A highly robust cluster-based indium(III)-organic framework with efficient catalytic activity in cycloaddition of CO 2 and Knoevenagel condensation.

The efficient catalytic performance displayed by MOFs is decided by an appropriate charge/radius ratio of defect metal sites, large enough solvent-accessible channels and Lewis base sites capable of polarizing substrate molecules. Herein, the solvothermal self-assembly led to a highly robust nanochannel-based framework of {[In 4 (CPDD) 2 (μ 3 -OH) 2 (DMF)(H 2 O) 2 ]·2DMF·5H 2 O} n (NUC-66) with a 56.8% void volume, which is a combination of a tetranuclear cluster [In 4 (μ 3 -OH) 2 (COO) 10 (DMF)(H 2 O) 2 ] (abbreviated as {In 4 }) and a conjugated tetracyclic pentacarboxylic acid ligand of 4,4'-(4-(4-carboxyphenyl)pyridine-2,6-diyl)diisophthalic acid (H 5 CPDD). To the best of our knowledge, NUC-66 is a rarely reported {In 4 }-based 3D framework with embedded hierarchical triangular-microporous (2.9 Å) and hexagonal-nanoporous (12.0 Å) channels, which are shaped by six rows of {In 4 } clusters. After solvent exchange and vacuum drying, the surface of nanochannels in desolvated NUC-66a is modified by unsaturated In 3+ ions, N pyridine atoms and μ 3 -OH groups, all of which display polarization ability towards polar molecules due to their Lewis acidity or basicity. The catalytic experiments performed showed that NUC-66a had high catalytic activity in the cycloaddition reactions of epoxides with CO 2 under mild conditions, which should be ascribed to its structural advantages including nanoscale channels, rich bifunctional active sites, large surface areas and chemical stability. Moreover, NUC-66a, as a heterogeneous catalyst, could greatly accelerate the Knoevenagel condensation reactions of aldehydes and malononitrile. Hence, this work confirms that the development of rigid nanoporous cluster-based MOFs built on metal ions with a high charge and large radius ratio will be more likely to realize practical applications, such as catalysis, adsorption and separation of gas, etc .