[Ag<sub>7</sub>(H){E<sub>2</sub>P(OR)<sub>2</sub>}<sub>6</sub>] (E = Se, S): Precursors for
the Fabrication of Silver Nanoparticles
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Abstract
Reactions of Ag(I) salt, NH<sub>4</sub>(E<sub>2</sub>P(OR)<sub>2</sub>) (R = <sup>i</sup>Pr, Et; E = Se, S), and NaBH<sub>4</sub> in a 7:6:1 ratio in CH<sub>2</sub>Cl<sub>2</sub> at room
temperature, led to the formation of hydride-centered heptanuclear
silver clusters, [Ag<sub>7</sub>(H){E<sub>2</sub>P(OR)<sub>2</sub>}<sub>6</sub>] (R = <sup>i</sup>Pr, E = Se (<b>3</b>): R =
Et; E = S(<b>4</b>). The reaction of [Ag<sub>10</sub>(E){E<sub>2</sub>P(OR)<sub>2</sub>}<sub>8</sub>] with NaBH<sub>4</sub> in CH<sub>2</sub>Cl<sub>2</sub> produced [Ag<sub>8</sub>(H){E<sub>2</sub>P(OR)<sub>2</sub>}<sub>6</sub>](PF<sub>6</sub>) (R = <sup>i</sup>Pr, E = Se
(<b>1</b>): R = Et; E = S(<b>2</b>)), which can be converted
to clusters <b>3</b> and <b>4</b>, respectively, via the
addition of 1 equiv of borohydride. Intriguingly clusters <b>1</b> and <b>2</b> can be regenerated via adding 1 equiv of Ag(CH<sub>3</sub>CN)<sub>4</sub>PF<sub>6</sub> to the solution of compounds <b>3</b> and <b>4</b>, respectively. All complexes have been
fully characterized by NMR (<sup>1</sup>H, <sup>77</sup>Se, <sup>109</sup>Ag) spectroscopy, UV–vis, electrospray ionization mass spectrometry
(ESI-MS), FT-IR, thermogravimetric analysis (TGA), and elemental analysis,
and molecular structures of <b>3</b><sub><b>H</b></sub> and <b>4</b><sub><b>H</b></sub> were clearly established
by single crystal X-ray diffraction. Both <b>3</b><sub><b>H</b></sub> and <b>4</b><sub><b>H</b></sub> exhibit
a tricapped tetrahedral Ag<sub>7</sub> skeleton, which is inscribed
within an E<sub>12</sub> icosahedron constituted by six dialkyl dichalcogenophosphate
ligands in a tetrametallic-tetraconnective (μ<sub>2</sub>, μ<sub>2</sub>) bonding mode. Density functional theory (DFT) calculations
on the models [Ag<sub>7</sub>(H)(E<sub>2</sub>PH<sub>2</sub>)<sub>6</sub>] (E = Se: <b>3′</b>; E = S: <b>4′</b>) 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 <b>3′</b> and 378 nm for <b>4′</b>. Intriguingly further reactions of [Ag<sub>7</sub>(H){E<sub>2</sub>P(OR)<sub>2</sub>}<sub>6</sub>] with 8-fold excess amounts of NaBH<sub>4</sub> 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
