Rhodium Carbonyl Complexes within the Tunable Microenvironments of UiO-66 Analogues

Herein, UiO-66 variants with varying microenvironments were prepared to investigate their influence on the chemistry of Rh carbonyl complexes bound to vacant MOF node sites. The complexity observed in Rh carbonyl-supported species across different UiO-66 variants prompted detailed structural characterization, including defect site analysis. Metal loading was found to be governed by three main factors: the defect content, type of metal node, and nature of functional groups on the linkers. We identified monomeric <i>gem</i>-Rh(CO)₂ and dimeric [Rh₂(μ-Cl)₂(CO)₄] species supported on the node as the main entities with varying concentrations of both species and slight shifts in CO stretches in different UiO-66 variants, indicating subtle changes in their electronic environments. The dimeric Rh species was found to be sensitive to moisture, and in UiO-66-NH₂ and UiO-66-(OH)₂, additional Rh carbonyl species were observed, with no dimeric Rh species being observed when the amine-functionalized support was used. Rh-loaded Ce-UiO-66, UiO-66, and UiO-66-NH₂ were exposed to syngas at 100 °C for 3 h. While no significant changes were observed for others, Rh bound within Ce-UiO-66 underwent a change in speciation, with a Rh₆(CO)₁₆ cluster becoming the dominant species. These studies demonstrated significant complexity in using UiO-66 topology MOFs as "privileged ligands" for supporting potential heterogeneous catalysts.