Abstract
13 min readAn almost spherical cluster with a diameter of about 2.5 nm is formed by Ag atoms (blue), S2− ligands (yellow), and P atoms (green) of PnPr3 ligands in [Ag188S94(PnPr3)30] (see picture). The phosphane ligands from the spherical Ag2S cluster core point out like the spines of a hedgehog. The sulfur substructure is constructed from three almost spherical shells. Many investigations towards synthesis and structural characterization of large metal-rich clusters have been reported over the last few years. Particularly noteworthy are the oxometallates of molybdenum described by Müller et al.; only recently Na48[HxMo368O1032(H2O)240(SO4)48]⋅n 1000 H2O (x≈16; n≈1000), the largest known derivative of a heteropolyacid was structurally characterized.1 Other examples of large cluster complexes are [Pd145(CO)x(PEt3)30] (x≈60) made by Dahl et al.2 and [Ga84{N(SiMe3)2}20Li6Br2(thf)20] reported by Schnöckel and Schnepf.3 The synthesis of the larger Ag–S cluster complexes [Ag14S(SPh)12(PPh3)8]4 and [HNEt3]4[Ag50S7(SC6H4tBu)40]5 was recently achieved by Jin et al. Over the last few years one of our major fields of interest was the synthesis of transition-metal clusters containing S, Se, Te, P, As, and Sb as bridging ligands, for example, copper-chalcogenide clusters stabilized by tertiary phosphane ligands.6 Reactions of transition-metal salts with Group 16 silyl derivatives E(SiMe3)2 or RESiMe3 (R=alkyl- or aryl-; E=S, Se, Te) allows access to these clusters.7 Addition of tertiary phosphanes to these reactions prevents the formation of the thermodynamically stable binary phase.8 The molecular structures of complexes 1 and 2 in the solid-state were determined by X-ray crystallography.15 Compound 1 crystallizes in the space group P with two molecules in the unit cell. The molecular structure is shown in Figure 1. The cluster core consists of an arrangement of 70 silver and 50 sulfur atoms protected by four triphos ligands and 34 phenyl rings of SPh− groups. Four benzoate ligands make up the additional charge to give a neutral cluster by coordinating by their oxygen centers to the surface of the cluster core. Molecular structure of 1 (phenyl rings of the phosphane, benzoate, and SPh− are omitted for clarity; P (green), S2− ions (orange), S atoms of PhS− ligands (yellow), O (red), Ag (blue), both positions of Ag70 (light blue)). Selected bond lengths [±0.5 pm] and angles [±0.2°]: Ag8-Ag13 288.9, Ag19-Ag22 338.0, Ag70-Ag70′ 132.8, Ag1-S43 237.0, Ag7-S43 289.6, Ag13-S41 251.3, Ag17-S29 275.3, Ag17-S42 260.3, Ag22-S39 242.3, Ag28-S46 292.7, Ag39-S29 279.3, Ag46-S5 242.0, Ag64-S22 249.1, Ag64-S48 292.2, Ag67-S2 246.4, Ag69-S27 266.0, Ag69-S31 244.4, Ag36-P2 239.0, Ag52-P4 246.3, Ag34-O8 258, Ag43-O6 251, Ag64-O7 229, Ag69-O5 233; S43-Ag1-S48 174.8, S44-Ag4-S46 155.3, S38-Ag10-S46 160.4, S25-Ag28-S46 81.6, S28-Ag53-S46 89.2, S20-Ag59-S23 137.5, P6-Ag22-S39 147.8, P10-Ag37-S10 94.1, O8-Ag23-S6 89.5, O5-Ag69-S31 125.0, Ag41-S4-Ag68 92.4, Ag46-S5-Ag66 85.1, Ag23-S26-Ag29 70.2, Ag49-S32-Ag60 163.3, Ag67-S33-Ag70 66.0, Ag38-S36-Ag52 68.0, Ag4-S39-Ag10 61.5, Ag4-S39-Ag22 159.8, Ag13-S41-Ag67 164.6, Ag8-S45-Ag12 63.4, Ag26-S45-Ag42 158.2. The sulfur atoms are arranged in a highly distorted a layer substructure (Figure 2) with no bonding interactions. The sulfur atoms of the SPh− groups (S1–S34; yellow) are located on the surface of the cluster and S2− ligands (S35–S50; orange) are part of the