How Does CO₂ React with Styrene Oxide in Co-MOF-74 and Mg-MOF-74? Catalytic Mechanisms Proposed by QM/MM Calculations

Abstract

We report a QM/MM computational study of the cycloaddition between carbon dioxide (CO2) and styrene oxide in two different metal–organic frameworks (MOFs), Co-MOF-74 and Mg-MOF-74, to obtain atomic-level insights into the catalytic mechanism. The results suggest that both reactions begin by forming Lewis acid–base pairs between the epoxide and an open metal site and between CO2 and a phenolate moiety in the linker. Consequently, higher electrophilic and nucleophilic reactivities are conferred on the epoxide and CO2 (CO2(A)), respectively, thereby facilitating the initial ring-opening of the epoxide moiety. The ring-opening process is followed by the adsorption of a second CO2 molecule (CO2(B)), which is necessary for the subsequent ring closure to occur. In the case of Co-MOF-74, the binding of CO2(B) to the alkoxide oxygen increases the flexibility of the substrate moiety, enabling the cyclization pathway in which CO2(A) is incorporated into the final product. In contrast, the carbonate intermediate in the Mg-MOF-74-catalyzed reaction undergoes an intramolecular nucleophilic attack to form a 5-membered cyclic product. During this step, CO2(A) behaves essentially as a cocatalyst and is released back into the framework upon product formation. Our QM/MM results also suggest that the Lewis acid site has somewhat different coordination geometries in Co-MOF-74 and Mg-MOF-74 during the final ring-closure step.