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)]{V2O6}(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 {V2O6}(n)(2n-) 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 {V2O6}(n)(2n-) polyanion as a Lewis base catalyst can activate methylene or cyanating reagents through the nucleophilic reaction. In addition, the connection mode of {V2O5}-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.

Automated summary

The zinc-organic framework V-Zn-MOF combines rich Lewis acid and base sites, efficiently catalyzing Knoevenagel condensation and cyanosilylation of aromatic aldehydes. The synergetic effect of open zinc sites as Lewis acid and {V2O6}(n)(2n-) polyanion as Lewis base contributes to the selective catalytic activity. The connection mode of {V2O5}-O-Zn enhances efficiency by reducing the molecular size distance between substrates.