Ru clusters confined in Hydrogen-bonded organic frameworks for homogeneous catalytic hydrogenation of N-heterocyclic compounds with heterogeneous recyclability

• Hydrogen-bonded organic frameworks (HOFs) with cage molecules were prepared. • Ru clusters with diameter of about 0.47 nm were confined in HOFs to form Ru@HOF. • Ru@HOF catalyst can well disperse in water that similar as a homogeneous catalyst. • Ru@HOF can efficiently hydrogenation of N-heteroarenes in aqueous solution. • Ru@HOF can be recycled easily by addition of acetone in aqueous reaction mixture. Combining the advantages of homogeneous catalysis and the recyclability of heterogeneous catalysts is highly desired for sustainable catalysis but remains a challenging goal. In the current work, hydrogen-bonded organic frameworks (HOFs) that consist of cage molecules with amino groups were prepared. The HOFs with heteroatom-containing porous cavities and stable backbone confine ultrafine Ru clusters with diameter of about 0.47 nm and high Ru loading of 30 wt%. The obtained Ru@HOF catalyst can well disperse in water in the form of discrete cage-confined Ru clusters similar to a homogeneous catalyst, and can be recycled easily through simple precipitation by the addition of acetone due to the formation of hydrogen bonds between discrete cages. The Ru@HOF was used as a homogenous catalyst for efficiently selective catalytic hydrogenation of N-heteroarene compounds, such as quinolines and indoles in aqueous solution. Especially quinoline compounds can be fully hydrogenated to decahydroquinoline with Ru@HOF catalyst, which is not easy to be implemented with other Ru nanoparticle-based catalysts. The high catalytic performance of Ru@HOF is attributed to the homogeneously dispersed and accessible active sites of the ultrafine Ru clusters. This study provides a novel strategy for fabricating ultrafine noble metal clusters-based catalysts with homogeneous catalytic performance and heterogeneous recyclability.