DALTON FULL PAPER J. Chem. Soc., Dalton Trans., 1998, 2969–2972 2969 Syntheses and structures of new copper(I) selenide clusters [Cu32Se16(PPh3)12], [Cu52Se26(PPh3)16] and [Cu72Se36(PPh3)20] † Andreas Eichhöfer and Dieter Fenske * Institut für Anorganische Chemie, Universität Karlsruhe, Engesserstraße Geb. Nr. 30.45, D-76128 Karlsruhe, Germany Received 15th May 1998, Accepted 20th July 1998 The reaction of Se(SiMe3)2 with copper(I) acetate in the presence of PPh3 yielded the new clusters [Cu32Se16(PPh3)12] 2, [Cu52Se26(PPh3)16] 3 and [Cu72Se36(PPh3)20] 4 depending on the conditions.The molecular structures of 2, 3 and 4 have been characterised by X-ray crystallography and compared with the known cluster [Cu146Se73(PPh3)30] 1 obtained from the same reaction under diVerent conditions. Introduction The synthesis of large metal-containing clusters is of considerable interest because these compounds are good model systems for investigating the chemical and physical properties of solid state materials. This is particularly important for nanoparticulate binary metal chacogenides which show quantum eVects such as size-dependent band gaps.1 We have been successful in preparing a large number of copper selenide clusters using the reaction of CuX (X = Cl or OAc) with Se(SiMe3)2 and PR3 (R = organic group) outlined in eqn.(1). Some of the compounds we have structurally charac- 2CuX1yPR31Se(SiMe3)2 THF or Et2O 22SiMe3X [Cu2Se(PR3)y]n (1) terised are as follows: [Cu12Se6(PnPr3)8], [Cu26Se13(PEt2Ph)14], [Cu30Se15(PiPr3)12], [Cu36Se18(PtBu3)12], [Cu44Se22(PEt2Ph)18] and [Cu70Se35(PEt3)22].A noteworthy example is the structure of [Cu146Se73(PPh3)30] 1 one of the largest known structurally characterised clusters.2b From the range of products listed above it can clearly be seen that the phosphine has a profound role in determining the size of the copper selenide cluster. This may be attributed both to steric eVects due to the characteristic Tolman cone angle and to kinetic factors arising from stabilisation of copper/phosphine/ acetate clusters.We now wish to report that other factors such as ligand concentration, temperature and solvent also influence cluster formation. For this study we have conducted a detailed investigation of the CuOAc/Se(SiMe3)2/PPh3 system from which we have previously isolated 1. Experimental Standard Schlenk techniques were employed throughout the syntheses using a double-manifold vacuum line with high purity dry nitrogen.The solvents diethyl ether, tetrahydrofuran and diglyme were dried over sodium–benzophenone and distilled under nitrogen. CuOAc3 and Se(SiMe3)2 4 were prepared according to standard literature procedures. Syntheses [Cu32Se16(PPh3)12] 2. PPh3 (1.73 g, 6.6 mmol) was added to a suspension of copper(I) acetate (0.27 g, 2.2 mmol) in THF (40 † Dedicated to Professor Brian F. G. Johnson on the occasion of his 60th birthday.mL). A clear colourless solution was formed. Upon the addition of Se(SiMe3)2 (0.25 mL, 1.1 mmol) the solution gradually turned yellow, and subsequently dark brown. After 1 d, 20 mL of diethyl ether were added to a solution cooled to 4 8C. After 3 d at this temperature, dark yellow crystalline plates of 2 were formed. Yield: 54% (Found: C, 39.02; H, 2.90. C216H180Cu32- P12Se16 requires C, 40.26; H, 2.80%). [Cu52Se26(PPh3)16] 3. PPh3 (1.73 g, 6.6 mmol) was added to a suspension of copper(I) acetate (0.27 g, 2.2 mmol) in THF (40 mL).A clear colourless solution was formed. Upon addition of Se(SiMe3)2 (0.25 mL, 1.1 mmol), the solution gradually turned