inner core. The four sulfur layers are highly corrugated with a pseudo S4 axis through the midpoints of S39–S47 and S14–S45 (in Figure 2 from bottom to top). The top and bottom sulfur layers shown contain eight S atoms and are twisted by 90° relative to each other. Each of these layers contains two S2− ligands (S39, S47 and S35, S45) and six SPh− ligands (S17, S18, S21, S24, S25, S30 and S4, S8, S11, S14, S16, S29, respectively). The two inner sulfur layers contain 17 S atoms each, six S2− ligands (S36, S38, S40, S44, S46, S48 and S37, S41, S42, S43, S49, S50, respectively), which are surrounded by rings of 11 thiolato ligands (S1, S5, S7, S9, S10, S19, S20, S22, S27, S28, S34 and S2, S3, S6, S12, S13, S15, S23, S26, S31, S32, S33). The sulfur substructure of 1 S atoms of PhS− (yellow) and polyhedral arrangement of S2− ions (orange). The S atoms in the SPh− ligands are μ2-, μ3-, and μ4 bridging, whereas central S2− ions act as μ4 to μ7 bridges between Ag atoms. The P-bonded Ag atoms exhibit either a trigonal-planar (Ag14, Ag16, Ag22, Ag25, Ag32, Ag36, Ag37, Ag39, Ag52, Ag55) or a distorted-tetrahedral (Ag17, Ag21) coordination geometry. Ag atoms coordinated by O (benzoate) atoms have as analogous coordination geometry (trigonal planar for Ag23, Ag43, Ag54, Ag65, Ag69; tetrahedral for Ag20, Ag34, Ag42, Ag64). The remaining 49 Ag atoms are solely coordinated by S2− ligands and have coordination numbers from two to four. Observed S-Ag-S angles for the Ag atoms range from 137.5(2)–174.8(2)° (non-linear), 89.2(2)–160.4(2)° (trigonal planar with ∑S-Ag-S=348.5–360.0°) and 81.6(2)–155.3(2)° (tetrahedral). As a consequence of these different Ag coordination environments, significantly shorter Ag–S bonds (237.0(4)–251.3(4) pm) are observed for coordination number two in comparison to trigonal planar coordination (242.0(5)–289.6(4) pm) or tetrahedral coordination (246.4(7)–292.7(4) pm). Ag70 is disordered over two sites and is coordinated by S1, S33 and S9 or S12. Assuming that in 1 only S2−, SPh−, and monoanionic benzoate ligands are present, all the Ag atoms have the formal charge +1 and thus have a d10 configuration. Ag–Ag separations within the cluster core are 288.9(2)–338.0(2) pm and indicate that there are no Ag–Ag interactions. Compound 2 crystallizes in the space group P with one molecule in the unit cell. The molecular structure shows 2 has an almost spherical structure with symmetry (Figure 3). The surface is protected by 30 PnPr3 ligands. The diameter of the Ag2S core is 1.8—2.0 nm. On including the bound P atoms the diameter increases to 2.25–2.50 nm and when also the nPr groups are included the particle size is 3.2 nm. This size corresponds well with the hydrodynamic diameter of the particle determined by dynamic light-scattering measurements. Molecular structure of 2 (nPr groups atoms have been omitted for clarity P (green), S2− ions (yellow), Ag (blue)). Selected bond lengths [±0.8 pm] and angles [±0.3°]: Ag1-Ag25 307.3, Ag14-Ag47 287.2, Ag1-S37 244.3, Ag28-S9 239.2, Ag64-S39 265.1, Ag89-S47 242.1, Ag90-S41 272.7, Ag92-S47 280.2, Ag5-P5 236.5, Ag3-P3 241.2; S12-Ag16-S11 171.8, S21-Ag44-S7 162.0, S23-Ag53-S28 109.5, S18-Ag55-S17 137.1, P3-Ag3-S40 163.9, P11-Ag11-S32 176.1, Ag41-S1-Ag63 69.9, Ag16-S11-Ag17 99.9, Ag24-S16-Ag22 108.9, Ag77-S39-Ag76 130.4. The structure of 2 is highly complex and in addition the Ag and S atoms in the cluster center are disordered. The extent of