Surface Modifications of eight-electron Pd/Ag Superatomic Alloys

doi:10.21203/rs.3.rs-1868582/v1

Atomically precise thiolate-protected noble metal nanoclusters and their alloys are far more numerous than their selenium congeners, the synthesis of which remains extremely challenging. Herein, we account the synthesis of a series of atomically defined dithiophosph(in)ate protected eight-electron superatomic Pd/Ag nanoalloys [PdAg₂₀{S₂PR₂}₁₂], 2a-c (where R = OⁱPr, a; OⁱBu, b; Ph, c) via ligand exchange and/or co-reduction methods. The ligand exchange reaction on [PdAg₂₀{S₂P(OⁿPr)₂}₁₂], 1, with [NH₄{Se₂PR₂}₁₂] (where R = OⁱPr, or OⁿPr) leads to the formation of [PdAg₂₀{Se₂P(OⁱPr)₂}₁₂] (3) and [PdAg₂₀{Se₂P(OⁿPr)₂}₁₂] (4), respectively. Solid state structures of 2a, 2b, 3 and 4 unravel different PdAg₂₀ metal frameworks from their parent cluster, originating from the different distributions of the eight-capping silver(I) atoms around a Pd@Ag₁₂ centered icosahedron with C_2, D_3, T_h and T_h symmetries, respectively. Surprisingly ambient temperature crystallization of the reaction product 3 obtained by the ligand exchange reaction on 1 has resulted in the co-crystallization of two isomers in the unit cell with overall T (3a) and C₃ (3b) symmetries, respectively. This is the first ever characterized isomeric pair among the selenolate-protected NCs. Density functional theory (DFT) studies further rationalize the preferred geometrical isomerism of the PdAg₂₀ core.

Silver

palladium

selenium

alloy

nanocluster

isomer

symmetry

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 syntheses.^1–9 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 noble 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 combinations.^1,10−16 In contrast, structurally precise Au and Ag clusters co-protected by selenols are much rarer, with approximately twenty reported examples.^17–22 In order to illuminate the effects of surface functionalization of nanoparticles, recent reports have demonstrated the fabrication of novel 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 species. Thus, the syntheses of selenolate-protected NCs are of paramount importance.

The first reported 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 NaBH₄.²³ Subsequently, the same group also synthesized Au₃₈(SeC₁₂H₂₅)₂₄ from Au₃₈(SR)₂₄ via a ligand exchange method.²⁴ Zhu et al demonstrated that selenophenolate-protected Au₁₈²⁵ and Au₂₅¹¹ NCs exhibit different optical properties from those of their thiolate homologues. Later, studies on Au₂₄(ER)₂₀ (E = Se or S) NCs unraveled different structures and optical properties between both families of chalcogenolates.²⁶ In 2013, Pradeep and his co-workers reported the first atomically precise silver NC stabilized by selenolates, Ag₄₄(SePh)₃₀, which revealed similar properties as its thiolate counterpart.²⁷ More recently, Bootharaju et al. reported a Cd-doped silver NC protected by selenophenolates, namely Cd₁₂Ag₃₂(SePh)₃₆, which exhibits characteristic UV-vis absorption features and rare near-infrared (NIR) photoluminescence at ∼1020 nm.¹⁸

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],^28a [Ag₁₀(Se){Se₂P(OⁱPr)₂}₈],^28b [Ag₈(X){Se₂P(OⁱPr)₂}₆]⁺ (X = H, Cl or Br),²⁹ [Ag₁₁(µ₉-Se)(µ₃-X)₃{Se₂P(OⁱPr)₂}₆] (X = I,^30a Br^30b), [Ag₁₁(µ₉-I)(µ₃-I)₃{Se₂P(OⁱPr)₂}₆]⁺,^30c [Ag₁₂(µ₅-X)₂ {Se₂P(OEt)₂}₁₀] (X = Br, I)³¹ [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 are currently interested in developing our chemistry of dsep-protected alloy NCs. Among the several synthetic methods available, the ligand exchange method 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.

