A New Synthetic Methodology in the Preparation of Bimetallic Chalcogenide Clusters via Cluster-to-Cluster Transformations

A decanuclear silver chalcogenide cluster, [Ag10(Se){Se2P(OiPr)2}8] (2) was isolated from a hydride-encapsulated silver diisopropyl diselenophosphates, [Ag7(H){Se2P(OiPr)2}6], under thermal condition. The time-dependent NMR spectroscopy showed that 2 was generated at the first three hours and the hydrido silver cluster was completely consumed after thirty-six hours. This method illustrated as cluster-to-cluster transformations can be applied to prepare selenide-centered decanuclear bimetallic clusters, [CuxAg10-x(Se){Se2P(OiPr)2}8] (x = 0–7, 3), via heating [CuxAg7−x(H){Se2P(OiPr)2}6] (x = 1–6) at 60 °C. Compositions of 3 were accurately confirmed by the ESI mass spectrometry. While the crystal 2 revealed two un-identical [Ag10(Se){Se2P(OiPr)2}8] structures in the asymmetric unit, a co-crystal of [Cu3Ag7(Se){Se2P(OiPr)2}8]0.6[Cu4Ag6(Se){Se2P(OiPr)2}8]0.4 ([3a]0.6[3b]0.4) was eventually characterized by single-crystal X-ray diffraction. Even though compositions of 2, [3a]0.6[3b]0.4 and the previous published [Ag10(Se){Se2P(OEt)2}8] (1) are quite similar (10 metals, 1 Se2−, 8 ligands), their metal core arrangements are completely different. These results show that different synthetic methods by using different starting reagents can affect the structure of the resulting products, leading to polymorphism.


Synthetic Strategy
In our previous study, a decanuclear silver cluster, [Ag 10 (Se){Se 2 P(OEt) 2 }] 8 (1), was isolated from the reaction of [Ag(CH 3 CN) 4 ]PF 6 and NH 4 [Se 2 P(OEt) 2 ] in a 1:1 molar ratio at −20 • C for 24 h (Scheme 1a) [19]. The encapsulated selenide anion was generated from the slow decomposition of dsep ligands. Herein we introduced a new strategy, which is inspired by the thermal-induced self-redox reaction of [  (2). The composition of 2 (10 Ag + + 1 Se 2− + 8 dsep ligands) has been characterized by X-ray diffraction, which is the same as 1 but with a completely different solid-state structure. At 60 • C the cleavage of P-Se bond of dsep ligands occurs to generate Se, which can be reduced to Se 2− by the interstitial hydride in [Ag 7 (H){Se 2 P(O i Pr) 2 } 8 ]. Unlike the reactions used silylated chalcogen reagents to generate considerable amount of chalcogenide anions, this new method can control the Se 2− ratio in a relatively small range so that smaller size of metal chalcogenide clusters can be generated.
inverse coordination, an emerging concept coined by Ionel Haiduic [23,24]. Nevertheless, efficient controls on both the amount of chalcogenide generated in situ and the size of clusters remain challenging. Herein, we report a new synthetic pathway leading to the formation of M10(Se)L8 (L = diisopropyl diselenophosphate, dsep) via a cluster-to-cluster transformation. In addition, intriguing structural isomers identified in the M10(Se) core are also presented. . This methodology provides a facile route to produce anion-encapsulated heterometallic clusters via a cluster-to-cluster transformation. However, this method cannot predict the exact position where the heterometals will possibly occupy. Nevertheless, it opens up many possibilities to generate structural isomers. Compound 3, which the entire metal core is completely different from that of 1 and 2, has been structurally characterized by XRD. The structural analysis will be discussed in the following section.  . This methodology provides a facile route to produce anionencapsulated heterometallic clusters via a cluster-to-cluster transformation. However, this method cannot predict the exact position where the heterometals will possibly occupy. Nevertheless, it opens up many possibilities to generate structural isomers. Compound 3, which the entire metal core is completely different from that of 1 and 2, has been structurally characterized by XRD. The structural analysis will be discussed in the following section.

