Supramolecular Preorganization Rhodium and Iridium Metal Complexes Within M12L24 Self-Assembled Nanospheres for the Confined Synthesis Rh/Ir Alloyed Nanoparticles

The cage-templated formation of iridium-rhodium alloyed nanoparticles is reported. It is based on molecular complexes with hydrogen bonding units complementary to those of the inner cage, leading to controlled nanoparticle formation under reductive conditions. The activity and selectivity of the nanoparticles in reductive catalysis depends on the ratio.

Controlling the size and composition of metal nanoparticles is of considerable interest, as these are essential to their catalytic properties. Recently, our group has developed a preorganization strategy for controlled Ir nanoparticle synthesis inside Pt₁₂L₂₄nanospheres. In the current contribution, we expand this method to the controlled synthesis of Rh nanoparticles. The encapsulated Rh^I complexes (Rh-s @ G-sphere) led to reasonable size control (2.8 ± 0.9 nm). Next, we demonstrated the formation of Rh-Ir alloyed nanoparticles with varying Rh/Ir compositions, by preorganization of the respective metal complexes inside Pt₁₂L₂₄ nanospheres based on complementary hydrogen bonds before the reduction step that leads to nanoparticle formation. These heterometallic particles were evaluated in the hydrogenation of cinnamaldehyde (7) as a probe reaction. Besides a high activity in this probe reaction, the Rh particles also catalyzed the conversion of the solvent (CH₃CN). The formed basic amine leads to follow-up reactions of the product and compatibility issues with the hosting nanosphere. The solvent hydrogenation was effectively suppressed by using the Rh:Ir alloyed nanoparticles, provided that they contain > 66% Ir. Compared to the monometallic Ir particles (Ir-s @ G-sphere), the Rh:Ir alloyed nanoparticles displayed higher catalytic activity, reaching optimal selectivity and activity at an 8:16–Rh:Ir ratio. The combined catalytic results illustrate that preorganization of the metal complexes in the nanosphere before the reduction with hydrogen effectively facilitates the formation of Rh:Ir alloyed nanoparticles, which allows for tuning a catalyst to create a more active and selective catalyst compared to monometallic or nonencapsulated Rh/Ir particles.