag clusters co-protected by selenols are much rarer, with approximately twenty reported examples17,18,19,20,21,22. In order to illuminate the effects of surface functionalization of nanoparticles, recent reports have demonstrated the fabrication of several stable functional nanomaterials by using selenolates in the place of thiolates as protecting ligands. Unlike in thiolate-protected NCs, true structural isomerism, which is an interesting feature for fine tuning many properties, has not been reported so far for selenolate-protected NCs. Thus, the syntheses of selenolate-protected NCs are of paramount importance.

The initial attempt in the synthesis of a selenolate protected cluster, namely Au25(SeC8H17)18−, was made in 2011 by Negeshi and his coworkers via the reduction of an Au(I) selenolate complex by NaBH423. Subsequently, the same group also synthesized Au38(SeC12H25)24 from Au38(SR)24 via a ligand exchange (LE) method24. Zhu et al. demonstrated that selenophenolate-protected Au1825 and Au2511 NCs exhibit different optical properties from those of their thiolate homologs. Later, studies on Au24(ER)20 (E = Se or S) NCs unraveled different structures and optical properties between both families of chalcogenolates26. In 2013, Pradeep and his co-workers reported the first atomically precise silver NC protected by selenolates, Ag44(SePh)30, which revealed similar properties as its thiolate counterpart27. More recently, Bootharaju et al. reported a Cd-doped silver NC protected by selenophenolates, namely Cd12Ag32(SePh)36, which exhibits rare near-infrared (NIR) photoluminescence at ∼1020 nm18.

As part of our research efforts in the synthesis of selenolate-protected Ag clusters, we have isolated a series of diselenophosphate (dsep) protected mono- and bimetallic silver clusters such as [Ag7(H){Se2P(OiPr)2}6]28, [Ag10(Se){Se2P(OiPr)2}8]29, [Ag8(X){Se2P(OiPr)2}6]+ (X = H, Cl or Br)30, [Ag11(µ9-Se)(µ3-X)3{Se2P(OiPr)2}6] (X = I31, Br32), [Ag11(µ9-I)(µ3-I)3{Se2P(OiPr)2}6]+ 33, [Ag12(μ5-X)2{Se2P(OEt)2}10] (X = Br, I)34 [Ag20{Se2P(OiPr)2}12]20, [Ag21{Se2P(OEt)2}12]+ 20, [PtAg20{Se2P(OR)2}12] (R = iPr or nPr)21 and [AuAg20{Se2P(OEt)2}12]+ 20. In fact the latter species, fabricated via a ligand exchange method, is the first structurally characterized alloy NC entirely covered by a Se ligand shell. We have produced several Pt/Pd doped Ag NCs of late21,35,36, among them the M21 core metallic system predominates (See Supplementary Table S1). Thus, we intended to outspread our approach in the development of dsep-protected PdAg20 alloy NCs. Amid the several synthetic methods available, the ligand exchange method37,38,39,40 is one of most fruitful strategies to yield molecularly pure NCs stabilized by dsep ligands. Herein we report the isolation of a series of 8-electron superatomic, dichalcogenolate-protected PdAg20 alloy NCs that include a pair of selenolate-protected isomers.

Results and discussion

Synthesis and characterization of [PdAg20{S2PR2}12], R = OiPr (2a), R = OiBu (2b), R = Ph (2c)

Beforehand, we have synthesized and structurally characterized the thermodynamically stable alloy [PdAg20{S2P(OnPr)2}12] (1). This cluster can be formally regarded as a centered icosahedral [Pd@Ag12]4+ 8-electron superatomic core (with 1S2 1P6 1D0 superatomic configuration) passivated by an outer shell made of eight Ag+ and twelve monoanionic dithiophosphate (dtp) ions in such a way the complete PdAg20 metallic kernel is lowered to ideal C2 symmetry and the whole NC to C136. NC 1 is robust, yet the liability of its protecting ligands tempted us to study its ligand exchange (LE) behavior. As shown in the Fig. 1, the treatment of 1 with 12 equivalents of NH4[S2P(OiPr)2] at −20 °C in tetrahydrofuran (THF) led to the formation of [PdAg20{S2P(OiPr)2}12] (2a) in 70% yield within an hour. There was no obvious color change observed when NH4[S2P(OiPr)2] was added to the brown red solution of 1 in THF during the course of reaction, thus the progress of reaction was monitored by thin layer chromatography (TLC). Alternatively, the same compound was produced more conveniently by direct archetypal one pot synthetic method in moderate yield (41 %) (See Experimental methods). In parallel to the synthesis of 2a, compound [PdAg20{S2P(OiBu)2}12] (2b) was synthesized via co-reduction method in 40% yield. Compound [PdAg20{S2P(OPh)2}12] (2c) was synthesized via the LE method in 65% yield (Experimental methods). All NCs (2a–c) have been characterized by positive ion mode electrospray ionization mass spectrometry (ESI-MS) and nuclear magnetic resonance (NMR) spectroscopy. ESI mass spectra of 2a–c have been provided in Table 1, Fig. 2 and Supplementary Figs. S1 and S2. 31P{1H} and 1H NMR spectra of 2a–c have been provided in Table 1 and Supplementary Data 1, Figs. 1–5.

Fig. 1: Synthesized compounds.
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Synthesis of dichalcogenolate protected Pd-Ag alloy NCs 2a, 3 and 4 via ligand exchange reaction.

