Solvent-free heterogeneous catalysis for cyanosilylation in a modified sodalite-type Cu(ii)-MOF

Abstract

Solvothermal reaction of Cu(OAc)₂ and H₃BTT·2HCl generated a sodalite-type metal–organic framework [(Cu₄O_0.27Cl_0.73)₃(H_0.5BTT)₈(H₂O)₁₂]·3MeOH·9DMF (1) (BTT³⁻ = 1,3,5-benzene tristetrazolate), which features a porous 3D (3,8)-connected framework constructed by square [Cu₄(μ₄-O/Cl)] units and triangular BTT ligands and can be dehydrated to form [(Cu₄O_0.27Cl_0.73)₃(H_0.5BTT)₈] (1′) with coordinatively unsaturated Cu²⁺ centers. Compared with related M–BTT MOFs, partial replacement of μ₄-Cl with μ₄-O at the square Cu₄ cluster and the absence of irremovable guest [M(DMF)₆]²⁺ cations in 1′ enhance the Lewis acidity of coordinatively unsaturated metal centers and the effective pore volume. Open Cu(II) sites with stronger Lewis acidity give rise to 1′ being an active, stable and reusable solvent-free heterogeneous catalyst for C–C bond-forming reaction by cyanosilylation of carbonyl compounds. The loading of 1 mol% of catalyst 1′ is as low as one eleventh of that used in related Mn–BTT but leads to as high as 96% conversion of benzaldehyde, indicating that the catalytic activity of M–BTT MOFs was significantly improved via post-modification. In addition, the larger pore volume makes 1′ selectively adsorb N₂ and O₂ gases with hysteresis loops over CO₂ and H₂ gases without hysteresis loops, which did not appear in related M–BTT MOFs.