NMR characterization of ligand binding and exchange dynamics in triphenylphosphine-capped gold nanoparticles

Triphenylphosphine (PPh₃)-capped 1.8 nm diameter gold nanoparticles (AuNPs) are characterized by a combination of ¹H, ²H, and ³¹P solution-and solid-state NMR. The ³¹P{¹H} NMR resonance associated with the surface-bound PPh₃ is clearly identified and is present as a broad peak centered at 56 ppm. ³¹P and ¹H hole burning NMR experiments show that the line broadening associated with the surface-bound PPh₃ is primarily due to a variety of different chemical shift environments at the surface of the nanoparticles. The surface bound PPh₃ can be displaced with either d₁₅-PPh₃ or Au(d₁₅-PPh ₃)Cl in CD₂Cl₂ solution. In both cases, exchange results in loss of Au(PPh₃)Cl from the nanoparticle surface, with no evidence for loss of the PPh₃ 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(PPh₃)Cl from the surface with either d₁₅-PPh₃ or Au(d ₁₅-PPh₃)Cl. The observed ³¹P chemical shift of surface-bound PPh₃ is consistent with mixed valence Au(0) and Au(I) at the nanoparticle surfaces, suggesting the presence of surface-bound Au complexes.