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Table 1 Spectroscopic data of compounds for 1, 2(a–c), 3 and 4.
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Fig. 2: Mass spectrometry.
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ESI-MS (Positive mode) of [2a + Ag]+. Insets: experimental (black) and simulated (red) mass spectra.

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Fig. 3: Molecular structures of 1, 2a and 2b.
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a, b Total structure of 1 (propoxy groups omitted for clarity) and its Pd@Ag20 metallic core with C2 symmetry, respectively. c, d Total structure of 2a (isopropoxy groups omitted for clarity) and its Pd@Ag20 metallic core with C2 symmetry, respectively36. e, f Total structure of 2c (isobutoxy groups omitted for clarity) and its Pd@Ag20 metallic core with D3 symmetry, respectively. (color code. Pd: orange; Agico: pink, Agcap: green; S: yellow; P: sky blue).

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Fig. 4: Molecular structure of 3(Pn).
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a Total structure of [PdAg20{Se2P(OiPr)2}12] (3(Pn)) (isopropoxy groups omitted for clarity), b Illustration of the Pd@Ag20 metallic core in 3(Pn) with Th symmetry, c A view of the Pd@Ag12 centered icosahedron core with its 8 capping Ag atoms, d The centered Pd@Ag12 icosahedron inscribed in Ag8 cube (color code. Pd: orange red; Agico: pink, Agcap: green; Se: light orange; P: light blue).

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Fig. 5: The molecular structure of 3a and 3b.
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a The two co-crystallized structures of [PdAg20{Se2P(OiPr)2}12], 3a (left) and 3b (right). Isopropoxy groups have been omitted for better clarity. b The Th Pd@Ag20 metallic core of 3a. c The C3 Pd@Ag20 metallic core of 3b. (color code. Pd: orange; Agico: blue, Agcap: pink; Se: light orange; P: dark green).

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Upon the replacement in 1 of the dithiophosphates ligands having linear alkyl chain (n-propyl) with their branched derivatives (di-isopropyl dithiophosphates), the 31P{1H} NMR spectrum in CDCl3 displays a signal shift from 104.9 ppm to 101.66 ppm at room temperature (Supplementary Fig. S3). The 1H NMR spectrum of 2a in CDCl3 shows two set of signals with a multiplet ranged at δ = 4.97–4.85 ppm (corresponding to the –OCH groups) and a doublet ranged at δ = 1.35–1.33 ppm (corresponding to –(CH3)2), in an integration ratio of 1:6 which is clearly attributed to the iPr groups (Supplementary Data 1, Fig. S2).

The ESI mass spectrum (positive-ion mode) was recorded to identify the molecular formula of 2a. The spectrum reveals two prominent bands corresponding to [2a + Ag]+ at m/z 4931.15 (calcd. 4930.97), and [2a + Ag+H]2+ at m/z 2465.59 (calcd. 2465.95). Their simulated isotopic distributions are in good agreement with the experimental results (Fig. 2). The position of the molecular ion peak in 2a matches exactly with its parent NC 1, signifying the retention metal atomicity upon LE. Moreover, the UV–vis absorption spectrum of 2a features the same absorption pattern (384, 436 nm) as its parent NC 1. Thus, from the above spectroscopic data obtained in solution state, one would presume that the structure of 2a is the same as 1.

Similarly, the 31P{1H} NMR spectrum displays one type of resonance for 2b (δ = 103.8 ppm) and for 2c (δ = 63.1 ppm). The 1H NMR spectra of 2b and 2c displayed three and two types of resonances, corresponding to iso-butoxy and phenyl groups, respectively. Further, the positive ion mode ESI mass spectra of 2b and 2c show prominent bands corresponding to [M + Ag]+ at m/z 5267.64 (Calcd. 5267.62) and [M + 2Ag]2+ at m/z 2735.7615 (Calcd. 2735.8916), respectively. In order to elucidate the structure of these nanoalloys (2a–c), single crystals X-ray diffraction studies were undertaken. We were successful to crystallize 2a and 2b. The details of their X-ray structural analysis were discussed below. All of our attempts to crystallize 2c were failed.

Single crystals of suitable quality for X-ray diffraction for 2a and 2b were grown by crystallization from diffusion of hexane into a concentrated dichloromethane solution at −4 °C within couple of weeks. Surprisingly, the resulting solid-state structures unveil different configuration of the outer shell which protects the 8-electron [Pd@Ag12]4+ core, as illustrated in Fig. 3 compared to that of 1. In particular, the arrangements of the 8 Ag+ capping atoms around the centered icosahedral core differs from that in 1, as one can see in Fig. 3 (from Fig. 3b→3d→3f). Whereas in both 1 and 2a NCs the PdAg20 skeleton adopts pseudo-C2 symmetry; that of 2b the PdAg20 skeleton adopts pseudo-D3 symmetry (Supplementary Fig. S4). The twelve dtp ligands in 2a are equally distributed on both sides of the pesudo-C2 axis (Supplementary Fig. S4). These dtp ligands are coordinated to both icosahedral and capping silver atoms (Agico and Agcap, respectively) in five different binding modes bimetallic biconnectivity (η2: µ1, µ1), bimetallic triconnectivity (η2: µ2, µ1), trimetallic triconnectivity (η3: µ2, µ1), trimetallic tetraconnectivity (η3: µ2, µ2) and tetrametallic tetraconnectivity (η4: µ2, µ2) (Supplementary Fig. S5) in a ratio of 1:1:7:1:2. Further the seven dtp ligands with trimetallic triconnectivity (η3: µ2, µ1), differ in the coordination to different combination of Agcap and Agico atoms except for a couple of dtp ligands (red box, in Supplementary Fig. S5). As in any 8-electron dtp- or dsep-protected M21 NC characterized so far, the eight Ag+ capping atoms in 2a lie in a nearly planar AgSe3 coordination mode, making locally stable 16-electron metal centers. With the 12 protecting ligands around the PdAg20 metal skeleton (Fig. 3c), the entire molecular symmetry of 2a is C1. On the other hand, the total twelve dtp ligands in 2b are distributed in three spherical rows around the pseudo-C3 axis in 3:6:3 ratios (Supplementary Fig. S4). They bind to both capping and icosahedral silver atoms only through two coordination patterns that are trimetallic triconnectivity and trimetallic tetraconnectivity (Supplementary Fig. S6), in such a way the whole NC ideal symmetry is reduced to C3. A similar C3 arrangement has been described in the related 8-electron NC [Ag21{S2P(OiPr)2}12]+ 41.

