Highly Stable Polyoxovanadate-Based Zn–MOF with Dual Active Sites as a Solvent-free Catalyst for C–C Bond Formation

A polyoxovanadate-based zinc–organic framework, [Zn(bix)]{V 2 O 6 }(V-Zn-MOF, bix = 1,4-bis(imidazole-1-ylmethyl)benzene), was successfully synthesized under hydrothermal conditions. Single-crystal X-ray analysis reveals that the V-Zn-MOF contains {V 2 O 6 } n 2 n – polyanion chains and [Zn(bix)] 2+ cation frameworks with open zinc metal sites, which endows V-Zn-MOF-rich Lewis base and acid sites. Considering that Lewis acid and base can usually be utilized as active centers for catalytic reactions, we explore the catalytic activity of the V-Zn-MOF for Knoevenagel condensation and cyanosilylation of aromatic aldehydes. The experimental results indicate that the V-Zn-MOF can efficiently and selectively catalyze both reactions of the Knoevenagel condensation and cyanosilylation of aromatic aldehydes under solvent-free conditions, which is attributed to the two-site synergetic effect: (1) the open zinc site as a Lewis acid catalyst can electrophilically activate the carbonyl group of aldehydes; (2) the {V 2 O 6 } n 2 n – polyanion as a Lewis base catalyst can activate methylene or cyanating reagents through the nucleophilic reaction. In addition, the connection mode of {V 2 O 5 }–O–Zn closes the molecular size distance between the two activated substrates, thereby improving efficiency of the catalytic reaction. More importantly, the V-Zn-MOF exhibits excellent sustainability, no obvious decrease in the catalytic activity is observed in both catalytic reactions after 10 cycles. These results manifest that the V-Zn-MOF has great potential for practical application in C–C bond formation.