Bis(dimethylphosphino)methane-ligated silver chloride, cyanide and hydride cluster cations: Synthesis and gas-phase unimolecular reactivity

Abstract

Electrospray ionization mass spectrometry (ESI/MS) of mixtures of AgNO3 or AgBF4 with the capping ligand Me2P(CH2)PMe2 (dmpm, L) in a solution of 1:1 methanol:chloroform or acetonitrile revealed the formation of: dinuclear clusters [Ag2L2]2+, [Ag2(LH)L]+, and [Ag2ClL2]+; trinuclear clusters primarily as [Ag3Cl2L3]+; and the tetranuclear cluster [Ag4Cl3L3]+. Addition of NaBH4 to these solutions of dmpm-protected clusters results in the formation of the silver hydride cluster cations [Ag2HL2]+, [Ag3HL3]2+, [Ag3HL4]2+, [Ag3H(CN)L3]+ and [Ag3HClL3]+ as determined by ESI/MS. Use of NaBD4 confirmed that the borohydride is the source of the hydride in these clusters.The gas-phase unimolecular chemistry of selected clusters was examined in a LTQ-FT hybrid linear ion trap (LIT) mass spectrometer. Low-energy collision-induced dissociation (CID) was performed on the clusters [Ag3Cl2L3]+, [Ag3HClL3]+, [Ag3H(CN)L3]+, [Ag3HL3]2+, [Ag3HL4]2+ and their nascent fragments. Neutral ligand loss, core fission and ligand activation were observed to depend on the cluster stoichiometry. Thus, [Ag3XYL3]+ (X, Y=Cl; X=H; Y=Cl or CN) mainly fragment via ligand loss, [Ag3XYL2]+ undergoes both ligand loss and core fission, [Ag3XYL]+ undergoes core fission and [Ag2YL]+ undergoes ligand activation. Small yields of reductive elimination of HY are observed for [Ag3HYL2]+ (Y=Cl and CN). Density functional theory (DFT) calculations were used to calculate the energetics of the optimised structures for all parent, fragment ions and neutrals and to estimate the reaction endothermicities. Generally there is a reasonable agreement between the most abundant product ion formed and the predicted endothermicity, except for the reductive elimination reactions. Finally, electron-induced dissociation (EID) of [Ag3HL3]2+ and [Ag3HL4]2+ gave similar fragmentation channels to CID.