Structural Analyses
All three M 10 (Se)L 8 clusters (1-3) are composed of ten metal atoms, one selenide anion and eight dsep ligands. Although three structures have similar compositions, their geometries are strikingly different. In the previously published structure 1, the selenide anion is encapsulated in a distorted, cis-bicapped trapezoidal-prismatic Ag 10 framework ( Figure 1a), which is surrounded by eight dsep ligands (Figure 1b) [19]. Short contacts are observed between the encapsulated Se 2− (Se encap ) and ten peripheral silver atoms where Se encap -Ag distances are in the range of 2.6312(19)-3.187(2) Å (avg. 2.939(2) Å).
In structure 2, the metal array is quite different from 1. There are two clusters in the asymmetric unit and their structures are slightly different. Considering the two Ag 10 (Se) cores in crystal 2 as cluster A (Figure 1c) and cluster B (Figure 1d), both clusters have six out of ten Ag atoms (Ag chair ) arranged in a chair-like metallo-ring conformation with one Se 2− anion sitting above its center. The rest of four Ag atoms are located above the (Ag chair ) 6 Se unit to constitute the whole Ag 10 (Se) framework. Since the positions of the ten silver atoms in clusters A and B are slightly different, two Ag 10 (Se) cores are pseudo-mirror images. This also affects the distances of Ag-Se encap , resulting in different coordination modes of the central Se atom in clusters A and B. Se1 has eight short contacts to the adjacent Ag atoms ranging from 2.5113 (13)  However, their coordination patterns at relatively similar position are not the same due to the different metal sites in each Ag 10 (Se) core. For example, the dsep ligand on P8 is trimetallic (Ag1, Ag10, Ag9) triconnectivity, but P10 is tetrametallic (Ag15, Ag18, Ag13, Ag12) tetraconnectivity. Nine of the ten Ag atoms are three-coordinated and one Ag atom (Ag3) are two-coordinated to Se on the dsep ligands in cluster A; eight of the ten Ag atoms are trigonal-and two Ag atoms (Ag14 and Ag16) are digonal-coordinated in cluster B. There are 29 and 28 connectivities between dsep ligands and Ag 10 (Se) core in clusters A and B, respectively, which Se-Ag distances are in the range of 2.5368 (13) Table 1.

ESI Mass Spectroscopy
The

NMR Spectroscopy
The fact that a single resonance centered at 75.3 ppm flanked by one set of satellite peaks ( 1 JPSe = 669 Hz) in the 31 P{ 1 H} NMR spectrum ( Figure S3) coupled with one set of isopropoxy resonance (4.88 and 1.36 ppm) observed in the 1 H NMR spectrum of 2 at ambient temperature ( Figure S6) strongly suggests its non-rigid structure in solution, which

