Electronic Activation versus Steric Protection of a Rh(I) Pincer Complex inside a Supramolecular Metallobox.

We report the formation of a highly stable host-guest assembly between a nanosized iridium(I)-based metallobox and the square-planar pincer complex [RhI(CNC)(CO)]+ (with CNC being a planar pyridine-centered di-N-heterocyclic carbene pincer ligand). NMR and UV-vis studies reveal an exceptionally high binding affinity (Ka = 2.5 × 106 M-1) in methylene chloride. Infrared spectroscopy shows that encapsulation within the electron-rich cavity increases the electron density at the rhodium center, as evidenced by a lowering of the CO stretching frequency. Despite this electronic enrichment, the confined Rh(I) complex becomes remarkably resistant toward oxidative addition with MeI, demonstrating the strong protective effect exerted by the host cavity. These findings highlight how supramolecular confinement can simultaneously tune the electronic structure and reactivity of transition-metal complexes, offering new perspectives for the design of confined catalytic systems.