Elucidating Cooperative Interactions between Grafted Amines and Tin or Titanium Sites on Silica

The efficient promotion of cooperative catalytic interactions on solid surfaces can be of great benefit for a range of important reactions. Herein, we demonstrate that the cooperative interactions of isolated tin (Sn) and titanium (Ti) sites on silica with grafted primary amines (NH2) can be tuned by changing the immediate chemical environment of the metal sites (M). We show that, by tethering various size organic ligands (RO) to the M sites, we can govern the interactions between the sites as measured by the presence of NH3+. We show that the concentration of NH3+ is directly correlated with the activity of the model Henry reaction. We further find that the selectivity to the olefinic product increased from 59% for the cooperative interactions of grafted NH2 and surface silanols to 84-92% for the cooperative interactions between grafted NH2 and the isolated Sn or Ti sites. An analysis by DFT shows that these cooperative interactions are enabled by the presence of a trace amount (two molecules per M site) of water near the metal sites and a resulting hydrolysis, which depends on the hydrophobicity of the RO group and the nature of the metal. Hence, the current work provides advanced molecular-level insights into the underlying principles of cooperative interactions on a solid surface and guidance for governing such interactions by tuning the chemical environment.