Synthesis and Characterization of [PdAg ₂₀ {S ₂ PR ₂ } ₁₂ ], R = O ⁱ Pr (2a), R = O ⁱ Bu (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 C₁.³² 1 is robust, yet the liability of its protecting ligands tempted us to study the ligand exchange (LE) behaviour in order to produce new NCs. As shown in the Scheme 1, the treatment of 1 with 12 equivalents of NH₄[S₂P(OⁱPr)₂] at -20 ºC in tetrahydrofuran (THF) for 2 minutes led to the formation of [PdAg₂₀{S₂P(OⁱPr)₂}₁₂] (2a) in 70% yield within an hour. Alternatively, the same compound was produced more conveniently by direct archetypal one pot synthetic method in moderate yield (41%) (See Experimental section). In parallel to the synthesis of 2a, compound [PdAg₂₀{S₂P(OⁱBu)₂}₁₂] (2b) was synthesized via co-reduction methods in 40% yield. Compound [PdAg₂₀{S₂P(OPh)₂}₁₂] (2c) was synthesized via the LE method in 65% yield (Experimental section). All NCs (2a-c) have been characterized by positive-mode ESI mass spectrometry, ³¹P{¹H} and ¹H NMR (see Supporting information, Figure S1-S7).

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} nuclear magnetic resonance (NMR) spectrum in CDCl₃ displays a signal shift from 104.9 ppm to 101.66 ppm at room temperature (Figure S1 and S8). 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 (Figure S2).

The ESI mass spectrum (positive-ion mode) was recorded to identify the molecular formula of 2a. The spectrum reveals a prominent band corresponding to [2a + Ag]⁺ at m/z 4931.19 (calcd. 4930.97), and its simulated isotopic pattern is in good agreement with the experimental one (Fig. 1). 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 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 2628.02 (Calcd. 2628.37), 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 failed.

Single crystals of suitable quality for X-ray diffraction for 2a and 2b were grown by crystallization from diffusion of hexanes 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. 2 compared to that of 1.³² 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. 2 (from Fig. 2b → 2d → 2f). Whereas in both 1 and 2a NCs the PdAg₂₀ skeleton adopts pseudo-C₂ symmetry; that of 2b the PdAg₂₀ skeleton adopts pseudo-D₃ symmetry (Figure S9). The twelve dtp ligands in 2a are equally distributed on both sides of the C₂ axis (Figure S9). They are coordinated to both capping and icosahedral silver atoms (Ag_cap and Ag_ico, respectively) in five different modes in a ratio of 1:1:7:1:2; 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) (Figure S10). Further the seven dtp ligands with trimetallic triconnectivity (η³: µ₂, µ₁), differ in the coordination to different combination of Ag_cap and Ag_ico atoms except for a couple of dtp ligands (red box, in Figure S10). 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. 2c), the whole molecular symmetry of 2a is C₁. On the other hand, the twelve dtp ligands in 2b are distributed in three spherical rows around the pseudo-C₃ axis in 3:6:3 ratios (Figure S9). They bind to both capping and icosahedral silver atoms only through two coordination patterns: trimetallic triconnectivity and trimetallic tetraconnectivity (Figure S11), 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]⁺.³⁴

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 Ag_ico-Ag_ico and Ag_ico-Ag_cap 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 1

Spectroscopic data of compounds for 1, 2(**a-c**), 3 and 4.
Compound	³¹P NMR(ppm)		Emission (nm)^a	ESI-MS(m/z, M⁺)
1³²	104.9	384, 436	748	4930.8
2a	101.6	384, 436	741	4931.19
2b	105.0	384, 436	732	5267.19
2c	63.14	419, 486	669	2628.02 ^b
3	67.17	410, 501	712	6055.77
4	74.8	408, 498	702	6055.88
^aPhotoluminescence recorded in 2-Methyl tetrahydrofuran at 77K. ^bESI-MS peak corresponds to (M + 2Ag)²⁺ peak.

[PdAg ₂₀ {Se ₂ P(O ⁱ Pr) 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 analogues via LE reactions. Compound 1 was treated with NH₄[Se₂P(OⁱPr)₂] at -20 ºC in THF (Scheme 1). The reaction proceeded immediately as the color of the reaction mixture changed from brown red to purple (Figure S12). The positive-ion ESI-MS spectrum of reaction mixture displays a prominent band for molecular ion peak at m/z = 6055.77 (calcd. 6055.23) corresponding to [PdAg₂₀{Se₂P(OⁱPr)₂}₁₂ + Ag]⁺ (Figure S13).

