Three-Dimensional Co(II)-Metal–Organic Frameworks with Varying Porosities and Open Metal Sites toward Multipurpose Heterogeneous Catalysis under Mild Conditions

In recent years, heterogeneous catalysis has become one of the most active domains in the research of metal–organic frameworks (MOFs). Here, two three-dimensional (3D) Co­(II)-MOFs with open metal sites and exposed azo functionality on the MOF backbone have been constructed via mixed ligand assembly. Both the MOFs, 1 {[Co₂(1,4-NDC)₂(L)­(H₂O)₂(μ₂-H₂O)]·(DMF)₂(H₂O)}_n and 2 {[Co­(fma)­(L)­(H₂O)₂]·S}_n [1,4-NDC = 1,4-naphthalene dicarboxylic acid, fma = fumaric acid, L = 3,3′-azobis pyridine and S = disordered solvents] exhibit 3D frameworks with metal-bound aqua ligands. These metal-bound aqua ligands, as well as the lattice solvent molecules, could simply be removed upon activation affording the desolvated frameworks 1a and 2a respectively maintaining the original crystallinity with a varying number of open metal sites. Although crystallographic analysis revealed a porous structure for both the MOFs (34.4% and 14.3% void volume for 1 and 2, respectively), 1 showed a permanently microporous nature with a Brunauer–Emmett–Teller surface area of 197 m² g^–1 and moderate CO₂ uptake capacity as established through a gas sorption study. Both MOFs exhibit efficient catalytic activity for the chemical fixation of CO₂ to cyclic carbonate in the presence of a cocatalyst, cyanosilylation reaction, and Knoevenagel condensation under solvent-free and mild conditions and thus demonstrating their multipurpose heterogeneous catalytic nature. The limited pore space decorated with the exposed metal sites and the functional azo groups were efficient for size-selective heterogeneous catalysis with varying catalytic efficiencies. A systematic comparison in their catalytic performances could be made with the establishment of a structure–function relationship. Besides, both MOFs can easily be separated out from the reaction mixtures and reused for at least four cycles without any loss of catalytic activity and structural integrity.