Role of sterics in phosphine-ligated gold clusters

Abstract

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.