Abstract
Heterogeneous catalysis in water has not been explored beyond certain advantages like recyclability and recovery of the catalysts from the reaction medium. In doing so, they often fail to address other authentic pitfalls of homogeneous catalysis. Moreover, poor yield, extremely low selectivity, and active catalytic sites' deactivation further underrate the heterogeneous catalysis in water. On the other hand, most of the synthetically useful homogeneous catalysts are either water intolerant or remain catalytically inactive in water. Considering these facts, we have rationally designed and synthesized solution dispersible porous covalent organic framework (COF) nano-spheres to utilize their distinctive morphology and dispersibility to bridge between homogeneous and heterogeneous catalysis. The success has further been extended in fabricating catalyst immobilized COF thin-films via covalent self-assembly for the very first time. We have used these catalyst immobilized COF thin-films to develop a general methodology for the C-H functionalization of organic substrates in water. This unique covalent self-assembly occurs through the protrusion of the fibers/threads at the interface of two nano-spheres, transmuting the catalytic spheres into films without any leaching of catalyst molecules, which was hitherto unheard of. The catalyst immobilized porous COF thin-films' chemical functionality and hydrophobic environment stabilizes the high valent transient active oxoiron(V) intermediate in water and restricts the active catalytic site's deactivation. An elevated catalytic yield and high selectivity (3°:2°) have been achieved in open-air conditions at room temperature, accompanying the elemental feature of heterogeneous catalysis, i.e., the recyclability. These COF films functionalized the unactivated C-H bonds in water with a high catalytic yield (45-99%) and with a high degree of selectivity (cis:trans=155:1; 3°:2°=257:1 in case of cis-1,2-dimethylcyclohexane). To establish the "practical implementation" of this approach, we conducted the inflow catalysis (Turnover Number = 355±5) using catalyst immobilized COF films fabricated on a macroporous polymeric support.
Cover Picture: Molecular Heterogeneous Catalysis: A Single-Site Zeolite-Supported Rhodium Complex for Acetylene Cyclotrimerization (Chem. Eur. J. 26/2007)
