(521d) Designing Single-Atom Sites for Regio- and Stereo-Selective Conversions By Self-Assembled Monolayers of Phosphonic Acids

Atomically disperse catalysts have gained interest due to their structural homogeneity, resulting in higher overall selectivity when compared to nanoparticle counterparts; however, there is still room for improvement for highly regioselective and stereoselective chemical conversions over these sites, such as hydroformylation and directed hydrogenations. Significant research has been conducted to make highly selective and active organometallic catalytic systems, providing a variety of techniques to cater active sites through steric and electronic modification. These techniques lend themselves to be translated to modification of isolated sites through self-assembled monolayers on the support, directing reaction pathways through adsorbate-ligand interactions as well as metal-ligand interactions. Previously, we modified site-isolated Rh₁/TiO₂ with a series of substituted benzyl phosphonic acids. We found that modification of the support by phosphonic acids reduced increased absolute hydroformylation activity by up to 18× compared to the unmodified case at 150°C. The bite angle of adsorbed carbon monoxide, as determined from infrared spectra, correlated with hydroformylation activity – indicating a local confinement effect. These findings demonstrate that Rh₁ active sites are highly sensitive to modification of the support.

Here, we explore the use of phosphonic acid ligands to confine the active site and induce regioselective hydroformylation of styrene and stereoselective hydrogenation of 4-terpeninol. Preliminary results show that introduction of phosphonic acids to the surface shifts selectivity to products that are sterically favorable. In both hydroformylation and hydrogenation, these impacts are likely due to limiting the modes in which the large substrate can approach the narrow active site, causing modified catalysts to prefer “upright”, sterically-favored products.