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 ¹H{³¹P} 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 (¹J_P−Se = 604.51 and 710.40 Hz) (Figure S14). ¹H{³¹P} NMR spectrum shows characteristic signals of isopropyl ligands of di-isopropyl diselenophosphates (Figure S15).

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. 3 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 T_h 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 ºC in THF was performed (Scheme 1). Likewise, in the transformation from 1 to 3 the solution undergoes an instant colour 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 NMR spectroscopy.

The 3( P 3 ₁ c ) solid-state structure of [PdAg ₂₀ {Se ₂ P(O ⁱ Pr) ₂ } ₁₂ ]

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. 4).³³ 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 = O^i/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 T_h symmetry, is shown in Fig. 3b-d and 4b. Their twelve dsep ligands display trimetallic triconnectivity (η³: µ₂, µ₁) bridging pattern with two capping Ag atoms (Ag_cap) and one icosahedral Ag atom (Ag_ico), 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]⁺.³⁴ The differences in the positions of the outer capping Ag atoms in 3a and 3b, and their possible interchange pathway, is illustrated in Figure S16. 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-Ag_ico average distances in 3a and 3b are equivalent (2.758(10) Å and 2.754(10) Å, respectively), as well as their average Ag_ico-Ag_ico 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 (see Table 2). The average Ag_ico@Se distance in both 3a and 3b are larger than the Ag_cap@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. 5a). The T and C₃ layer are alternately stacked along the [001] direction (Fig. 5b). 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 (LEIST) route it is indeed inevitable not to synthesize another normal propyl alkyl chain analogue. Note that the precedence of structurally precise selenium protected alloy clusters is awfully inadequate. Thus, as shown in scheme 1, we have endeavoured 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 (¹J_P−Se = 616.41 and 719.23 Hz) at room temperature (Figure S17). 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 intigration ratio of 1:1:1.5, which is clearly attributed to ⁿPr group of di-propyl diselenophosphate ligands (Figure S18). The ESI mass (positive-ion mode) spectrum shows a prominent band corresponding to [4 + Ag]⁺ at m/z = 6055.88 (calcd. 6055.23), and its simulated isotopic pattern is in good agreement with the experimental one (Figure S19). 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. 3 and S20). 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.

Table 2

Selected structural metrics (average (top line) and ranges (bottom line)) of compounds for 1, **2a-b**, **3, 3a-b** and 4.
Entry		Ag_ico-Ag_ico	Ag_ico-Ag_cap	Ag_ico-E	Ag_cap-E	E-E
1		2.897	2.971	2.687	2.540	3.414
1		2.827–2.987	2.856–3.346	2.471–3.047	2.480–2.726	2.772–2.747
2a		2.896(7)	3.020(7)	2.605(14)	2.542(19)	3.399
2a		2.807(1)- 2.997(1)	2.863(1)-3.287(1)	2.486(4)-2.933(4)	2.472(5)-2.651(4)	3.304–3.456
2b		2.893(6)	3.014(5)	2.542(2)	2.506(1)	3.400 (3)
2b		2.829(8)- 2.930(1)	2.847(9)-3.204(7)	2.480(3)-2.590(2)	2.330(1)-2.590(3)	3.380(4)-3.430(2)
3 (Pn)		2.902(11)	2.949(100)	2.683(10)	2.614(15)	3.688
3 (Pn)		2.845(2)-2.969(2)	2.899(2)-3.005(2)	2.666(3)-2.696(3)	2.595(3)-2.639(3)	3.655–3.704
3(P3₁c)	3a	2.901(9)	2.950(8)	2.668(7)	2.607(10)	3.665
	3a	2.841(2)-2.957(3)	2.901(3)-2.989(2)	2.662(4)-2.674(3)	2.600(3)-2.617(3)	3.646–3.691
	3b	2.896(10)	2.965(9)	2.739(8)	2.607(12)	3.668
	3b	2.845(3)-2.945(2)	2.880(3)-3.123(3)	2.628(3)-3.104(4)	2.576(3)-2.673(3)	3.653–3.694
4		2.893(2)	2.949(2)	2.668(2)	2.618(2)	3.698
4		2.833(2)-2.961(2)	2.902(2)-3.009(2)	2.630(2)-2.699(2)	2.607(2)-2.637(2)	3.677–3.721

