On the interaction of phosphines with high surface area mesoporous silica

To increase the efficiency and selectivity of homogeneous catalysts, particularly useful in the synthesis of fine chemicals and drugs, fine tuning of the steric and electronic properties of the complexes can be achieved by modification of the ligands in the coordination sphere of the metal center. Considerable efforts have been devoted in order to immobilize such well-defined catalysts on solid substrates, e.g., silica, to facilitate catalysts’ recovery and to reduce contamination of desired products by metallic impurities. However, the presence of the silica surface can play a very important role in tuning the electronic properties of the metal, its steric environment, or in participating in the reactivity of the complex. In this context, several moieties have been used to anchor metallic catalysts on surfaces, but one of the most interesting is phosphine. Herein, we report on the addition of PPh2Cl which leads to the grafting and the oxidation of the phosphine species, even in absence of oxygen, and that the nature of the surface plays an important role in secondary interactions, e.g., hydrogen bonding, and modifies the spectroscopic properties of the functional groups on the surface. In particular, the chemical shift of the phosphorous resonance in the P NMR spectra is altered by hydrogen bonding between available silanol or water molecules present on the silica surface and the phosphorous oxide. The DFT models developed for this process are in direct accordance with the experimental results and demonstrate firmly that the oxidation of the phosphine after grafting of ClPR2 is highly favored thermodynamically and occurs with the formation of Si-Cl bonds on the surface. Passivation of the surface with hexamethyldisilazane limits the extent of the H-bonding between the surface and the oxide, but also leads to some substitution reaction between bound phosphorous species and the trimethylsilyl (TMS) moieties. These findings offer new knowledge critical to fully ascertain the environment and the stability of immobilized phosphine-containing catalytic systems and, thus, further broaden the range of their reactivity