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Scientists Develop Efficient Metal-organic Framework Supported Non-precious-metal Catalyst for Intramolecular Hydroamination of o-Alkynylanilines

Indole moieties play a prominent role in biological and pharmaceutical areas, and the cyclization of o-alkynylanilines provides one useful synthetic method for indole moieties. However, the synthetic processes involve noble metals catalysts, such as Au, Ag or Pt. Therefore it is essential to explore new noble-metal-free, reusable and efficient catalyst. 

In a study published in Angew. Chem. Int. Ed., a research group led by Prof. YUAN Daqiang from Fujian Institute of Research on the Structure of Matter (FJIRSM) of Chinese Academy of Sciences reported a reusable and robust single-site Zn(II) catalyst supported on metal-organic framework for efficiently catalyzing the cyclization of o-alkynylanilines. 

Metal-Organic Frameworks (MOFs) is a class of crystals with porosity and stability. And MOFs can produce uniform catalytic microenvironments which are not available to molecular catalysts. MOFs have been believed to be a promising platform as single-site heterogeneous catalysts. 

Prof. YUAN’s group revealed that single-site Zn(II) based on MOFs is an efficient and reusable catalyst for intramolecular hydroamination of o-alkynylanilines. The catalyst had been synthesized through postsynthetic functionalization.

Through transmission electron microscopy (TEM) and aberration-corrected high-angle annular dark-field scanning TEM (HAADF-STEM) images, researchers found that the Zn element is isolated inside the MOFs. Throgh the X-ray photoelectron spectroscopy (XPS) and X-ray absorption near-edge structure (XANES) analyses, they verified the Zn-UiO-67-BPY (H2BPY was short for 2, 2’-bipyridine-5,5’-dicarboxylic acid) only held divalent Zn(II) ions.

The best fit of the extended X-ray adsorption fine structure (EXAFS) data revealed that the coordination environment of Zn(II) might be two Zn-N (two N from one BPY) and three Zn-O. 

Besides, researchers found that Zn-UiO-67-BPY could efficiently catalyze the intramolecular hydroamination of 2-(phenylethynyl)aniline through adjusting the solvent, temperature and amount of catalyst. The control reactions with UiO-67-BPY alone and a hot filtration experiment revealed that the cycloisomerization reaction was catalyzed by isolated Zn(II) inside the MOFs. However, they also found 2-(phenylethynyl)aniline mainly proceed the hydration of the alkyne in a homogeneous catalytic system. 

Moreover, they selected several representative substrates to explore the scope of the cycloisomerization reaction catalyzed by Zn-UiO-67-BPY. Most of them obtained good yields (over 92 %). 

The Zn-UiO-67-BPY was recycled and reused after catalysis. Scanning electron microscopy (SEM), TEM, the powder X-ray diffraction (PXRD), inductively coupled plasma (ICP) and XPS proved the catalyst held the stability and structural integrity. 

The system of Zn(II)-catalyzed hydroamination reaction does not have additional ligands, base or acid, which is very clean and mild to natural environment.  

 

The brief process of catalyzing hydroamination by open single Zn(II) site supported on MOFs. (Image by Prof. YUAN’s Group)

  

Contact: 

Prof. YUAN Daqiang 

Fujian Institute of Research on the Structure of Matter 

Chinese Academy of Sciences 

Email: ydq@fjirsm.ac.cn 

 

 


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