yellow, and subsequently dark brown. After 1 d, 20 mL of diethyl ether were added and the solution was kept at room temperature. After 2 d, dark yellow crystalline plates of 2 were formed in low yield, and after 2 additional days, black crystals of 3 began to grow as the major product of the reaction.Yield: 50% (Found: C, 37.1; H, 2.80. C288H240Cu52P16Se26 requires C, 36.2; H, 2.5%). [Cu72Se32(PPh3)20] 4. PPh3 (1.28 g, 4.9 mmol) was added to a suspension of copper(I) acetate (0.20 g, 1.63 mmol) in diglyme (30 mL). On addition of Se(SiMe3)2 (0.25 mL, 1.1 mmol) the solution gradually turned clear, changing colour from yellow to dark brown. After 3 d, black crystals of 4 were formed. Yield: 51% (Found: C, 35.49; H, 3.33. C360H300Cu72P20Se36 requires C, 34.15; H, 2.37%).Two diglyme molecules were localised and refined in the X-ray structure analysis, but there is evidence for the existence of other highly disordered solvent molecules which may be responsible for the high carbon and hydrogen content found in the elemental analysis. X-Ray structural analyses Single crystal X-ray structural analyses of compounds 2–4 were performed using a Stoe-IPDS diVractometer (Mo-Ka radiation) equipped with an image plate area detector and a rotating anode.Structure solution and refinement were carried out using SHELXS 865 and SHELXL 936 software using direct methods techniques. The weighting scheme applied was of the form w = 1/[s2(F2) 1 (aP)2 1 bP] [a,b = refined variables, P = 1 3 – (Fo 2,0) 1 2 3 – Fc 2]. All calculations were performed on a Silicon Graphics INDY computer. Molecular diagrams were prepared using the SCHAKAL 92 program.7 Compound 2. C216H180Cu32P12Se16?4C4H8O, M = 6730.6, triclinic, space group P1� , a = 19.319(4), b = 21.531(4), c = 33.931(7) Å, a = 79.58(3), b = 80.11(3), g = 74.37(3)8, U =2970 J.Chem. Soc., Dalton Trans., 1998, 2969–2972 13254(5) Å3, T = 200 K, Z = 2, Dc = 1.675 g cm23, F(000) = 6496, m(Mo-Ka) = 4.817 mm21, 2qmax = 558, 31788 independent reflections measured (Rint = 0.0859) and 22364 I > 2s(I). The structure was solved by direct methods and refined on F2. All Se, Cu and P atoms were refined anisotropically, all C atoms isotropically and positions for H atoms were calculated except those of THF to yield R = 0.0823, wR = 0.2275, S = 0.991.Compound 3. C288H240Cu52P16Se26?2.5C4H8O, M = 9721.53, triclinic, space group P1� , a = 22.336(5), b = 23.203(5), c = 37.894(8) Å, a = 91.00(3), b = 106.95(3), g = 99.06(3)8, U = 18510(6) Å3, T = 200 K, Z = 2, Dc = 1.741 g cm23, F(000) = 9350, m(Mo-Ka) = 5.580 mm21, 2qmax = 51.88, 57286 independent reflections measured (Rint = 0.0892) and 39806 I > 2s(I). The structure was solved by direct methods and refined on F2.All Se, Cu and P atoms were refined anisotropically, all C atoms isotropically and positions for H atoms were calculated except those of THF to yield R = 0.0823, wR = 0.2373, S = 1.023. Compound 4. C360H300Cu72P20Se36?2C6H16O3, M = 12931.18, triclinic, space group P1� , a = 25.632(5), b = 33.806(7), c = 34.904(7) Å, a = 72.69(2), b = 87.16(2), g = 72.64(2)8, U = 27534(1) Å3, T = 190 K, Z = 2, Dc = 1.560 g cm23, F(000) = 12440, m(Mo-Ka) = 5.185 mm21, 2qmax = 458, 66463 independent reflections measured (Rint = 0.073) and 45921 I > 2s(I).The structure was solved by direct methods and refined on F2. All Se, Cu and P atoms were refined anisotropically, all C atoms isotropically to yield R = 0.0829, wR = 0.2271, S = 0.939. CCDC reference number 186/1098. See http://www.rsc.org/suppdata/dt/1998/2969/ for crystallographic files in .cif