Role of sterics in phosphine-ligated gold clusters

This study examined the solution-phase exchange reactions of triphenylphosphine (PPh3) ligands on Au8L72+ (L = PPh3) gold clusters with three different tolyl ligands using electrospray ionization mass spectrometry to provide insight into how steric differences in the phosphines influence the extent of ligand exchange and the stability of the resulting mixed-phosphine clusters. The size distributions of tolyl-exchanged gold clusters were found to depend on the position of the methyl group in the tri(tolyl)phosphine ligands (-ortho, -meta, and -para). Due to different sterics, the tri(m-tolyl)phosphine (TMTP) and tri(p-tolyl)phosphine (TPTP) ligands exchanged efficiently onto the Au8L72+ (L = PPh3) clusters while the tri(o-tolyl)phosphine ligands did not exchange. In addition, while TPTP fully exchanged with all seven PPh3 on the Au8L72+ cluster, TMTP exchanged with only six PPh3 ligands. Employing collision-induced dissociation, the tolyl-exchanged mixed-ligand clusters were demonstrated to fragment through loss of neutral ligands and AuL2+. Comparison of the relative fragmentation yields of PPh3vs. TMTP and TPTP from the mixed-ligand clusters indicated that these tolyl ligands are more strongly bonded to the Au82+ gold core than PPh3. To provide molecular-level insight into the experimental results we also performed complementary electronic structure calculations using density functional theory at the B3LYP-D3/SDD level of theory on representative model 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 with PPh3. Our joint experimental and theoretical findings demonstrate the subtle interplay of steric and electronic factors that determine the size distribution, stability, and dissociation pathways of phosphine ligated gold clusters.