the disorder depends on the reaction conditions leading to the formation of 2 and on the temperature during data collection. Surprisingly, the disorder does not affect the atomic positions at the cluster periphery. In Ag2S—an ionic conductor—a similar disorder of the Ag+ ions occurs at temperatures above 150 °C.16 In the solid-state structure of Ag2S, however, only a few Ag+ ions show temperature-dependent disorder. Assuming that 2 is a nanoscopic section of the Ag2S structure then it should not be surprising to make similar observations. It is, however, unusual that this observation can be made for a cluster of the size of 2. To our surprise, it evolved that the structure of 2 cannot be interpreted as part of the Ag2S structure. Related selenido and selenato complexes of silver (with less than 112 Ag atoms) are known to have structures composed of several shells, for example, [Ag30Se8(SetBu14(PnPr3)8] and [Ag90Se38(SetBu)14(PEt3)22].10 However, the Se atoms in [Ag112Se32(SenBu)48(PtBu3)12] and [Ag172Se40(SenBu)92(dppp)4], the largest Ag2Se cluster to be structurally characterized, adopt a distorted body-centered cubic arrangement in which the Ag atoms occupy the vacant positions and are surrounded by Se atoms in a distorted trigonal-planar or tetrahedral coordination environment. Disorder of Ag atoms, however, has also occurred in Ag90-, Ag112-, and Ag172 clusters and seems to be a common feature in the structures of these clusters. The structural change from complexes composed of several shells to structures that can be regarded as part of a solid-state structure of a binary compound very much depends on the size and properties of the cluster. These influences have already been discussed for ligand-protected M2E clusters (E=S, Se, M=Cu, Cd).7–9, 17, 18 It is surprising in this context that the number of Ag2E units present in a cluster does not allow any prediction of the structural change. Only recently we managed to demonstrate this for [Ag124Se97(SePtBu2)4Cl6(tBu2P(CH2)3PtBu2)12] in which the Se substructure consists of a central Se atom surrounded by a distorted Se16 Frank–Kasper polyhedron, which itself is surrounded by a Se44 polyhedron as the second shell.18 Similar structural properties are found in 2: 94 S atoms are distributed over three shells. The outer shell consists of 50 (S1–S25), the middle shell of 34 (S26–S42), and the inner shell of 10 (S43-S47) S atoms. Non-bonding S–S separations are in the range 450–500 pm. Figure 4 shows the arrangement of the sulfur polyhedra in 2. The S50 polyhedron (bright yellow) consists of 30 twisted S5 rings linked together by six S3 triangles. The middle shell (dark yellow) forms a previously unknown Δ3 polyhedron of 34 S atoms that itself contains an S10 polyhedron (orange). However, because of the disorder, the structure of the S10 polyhedron cannot be determined. During refinement comparatively high electron density remains within the cluster core; considering AgS bond lengths we believe that this observation can be explained by disordered Ag and S atoms. Hence, Ag atoms (Ag87–Ag94) in the vicinity of the S10 polyhedron show high temperature factors. For these reasons, the formula of 2 has to be considered as idealized, but coincides well with elemental analysis and also with interatomic distances commonly observed for AgS bonds. The edges of the S50 outer shell are occupied by Ag atoms (Ag16–Ag48; Figure 5) which act as