NMR Spectroscopy
The fact that a single resonance centered at 75.3 ppm flanked by one set of satellite peaks ( 1 J PSe = 669 Hz) in the 31 P{ 1 H} NMR spectrum ( Figure S3) coupled with one set of isopropoxy resonance (4.88 and 1.36 ppm) observed in the 1 H NMR spectrum of 2 at ambient temperature ( Figure S6) strongly suggests its non-rigid structure in solution, which Molecules 2021, 26, 5391 7 of 13 could be due to the labile Ag-Se bonds. The satellite peaks are due to the 31 P nuclei coupled with the 77 Se nuclei (I = 1/2) in diselenophosphate compounds [32] in which the natural abundance of 77 Se is only 7.56%. The cluster-to-cluster transformation process can be monitored by the time-dependent 31 P{ 1 H} and 1 H NMR spectroscopy. In the reaction shown in Scheme 1b, compound 2 which 31 P resonance at 75.3 ppm was generated at the first three hours (Figure 3a). The 31 P resonance of [Ag 7 (H){Se 2 P(O i Pr) 2 } 6 ] at 83.1 ppm gradually disappeared over time, then Ag 7 (H) was barely seen after 1.5 days. The consumption of Ag 7 (H) can also be followed by the time-dependent 1 H spectra (Figure 3b). That is, the pseudo octet peak of central hydride originated from 1 J(H, Ag) was gradually disappeared and the chemical shift of methine proton on isopropoxy groups displayed a down-field shift due to the different values of two species, Ag 7 H and Ag 10 Se. It can be assumed that the slow decomposition of Ag 7 (H) prevents Se 2− being released in large quantities at one time followed by the cluster-to-cluster transformation. The fact that the coordinated hydride as the reductant can be tracked by recognizing the in-situ generated H 2 , which resonates at 4.61 ppm ( Figure S7). Fragment peaks from dsep ligand decomposition can also be observed in the 31  could be due to the labile Ag-Se bonds. The satellite peaks are due to the 31 P nuclei coupled with the 77 Se nuclei (I = 1/2) in diselenophosphate compounds [32] in which the natural abundance of 77 Se is only 7.56%. The cluster-to-cluster transformation process can be monitored by the time-dependent 31 P{ 1 H} and 1 H NMR spectroscopy. In the reaction shown in Scheme 1b, compound 2 which 31 P resonance at 75.3 ppm was generated at the first three hours (Figure 3a). The 31 P resonance of [Ag7(H){Se2P(O i Pr)2}6] at 83.1 ppm gradually disappeared over time, then Ag7(H) was barely seen after 1.5 days. The consumption of Ag7(H) can also be followed by the time-dependent 1 H spectra (Figure 3b). That is, the pseudo octet peak of central hydride originated from 1 J(H, Ag) was gradually disappeared and the chemical shift of methine proton on isopropoxy groups displayed a down-field shift due to the different values of two species, Ag7H and Ag10Se. It can be assumed that the slow decomposition of Ag7(H) prevents Se 2-being released in large quantities at one time followed by the cluster-to-cluster transformation. The fact that the coordinated hydride as the reductant can be tracked by recognizing the in-situ generated H2, which resonates at 4.61 ppm ( Figure S7). Fragment peaks from dsep ligand decomposition can also be observed in the 31 [33] and phosphoric acid, respectively. It is noted that the diselenophosphates are thermally unstable where its sulfur analogue is not so sensitive to heat [34]. While the 31 P chemical shifts of both 1 and 2 are almost identical, their 77 Se{ 1 H} NMR ( Figure S4) spectra are different. A doublet at 103.3 ppm and a broad resonance at −1292.4 ppm ( Figure S5) corresponding to the resonance frequency of the dsep ligands and the encapsulated Se 2-, respectively, can be seen in the 77 Se spectrum of 2. The resonances are slightly different from that of 1 (108 ppm, dsep; −1395.4 ppm, Seencap) [19]. It could be due to the stronger interactions between Seencap and Ag atoms in 2, resulting in the downfield shift of the Seencap resonance. The 31 P{ 1 H} NMR spectrum of 3 shows multiple resonances overlapping together at around 73.9-75.2 ppm (Figure S8), indicating the multiple coordination environment of the dsep ligands. It is primarily arisen from the randomly doped Cu atoms on multiple positions in structure 3. Unfortunately, no satisfied signals can be detected in the 77 Se NMR spectrum of 3 at ambient temperature. While the 31 P chemical shifts of both 1 and 2 are almost identical, their 77 Se{ 1 H} NMR ( Figure S4) spectra are different. A doublet at 103.3 ppm and a broad resonance at −1292.4 ppm ( Figure S5) corresponding to the resonance frequency of the dsep ligands and the encapsulated Se 2− , respectively, can be seen in the 77 Se spectrum of 2. The resonances are slightly different from that of 1 (108 ppm, dsep; −1395.4 ppm, Se encap ) [19]. It could be due to the stronger interactions between Se encap and Ag atoms in 2, resulting in the downfield shift of the Se encap resonance. The 31 P{ 1 H} NMR spectrum of 3 shows multiple resonances overlapping together at around 73.9-75.2 ppm (Figure S8), indicating the multiple coordination environment of the dsep ligands. It is primarily arisen from the randomly doped Cu atoms on multiple positions in structure 3. Unfortunately, no satisfied signals can be detected in the 77 Se NMR spectrum of 3 at ambient temperature.

