Catalytic comparison of cyanosilylation/Knoevenagel condensation reactions by Pb-based heterometallic organic frameworks with different second metal ions

Abstract

In this work, four new examples of isostructural three-dimensional (3D) heterometallic organic frameworks (HMOFs) [PbM_0.5(L)(H₂O)]_n (M = Mn (1), Co (2), Zn (3), Cd (4)) have been successfully synthesized based on the 2-tetrazolium-terephthalic acid (H₃L) ligand. Due to the abundance of Lewis acid–base sites in the structure, they exhibit high catalytic activity for the cyanosilylation and Knoevenagel condensation reactions. The second ions (M) act as the main microenvironment for the Pb active sites to effectively regulate the catalytic activity of [PbM_0.5(L)(H₂O)]_n. The experimental results showed that the effect of second metal ions Mn, Co, Zn, and Cd on the catalytic activity of [PbM_0.5(L)(H₂O)]_n decreases sequentially. Among them, 1 was generally applicable to different substrates, the structure remained complete after five cycles and the conversion rate still reached about 97%. Combined with the catalytic mechanism, X-ray photoelectron spectroscopy (XPS) and kinetic analysis showed that the difference of electron cloud density around Pb(II) and uncoordinated nitrogen of the structure is the main reason for the different catalytic activity. Among the four example catalysts, the structure of 1 has the lowest binding energy for Pb 4f and the highest binding for N 1s. The difference in binding energy brings about a variation in the electron cloud density, thereby affecting the activity of the catalytic reaction. This study is expected to bring implications for the synthesis of HMOFs and the effect of second metal ions on catalytic performance.