Abstract
Heterogeneous catalysis in water has not been explored beyond certain advantages such as recyclability and recovery of the catalysts from the reaction medium. Moreover, poor yield, extremely low selectivity, and active catalytic site deactivation further underrate the heterogeneous catalysis in water. Considering these facts, we have designed and synthesized solution-dispersible porous covalent organic framework (COF) nanospheres. We have used their distinctive morphology and dispersibility to functionalize unactivated C-H bonds of alkanes heterogeneously with high catalytic yield (42-99%) and enhanced regio- and stereoselectivity (3°:2° = 105:1 for adamantane). Further, the fabrication of catalyst-immobilized COF nanofilms via covalent self-assembly of catalytic COF nanospheres for the first time has become the key toward converting the catalytically inactive homogeneous catalysts into active and effective heterogeneous catalysts operating in water. This unique covalent self-assembly occurs through the protrusion of the fibers at the interface of two nanospheres, transmuting the catalytic spheres into films without any leaching of catalyst molecules. The catalyst-immobilized porous COF nanofilms' chemical functionality and hydrophobic environment stabilize the high-valent transient active oxoiron(V) intermediate in water and restricts the active catalytic site's deactivation. These COF nanofilms functionalize 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, for cis-1,2-dimethylcyclohexane). To establish this approach's "practical implementation", we conducted the catalysis inflow (TON = 424 ± 5) using catalyst-immobilized COF nanofilms fabricated on a macroporous polymeric support.
Highlights
Direct and selective functionalization of inert C-H bonds as a latent functional group in water is a demanding endeavour,[1,2,3,4,5] enzymes do the same transformation under ambient conditions.[6,7,8] most enzymes are expensive and lose their catalytic activity with a slight alteration of reaction conditions.[9]
The TpDPP covalent organic framework (COF) nano-spheres were synthesized by the reaction between the 0.03 mmol triformylphloroglucinol (Tp), and 0.045 mmol 3,8-diamino-6-phenylphenathridine (DPP) in the presence of a catalytic amount (10-15mL) of trifluoroacetic acid
The dynamic light scattering (DLS) analyses revealed the as-synthesized COF nano-spheres¢ average size to be in the range of 590-610 nm (Figure 2e-f)
Summary
Direct and selective functionalization of inert C-H bonds as a latent functional group in water is a demanding endeavour,[1,2,3,4,5] enzymes do the same transformation under ambient conditions.[6,7,8] most enzymes are expensive and lose their catalytic activity with a slight alteration of reaction conditions.[9]. This shift in –C=O and C–N stretch bands indicates the presence of N─H···O hydrogen bonding between the free amine of the COF nano-sphere and the amide carbonyl of the immobilized catalyst.[36,37] The XPS analysis of the (Et4N)2[FeIII(Cl)bTAML] catalyst revealed the deconvoluted C 1s spectra, which indicates the presence of two distinct peaks at 286.1 and 287.1 eV corresponding to the amide carbon and the diamide carbon, respectively (Figure S32-33).
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