[Ag7(H){E2P(OR)2}6] (E = Se, S): Precursors for the Fabrication of Silver Nanoparticles

Reactions of Ag(I) salt, NH4(E2P(OR)(2)) (R = Pr-i, Et; E = Se, S), and NaBH4 in a 7:6:1 ratio in CH2Cl2 at room temperature, led to the formation of hydride-centered heptanuclear silver clusters, [Ag-7(H){E2P(OR)(2)}(6)] (R = Pr-i, E = Se (3): R = Et; E = S(4). The reaction of [Ag-10(E){E2P(OR)(2)}(8)] with NaBH4 in CH2Cl2 produced [Ag-8(H){E2P(OR)(2)}(6)](PF6) (R = Pr-i, E = Se (1): R = Et; E = S(2)), which can be converted to clusters 3 and 4, respectively, via the addition of 1 equiv of borohydride. Intriguingly clusters 1 and 2 can be regenerated via adding 1 equiv of Ag(CH3CN)(4)PF6 to the solution of compounds 3 and 4, respectively. All complexes have been fully characterized by NMR (H-1, Se-77, Ag-109) spectroscopy, UV-vis, electrospray ionization mass spectrometry (ESI-MS), FT-IR, thermogravimetric analysis (TGA), and elemental analysis, and molecular structures of 3(H) and 4(H) were clearly established by single crystal X-ray diffraction. Both 3(H) and 4(H) exhibit a tricapped tetrahedral Ag-7 skeleton, which is inscribed within an E-12 icosahedron constituted by six dialkyl dichalcogenophosphate ligands in a tetrametallic-tetraconnective (mu(2), mu(2)) bonding mode. Density functional theory (DFT) calculations on the models [Ag-7(H)(E2PH2)(6)] (E = Se: 3'; E = S: 4') yielded to a tricapped, slightly elongated tetrahedral silver skeleton, and time-dependent DFT (TDDFT) calculations reproduce satisfyingly the UV-vis spectrum with computed transitions at 452, and 423 nm for 3' and 378 nm for 4'. Intriguingly further reactions of [Ag-7(H){E2P(OR)(2)}(6)] with 8-fold excess amounts of NaBH4 produced monodisperse silver nanoparticles with an averaged particle size of 30 nm, which are characterized by scanning electron microscopy (SEM), energy dispersive X-ray (EDX) spectroscopy, X-ray diffraction (XRD), and UV-vis absorption spectrum.