Bis(diphenylphosphino)methane ligated gold cluster cations: Synthesis and gas-phase unimolecular reactivity

Abstract

Electrospray ionization mass spectrometry (ESI-MS) and UV–vis spectroscopy were used to monitor the condensed phase synthesis of gold cluster cations via NaBH4 reduction of Ph3PAuCl in the presence of Ph2P-(CH2)-PPh2 (dppm, L1) a capping ligand. ESI-MS highlights that upon mixing of Ph3PAuCl and dppm a range of ligated Au(I) cations are present, including [Au2L12]2+, [Au(PPh3)2]+, [Au2L13]2+, [AuL12]+ and [Au2ClL12]+. These cationic precursors to reduction can be controlled by the ratios of the two reactants. For a ratio of 1:2 of Ph3PAuCl and dppm, [AuL12]+ is a major ion, while [Au(PPh3)2]+, which has previously been postulated to be a key precursor to gold colloid formation (J.M. Pettibone, J.W. Hudgens, Small 8 (2012) 715) is essentially absent. Upon NaBH4 reduction of a solution of this stoichiometry (Ph3PAuCl:dppm, 1:2), the following types of gold nanocluster ions are observed via ESI-MS: [Au5L13(CH(PPh2)2)]2+, [Au9L14]3+, [Au9L15]3+, [Au10L14]2+, [Au11L15]3+, [Au11L16]3+, [Au13L16]3+ and [Au14L16(CH(PPh2)2)]3+. The gas phase unimolecular chemistry of these clusters and their fragment ions were examined in a LTQ FT Hybrid Linear Ion Trap (LIT) Mass Spectrometer. Low-energy collision induced dissociation (CID) produced a range of novel gas-phase clusters of the type, [AuxL1y]z+ (x=2, 3, 6–13; y=1–6 and z=1–3) and [AuxL1y(CH(PPh2)2)]z+ (x=5, 14; y=2, 5; z=2, 3), via ligand loss and core fission fragmentation pathways. In addition fragmentation channels due to CP bond activation were also observed. Electron capture dissociation (ECD) on the Au cluster cations resulted in either one or a combination of three fragmentation channels: (i) charge reduction, (ii) charge reduction accompanied by ligand loss and/or (iii) charge reduction accompanied by CP bond activation.