Polyoxometalate-based homochiral metal-organic frameworks for tandem asymmetric transformation of cyclic carbonates from olefins

Abstract

Currently, great interest is focused on developing auto-tandem catalytic reactions; a substrate is catalytically transferred through mechanistically distinct reactions without altering any reaction conditions. Here by incorporating a pyrrolidine moiety as a chiral organocatalyst and a polyoxometalate as an oxidation catalyst, a powerful approach is devised to achieve a tandem catalyst for the efficient conversion of CO2 into value-added enantiomerically pure cyclic carbonates. The multi-catalytic sites are orderly distributed and spatially matched in the framework. The captured CO2 molecules are synergistically fixed and activated by well-positioned pyrrolidine and amine groups, providing further compatibility with the terminal W=O activated epoxidation intermediate and driving the tandem catalytic process in a single workup stage and an asymmetric fashion. The structural simplicity of the building blocks and the use of inexpensive and readily available chemical reagents render this approach highly promising for the development of practical homochiral materials for CO2 conversion.

Figure 1. The structure and characterization of ZnW−PYIs.

(a) Circular dichroism spectra of bulk crystals of ZnW−PYI1 (black) and ZnW−PYI2 (red), respectively. (b) Plot of the one-dimensional connections between the bipyridine ligands and the zinc centres, showing the coordination mode of the zinc centres. (c) Perspective view of the two-dimensional sheet connected by the polyoxometalate clusters and the zinc centres, the bipyridine ligands were omitted for clarity. (d) 3D open network of ZnW−PYI1 viewed down the b axis. H atoms and solvent molecules are omitted for clarity. Carbon, nitrogen and zinc are drawn in gray (orange for PYI), blue, and cyan, respectively, with ZnW₁₂O₄₀ ⁶⁻ shown as polyhedra.

Figure 2. The design concept of achieving a tandem catalyst.

Synthetic procedure of the metal-organic framework, showing the constitutive/constructional fragments of the MOF; and the schematic representation of tandem catalysis for the asymmetric cyclic carbonate transformation from olefins and carbon dioxide.

Figure 3. The spectroscopic studies of ZnW−PYI1.

(a) Infrared (IR) spectra of fresh and TBHP-oxidized ZnW−PYI1. (b) IR spectra of fresh and cinnamaldehyde-incorporated ZnW−PYI1. (c,d) IR spectra and Raman spectra of fresh (dark line) and CO₂-adsorbed ZnW−PYI1 (blue line), respectively.

Figure 4. The spatial relationship of multiple catalytic sites and synergetic catalysis.

(a) Schematic diagram of the catalytic site distribution in ZnW−PYI1, showing the interactions of the catalytic sites. (b) Diagram of potential mechanism for the auto-tandem catalysis.