The inner icosahedral Pd@Ag12 cores of 2a and 2b are very similar to that of 1. The Pd-Ag radial bond distances average 2.755 Å, 2.767 Å for 2a and 2b, respectively (2.757 Å in 1) and the peripheral Agico-Agico and Agico-Agcap bond distances in 2a and 2b are also fairly similar to those of 1 (Table 2). Thus, the 8-electron nanoalloys 1, 2a and 2b whose compositions differ only by the nature of their alkyl substituents, can be considered as pseudo-isomers. The presence of different arrangements of their outer shells is likely the result of the slightly different steric factors of their alkyl chains in 1 and 2a.

Table 2 Selected structural metrics (average (top line) and ranges (bottom line)) of compounds for 1, 2a-b, 3, 3a-b and 4.
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[PdAg20{Se2P(OiPr)2}12], (3) and its 3(Pn) solid-state structure

After successful isolation of the 2a–c NCs, it was indeed obvious to attempt the synthesis of their diselenophosphate (dsep) protected analogs via LE reactions. Compound 1 was treated with NH4[Se2P(OiPr)2] at −20 °C in THF (Fig. 1). The reaction proceeded immediately as the color of the reaction mixture altered from brown red to purple which indicates replacement of surface dtp ligands by dsep ligands (Supplementary Fig. S7). The possibility of partial replacement of ligands cannot be excluded, however we have never encountered the partial replacement when we intend to produce dsep protected NCs from their dtp siblings via LE method20,21,22,28,29,30,31,32,33,34. In particular, most of the cases these reactions are associated with alteration of the metallic cores20,21,22,28,29,30,31,32,33,34. The change in metallic core is certainly a reason for the distinct color change, however we believe the change in the ligand environment plays a major role for the immediate color change. Note that, when we performed ligand exchange reaction onto 1 with n-propyl dithiophosphate surface ligand by iso-propyl dithiophosphate ligand, then there was no remarkable color changes observed, even though reaction culminated in the formation of altered metal core. The positive ion ESI mass spectrum of reaction mixture shows a prominent band or molecular ion peak at m/z = 6055.77 (calcd. 6055.23) corresponding to [PdAg20{Se2P(OiPr)2}12 + Ag]+ (Supplementary Fig. S8).

In order to determine its molecular structure, much effort was devoted to obtain suitable crystals for single crystal X-ray diffraction. Several sets of crystallization with varied solvent combinations and in mutable temperatures ended up producing extremely bad quality crystals. Nevertheless, diffusion of hexane into a saturated acetone solution of the reaction mixture kept at −10 °C yielded proper single crystals within a week. The crystals of 3 obtained from this low temperature crystallization were dissolved in d6-acetone and subjected to 31P{1H} and 1HH NMR studies. The 31P{1H} NMR spectrum at ambient temperature shows a single resonance at δ = 68.16 ppm (161.9 MHz, d6-acetone) flanked with two set of satellites (1JP-Se = 604.51 and 710.40 Hz) (Supplementary Data 1, Fig. 6). 1H NMR spectrum shows characteristic signals of isopropyl ligands of di-isopropyl diselenophosphates (Supplementary Data 1, Fig. S7).

Fig. 6: Illustration of packing diagrams of the co-crystallized NCs 3a and 3b with dsep ligands omitted for clarity.
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a View down the b axis. b View down the c axis (color code: T layers are in blue and C3 layers in pink.).

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Fig. 7: Photophysical properties.
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UV–vis spectrum (left) of 1, 2a, 2b and 2c in 1 ×10−5 M CH2Cl2 and the normalized emission spectrum (right) of 1, 2a, 2b and 2c in MeTHF at 77 K.

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Single crystals obtained were subjected to the X-ray diffraction study. Their analysis reveals that 3 crystallize in Pn space group. Its solid-state structure is labeled 3(Pn) in the following. It is shown in Fig. 4 and exhibits a Pd-centered icosahedral Ag12 core inscribed in a cube made up of 8 capping Ag atoms, in such a manner that the entire Pd@Ag12@Ag8 framework attains ideal Th symmetry. The whole metal kernel is protected by 12 dsep ligands situated on the 12 edges of the cube, in such a way that the whole NC ideal symmetry is reduced to T. The detailed molecular structure of 3(Pn) is identical to that of 3a, discussed in the next section below.

