On the Interaction of Phosphines with High Surface Area Mesoporous Silica

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• 作者：H茅l猫ne Staub ; Iker Del Rosal ; Laurent Maron ; Freddy Kleitz ; Fr茅d茅ric-Georges Fontaine
• 刊名：The Journal of Physical Chemistry C
• 出版年：2012
• 出版时间：December 13, 2012
• 年：2012
• 卷：116
• 期：49
• 页码：25919-25927
• 全文大小：487K
• 年卷期：v.116,no.49(December 13, 2012)
• ISSN：1932-7455

文摘

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鈥?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 to mesoporous silica, which leads to the grafting and the oxidation of the phosphine species, even in the 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 phosphorus resonance in the ³¹P NMR spectra is altered by hydrogen bonding between available silanol or water molecules present on the silica surface and the phosphorus 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鈥揅l bonds on the surface. Passivation of the surface with hexamethyldisilazane limits the extent of the H-bonding between the surface and the oxide and also leads to some substitution reaction between bound phosphorus 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.