[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 = iPr, 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, [Ag7(H)­{E2P­(OR)2}6] (R = iPr, E = Se (3): R = Et; E = S­(4). The reaction of [Ag10(E)­{E2P­(OR)2}8] with NaBH4 in CH2Cl2 produced [Ag8(H)­{E2P­(OR)2}6]­(PF6) (R = iPr, 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)4PF6 to the solution of compounds 3 and 4, respectively. All complexes have been fully characterized by NMR (1H, 77Se, 109Ag) spectroscopy, UV–vis, electrospray ionization mass spectrometry (ESI-MS), FT-IR, thermogravimetric analysis (TGA), and elemental analysis, and molecular structures of 3H and 4H were clearly established by single crystal X-ray diffraction. Both 3H and 4H exhibit a tricapped tetrahedral Ag7 skeleton, which is inscribed within an E12 icosahedron constituted by six dialkyl dichalcogenophosphate ligands in a tetrametallic-tetraconnective (μ2, μ2) bonding mode. Density functional theory (DFT) calculations on the models [Ag7(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 [Ag7(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.