Design Of Highly Stable COFs With Embedded Ni Active Center And Their Photocatalytic Performance For C-O/N Coupling

Compared with heterogeneous photocatalysts such as Ti O₂ and Cd S,covalent organic frameworks have shown great potential in the field of photocatalytic organic conversion due to their diverse structures,large specific surface area and strong light absorption capability,especially their long-range ordered structure are beneficial to the electron transfer.But the preparation of COFs with high stability and good catalytic activity was still difficult in research work.COFs constructed based on-C=C-bonds have shown excellent stability under acidic and basic conditions,and are able to overcome the weakening of in-planeπ-electron conjugation caused by the polarization of imine bonds,thus improving the mobility of photogenerated electrons and enhancing the photocatalytic efficiency.Therefore,in this paper,three COFs connected by-C=C-were prepared by Knoevenagel condensation reaction.The bipyridine unit was embedded in the skeleton through molecular design and could be combined with metal Ni to act as active sites of photocatalytic C-O/N coupling reaction.By introducing push-pull electron groups such as（-F/-OCH₃）into the framework,remote control of the active site was achieved by simple push-pull electron effect.The polarization of the interlayer electron cloud caused by the introduction of-F further enhanced the electron transfer between the layers,promoting the continuous transfer of photogenerated electrons to the active site,thus improving the photocatalytic C-O/N coupling reaction activity.The following are the specific research contents:1.Three-C=C-linked COFs（TFPPY-BPY-CN/TFPPY-F-BPY-CN/TFPPY-OCH₃-BPY-CN）were successfully synthesized by Knoevenagel condensation using solvothermal method.The Ni ions were chelated into the bipyridine by refluxing in methanol,which could serve as the active site for photocatalytic C-O coupling.The introduction of-F significantly enhanced the reactivity.TFPPY-F-BPY-CN-Ni could completely convert 4-bromobenzonitrile to 4-methoxybenzonitrile within 13 h under the irradiation of 6 W blue LED light.The practicability of the reaction was proved by the experiment of substrate expansion and gram-scale reaction.The possible mechanism of the enhanced photocatalytic C-O coupling activity was speculated:the introduction of-F polarized the interlayer electron cloud and enhanced the interlayer non-covalent interaction,promoting the migration of photogenerated electrons between and within the layer to the catalytic active center,thus improving the photocatalytic C-O coupling performance.2.Three imine bonds linked COFs（TFPPY-BPY-NH₂/TFPPY-F-BPY-NH₂/TFPPY-OCH₃-BPY-NH₂）were synthesized via Schiff base condensation.Similarly,the introduction of-F significantly enhanced the activity of photocatalytic C-N coupling.Under the irradiation of 18 W blue LED light,TFPPY-F-BPY-CN-Ni could completely convert 4-bromobenzonitrile to 4-（pyrrolidine-1-yl）benzonitrile within 6 h,and behaved excellent stability before and after the reaction.Compared with COFs based on imine bonds,-C=C-based COFs exhibited good thermochemical stability,and-C=C-could overcome the disadvantage of weak in-planeπ-electron conjugation caused by-C=N-polarization,which promoted the transfer of electrons between layers and the generation of active intermediates.Combined with the experimental results,a possible photocatalytic C-N coupling mechanism was proposed different from photocatalytic C-O coupling.