Synthesis, structure, and condensed-phase reactivity of [Ag3(μ3-H)(μ3-BH4)LPh3](BF4) (LPh = bis(diphenylphosphino)amine) with CS2.

Abstract

Electrospray ionisation mass spectrometry (ESI-MS) was used to monitor the reaction of AgBF4, bis(diphenylphosphino)amine (dppa = (Ph2P)2NH = LPh) and NaBH4 in acetonitrile and thereby direct the synthesis of the silver nanocluster [Ag3(μ3-H)(μ3-BH4)LPh3](BF4), 3b·BF4, formed via reaction of AgBF4, bis(diphenylphosphino)amine (dppa = (Ph2P)2NH = LPh) and NaBH4 in acetonitrile. The X-ray structure of 3b·BF4 highlights that the cation adopts a planar trinuclear Ag3 geometry surrounded by three dppa ligands and coordinated on the bottom face by a μ3-hydride and on the top face by a μ3-BH4. The solution phase structure of 3b·BF4 was characterised by multinuclear NMR and DOSY NMR, which showed that the borohydride anion remains bound in the [Ag3(μ3-H)(μ3-BH4)LPh3]+ cluster cation in solution. ESI-MS and in situ1H and HSQC NMR spectroscopy reveals that 3b·BF4 reacts with CS2 in solution at the BH4 site to yield [Ag3(H)(S2CH)LPh3]+, 4b, which has to date eluded structural characterisation via X-ray crystallography due to lack of formation of suitable crystals. The gas-phase ion chemistry of [Ag3(H)(S2CH)LPh3]+ was examined under multistage mass spectrometry conditions using collision-induced dissociation (CID) and compared to that of the previously examined copper analogue, [Cu3(H)(S2CH)LPh3]+. While both cluster cations fragment via ligand loss, the CID spectra of the resultant [M3(H)(S2CH)LPh2]+ are different. Unlike [Cu3(H)(S2CH)LPh2]+, which solely undergoes loss of thioformaldehyde to give [Cu3(S)LPh2]+, [Ag3(H)(S2CH)LPh2]+ gives a richer CID spectrum with fragmentation channels that include ligand loss, CH2S loss and reductive elimination of dithioformic acid. DFT calculations exploring rearrangement and fragmentation of the model systems [M3(H)(S2CH)LMe2]+ ((Me2P)2NH = dmpa = LMe) were used to suggest plausible mechanisms and examine the energetics of the three competing channels: ligand loss, CH2S loss and reductive elimination of dithioformic acid.