Effects of different ligand modifications on catalytic transfer hydrogenation of Lewis acid catalysts: Quantum chemical studies on the case of ZrCl2-Sal(ph)en

• Published in: Journal of the Taiwan Institute of Chemical Engineers Vol. 134; p. 104299
• Main Authors: Peng, Qingrui, Li, Lei, Hui, Wei, Hu, Aiyun, Wang, Xiang, Wang, Haijun, Xia, Yongmei
• Format: Journal Article
• Language: English
• Published: Elsevier B.V 01.05.2022
• Subjects: Electronic effects; Lewis acid catalysis; Ligand substitution; Schiff-base (Salen/Sal(ph)en) Zr complexes; γ-Valerolactone; Ligand substitution; Electronic effects; Lewis acid catalysis; γ-Valerolactone; Schiff-base (Salen/Sal(ph)en) Zr complexes
• ISSN: 1876-1070; 1876-1089
• DOI: 10.1016/j.jtice.2022.104299

•Hydrogenation of EL by ZrCl2-Sal(ph)en-Model was established.•Electronic effects of ligand substitution on Lewis acid catalysis was studied.•A linear LFER was found between Hammett σp of ligand and the rate.•Spatial effects of auxiliary ligands at two axial sites was discussed. The systematic description of the steric and electronic ligand effects on the reactivity of the catalysts is one of the main goals in homogeneous catalysis. The experiments and molecular modeling calculations were performed to establish the structure–reactivity relationship with various ligand substituents of ZrCl2-Sal(ph)en-X (X = H, CH3, OCH3, Br, NO2, Cl). A clear linear free-energy relationship (LFER, r2=0.97/0.93) was found between Hammett σp value of the phenoxyl side group substituent X and the rate KX or reaction barrier of the hydrogenation of ethyl levulinate. Lewis acid sites are stronger in catalysts with an electron-withdrawing group close to the Zr site. In addition, the auxiliary ligands at two axial sites connected to the Zr site have a more significant impact on the catalyst activity. Among the three axial ligands (Cl, OH and OiPr), the -OH ligand with a smaller size endows additional basicity of catalyst, which is beneficial to the activation and dissociation of the hydroxyl group in the 2-propanol at the Lewis acidic sites (Zr4+) and basic sites(O2−), thus increasing the reaction rate of the Meerwein-Ponndorf-Verley (MPV)reaction of ethyl levulinate. Graphical abstract [Display omitted] .