Development of Magnesium Oxide-Silver Hybrid Nanocatalysts for Synergistic Carbon Dioxide Activation to Afford Esters and Heterocycles at Ambient Pressure.

Multi-metallic hybrid nancatalysts consisting of a porous metal oxide host and metal satellite guests serve as a scaffold for multi-step transformations of divergent and energy-challenging substrates. Here we have developed a 3D porous MgO framework (Lewis basic host) with Ag nanoparticles (noble metal guest) for ambient pressure activation and insertion of CO into unsaturated alkyne substrates. The hybrid MgO@Ag-x (x = 2, 5, 7, 8 at % Ag) catalysts are synthesized by impregnating Ag ions in porous MgO cubes followed by reduction using NaBH. Morphological (SEM, TEM, EDX mapping) and structural (PXRD, XPS) characterization reveal that the micron-sized hybrid cubes derive from self-assembly of ~100 nm (edge length) MgO cubes decorated with ~ 5 to 25 nm Ag NPs. Detailed XPS analysis illustrates Ag is present in two forms, <10 nm NPs and ~25 nm aggregates. The MgO@Ag-7 catalyst is effective for inserting CO into aryl alkynes followed by S2 coupling with allylic chlorides to afford a wide range of ester and lactone heterocycles in excellent yields (61-93%) and with low E-factor (2.8). The proposed mechanism suggests a CO capture and substrate assembly role for 3D porous MgO while Ag performs the key activation of alkyne and CO insertion steps. The catalyst is recyclable (5x) with no significant loss of product yield. Overall, these results demonstrate viable approaches to hybrid catalyst development for challenging conversions such as CO utilization in a green and sustainable manner.