format. Results and discussion We have previously shown that copper(I) acetate reacts with 5 equivalents of PPh3 and 0.5 equivalent of Se(SiMe3)2 in THF at room temperature to yield [Cu146Se73(PPh3)30] 1,2b after a number of days.We now find that similar reactions with only 3 equivalents of PPh3 produce dark coloured solutions. Addition of diethyl ether and cooling to 4 8C leads to the crystallisation of [Cu32Se16(PPh3)12] 2. Keeping these mixed solvent solutions at room temperature leads to the formation of [Cu52Se26- (PPh3)16] 3. [Cu72Se36(PPh3)20] 4 crystallises from the same reactions if diglyme is used instead of THF as a solvent.Compound 2 crystallises as dark yellow plates in the triclinic Fig. 1 Molecular structure of [Cu32Se16(PPh3)12] 2. Cu11: blue; Se22: red; P: green. Cu–Cu contacts and carbon atoms are omitted for clarity. Cu, Se and P atoms are labelled with blue, red and green numbers respectively. space group P1� . Fig. 1 shows thure with the exception of the carbon atoms. The selenium atoms Se1–Se16 form a flattened polyhedron compromising of non-bonded Se3 triangles (Se–Se 3.984 to 4.645 Å). There are four distinct copper environments.Twenty-four of the copper atoms cap the selenium triangles, of these sixteen (Cu1, Cu2, Cu3, Cu4, Cu6, Cu11, Cu15, Cu16, Cu19, Cu21, Cu22, Cu24, Cu27, Cu28, Cu29, Cu31) are bonded only to selenium producing a distorted trigonal coordination geometry. These copper atoms lie below the plane formed by the selenium atoms to which they are coordinated such that they are within the selenium polygon.A further eight (Cu5, Cu8, Cu12, Cu14, Cu17, Cu18, Cu20, Cu25) are also bonded to a phosphine ligand (P2, P3, P4, P6, P7, P8, P9, P11) giving a tetrahedral environment with the copper atoms situated on the exterior of the selenium faces. Four copper atoms (Cu7, Cu10, Cu13, Cu23) are coordinated in a quasilinear fashion to two selenium atoms (Se3–Cu23–Se15 156.13, Se13–Cu7–Se10 159.28, Se1–Cu10–Se2 170.2, Se9–Cu13–Se14 170.528).The four remaining copper atoms Cu9, Cu26, Cu30 and Cu32 are bonded in a distorted trigonal environment to two selenium atoms and one phosphorus atom from PPh3. The Cu– Se distances range from 2.295 to 2.930 Å. For the selenium atoms, the coordination numbers five (Se3, Se4, Se10, Se11, Se12, Se13) and six (Se1, Se2, Se5, Se6, Se7, Se8, Se9, Se14) are found. Compound 3 forms large black or smaller brown crystals which crystallise in the triclinic space group P1� (Fig. 2).By analogy with 2, the framework in 3 can also be seen to be made up from edge-sharing Se3 triangles. The selenium atoms form an approximately spherical polyhedron with three additional atoms (Se1, Se8, Se18) located within the cavity. The Se–Se distances range from 3.803 to 4.707 Å and are essentially nonbonding. Thirty-six of the fifty-two copper atoms are bonded only to the selenium atoms (Cu–Se 2.19–2.91 Å) and are located within the lattice, exhibiting either a linear (Cu12, Cu28, Cu36, Cu41, Cu43), distorted trigonal (Cu1, Cu3, Cu4, Cu7, Cu10, Cu11, Cu15, Cu18, Cu21, Cu24, Cu25, Cu26, Cu29, Cu30, Cu33, Cu35, Cu38, Cu39, Cu40, Cu42, Cu44, Cu46, Cu47, Cu48, Cu49, Cu50, Cu52) or a distorted tetrahedral structure (Cu8, Cu13, Cu20, Cu51).Coordination numbers for the selenium atoms vary from four (Se26) to five (Se3, Se4, Se7, Se9, Fig. 2 Molecular structure of [Cu52Se26(PPh3)16] 3. Cu11: blue; Se22: red; P: green. Cu–Cu contacts and carbon atoms are omitted for clarity.Cu, Se and P atoms are labelled with blue, red and green numbers respectively.J. Chem. Soc., Dalton