Highly Active Urea-Functionalized Zr(IV)-UiO-67 Metal-Organic Framework as Hydrogen Bonding Heterogeneous Catalyst for Friedel-Crafts Alkylation

A new Zr(IV)-based UiO-67 metal-organic framework (1) was prepared with urea-functionalized biphenyl-4,4'-dicarboxylic acid (BPDC-urea) as the linker using conventional solvothermal technique and thoroughly characterized using X-ray powder diffraction (XRPD), Fourier transform infrared (FT-IR) spectroscopy, thermogravimetric (TG), and N-2 sorption analyses. The activated form of 1 (called 1') exhibited excellent BET surface area in spite of having a large functional moiety (urea) in the linker side. The activated form of this material (1') was successfully employed for the Friedel-Crafts alkylation of indole with beta-nitrostyrene to achieve 97% yield in toluene at 70 degrees C for 24 h. Furthermore, the catalyst was used for four cycles, with no significant loss in its activity, and the reaction was heterogeneous in nature. The activity of 1' was comparable to UiO-67-(NH2)(2), whereas the activity was 2-fold higher compared to the parent UiO-67. Further, the activity of the BPDC-urea linker was nearly 2-fold higher than that of ZrCl4, suggesting the crucial role played by the urea moiety than the metal node. In addition, the catalyst (1') exhibited a wide substrate scope, allowing the preparation of a series of compounds with moderate to high yields under the optimized reaction conditions. The roles of metal salt and linker in the catalysis have also been studied separately, and the mechanism for the catalysis has been clarified.