One-Pot Synthesis of Au₁₁(PPh₂Py)₇Br₃ for the Highly Chemoselective Hydrogenation of Nitrobenzaldehyde

Abstract

In this study, the gold clusters Au11(PPh3)7Cl3 and Au11(PPh2Py)7Br3 (PPh2Py = diphenyl-2-pyridylphosphine) are synthesized via a one-pot procedure based on the wet chemical reduction method. The Au11(PPh3)7Cl3 cluster is found to be active in the chemoselective hydrogenation of 4-nitrobenzaldehyde in the presence of hydrogen (H2) and a base (e.g., pyridine). Interestingly, the cluster with the functional ligand PPh2Py shows similar activity without losing catalytic efficiency in the absence of the base. The structure of the gold clusters and reaction pathway of the catalytic hydrogenation are investigated at the atomic/molecular level via UV–vis spectroscopy, electrospray ionization (ESI) mass spectrometry, and density functional theory (DFT) calculations. It is found that one ligand (PPh3 or PPh2Py) removal is the first step to expose the core of the gold clusters to reactants, providing an active site for the catalytic reaction. Then, the H–H bond of the H2 molecule becomes activated with the aid of either free amine (base) or ligand PPh2Py which is attached to the gold clusters. This work demonstrates the promise of the functional ligand PPh2Py in the catalytic hydrogenation to reduce the amount of materials (free base: e.g., pyridine) that ultimately enter the waste stream, thereby providing a more environmentally friendly reaction medium.