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 gem-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.