Synergistic catalysis between palladium-N-heterocyclic carbene and Lewis base in a metal-organic framework for efficient Knoevenagel condensation

• A 3D Zn-MOF was prepared using 1,3-bis(1,3-isophthalic acid)-1 H -imidazolium chloride. • Pd-NHC motif was grafted onto the Zn-MOF via post-synthetic modification, yielding Pd-NHC@Zn-MOF. • Pd-NHC@Zn-MOF achieved high yields in Knoevenagel condensations with record TON and TOF. • Synergy between Pd-NHC and uncoordinated carboxylate-O is crucial for promoting the condensation. In this study, the synergistic catalysis for Knoevenagel condensation between palladium-N-heterocyclic carbene (Pd-NHC) sites and the uncoordinated oxygen atoms of the carboxylate groups as Lewis base sites in a three dimensional metal-organic framework (MOF), namely n[NH 2 (CH 3 ) 2 ]·[Zn(L)·2H 2 O] n [Zn-MOF, H 4 L + Cl – = 1,3-bis(1,3-isophthalic acid)-1 H imidazolium chloride], was presented. The Pd-NHC motif was successfully embedded onto the Zn-MOF by reacting with PdCl 2 to yield the Pd-NHC@Zn-MOF, which were thoroughly characterized using a range of analytical methods. The Pd-NHC@Zn-MOF exhibited high activity and reusability in the Knoevenagel condensation reaction under mild reaction conditions, yielding 85-95% of the desired products at ppm-level Pd loadings, with the highest turnover number (TON) of 394,620 and turnover frequency (TOF) of 641,575 h –1 reported so far for this reaction.