Trans., 1998, 2969–2972 2971 Se11, Se12, Se14, Se15, Se16, Se17, Se21, Se22, Se23, Se24, Se25), six (Se5, Se8, Se10, Se13, Se19), seven (Se1, Se2, Se6, Se18) and eight (Se20). Those copper atoms which are bonded to the sixteen phosphine ligands are either coordinated in a nearly trigonal planar fashion, for those that are bonded to one phosphorus atom and two selenium atoms (Cu2, Cu5, Cu9, Cu14, Cu22, Cu23, Cu34, Cu37, Cu45), or in a distorted tetrahedral way for those that are coordinated by three selenium atoms and one phosphorus atom (Cu6, Cu16, Cu17, Cu19, Cu27, Cu31, Cu32).As in structure 2 copper atoms which are coordinated by three selenium atoms only are shifted to the interior of the cluster cage. Copper atoms additionally bonded to one phosphorus atom are located on the cluster surface. In contrast to the wide range of Cu–Se distances, the Cu–P bonds all lie within a narrow range (2.215–2.284 Å).Compound 4 forms black crystals in the triclinic space group P1� . The thirty-six selenium atoms build up a nearly trigonal prismatic polyhedron with trigonal selenium faces (Fig. 3). In contrast to 2 and 3, 4 possesses a symmetrical layer-type structure with three layers each containing ten, sixteen and ten selenium atoms. The packing of the layers is nearly hexagonal, of the type ABA.Only Se30 and Se32 do not follow this trend. The non-bonding Se–Se distances range from 3.81 to 4.71 Å, and the distance between the layers is approximately 3.6 Å. The holes in the selenium lattice are filled with seventy-two copper atoms (Fig. 4). As in 2 and 3, there are three coordination environments for the copper atoms bonded only to selenium. Cu4 is linearly coordinated, forty copper atoms (Cu1, Cu2, Cu3, Cu7, Cu8, Cu9, Cu11, Cu12, Cu13, Cu14, Cu15, Cu17, Cu19, Cu20, Cu21, Cu22, Cu23, Cu24, Cu25, Cu27, Cu29, Cu30, Cu34, Cu38, Cu41, Cu42, Cu43, Cu46, Cu47, Cu49, Fig. 3 View perpendicular (above) and parallel (below) to the hexagonal ABA layering of the selenium atoms in 4. Non-bonding Se–Se contacts are shown only in the layers. Distance between the layers ca. 3.61 Å. Cu50, Cu54, Cu55, Cu58, Cu59, Cu61, Cu63, Cu65, Cu71, Cu72) exhibit a distorted trigonal environment and eleven copper atoms (Cu6, Cu16, Cu33, Cu36, Cu37, Cu39, Cu44, Cu51, Cu52, Cu57, Cu70) are coordinated in a distorted tetrahedral fashion (Cu–Se 2.354–2.921 Å).For those copper atoms bonded to selenium and phosphorus atoms, six copper atoms (Cu5, Cu18, Cu31, Cu32, Cu35, Cu69) exhibit the coordination number three (2 Se, 1 P atom), and fourteen copper atoms (Cu10, Cu26, Cu28, Cu40, Cu45, Cu48, Cu53, Cu56, Cu60, Cu62, Cu64, Cu66, Cu67, Cu68) are coordinated in a distorted tetrahedral geometry by three selenium atoms and one phosphorus atom of the PPh3 ligands (Cu–P 2.211–2.242 Å).The copper atoms coordinated to phosphine ligands are shifted to the surface of the cluster in contrast to pure trigonal coordinated ones, as was also observed for 2 and 3. If we assume a maximum Cu–Se distance for a bonding interaction of 2.92 Å we find a wide spread of selenium coordination numbers ranging from four to ten: four (Se11, Se4), five (Se1, Se5, Se7, Se10, Se22, Se26, Se27, Se31, Se36), six (Se2, Se3, Se4, Se6, Se8, Se9, Se24, Se28, Se29, Se30, Se32, Se33, Se34, Se35), seven (Se13, Se14, Se15, Se17, Se18, Se21, Se23, Se25), eight (Se16), nine (Se19) and ten (Se19).We have also previously observed a large range of selenium coordination numbers in other copper selenide clusters. The larger the cluster size the higher the average coordination