Boosting free radical type photocatalysis over Pd/Fe-MOFs by coordination structure engineering†
The development of novel heterogeneous photocatalytic systems, along with a deep understanding of the relationship between the catalytic center chemical environment and the catalytic performance, is of great significance. Herein, the surface microenvironment of Pd nanoparticles was modulated with engineered Fe-MOF coordination structures (octahedron MIL-100(Fe), concave octahedron MIL-101(Fe) and irregular lumpy MIL-53(Fe)). Two heterogeneous free radical photocatalytic organic transformations have been developed over Pd nanoparticle loaded Fe-MOFs (Pd/Fe-MOFs). The photocatalytic C–H arylation of thiazole and decarboxylation cross-coupling with cinnamic acid were investigated. Thiazole C–H arylation with halobenzenes was brought about through C–halogen bond activation with the photogenerated electron-rich Pd NPs, the aryl radical generation and the follow-up radical addition. The cinnamic acid decarboxylation cross-coupling was also achieved by means of C–halogen bond activation with photogenerated electron-rich Pd NPs. The base regulated the product stereoselectivity by affecting the balance between cinnamic acid and carboxylate anions, as well as the balance between aryl radicals and the coordination complex intermediates. The improvement of the heterogeneous photocatalytic performance for thiazole C–H arylation and cinnamic acid decarboxylation cross-coupling should be ascribed to the difference in the electron transfer efficiency to Pd NPs over various engineered Fe–O cluster coordination structures. This work highlights the importance of exploiting structure engineering for heterogeneous photocatalytic systems.