It should be also mentioned that the molecularly pure as-synthesized NC 2a also was used as starting precursor for LE reaction in order to produce 3. Therefore, the reaction of 2a with NH4[Se2P(OiPr)2] at −20 °C in THF was performed (Fig. 1). Likewise, in the transformation from 1 to 3 the solution undergoes an instant color change from brown to purple. The work up of the reaction mixture was done immediately. The 31P{1H} NMR spectrum of the reaction mixture exclusively shows single resonance at δ = 68.16 ppm in d6-acetone, the same resonance as observed in 31P{1H} NMR spectrum of 3. Thus, the transformation from 2a to 3 was confirmed by 31P{1H} NMR spectroscopy.

The solid-state structure of [PdAg20{Se2P(OiPr)2}12], 3(P31 c)

Single-crystals of 3 could also be obtained by slow diffusion in hexane into the saturated acetone solution of reaction mixture at ambient temperature. Their X-ray analysis reveals that in such conditions 3 crystallizes in the P31c space group. This solid-state phase is labeled 3(P31c) in the following. It reveals an isomeric NC pair of [PdAg20{Se2P(OiPr)2}12] clusters (3a and 3b), co-crystallized in the unit cell in a 1:1 ratio, with T and C3 pseudo-symmetry, respectively (Fig. 5)42. The molecular structure of 3a (T symmetry) is the same as that of 3(Pn), as well as that of the previously reported isoelectronic monocationic [MAg20{Se2P(R)2}12]q+ (M = Ag or Au; R = OEt; q = 1: M = Pt; R = Oi/nPr; q = 0)20,21. The structural metrics of 3a and 3(Pn), are similar (Table 2). Their icosahedral Pd@Ag20 core embedded within a cuboid made of eight capping Ag atoms, resulting in a PdAg20 framework of Th symmetry, is shown in Fig. 4b–d. Their twelve dsep ligands display trimetallic triconnectivity (η3: µ2, µ1) bridging pattern with two capping Ag atoms (Agcap) and one icosahedral Ag atom (Agico), reducing the whole ideal NC symmetry to T. The molecular structure of 3b (C3 symmetry) is similar to that of 2b (see above) and [Ag21{S2P(OiPr)2}12]+ 41. The differences in the positions of the outer capping Ag atoms in 3a and 3b, and their possible interchange pathway, is illustrated in Supplementary Fig. S9. To the best of our knowledge clusters 3a and 3b constitute the first pair of true isomers within the family of Se-protected NCs certified by X-ray crystallography. The Pd-Agico average distances in 3a and 3b are equivalent (2.758(10) Å and 2.754(10) Å, respectively), as well as their average Agico-Agico distance (2.901(9) Å and 2.896(10) Å, respectively). Thus, the structure of the Pd@Ag12 core in 3a and 3b is quite independent from the configuration of the outer sphere (Table 2). The average Agico@Se distance in both 3a and 3b are larger than the Agcap@Se distances. The Se···Se bite distances in 3a and 3b are fairly similar (Table 2) and slightly shorter than those observed in [Ag21{Se2P(OEt)2}12]20 (3.697 Å) and [AuAg20{Se2P(OEt)2}12]20 (3.697 Å).

The two isomers assemble in a layer-by-layer mode. Each layer consists of pure 3a(T) or 3b(C3) (Fig. 6a). The T and C3 layer are alternately stacked along the [001] direction (Fig. 6b). The three-fold rotational axes of 3a and 3b are parallel to the c axis of the trigonal lattice. Finally, it is worth mentioning at this point that the isomer selectivity of the low temperature crystallization (3(Pn), T isomer) facilitates its further spectroscopic characterizations.

[PdAg20{Se2P(OnPr)2}12], (4)

Given the synthesis of 2a-3 via ligand-exchange-induced structure transformation route it is indeed inevitable not to synthesize another normal propyl alkyl chain analog. Note that the precedence of structurally precise selenium protected alloy clusters is awfully inadequate. Thus, as shown in Fig. 1, we have endeavored the ligand replacement of dithiophosphates on 1 by diselenophosphates with linear alkyl chain (n-propyl). The reaction leads to the formation of [PdAg20{Se2P(OnPr)2}12] (4) in 75 % yield. Its 31P{1H} spectrum in CDCl3 displays a signal at δ = 73.59 ppm flanked with two set of satellites (1JP-Se = 604.51 and 710.40 Hz) at room temperature (Supplementary Data 1, Fig. 8). The 1H{31P} NMR spectrum of 4 in CDCl3 reveals three set of signals with multiplets ranged at δ = 4.03–4.02 ppm (corresponds to –OCH2 group), δ = 1.78–1.70 ppm (corresponds to –CH2) and at δ = 0.95–0.92 ppm (corresponds to –CH3) in an integration ratio of 1:1:1.5, which is clearly attributed to nPr group of di-propyl diselenophosphate ligands (Supplementary Data 1, Fig. S9). The ESI mass (positive-ion mode) spectrum shows a prominent band owing to [4 + Ag]+ at m/z = 6056.02 (calcd. 6056.49), and its simulated isotopic distribution is in good agreement with the experimental one (Supplementary Fig. S10). Based on these spectroscopic evidences, the molecular structure of 4 should adopt the same T arrangement as that of 3a. This is confirmed by the solid state structure of 4 obtained from single-crystal X-ray diffraction (Fig. 4 and Supplementary Fig. S11). Its structural metrics are similar to those of 3(Pn) and 3a (Table 2). Interestingly, 4 crystallize as a racemate in the P21 space group.

Fig. 8: Experimental and theoretical photophysical studies of title clusters.
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a UV–vis spectrum of 1, and 3 in 1 ×10−5 M CH2Cl2 (left) and emission spectrum of 1 and 3 in MeTHF at 77 K (right). b, c Time-resolved photoluminescence spectrum of 2a and 3 at 77 K. d The TD-DFT-simulated UV–vis spectrum of 2a’, 3a’ and 3b’.

