Domination of Local Environment Over Pore Confinement Effects on the Catalytic Performances of Single-Site Cp*IrIII-NHC Heterogeneous vs. Homogeneous H/D Exchange Catalysts

Three different tailored mesostructured hybrid organic-inorganic materials containing single site Ir-NHC moieties (NHC = N-heterocyclic carbene) were synthesized by a very convenient and attractive methodology: (i) the synthesis of mesostructured materials with different pore-sizes and containing two types of imidazolium units regularly distributed along their channel pores was achieved using a sol-gel process via a templating route; (ii) the in situ transformation of the imidazolium moieties to Ag-NHC units was quantitatively performed; (iii) the transmetallation of Ag-NHC groups with [(CpIrCl2)-Ir-center dot](2) was carried out successfully. Each step was controlled fully and evaluated by several analytical techniques (XRD, N-2 adsorption, elemental analysis, and solid state NMR). The catalytic properties of the resulting Ir-NHC-containing materials were studied in H/D-exchange reactions with various substrates, and compared to their homogeneous homologues. Their catalytic activity highlighted the following: (i) the substituents on the NHC ring have an important role; (ii) the size of the pores have no influence on the catalytic reactivity; (iii) the close environment around the metal center has a major influence on the catalytic reaction. This latter point suggests that the selective C-H bond activation by mesoporous catalysts with a mixture of reagents can be performed by fine tuning the steric hindrance around the metal centers, while keeping big pores for easy of diffusion of substrate and product molecules.