PNNL

Abstract

This study examined the solution-phase exchange reactions of triphenylphosphine (PPh3) ligands on Au8L7 2+ (L = PPh3) clusters with three different tolyl ligands using electrospray ionization mass spectrometry (ESI-MS) to provide insight into how steric changes to the phosphines influence the extent of ligand exchange and the stability of the resulting mixed phosphine clusters. The size distribution of tolyl-exchanged gold clusters was found to depend on the position of the methyl group in the tri(tolyl)phosphine ligands (-orth, -meta, and -para). Due to different steric effects, the tri(m-tolyl)phosphine (TMTP) and tri(p-tolyl)phosphine (TPTP) ligands exchanged efficiently on Physical Chemistry Chemical Physics Page 2 of 32 2 the Au8L7 2+ (L = PPh3) clusters while the tri(o-tolyl)phosphine ligand did not. In addition, while TPTP fully exchanged with all PPh3 on the Au8L7 2+ cluster, TMTP exchanged with only six ligands. Employing collision-induced dissociation (CID) experiments, the tolyl-exchanged clusters were demonstrated to fragment through loss of neutral ligands and AuL2 +. A comparison of the relative fragmentation yield of PPh3 vs. TMTP and TPTP on the mixed ligand clusters indicated that the tolyl ligands are more strongly bonded to the Au8 2+ gold core than PPh3. We also performed complementary electronic structure calculations using density functional theory (DFT) at the B3LYP/SDD and M06-2X/SDD level on representative model cluster systems. These computations revealed that steric interactions of the CH3 group on the tri(o-tolyl)phosphine ligand are responsible for the lack of ligand exchange in solution. Our joint experimental and theoretical findings demonstrate the subtle interplay of steric and electronic considerations in determining the size distribution, stability, and dissociation pathways of tolyl-phosphine ligated gold clusters.