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 (see Table 1, Fig. 6a). 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. 6b). 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 solution at 77K. Their emission maxima in 2-methyl tetrahydrofuran (MeTHF) occur at λ_max = 748 nm, 741 nm and λ_max = 732 nm, respectively (Fig. 5a and Figure S21-S23). Cluster 2c is also emissive in solution state at 77K. Its emission maximum appears at 669 nm in MeTHF (Fig. 6b and Figure S23) which is blue shifted to its parent cluster 1.

The UV-vis spectra feature two major broad absorption bands for 3 (λ_max = 410 and 501 nm) and 4 (λ_max = 408 and 498 nm) (Figure S24) which are red shifted with respect to those observed in their parent cluster 1 (λ_max = 384 and 436 nm) (Fig. 7a). Cluster 3 and 4 displays photoluminescence in solution at 77K 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 its dithiophosphate analogue 1 (Fig. 7a and Figure S25).

Computational Studies of title NCs

In a recent DFT investigation on the alloying of dichalcogenolate-protected Ag₂₁ species,³⁵ 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 centre 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 C₁³² 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 favoured 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, 3a and 3b, provided the simulated UV-Vis spectra shown in Fig. 7b. They are in good agreement with their experimental counterparts (Fig. 7a). The low-energy band is of 1P → 1D nature and a comparison of Figs. 7a and 7b let to suggest that the T isomer of 3 might be the dominant species in solution.

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, T_h, 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: T-symmetric (3a, T symmetry) and 3b (C₃ symmetry). 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.

General Procedures and Instrumentation. The reactions were carried out under N₂ atmosphere by using standard Schlenk techniques. 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),^36–38 [Ag(CH₃CN)₄]PF₆³⁹ and Pd[S₂P(OⁿR')₂]₂ (R' = OⁿPr, OⁱPr or OⁱBu),⁴⁰ were synthesized as described in the literature. All the solvents used in this work were distilled under N₂ atmosphere. ESI-mass spectrum recorded on a Fison Quattro Bio-Q (Fisons Instruments, VG Biotech, U. K.). NMR spectra were recorded on Bruker Advance DPX300 FT-NMR spectrometer that operates at 300 and 400 MHz while recording ¹H, 121.49, and 161.9 MHz for ³¹P, 100 MHz for ¹³C and 46.1 MHz for ²H. 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.

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

(a) 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 ºC for 15 minutes. Then twelve equivalents of NH₄[S₂P(OⁱPr)₂] (0.057 g, 0.2484 mmol) was added to the solution. The resulting mixture was stirred for 1 hour 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 ether. The wine-red solution obtained was dried which yielded (0.070 g, 70.17% based on Pd) [PdAg₂₀{S₂P(OⁱPr)₂}₁₂] (2a).

(b) 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 low temperature bath at -20 ºC for 15 minutes. LiBH₄∙THF (0.2 mL, 0.8 mmol) was added slowly via syringe to the reaction mixture and the orange color solution immediately turned to black after the LiBH₄∙THF addition. The reaction was aged 24 hours at the same temperature. Then 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. Further the brown-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 (2a) in 41 % yield (0.124 g based on Pd).

2a. ESI-MS = m/z 4931.19 [M + Ag]⁺ (Calcd. 4930.97). ¹H{³¹P} 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 [λ_max in nm, (ε in M^− 1cm^− 1)] 384 (71254), 436 (106058).

Synthesis of [PdAg ₂₀ {S ₂ P(O ⁱ Bu) ₂ } ₁₂ ] (2b): 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 ºC for 15 minutes. 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 hours. The orange colored solution instantaneously turned to black after the addition of LiBH₄∙THF. The solvent was evaporated under vacuum. The residue was thoroughly washed 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).