Photophysical Properties
The absorption spectrum of 2 exhibits a single low-energy LMCT band at 402 nm, which is very close to 396 nm of 3 (Figure 4a). The doped Cu atoms found in the bimetallic decanuclear cluster seem not to significantly perturb the characteristic absorption band. It is assumed that both clusters 2 and 3 might have similar structures as that observed in cluster 1 in solution leading to similar electronic transition energy even though their solidstate structures are different (vide supra). Both compounds are not emissive at ambient temperature but show orange emission at 77 K. Figure 4a depicts the emission spectra of 2 and 3 in 2-MeTHF glass, while their photophysical data are briefly summarized in Table 2. Both emission profiles are structureless and the photoluminescence of 2 centered at 666 nm is slightly blue-shifted, compared to 3 centered at 707 nm. The photoluminescence decay curves of 2 ( Figure 4b) and 3 (Figure 4c) can be well-fitted to a single exponential decay function (red curve). The lifetimes of 2 (λ ex = 401 nm) and 3 (λ ex = 396 nm), 16 µs and 22 µs, together with large Stokes shift (~10,000 cm −1 ), reflect their triplet excited state nature, a spin-forbidden phosphorescence.

Photophysical Properties
The absorption spectrum of 2 exhibits a single low-energy LMCT band at 402 nm, which is very close to 396 nm of 3 (Figure 4a). The doped Cu atoms found in the bimetallic decanuclear cluster seem not to significantly perturb the characteristic absorption band. It is assumed that both clusters 2 and 3 might have similar structures as that observed in cluster 1 in solution leading to similar electronic transition energy even though their solidstate structures are different (vide supra). Both compounds are not emissive at ambient temperature but show orange emission at 77 K. Figure 4a depicts the emission spectra of 2 and 3 in 2-MeTHF glass, while their photophysical data are briefly summarized in Table  2. Both emission profiles are structureless and the photoluminescence of 2 centered at 666 nm is slightly blue-shifted, compared to 3 centered at 707 nm. The photoluminescence decay curves of 2 ( Figure 4b) and 3 (Figure 4c) can be well-fitted to a single exponential decay function (red curve). The lifetimes of 2 (λex = 401 nm) and 3 (λex = 396 nm), 16 μs and 22 μs, together with large Stokes shift (~10,000 cm −1 ), reflect their triplet excited state nature, a spin-forbidden phosphorescence.

General Remarks
All chemicals were purchased from commercial sources and used as received. Solvents were purified following standard protocols [35]. All reactions were performed in oven-dried Schlenk glassware using standard inert atmosphere techniques. All reactions were carried out under N 2 atmosphere by using standard Schlenk techniques.

X-ray Crystallography
Single crystals suitable for X-ray diffraction analysis of 2 and [3a] 0.6 [3b] 0.4 were obtained by slow evaporation of acetone solution at ambient temperature within a week. Single crystals were mounted on the tip of glass fiber coated in paratone oil and then transferred into the cold N 2 gas stream. Data were collected on a Bruker APEX II CCD diffractometer (Bruker AXS Inc., WI, USA) using graphite monochromated Mo Kα radiation (λ = 0.71073 Å) at 100K. Absorption corrections for area detector were performed with SADABS [36] and the integration of raw data frame was performed with SAINT [37]. The structure was solved by direct methods and refined by least-squares against F 2 using the SHELXL-2018/3 package [38], incorporated in SHELXTL/PC V6.14 [39]. All non-hydrogen atoms were refined anisotropically. The detailed refinements of occupancy ratio on each atomic site are listed in Figure S1. Selected X-ray crystallographic data are listed in Table 3.  6 ], under thermal condition. The newly developed synthetic methodology provides a facile way to produce both homometallic and heterometallic chalcogenide clusters. Surprisingly, although the composition of [Ag 10 (Se){Se 2 P(O i Pr) 2 } 8 ] (2) is almost identical to the previously published [Ag 10 (Se){Se 2 P (OEt) 2 } 8 ] (1), the whole molecular shape in 2 is completely different from 1. This can be attributed to the use of different synthetic methods resulting in structural diversity. We also identify polymorphism on 2, where two pseudo symmetric [Ag 10 (Se){Se 2 P(O i Pr) 2 } 8 ] clusters are co-existed in the same asymmetric unit. Furthermore, a bimetallic chalcogenide cluster 3 can be elegantly generated by using a bimetallic hydride as the precursor. It is anticipated that a variety of metal chalcogenide clusters can be synthesized via a cluster-to-cluster transformation in the near future.