Domination of local environment over pore confinement effects on the catalytic performances of single-site Cp*Ir^III-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 [Cp*IrCl ₂]₂ was carried out successfully. Each step was controlled fully and evaluated by several analytical techniques (XRD, N₂ 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. Highly active and single-site Ir-NHC-containing materials have been synthesized and characterized. Their catalytic performances in C-H bond activation of several substrates have been studied, and it has been shown that the local confinementaround the metal centre (size of the NHC ligand, proximity to the surface and size of the passivating groups), and not the pore size, has a major effect.