Abstract
Tritylium salts have been used as Lewis acid catalysts in organic synthesis for a long time. In this work, we found that the Lewis acid catalytic activity of tritylium ions at the node of a tensile framework is significantly improved compared to that of the free tritylium salts. The tritylium-based framework, PAF-201 (PAF, porous aromatic framework), was prepared by acidification of a semi-rigid triphenylcarbinol-based parent framework, PAF-200. When PAF-200 was alternately exposed to HCl and NH3 gas, a fast allochroic cycle was observed due to repeated formation of tritylium species. Interestingly, the pseudo-first-order reaction rate of a Povarov model reaction catalyzed by PAF-201 as a Lewis acid was ∼3.7 times and ∼4.7 times as those of tritylium tetrafluoroborate and tri(4-biphenyl)carbonium tetrafluoroborate, respectively. Theoretical calculations revealed that the tritylium ion at the node of PAF-201 has a quasi-planar structure. The transformation of triphenylcarbinol in PAF-200 to tritylium in PAF-201 can make the framework taut, and the rebounding force toward the tetrahedral structure is stored. This is favorable for tritylium to activate the imine substrate along with a deformation of the quasi-plane to tetrahedron. PAF-201 could be easily recycled at least three times without evident loss of catalytic activity. This work presents the catalytic activity of the tritylium ion under stress.
Highlights
Porous organic polymers (POPs) have been developed rapidly owing to their signi cant advantages, such as large speci c surface area and high stability.[1,2,3] In particular, introducing an ionic structure into the polymer backbone endows POPs with new functionalities that may expand their applications.[4,5,6,7] These backbones with negative or positive charges have been applied to different aspects, such as transport, absorption, separation, or catalysis.[4,5,6,7] Notably, in terms of catalysis, most ionic frameworks have been used to carry active ionic counterparts or metal nanoparticles, and have seldom been used as catalytic sites themselves
Some of the triphenylcarbinol units of porous aromatic frameworks (PAFs)-200 were transformed into tritylium ions, affording the taut PAF-201 framework
A fast allochroic cycle was observed when PAF-200 was alternately exposed to HCl and NH3 gas
Summary
Porous organic polymers (POPs) have been developed rapidly owing to their signi cant advantages, such as large speci c surface area and high stability.[1,2,3] In particular, introducing an ionic structure into the polymer backbone endows POPs with new functionalities that may expand their applications.[4,5,6,7] These backbones with negative or positive charges have been applied to different aspects, such as transport, absorption, separation, or catalysis.[4,5,6,7] Notably, in terms of catalysis, most ionic frameworks have been used to carry active ionic counterparts or metal nanoparticles, and have seldom been used as catalytic sites themselves. Some triphenylcarbinol-based porous materials have been reported,[15–16] but the corresponding tritylium-based frameworks as Lewis acids have not been introduced so far. It was found that the tritylium ions at the node of the tensile framework exhibited better Lewis acid catalytic ability than the free ones.
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