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Optical properties of the title NCs

It is interesting to note that the side chain in dithiophosph(in)ate ligands can lead to the variance of photoluminescence properties. The differed alkyl chains such as n-propyl (1), i-propyl (2a), i-butyl (2b), have least variance and look reddish while the phenyl derivative (2c) which was only obtained by ligand exchange appear to be orange to the naked eye. The UV–Vis spectra of 1, 2a and 2b show similar broad optical absorption bands at 384 and 436 nm, the latter band being intense (Table 1, Fig. 7). On the other hand, the phenyl derivative 2c features different absorption bands (419 and 486 nm) where the former is found to be more intense (Fig. 7). The absorption bands in 2c are red-shifted to their alkyl relatives. The change from alkyl to phenyl of the dtp substituents can alter the photoluminescence intensity. Compounds 1, 2a, and 2b show photoluminescence in their solution state at 77 K. Their emission maxima in 2-methyl tetrahydrofuran (MeTHF) occur at λmax = 748 nm, 741 nm and λmax = 732 nm, respectively (Fig. 7 and Supplementary Figs. S12 and S13). Cluster 2c is also emissive in solution state at 77 K. Its emission maximum appears at 669 nm in MeTHF (Fig. 7 and Supplementary Fig. S13) which is blue shifted to its parent cluster 1.

The UV–vis spectra feature two major broad absorption bands for 3 (λmax = 410 and 501 nm) and 4 (λmax = 408 and 498 nm) (Supplementary Fig. S14) which are red shifted with respect to those observed in their parent cluster 1 (λmax = 384 and 436 nm) (Fig. 8a). Cluster 3 and 4 displays photoluminescence in solution at 77 K where the emission maximum in 2-methyl tetrahydrofuran occurs at 712 and 702 nm, respectively which are slightly blue shifted with respect to those of their dtp analogs 1 or 2a (Fig. 8a and Supplementary Fig. S15). The time resolved photoluminescence spectra (77 K) of 2a–c, 3 and 4 exhibits a single exponential decay curve (Fig. 8b, c and Supplementary Figs. S16–18). The observed emission lifetimes (τ) of the dithiophosphate analogs 2a-b (2a: τ = 235.3 µs and 2b: τ = 198.8 µs) are comparatively longer than their diselenophosphate counterparts (3: τ = 82.8 µs and 4: τ = 82.1 µs). Further the lifetime of that of dithiophosphinate analog 2c is of 60 µs which is shorter compared to its dithiophosphate and diselenophosphate relatives. The emission lifetimes in the order of microseconds for NCs 2a–c, 3 and 4 indicate the occurrence of phosphorescence in each case.

Computational studies of title NCs

In a recent DFT investigation on the alloying of dichalcogenolate-protected Ag21 species43, we have shown that in the case of 8-electron NCs of the type [MAg20{dtp/dsep}12]±q (M = group 9 to group 12 metal), when M is a group 9 or 10 metal, it strongly prefers occupying the center of the icosahedron, i.e., [M@Ag20{dtp/dsep}12]±q. The reason lies in the involvement of the nd(M) valence orbitals in the metal-metal bonding through their stabilization by the vacant superatomic 1D shell. Calculations on the T, C3 and C136 structures of [M@Ag20{dtp/dsep}12]±q indicated also a small energy differences between these structures, in particular between T and C3, independently from the nature of M. Calculations on the simplified model [PdAg20{Se2PH2}12] with a slightly different basis set as previously35 found the T isomer to be slightly more stable, both in total energy (ΔE = 3.7 kcal/mol) and free energy (ΔG = 0.2 kcal/mol), this last value being not significantly different from zero. Calculations on the less simplified model [PdAg20{Se2P(OMe)2}12] found similar results with ΔE = 3.7 kcal/mol and ΔG = 2.7 kcal/mol. Although calculations on the real clusters 3a and 3b were not performed owing to their large size, these results confirm our previous finding that the T and C3 structures are close in energy, with the T isomer tending to be slightly more favored in the case of diselenolate ligands. The very small computed energy difference between the two isomers is fully consistent with their observation as co-crystallized species. The C1 structure adopted by compound 2a was also calculated in the case of the [PdAg20{Se2PH2}12] model. It was also found less stable than its T isomer (ΔE = 10.3 kcal/mol and ΔG = 4.8 kcal/mol). In the case of the dithiolate model [PdAg20{S2PH2}12], this energy difference is reduced (ΔE = 4.6 kcal/mol and ΔG = 0.0 kcal/mol), in agreement with the observation of 2a. They illustrate close similarities in the bonding situation of the various isomers, in full consistency with their closeness in energy. All these computed species have their three highest occupied orbitals of 1P nature, whereas the 1D level correspond the lowest vacant orbitals.

TD-DFT calculations on [PdAg20{S2PH2}12] (C1) and [PdAg20{Se2PH2}12] (T and C3), as models for 2a(2a’), 3a(3a’) and 3b(3b’), provided the simulated UV–Vis spectra shown in Fig. 8d. They are in good agreement with their experimental counterparts (Fig. 8d). The low-energy band is of 1P → 1D nature and a comparison of Figs. 8a and 8d let to suggest that the T isomer of 3 might be the dominant species in solution.

