(86f) Tuning Metal Nanoparticle Surface Site Accessibility, Electronic State, and Reaction Microenvironment Using Bound Organic Ligands

Synthesis of metallic nanoparticles in solution approaching 1 nm in diameter which contain accessible surface sites with covalently bound organic ligands remains a challenge. We demonstrate the synthesis of a series of gold nanoparticles, ≤ 2 nm in diameter, using bulky bisphosphine ligands, which impart stability to the nanoparticles. Au nanoparticles stabilized by bis(diphenylphosphino)methane (dppm) exhibited the highest accessibility, where 61% of the total gold atoms bind 2-napthalenethiol and this represents the highest fraction of accessible atoms reported for an organic ligand-stabilized metallic nanoparticle in solution. The Au nanoparticles synthesized with bisphosphine ligands are all active for resazurin reduction reaction, and the reaction rate scales with the number of binding sites on the nanoparticle surface.

There is additional potential in modulating the electronic characteristics of the metallic nanoparticles through bound organic ligands with different functional groups, such as electron-donating phosphines and electron-withdrawing thiols. We synthesize and compare ligand-bound Au nanoparticles of the same size with structurally similar ligands but with thiols (triphenylmethyl mercaptan, TPMT) and phosphines (triphenylphosphine, TPP) to assess the electronic effects of the ligand. The similarly sized nanoparticles showed a similar number of accessible sites and exhibit comparable catalytic activities for benzyl alcohol oxidation and CO oxidation. However, when the TPP- and TPMT-stabilized Au nanoparticles are used in resazurin reduction, the TPP-stabilized nanoparticles are active, while the TPMT-stabilized nanoparticles are inactive. It is hypothesized that this on/off behavior in activity, which is not due to differences in nanoparticle size or number of active sites, originates in electronic differences of the Au nanoparticle surfaces as supported by evidence from X-ray photoelectron spectroscopy and FTIR of bound CO molecules.

To further investigate the role of these ligands in modulating the electronic properties and reaction microenvironment of metal nanoparticles, we synthesize ruthenium nanoparticles using phosphine (both TPP and dppm) and thiol (TPMT) ligands. The average diameter of the Ru nanoparticles is approximately 0.8 nm and these are believed to be the smallest ligand-stabilized Ru nanoparticles synthesized in solution in the literature. The similarly sized Ru nanoparticles showed a similar catalytic activity for CO oxidation. However, in H₂O₂ decomposition, the identity of the ligand (monodentate vs.bidentate) does play an important role in this reaction. It is observed that the hydrophobicity/hydrophilicity of the ligand has a significant impact on the H₂O₂ decomposition rate, with a more hydrophobic ligand layer leading to reduced H₂O₂ decomposition rates. Thus, this contribution demonstrates the potential of utilizing bound organic ligands to control H₂O₂ decomposition rates and other reactions which are sensitive to hydrophobic/hydrophilic environments.