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}n2n– 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}n2n– 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.