non-linear bridges between two S atoms (Ag–S 236.1(9)–244.2(8) pm, S-Ag-S: 155.3(2)–172.2(3)°). The sulfur substructure in 2. The central S10 polyhedron (orange) is surrounded by a S34 polyhedron (dark yellow) which itself is encapsulated by a S50 polyhedron (bright yellow). The outer shell of 2 consisting of 114 Ag atoms (blue), 50 S atoms (yellow), and P atoms (green; PnPr3). The shortest Ag–S separations are observed in the middle S34 shell are in the range 333–388 pm. These very weak interactions could be the reason for the uncommon S-Ag-S bond angles in the outer shell. The broad range of Ag–E distances (E=S, Se, Te), generally complicates the structural discussion of Ag-chalcogenide clusters. The atoms Ag49–Ag57 are each coordinated by two outer-shell S atoms and act as bridging atoms between the outer shell (S50 polyhedron) and the middle shell (S34 polyhedron). Thus the atoms Ag49–Ag57 are surrounded by S atoms in a distorted trigonal-planar or tetrahedral fashion (S-Ag-S 104.3(2)–137.1(3)°, ∑ S-Ag-S 353.2–357.9°). The AgS bond lengths are between 247.2(8) and 283.5(8) pm. Additional weak interactions with outer-shell S atoms range from 301–309 pm. Figure 5 shows an Ag114S50 polyhedron (Ag1–Ag57 and S1–S25) constructed from 30 Ag5S5-ten-membered rings. One Ag atom (Ag1–Ag15), stabilized by a phosphane (PnPr3) ligand, caps each ring, these Ag atoms are not coordinated by S atoms of the S50 shell but to 30 S atoms of the middle S34 polyhedra. (Figure 6 shows the distorted linear coordination of Ag1–Ag15 by the S atoms the PnPr3 ligands Ag–P 236.5(7)–241.2(7) pm, Ag–S 239.1(6)–244.7(5) pm, P-Ag-S 163.9(3)–176.1(2)°). There are no interactions between Ag1–Ag15 and the Ag atoms and S atoms located on the Ag114S50 cluster surface (Ag⋅⋅⋅Ag > 297 pm, Ag⋅⋅⋅S 327—418 pm). The middle shell in 2 consists of 34 S2− ligands bound to P-coordinated Ag atoms; P (green), S2− ions (yellow), Ag (blue). The atoms Ag58–Ag64 connect the outer with the middle shell. Each one of these Ag atoms is coordinated by one S atom of the outer S50 shell and two S atoms of the middle S34 shell and has a distorted trigonal-planar coordination geometry (Ag–S 258.1(6)–294.7(7) pm, S-Ag-S 102.3(2)–145.9(2)°, ∑S-Ag-S 358.5–360°). Ag62–Ag64 occupy vacant tetrahedral positions located between both S polyhedra (Ag-S 261.4(6)–293.9(6) pm). The atoms Ag65 and Ag66 are solely coordinated by S atoms of the middle S34 shell in a distorted linear fashion (Ag-S 250.7(5)–257.9(6) pm, S-Ag-S 147.8(2)–161.4(2)°). Ag67–Ag80 are coordinated by three S atoms belonging to the middle S34 polyhedra and have a distorted trigonal-planar coordination geometry (Ag–S 256.1(6)–274.4(6) pm, S-Ag-S 101.6(2)–126.3(2)°, ∑S-Ag-S 336.5–349.8°). Ag81–Ag94 act as bridging atoms between the middle S34 and the inner S10 polyhedron. Ag81 and Ag82 are coordinated by two S atoms (Ag–S 255.1(6)–262.4(6) pm, S-Ag-S 172.3(3)°) in a slightly bent arrangement. Ag83–Ag89 show a distorted trigonal-planar coordination (Ag–S 242.1(6)–287.1(8) pm, S-Ag-S 83.2(2)–163.6(3)°, ∑S-Ag-S 353.3–359.8°). Distorted tetrahedral coordination is observed for Ag90–Ag94 (Ag–S 251.7(9)–280.2(7) pm). As a result, the middle shell (diameter 1.1–1.5 nm) is formed by S26–S42, Ag58–Ag80, Ag81 and Ag83 and has the formula Ag50S34 (Figure 7). Ag82 and Ag84–Ag94 connect the middle shell with the inner S10 polyhedron. Ag49–Ag57 connect the Ag114S50 outer shell through AgS bonds with the