Imine bond-directed assembly of polyoxometalate-based metal-organic frameworks

Polyoxometalate-based metal-organic frameworks (POMOFs) are highly effective heterogeneous catalysts that combine the catalytic activity of polyoxometalates (POMs) with the high surface area, tunable porosity, and structural diversity of MOFs. Nevertheless, there is still a lack of a general method to integrate POMs with various transition metal-based building units into POMOFs under mild conditions. In this work, we employed imine bonds to link amino-functionalized Anderson-type POMs with aldehyde-terminated divalent metal clusters, resulting in a series of isostructural POMOFs, M(II)-POMOFs (M = Zn, Co, Mg, or Mn). Furthermore, we used post-synthetic metal exchange and oxidation to transform Zn-POMOF into Fe(III)-POMOF with strong Lewis acidic Fe³⁺ sites. Notably, both the synthesis and post-synthetic modifications were performed under mild conditions (room temperature, acid-free), preventing the decomposition of the POMs. Compared to M(II)-POMOFs or MOFs without POMs, the combination of Lewis acidic Fe³⁺ and POMs enhanced its catalytic activity for CO₂ cycloaddition with epoxides, enabling efficient synthesis of cyclic carbonates. This versatile synthetic method could broaden the scope of POMOFs, extending their applications in catalysis and beyond.