Benzothiadiazole Conjugated Metal-organic Framework for Organic Aerobic Oxidation Reactions under Visible Light

In the past several years, visible light induced organic transformations via photoredox catalysis have attracted increasing attention from chemists, owing to their mild, environmentally benign and low cost characteristics. Photoredox catalysts including noble metal complexes as well as some organic dyes are often used to promote the transformations under visible light irradiation. However, most of the reactions were conducted in homogeneous system, which makes it difficult to recycle the catalysts for reuse. From a sustainable viewpoint, an ideal photocatalyst should be easily recoverable, reusable and free of precious metals. To this end, photoactive metal-organic frameworks (MOFs) demonstrate unique advantageous features working as novel heterogeneous photocatalytic systems, yet their utilization toward organic transformations promoted by visible light has been limited. Herein we designed and synthesized a benzothiadiazole functionalized TPDC ligand H(2)1 (TPDC = terphenyl-4,4"-dicarboxylic acid). Briefly, a Suzuki reaction of 4,7-dibromo-2,1,3-benzothiadiazole with 4-(methoxycarbonyl)phenylboronic acid yielded methyl ester precursor, which was hydrolysed by KOH to get the ligand H(2)1 in a high yield. Dimethyl-substituted TPDC H(2)2, on account of its better solubility, was synthesized to replace the original TPDC for preparation of MOF UiO-68 framework. Due to the same length of the two ligands, the mix-and-match synthetic strategy was utilized to construct the benzothiadiazole functionalized UiO-68 topological framework (i.e. MOF UiO-68-S). UiO-68-S was synthesized by heating the mixture of ZrCl4 and a combination of ligands H(2)1 and H(2)2 (1 : 1 molar ratio) in N,N'-dimethylformamide (DMF) using HAc as an additive at 100 degrees C for 2 days. Powder X-ray diffraction (XRD) was employed to confirm its crystalline nature and isostructural with the parent UiO-68 framework. Nitrogen sorption experiment at 77 K revealed a typical type I reversible isotherm with Brunauer-Emmett-Teller (BET) surface area up to 1135 m(2.)g-(1), indicating its high porosity. Moreover, the MOF can serve as a highly active photocatalyst for visible light promoted aerobic oxidation reactions, including the selective oxygenation of sulfides and oxidative hydroxylation of arylboronic acids. In addition, UiO-68-S can be recycled at least 5 times without significant loss of catalytic activity and its framework is maintained following the catalytic reaction.