Functionalized melamine-based Dendron-OMS hybrids as highly-efficient catalysts for the nitroaldol (Henry) reaction

Controllable synthesis of organic–inorganic hybrid materials with functional sites and tunable structures is of great significance. Herein, novel melamine-based G_n-Dendron-OMS (G represents Generation; n = 1, 2, 3; Dendron = DMEDA, PIP, AMP, TMDP; OMS = FDU-12, MCM-41, SBA-15, KIT-6) hybrids with various terminal diamine groups, Dendron generations and ordered mesoporous silica hosts were synthesized, characterized and used to catalyze the nitroaldol (Henry) reaction. Among these hybrids, G₁-DMEDA-FDU-12 shows outstanding catalytic performance with 4-nitrobenzaldehyde conversion of 96 %, β-nitroalcohol selectivity of 94 % and TOF of 20.4 h⁻¹ under mild and solvent-free conditions. HMDS and HCl intervening active-site deactivation, in situ Raman spectra, Mulliken charge distribution and DFT calculations suggest that the triazine N atom (N_T) in the Dendron chain functions as the active site, the dehydrogenation of nitromethane is the rate determining step (RDS) and the Gibbs free energy change of the RDS on the triazine N atom decreased to as low as 30.2 kcal mol⁻¹. The formation of the deuterated β-nitroalcohol (R-OD) detected by FTIR and NMR indicates that nitroaldol (Henry) reaction undergoes an ion-pair mechanism. This work for the first time reveals the triazine-based active site for the nitroaldol (Henry) reaction and highlights a novel strategy to construct uniform, functional and tunable active sites for potential applications in catalysis, CO₂ capture and pollutant removal.