2b. ESI-MS = m/z 5267.64 [M + Ag]⁺ (Calcd. 5267.62). ¹H{³¹P} 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 [λ_max 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 (0.025g, 0.005 mmol) was dissolved in THF (10 mL) and was placed at -20 ºC for 5 minutes. Then K[S₂PPh₂] (0.019 g, 0.065 mmol) was added to the solution. The resulting mixture was stirred for 2 minutes in the same temperature. The solvent was evaporated under vacuum and 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 2628.02 [M + 2Ag]²⁺ (Calcd. 2628.37). 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 [λ_max 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 ºC for 15 minutes. Twelve equivalents of NH₄[Se₂P(OⁱPr)₂] (0.080g, 0.2484 mmol) was added to that solution. The resulting mixture was stirred for two minutes in the same temperature. The solvent was evaporated 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 evaporated for further analysis. Dark single crystals suitable for X-ray diffraction were grown 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 6055.77 [M + Ag]⁺ (Calcd. 6056.48). 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 NMR (22°C, 161. 9 MHz, CDCl₃, δ, ppm): 68.16 (J_P−Se = 604.51 and 710.40 Hz. UV-vis [λ_max 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 ºC for 15 minutes. 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 two minutes 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 compound 3 in 73% yields (0.090 g based on Pd).

3 or 3a. ESI-MS = m/z 6055.77 [M + Ag]⁺ (Calcd. 6056.48). 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 NMR (22°C, 161. 9 MHz, CDCl₃, δ, ppm): 68.16 (J_P−Se = 604.51 and 710.40 Hz. UV-vis [λ_max 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 ºC for 15 minutes. 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 two minutes 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 6055.77 [M + Ag]⁺ (Calcd. 6055.23). 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 NMR (22°C, 161.9 MHz, d₆-acetone, δ, ppm): 73.59 (¹J_P−Se = 616.41 and 719.23 Hz). UV-vis [λ_max 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 150K (2a, 3, (3a and 3b)) and 100K (2b and 4). Absorption corrections for area detector were performed with SADABS⁴¹ and the integration of raw data frame was performed with SAINT.⁴² The structure was solved by direct methods and refined by least-squares against F² using the SHELXL-2018/3 package,^43,44 incorporated in SHELXTL/PC V6.14.⁴⁵ All non-hydrogen atoms were refined anisotropically. The compound 2a was crystallized in P\(\stackrel{-}{1}\) 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 structures reported herein have been deposited at the Cambridge Crystallographic Data Centre, CCDC no. 1985874 (2a), 2189558 (2b), 1985873 (3), 1985872 (3a-b), and 1985875 (4).

Table 3

Crystallographic data for compounds **2a-b**, 3, **3a-b**, and 4
Compound	2a	2b	3	3a and 3b	4
Empirical Formula	C₇₂H₁₆₈Ag₂₀O₂₄P₁₂PdS₂₄	C₉₆H₂₁₆Ag₂₀O₂₄P₁₂Pd S₂₄	C₇₂H₁₆₈Ag₂₀O₂₄P₁₂Pd Se₂₄	C₇₂H₁₆₈Ag₂₀O₂₄P₁₂PdSe₂₄	C₇₂H₁₆₈Ag₂₀O₂₄P₁₂PdSe₂₄
Crystal system, Space group	Triclinic, P\(\stackrel{-}{1}\)	Hexagonal, P 6₃	Monoclinic, Pn	Trigonal, P3₁c	Monoclinic, P2₁
a (Å)	15.8599(12)	19.2447(7)	17.9414(11)	22.5252(9)	15.9494(6)
b (Å)	18.0946(14)	19.2447(7)	26.7432(15)	22.5252(9)	29.3080(12)
c (Å)	29.382(2)	26.1772(12)	18.8174(11)	40.401(2)	16.4792(7)
α (°)	82.1487(15)	90	90	90	90
β (°)	77.2002(15)	90	113.2950(10)	90	107.4809(10)
γ (°)	66.2229(13)	120	90	120	90
V(Å³)	7514.0(10)	8396.1(7)	8292.8(8)	17752.5(17)	7347.4(5)
Z	2	2	2	4	2
ρ_calcd, g·cm^− 3	2.132	2.041	2.382	2.226	2.689
µ, mm^− 1	3.166	2.841	7.834	7.319	8.842
Temperature, K	150(2)	100(2)	150(2)	150(2)	100(2)
θ_max, deg. / completeness, %	25.00 / 95.8	24.999 / 100	25.00 / 99.6	24.99 / 99.9	24.99 / 99.9
Reflections collected / unique	44057 / 25382 [R(int) = 0.0237]	58687 / 7962 [R(int) = 0.1157]	48427 / 22452 [R(int) = 0.0217]	113818 / 20789 [R(int) = 0.0863]	48724 / 25686 [R(int) = 0.0512]
Restraints / parameters	459 / 1432	582 /680	818 / 1550	712 / 967	456 / 1440
R1^a, wR2^b [I > 2σ(I)]	0.0668,0.1355	0.0798, 0.1909	0.0342, 0.0887	0.1125,0.3046	0.0399,0.0971
R1^a, wR2^b (all data)	0.0867,0.1511	0.1036, 0.2320	0.0387, 0.0919	0.1291,0.3191	0.0477,0.1046
Absolute structure parameter	-	0.40(18)	0.037(9)	0.18(3)	0.327(9)
Goodness of fit on F²	1.084	1.102	1.031	1.037	1.023
Largest diff. peak and hole, e/Å ³	1.897 and − 1.680	1.686 and − 1.393	1.572 and − 0.866	2.540 and − 2.782	1.750 and − 1.625

