NMR Characterization of Ligand Binding and Exchange Dynamics in Triphenylphosphine-Capped Gold Nanoparticles

Abstract

Triphenylphosphine (PPh 3 )-capped 1.8 nm diameter gold nanoparticles (AuNPs) are characterized by a combination of 1 H, 2 H, and 31 P solution- and solid-state NMR. The 31 P{ 1 H} NMR resonance associated with the surface-bound PPh 3 is clearly identified and is present as a broad peak centered at 56 ppm. 31 P and 1 H hole burning NMR experiments show that the line broadening associated with the surface-bound PPh 3 is primarily due to a variety of different chemical shift environments at the surface of the nanoparticles. The surface bound PPh 3 can be displaced with either d 15 -PPh 3 or Au( d 15 -PPh 3 )Cl in CD 2 Cl 2 solution. In both cases, exchange results in loss of Au(PPh 3 )Cl from the nanoparticle surface, with no evidence for loss of the PPh 3 ligand alone. Solution-state NMR was used to determine the room temperature rate constants for these exchange processes, with values of 0.17 and 0.20 min −1, respectively. Thus, essentially the same rate is observed for displacement of Au(PPh 3 )Cl from the surface with either d 15 -PPh 3 or Au( d 15 -PPh 3 )Cl. The observed 31 P chemical shift of surface-bound PPh 3 is consistent with mixed valence Au(0) and Au(I) at the nanoparticle surfaces, suggesting the presence of surface-bound Au complexes.