Conclusions

In summary, we have isolated and characterized a series of dtp- and dsep-protected 8-electron superatomic Pd doped silver NCs, of which several were structurally characterized. These structurally precise NCs feature a Pd-centered Ag12 icosahedron capped by 8 silver(I) atoms and 12 dichalcogenolate ligands with metallic PdM20 frameworks of ideal C2, Th, and D3 symmetries, respectively, which reduce to C1, T, and C3, respectively, when the 12 ligands are considered. Selenium-ligand exchange on 1 (C1 symmetry) induced the formation of a pair of [PdAg20{Se2P(OiPr)2}12] structural isomers that are T-symmetric (3a, T symmetry) and 3b (C3 symmetry). To our knowledge this is the first ever reported isomeric pair of selenium-protected NCs existed in a unit cell. In fact, these are the rare evidence of structurally characterized alloy cluster completely covered by Se shell. Overall this work demonstrates that the ligand exchange synthetic method indeed provides insights to the development of both pseudo and true structural isomeric alloy NCs.

Experimental methods

Reagents and Instrumentation

The reactions were carried out by using standard Schlenk techniques under N2 atmosphere. LiBH4 (2M in THF) and other chemicals were purchased from different commercial sources available and were used as received. The ligands and metal precursors used in this work, NH4[E2P(R)] (R = OnPr, OiPr, OiBu, or Ph; E = S or Se)44,45,46, [Ag(CH3CN)4]PF647 and Pd[S2P(OnR’)2]2 (R’ = OnPr, OiPr or OiBu)48, were synthesized as described in the literature. All the solvents used in this work were distilled under N2 atmosphere. ESI-mass spectra were recorded on a Fison Quattro Bio-Q (Fisons Instruments, VG Biotech, U. K.). Bruker Advance DPX300 FT-NMR spectrometer was used to record NMR spectra that operate at 300 and 400 megahertz (MHz) while recording 1H, 121.49, and 161.9 MHz for 31P and 100 MHz for 13C. Residual solvent protons were used as a reference (δ, ppm, CDCl3, 7.26). 31P{1H} NMR spectra were referenced to external 85% H3PO4 at δ 0.00. UV–Visible absorption spectra were measured on a Perkin Elmer Lambda 750 spectrophotometer using quartz cells with path length of 1 cm. The elemental analyses were done using a Perkin-Elmer 2400 CHN analyzer. Photoluminescence spectra and lifetime measurements were carried out using an Edinburgh FLS920 fluorescence spectrometer.

Synthesis of [PdAg20{S2P(OiPr)2}12] (2a)

Ligand exchange method

In an oven-dried Schlenk tube, [PdAg20{S2P(OnPr)2}12], 1 (0.100 g, 0.0207 mmol) was dissolved in THF (5 mL) and was placed at −20 °C for 15 min. Then twelve equivalents of NH4[S2P(OiPr)2] (0.057 g, 0.2484 mmol) were added to the solution. The reaction mixture was stirred for 1 h in the same temperature. The solvent was dried under vacuum and the obtained residue was extracted with hexane (3×5 mL) and filtered to remove decomposed impurities from ligand. In this reaction NH4[S2P(OnPr)2] has been isolated as byproduct. The hexane solution was passed through Al2O3 column followed by ether. The brown-red solution obtained was dried which yielded (0.070 g, 70.17% based on Pd) [PdAg20{S2P(OiPr)2}12] (2a).

  1. (a)

    Direct method. In an oven-dried Schlenk flask, [Ag(CH3CN)4](PF6) (0.5 g, 1.2 mmol) was suspended in THF (15 mL). To this NH4[S2P(OiPr)2] (0.14 g, 0.6 mmol), and [Pd{S2P(OiPr)2}] (0.020 g, 0.0626 mmol) were added sequentially. Then the reaction flask was placed in a low temperature bath at −20 °C for 15 min. LiBH4 ∙ THF (0.2 mL, 0.8 mmol) was added slowly via syringe to the reaction mixture and the orange colored solution immediately turned to black after the LiBH4 ∙ THF addition. The reaction was aged 24 h at the same temperature. The solvent was evaporated under vacuum. In order to eliminate decomposed impurities from ligand, the residue was thoroughly washed with deionized water and was subsequently extracted in CH2Cl2. The resulting CH2Cl2 solution was dried and then the residue was further dissolved in hexane. Then solution in hexane was passed through the Al2O3 column. Then the column was run by hexane/ether (80:20 v/v) mixture which resulted to brown-red solid. The brown-red solid was then re-dissolved in CH2Cl2 and was chromatographed on silica gel TLC plates. Elution with a hexane/CH2Cl2 (40:60 v/v) mixture which yielded molecularly pure compound (2a) in 41 % yield (0.124 g based on Pd).

2a. ESI-MS(m/z) [M + Ag]+ calcd. for C72H168Ag21O24P12PdS24, 4930.97; found, 4931.15; 1H NMR (22 °C, 300 MHz, CDCl3, δ, ppm): 4.97–4.85 (m, 24H, OCH), 1.35–1.33 (d, 144 H, CH3); 31P{1H} NMR (22 °C, 121.49 MHz, CDCl3, δ, ppm): 101.66; UV–vis [λmax in nm, (ε in M−1cm−1)]: 384 (71254), 436 (106058).