^aR1 = Σ｜︱F_o︱-︱F_c︱｜/ Σ︱F_o︱. ^bwR2 = { Σ[w(F_o² - F_c²)²] / Σ[w(F_o²)₂] }^1/2

Computational details.

DFT calculations were carried out on simplified model clusters with the Gaussian 16 package.⁴⁶. The considered ligand simplifications allow to save a huge amount of CPU time and its validity has been proven in many occasions.^{20–22,32−35} The BP86 functional^47,48 was used together with the general triple-ζ polarized Def2-TZVP basis set from EMSL Basis Set Exchange Library.^49,50 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 program.⁵¹ 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 functional⁵² and the LanL2DZ + pol^53–55 basis set. The UV-visible spectra were simulated from the computed TD-DFT transitions and their oscillator strengths by using the SWizard code,⁵⁶ each transition being associated with a Gaussian function of half-height width equal to 1000 cm^− 1.

ASSOCIATED CONTENT

Supporting Information

Spectroscopic data of 2a-c, 3, 3a, 3b and 4 and theoretical data of 2a, 3a and 3b.

Funding Sources

MOST 111-2123-M-259-002

Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENTS

This work was supported by the Ministry of Science and Technology in Taiwan (MOST 111-2123-M-259-002) and the France-Taiwan ANR-MOST 2018 program (project Nanoalloys).

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Scheme 1 is available in Supplementary Files section.

There is NO Competing Interest.

SI.docx
TableofContentEntryOnly.docx
combinedcheckcif.pdf
Scheme1.jpg
Scheme 1. Synthesis of dichalcogenolate protected Pd-Ag alloy clusters via ligand exchange reaction. (a) THF, -20 °C, 1h. (b) THF, -20 °C, 2 min. (c) THF, -20 °C, 2 min.

Surface Modifications of eight-electron Pd/Ag Superatomic Alloys

Status:

Journal Publication

Version 1

Abstract

Figures

Introduction

Results And Discussion

[PdAg₂₀{Se₂P(OⁿPr)₂}₁₂], (4)

Optical Properties of the title NCs

Computational Studies of title NCs

Conclusions

Experimental Section

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

Synthesis of [PdAg₂₀{S₂PPh₂}₁₂] (2c)

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

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

Declarations

References

Scheme

Additional Declarations

Supplementary Files

Status:

Journal Publication

Version 1

Surface Modifications of eight-electron Pd/Ag Superatomic Alloys

Status:

Journal Publication

Version 1

Abstract

Figures

Introduction

Results And Discussion

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

Optical Properties of the title NCs

Computational Studies of title NCs

Conclusions

Experimental Section

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

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

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

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

Declarations

References

Scheme

Additional Declarations

Supplementary Files

Status:

Journal Publication

Version 1

[PdAg₂₀{Se₂P(OⁿPr)₂}₁₂], (4)

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

Synthesis of [PdAg₂₀{S₂PPh₂}₁₂] (2c)

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

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