Fabricating Multivariate Metal–Organic Frameworks by Metal Site Reduction as Green Catalyst for CO₂ Conversion

Multivariate metal–organic frameworks (MTV-MOFs) have the potential to incorporate multiple metal active sites into a single framework, and the synergistic effect between the constituent metals endow porous materials with significant properties for specifically targeted applications. In this work, a novel reduction engineering strategy is described to extend the system of MTV-MOFs, producing multimetallic MTV-MOFs with a more diverse and desired combination of metal elements. This strategy is based on precisely modulating the valence states of specific metal nodes within bimetallic MOFs to accomplish the synthesis of multimetallic MTV-MOFs with the incorporation of more types metal combinations previously inaccessible with traditional one-pot methods. The resulting multimetallic MTV-MOF, CuICuIIZn-DOBDC, which features ternary active sites of CuI, CuII, and ZnII ions, as well as a high density of uncoordinated hydroxyl groups, constructs a noble-metal free heterogeneous catalyst with multiple synergistic catalytic effects. Notably, CuICuIIZn-DOBDC can effectively catalyze the green cyclization of propargylic amines with CO2 under room temperature and atmospheric pressure, without the need for co-catalysts or solvents. Most importantly, this work provides new ideas for extending the scope and tunability of multimetallic MTV-MOFs to incorporate more types of metal ion combinations, while also providing a new pathway for green conversion of CO2.