Functionalization of Magnetic UiO-66-NH<sub>2</sub> with a Chiral Cu(<scp>l</scp>-proline)<sub>2</sub> Complex as a Hybrid Asymmetric Catalyst for CO<sub>2</sub> Conversion into Cyclic Carbonates
Sobhan Rezayati, Ali Morsali
Abstract
In this study, a chiral [Cu( l -proline) 2 ] complex-modified Fe 3 O 4 @SiO 2 @UiO-66-NH 2 (Zr) metal–organic framework [Fe 3 O 4 @SiO 2 @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, Fe 3 O 4 @SiO 2 @UiO-66-NH 2 has been synthesized via Fe 3 O 4 modification with tetraethyl orthosilicate and subsequently with ZrCl 4 and 2-aminoterephthalic acid. The presence of the silica layer helps to stabilize the Fe 3 O 4 core, while the bonding between Zr 4+ and the –OH groups in the silica layer promotes the development of Zr-MOFs on the Fe 3 O 4 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 CO 2 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.