Synthesis of [PdAg20{S2P(OiBu)2}12] (2b)

  1. (a)

    Direct method. In an oven-dried Schlenk flask, [Ag(CH3CN)4](PF6) (0.5 g, 1.2 mmol) was suspended in THF (15 mL). To this NH4[S2P(OiBu)2] (0.155 g, 0.6 mmol), and [Pd{S2P(OiBu)2}] (0.030 g, 0.086 mmol) were added one after another. Then the reaction flask was placed in low temperature bath at −20 °C for 15 min. LiBH4 ∙ THF (0.2 mL, 0.8 mmol) was added slowly via syringe to the reaction mixture and then the resulting solution was stirred at the same temperature for 24 h. The orange colored solution instantaneously turned to black after the addition of LiBH4 ∙ THF. The solvent was dried completely under vacuum. Then the residue was washed thoroughly with deionized water followed by the extraction in CH2Cl2. The resulting CH2Cl2 solution was dried and then the residue was further dissolved in hexane. That solution in hexane was passed through the Al2O3 column. Then the column was run by hexane/ether (80:20 v/v) mixture which resulted to red solid. Moreover the red solid was re-dissolved in CH2Cl2 and was chromatographed on silica gel TLC plates. Elution with a hexane/CH2Cl2 (40:60 v/v) mixture which yielded molecularly pure compound (2b) in 40 % yield (0.177 g based on Pd). Compound 2b can also be synthesized via ligand replacement method but the crystalline materials can only be obtained by a co-reduction method. Note that in this metiod, compound 1 was treated with twelve equivalents of NH4[S2P(OiBu)2] in THF at ambient temperature for 10 min.

2b. ESI-MS(m/z) [M + Ag]+ calcd. for C96H216Ag21O24P12PdS24, 5267.62; found, 5267.64; 1H NMR (22 °C, 300 MHz, CDCl3, δ, ppm). 3.91 (t, J = 6.76 Hz, 48H), 2.00 (m, J = 6.08 Hz, 24H), 0.94 (d, J = 6.12 Hz, 144H); 31P{1H} NMR (22 °C, 161.9 MHz, CDCl3, δ, ppm); 103.85 UV–vis [λmax in nm, (ε in M−1cm−1)]: 384 (25994), 436 (47813).

Synthesis of [PdAg20{S2PPh2}12] (2c)

Ligand exchange method

In an oven-dried Schlenk tube, [PdAg20{S2P(OnPr)2}12], 1 (0.025 g, 0.005 mmol) was dissolved in THF (10 mL) and was placed at −20 °C for 5 min. Then K[S2PPh2] (0.019 g, 0.065 mmol) was added to the solution. The resulting reaction mixture was allowed to stir for 2 min at the same temperature. The solvent was then evaporated under vacuum. The obtained residue was extracted with hexane (3 × 5 mL) and filtered to eliminate by-product NH4[S2P(OnPr)2]. The hexane solution was passed through Al2O3 column followed by ether. Then the pure orange compound (2c) obtained was dried which produced 65 % yield (0.017 g based on Pd).

2c. ESI-MS(m/z) [M + 2Ag]2+ calcd. for C144H120Ag20P12PdS24, 2735.89; found, 2735.76; 1H NMR (22 °C, 300 MHz, CDCl3, δ, ppm). 7.44, 7.85(m,120 H, C6H5); 31P{1H} NMR (22 °C, 121.49 MHz, CDCl3, δ, ppm): 63.14; UV–vis [λmax in nm, (ε in M−1cm−1)]: 419 (44872), 486 (28290).

Synthesis of [PdAg20{Se2P(OiPr)2}12] (3, 3a and 3b)

Synthesis of 3a and 3b

In a Flame-dried Schlenk tube, [PdAg20{S2P(OnPr)2}12], 1 (0.100 g, 0.0207 mmol) was dissolved in THF (5 mL) and was placed at −20 °C for 15 min. Twelve equivalents of NH4[Se2P(OiPr)2] (0.080 g, 0.2484 mmol) was added to that solution. The resulting mixture was stirred for 2 min in the same temperature. The solvent was dried completely under vacuum and residue was extracted in hexane (3×5 mL) and filtered to remove decomposed impurities from ligand. In this reaction NH4[S2P(OnPr)2] has been isolated as byproduct. The solution was passed through Al2O3 followed by the addition acetone resulted the wine-red colored solution. Then the solution was then evaporated for further analysis. Suitable single crystals for X-ray diffraction were grown at low and ambient temperatures. The crystallization in the ambient temperature revealed a co-crystallization of 3a and 3b with chemical formula [PdAg20{Se2P(OiPr)2}12]. The yield of 3a·3b was 78 % (0.096 g based on Pd).

3a. ESI-MS(m/z) [M + Ag]+ calcd. for C72H168Ag21O24P12PdSe24, 6056.48; found 6055.77; 1H NMR (22 °C, 400 MHz, d6-Acetone, δ, ppm): 5.07–4.95 (q, 24H, OCH), 1.47–1.38 (m, 144H, CH2); 31P{1H} NMR (22 °C, 161. 9 MHz, CDCl3, δ, ppm): 68.16 (JP-Se = 604.51 and 710.40 Hz; UV–vis [λmax in nm, (ε in M−1cm−1)]: 410 (1033762), 501 (690654).

Alternative synthesis of 3 or 3a

In a Flame-dried Schlenk tube, [PdAg20{S2P(OiPr)2}12], 2a (0.100 g, 0.0207 mmol) was dissolved in THF (5 mL) and was placed at −20 °C for 15 min. Then twelve equivalents of NH4[Se2P(OiPr)2] (0.080 g, 0.2484 mmol) was added to the solution. The resulting mixture was stirred for 2 min at the same temperature. The solvent was dried under vacuum and residue was then extracted with hexane (3 × 5 mL) and filtered to remove decomposed impurities from ligand. In this reaction NH4[S2P(OnPr)2] has been isolated as byproduct. The hexane solution was passed through Al2O3 column followed by acetone which exclusively led to the isolation of compound 3 in 73% yields (0.090 g based on Pd).

