Functionalization of Magnetic UiO-66-NH₂ with a Chiral Cu(l-proline)₂ Complex as a Hybrid Asymmetric Catalyst for CO₂...

In this study, a chiral [Cu(l-proline)₂] complex-modified Fe₃O₄@SiO₂@UiO-66-NH₂(Zr) metal-organic framework [Fe₃O₄@SiO₂@UiO-66-NH-Cu(l-proline)₂] via multifunctionalization strategies was designed and synthesized. One simple approach to chiralize an achiral MOF-structure that cannot be directly chiralized using a chiral secondary agent like 4-hydroxy-l-proline. Therefore, this chiral catalyst was synthesized with a simple and multistep method. Accordingly, Fe₃O₄@SiO₂@UiO-66-NH₂ has been synthesized via Fe₃O₄ modification with tetraethyl orthosilicate and subsequently with ZrCl₄ and 2-aminoterephthalic acid. The presence of the silica layer helps to stabilize the Fe₃O₄ core, while the bonding between Zr⁴⁺ and the -OH groups in the silica layer promotes the development of Zr-MOFs on the Fe₃O₄ surface, and then the surfaces of the synthesized magnetic MOFs composite are functionalized with 1,2-dichloroethane and Cu(II) complex with 4-hydroxy-l-proline, [Cu(l-proline)₂] to afford the magnetically chiral nanocatalyst. Multiple techniques were employed to characterize this magnetically chiral nanocatalyst such as Fourier transform infrared (FT-IR), X-ray photoelectron spectroscopy (XPS), field-emission scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM), energy-dispersive X-ray spectrometry (EDX), powder X-ray diffraction (PXRD), circular dichroism (CD), inductively coupled plasma (ICP), thermogravimetric analysis (TGA), vibrating-sample magnetometry (VSM), and Brunauer-Emmett-Teller (BET) analyses. Moreover, a magnetically chiral nanocatalyst shows the asymmetric CO₂ fixation reaction under solvent-free conditions at 80 °C and in ethanol under reflux conditions with up to 99 and 98% ee, respectively. Furthermore, the reaction mechanism was illustrated concerning the total energy of the reactant, intermediates and product, and the structural parameters were analyzed.