numbers for selenium atoms leading to more dense copper selenium packing. The [Cu70Se35(PEt3)21] 2a structure is related to 4 simply by removal of a Cu2Se unit (Cu18, Cu35, Se30) which caps one of the corners of the CuSe polyhedron.Furthermore the two selenium atoms (Se30 and Se32) in this fragment are not part of the close-packed selenium array. This structural motif, in which a rectangular selenium frame is grafted onto one edge of the triangle is also present in 1 therefore 4 is in fact a fragment of the larger cluster. Conclusion In summary the Se cages for 2 and 3 diVer significantly from the symmetrical ones found in 4 and 1. This suggests that 2 and 3 cannot be intermediate structures in the formation of the layered clusters 4 and 1.Whilst there is a clear similarity between the structures of 4 and 1 no relationship between 2 and 3 is evident. Further, one can find analogies between all of the Fig. 4 Molecular structure of [Cu72Se36(PPh3)20] 4. Cu11: blue; Se22: red; P: green. Cu–Cu contacts and carbon atoms are omitted for clarity. Cu, Se and P atoms are labelled with blue, red and green numbers respectively.2972 J.Chem. Soc., Dalton Trans., 1998, 2969–2972 clusters in the coordination modes of the copper atoms. In common with other copper selenide clusters, the copper atoms prefer a distorted trigonal planar coordination of selenium atoms. At the cluster surfaces the copper atoms coordinate the selenium atoms in a slightly distorted triangular fashion, with some shifts towards the interior of the selenium polyhedron. Copper atoms that are linked to a phosphine ligand adopt the predicted tetrahedral geometry and are located well outside the Se lattice.The preference of trigonal coordination can also be seen from the fact that copper atoms in b-Cu2Se leave their ideal tetdral sites in the transition from a cubic high temperature phase to the lower symmetry a-Cu2Se phase to take up a more trigonal coordination.8 Although the final structure determination of the low temperature a-Cu2Se phase is still a point of discussion the best refinement of the data is achieved by assuming a cubic lattice of selenium atoms with the copper atoms occupying the tetrahedral and trigonal holes in a ratio 0.37 to 0.63.9 In contrast, 4 and 1 possess a hexagonal lattice of selenium atoms, a structure type known for the low temperature phase of Cu2S.10 Therefore it is possible that these cluster molecules display a new structure type of an as yet unknown Cu2Se polymorph. Hence cluster formation of 2 and 3 can be seen to be more like an unordered spontaneous aggregation process whereas the crystallisation of 4 and 1 is comparable to the formation of an ordered solid state structure.The results presented herein show the complex dependence of these reactions on a range of parameters. This knowledge will assist us in the design of rational syntheses for even larger clusters. Acknowledgements We are grateful to the Deutsche Forschungsgemeinschaft (SFB 195) and to the Bundesministerium für Bildung und Forschung (BMBF) for financial support of this work. References 1 H. Weller, Angew. Chem., 1996, 108, 1159; A. P. Alivisatos, Science, 1996, 271, 933; E. Kim, Y. Xia and G. M. Whitesides, Nature (London), 1995, 376, 581; M. Trau, D. A. Saville and I. A. Aksay, Science, 1996, 272, 706. 2 (a) D. Fenske and H. Krautscheid, Angew. Chem., 1990, 102, 1513; (b) D. Fenske, H. Krautscheid, G. Baum and M. Semmelmann, Angew. Chem., 1993, 105, 1364; (c) S. Dehnen, A. 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