3 or 3a. ESI-MS(m/z) [M + Ag]+ calcd for C72H168Ag21O24P12PdSe24, 6056.48; found 6055.77; 1H NMR (22 °C, 400 MHz, d6-Acetone, δ, ppm): 5.07–4.95 (q, 24H, OCH), 1.47–1.38 (m, 144H, CH2); 31P{1H} NMR (22 °C, 161. 9 MHz, CDCl3, δ, ppm): 68.16 (JP-Se = 604.51 and 710.40 Hz); UV–vis [λmax in nm, (ε in M−1cm−1)]: 410 (1033762), 501 (690654).

Synthesis of [PdAg20{Se2P(OnPr)2}12] (4)

In an oven-dried Schlenk tube, [PdAg20{S2P(OnPr)2}12], 1 (0.025 g, 0.005 mmol) was dissolved in THF (5 mL) and was placed at −20 °C for 15 min. Then twelve equivalents of NH4[Se2P(OnPr)2] (0.021 g, 0.065 mmol) was added to the solution. The resulting mixture was stirred for 2 min in the same temperature. The solvent was evaporated under vacuum and residue was extracted with hexane (3 × 5 mL) and filtered to remove decomposed impurities from ligand. In this reaction NH4[S2P(OnPr)2] has been isolated as byproduct. The hexane solution was passed through Al2O3 column followed by acetone which exclusively led to the isolation of [PdAg20{Se2P(OnPr)2}12] (4) in 75% yields (0.022 g based on Pd).

4: ESI-MS(m/z) [M + Ag]+ calcd for C72H168Ag21O24P12PdSe24, 6056.48; found 6056.02; 1H NMR (22 °C, 400 MHz, d6-acetone, δ, ppm): 0.90 (t, 72H, CH3), 1.59 (q, 48H, CH2), 3.85(m, 48, CH2); 31P{1H} NMR (22 °C, 161.9 MHz, d6-acetone, δ, ppm): 73.59 (1JP-Se = 604.51 and 710.40 Hz); UV–vis [λmax in nm, (ε in M−1cm−1)]: 408(95700), 498(68700).

Single crystal X-ray structure determination

Single crystals suitable for X-ray diffraction analysis of 2a, 2b, 3, (3a and 3b) and 4 were obtained by diffusing hexane into concentrated CH2Cl2 or acetone solution at room and (or) low temperature within one or two weeks. The single crystals were mounted on the tip of glass fiber coated in paratone oil, and then frozen. Data were collected on a Bruker APEX II CCD diffractometer using graphite monochromated Mo Kα radiation (λ = 0.71073 Å) at 150 K (2a, 3, (3a and 3b)) and 100 K (2b and 4). Absorption corrections for area detector were performed with SADABS49 and the integration of raw data frame was performed with SAINT50. The structure was solved by direct methods and refined by least-squares against F2 using the SHELXL-2018/3 package51,52, incorporated in SHELXTL/PC V6.1453. All non-hydrogen atoms were refined anisotropically. The compound 2a was crystallized in P\(\bar{1}\) space group; one of the isopropyl groups (C4-C6) was disordered over two positions with the same occupancy. The compound 3 is crystallized in Pn space group and one of the isopropyl groups (C16-C18) was disordered over two positions, refined with the same occupancy; 3a and 3b were co-crystallized in P31c space group where two Se atoms (Se9 and Se14) of the ligands were disordered over two positions with 90% and 10% occupancy and one of the isopropyl groups (C13-C15) was disordered over two positions with the same occupancy. Crystallographic data for compounds 2a-b, 3, 3a-b, and 4 have been listed below in Table 3. The X-ray crystallographic coordinates for structures reported in this Article have been deposited at the Cambridge Crystallographic Data Center (CCDC), under deposition number CCDC 1985874 (2a), CCDC 2189558 (2b), CCDC 1985873 (3), CCDC 1985872 (3a-b), and CCDC 1985875 (4). These data can be obtained free of charge from The Cambridge Crystallographic Data Center via www.ccdc.cam.ac.uk/data_request/cif.” The crystallographic information files for compounds 2a-b, 3, 3a-b, and 4 are provided as Supplementary Data 2.

Table 3 Crystallographic data for compounds 2a-b, 3, 3a-b, and 4.
Full size table

Computational details

DFT calculations were carried out on simplified model clusters with the Gaussian 16 package54. The considered ligand simplifications allow to save a huge amount of CPU time and its validity has been proven in many occasions20,21,22,36,41,43. The BP86 functional55,56 was used together with the general triple-ζ polarized Def2-TZVP basis set from EMSL Basis Set Exchange Library57,58. All the optimized geometries were ascertained as true minima on the potential energy surface by performing vibrational frequency calculations. The natural atomic orbital (NAO) charges were computed with the NBO 6.0 program59. The UV–visible transitions were calculated on the above-mentioned optimized geometries by means of time-dependent DFT (TD-DFT) calculations, with the CAM-B3LYP functional60 and the LanL2DZ + pol61,62,63 basis set. The UV–visible spectra were simulated from the computed TD-DFT transitions and their oscillator strengths by using the SWizard code64, each transition being associated with a Gaussian function of half-height width equal to 1000 cm−1. The Cartesian coordinates for computed compounds are provided as Supplementary Data 3.