Using electrospray ionization-tandem mass spectrometry to explore formation and gas-phase chemistry of silver nanoclusters generated from the reaction of silver salts with NaBH4 in the presence of bis(diphenylarsino)methane

Electrospray ionization‐mass spectrometry (ESI‐MS) of mixtures of AgBF₄ or AgNO₃ with the capping ligand bis(diphenylarsino)methane ((Ph₂As)₂CH₂ = dpam) in a solution of acetonitrile revealed the formation of the following cations: [Ag(CH₃CN)(dpam)]⁺, [Ag(dpam)₂]⁺, [Ag₂(Cl)(dpam)₂]⁺, and [Ag₃(Cl)₂(dpam)₃]⁺. Addition of NaBH₄ to these solutions results in the formation of the cluster cations [Ag₂(BH₄)(dpam)₂]⁺, [Ag₂(BH₄)(dpam)₃]⁺, [Ag₃(H)(BH₄)(dpam)₃]⁺, [Ag₃(BH₄)₂(dpam)₃]⁺, [Ag₃(H)(Cl)(dpam)₃]⁺, and [Ag₃(I)(BH₄)(dpam)₃]⁺, as established by ESI‐MS. Use of NaBD₄ confirmed that borohydride is the source of the hydride in these clusters. An Orbitrap Fusion LUMOS mass spectrometer was used to explore the gas‐phase unimolecular chemistry of selected clusters via multistage mass spectrometry (MSⁿ) experiments employing low‐energy collision‐induced dissociation (CID) and high‐energy collision‐induced dissociation (HCD) experiments. The borohydride containing clusters fragment via two competing pathways: (i) ligand loss and (ii) B–H bond activation involving BH₃ loss. Density functional theory (DFT) calculations were used to calculate the energetics of the optimized structures for all precursor ions, fragment ions, and neutrals and to estimate the reaction endothermicities. Generally, there is reasonable agreement between the most abundant product ion formed and the predicted endothermicity of the associated reaction channel. The DFT calculations predicted that the novel dimer [Ag₂(BH₄)(dpam)₂]⁺ has a paddlewheel structure in which the dpam and BH₄⁻ ligands bridge both silver centers.