Ag50S34 middle shell (AgS > 302 pm). Spherical structure of the middle shell Ag50S34 S2− ions (yellow), Ag (blue) in 2. The S2− ions in 2 adopt various coordination modes. S1–S16 act as μ3 bridging ligands coordinating three Ag+ ions each (Ag-S-Ag 63.2(2)–115.5(3)°). S17–S25 are μ4 bridging and each coordinate four Ag+ ions of the outer Ag114S50 shell. Weak Ag–S interactions to Ag+ ions belonging to the inner shell lead to distorted tetrahedral geometry for S1–S16 and distorted square-planar geometry for S17–S25 ((S1–S16)⋅⋅⋅Ag 290–312 pm, (S17–S25)⋅⋅⋅Ag 284–307 pm). The effect of these interactions is that the atoms S1–S25 are around 90–100 pm below the plane of the sulfur-bound Ag ions. Of the 34 sulfide ions (S26–S42) of the middle S34 polyhedron 30 coordinate atoms Ag1–Ag15, each of these sulfide ions is surrounded by a distorted octahedron of six Ag+ ions. The same coordination mode is found for S43, S44, and S46. Only S45 and S46 form μ4 bridges. So far we have assumed that AgS bonds are covalent. However, one must not overlook—the important!—ionic interactions between Ag+ and S2− ions. In an alternative approach 2 could be described as an uncharacteristic nanoparticle of a crystal with the ionic Ag2S structure. The formation of 2 instead of Ag2S occurs because of the presence of the protecting ligand sphere. Other silver chalcogenide clusters synthesized in our group all show the phenomenon of increased Ag disorder with increasing cluster size. This observation is in agreement with the fact that nanoparticles of the composition Ag2E generally show a higher degree of Ag disorder than the corresponding binary crystalline phases. Similar disorder has also been observed in the isostructural cluster core of 3. This suggests that the choice of cluster-stabilizing tertiary phosphane ligands does not effect the disorder within these Ag2S clusters. All experiments were carried out under exclusion of oxygen and moisture in an atmosphere of dry nitrogen. 1: S(Ph)SiMe3 (0.10 mL, 0.50 mmol) and S(SiMe3)2 (0.04 mL, 0.25 mmol) is added a rapidly stirred suspension of PhCO2Ag (silver benzoate; 230 mg, 1.00 mmol) and triphos (100 mg, 0.25 mmol) in diglyme (20 mL) at −20 °C. The reaction was then allowed to slowly warm to 0 °C. After 2 h at 0 °C the reaction mixture was allowed to warm to room temperature. Storage of the reaction for one month produced tiny red needles of 1 (yield 27 %). 2: CF3CO2Ag (silver trifluoroacetate; 230 mg, 1.00 mmol), PnPr3 (0.2 mL, 1.00 mmol), and S(SiMe3)2 (0.08 mL, 0.50 mmol) in diglyme (20 mL) were stirred at −40 °C. After 2 h the reaction was allowed to warm to room temperature. Storage of the reaction mixture for seven days produced black crystals of 2 (33 %). Changes to the reaction conditions causes precipitation of 2 and AgS as a black powder. 3: S(SiMe3)2 (0.08 ml, 0.50 mmol) was added to a suspension of CF3CO2Ag (230 mg, 1.00 mmol) and PnBu3 (0.25 ml, 1.00 mmol) in diglyme (25 ml) at −40 °C. After 3 days at this temperature the deep red solution was allowed to warm to −20 °C, then, after 1 day at this temperature, the reaction was allowed to warm to room temperature. Black crystals of 3 were formed on storing the reaction mixture at room temperature for 1 week. Correct elemental analyses (C, H, O, P, Ag) for 1–3 were obtained. Dedicated to Professor Achim Müller on the occasion of his 65th birthday
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