Catalytic performance of lanthanide molecular solids containing well-modified metal–organic octahedra

Abstract

Channel-like and cage-like porous molecular materials Tb–PT1 and Sm–PT1 were constructed through packing and stacking of the amide-containing lanthanide-based octahedra [Tb₆(H₃L)₄ (NO₃)₉-3H⁺]⁶⁺ and [Sm₆(H₃L)₄(NO₃)₁₂]⁶⁺, respectively, where H₃L is N′,N′,N′′-tris(pyridin-2-ylmethylene)benzene-1,3,5-tricarbohydrazide. Catalytic experiments on cyanosilylation reactions exhibited that the loading of only 1 mol% of Tb–PT1 leads to >90% conversion of the products with all the three nitrobenzaldehydes (NBA), but less than 10% conversion in the case of large size 3-formyl-1-phenylene-3 and 5-di-tert-butylbenzoate (BA) substrates. Adsorption of the substrates in the catalyst demonstrated a 3 : 1 mole ratio (per octahedron) of these nitrobenzaldehydes, but no adsorption of BA could be observed. Luminescent detection of the suspension of Tb–PT1 in CH₂Cl₂ solution exhibited that the presence of nitrobenzaldehydes quenched the emissions efficiently, whereas the addition of BA did not cause any significant spectra variations. All these results revealed that the catalytic performance of Tb–PT1 was size-selective and the interactions corresponding to the Tb³⁺ sites were dominant in activating the aldehydes. The catalytic experiments on aldol reactions between cyclohexanone and these aldehydes exhibited that the smallest 4-NBA substrate has the largest conversion (80%) over the others. The presence of cyclohexanone led to absorption spectral changes and luminescence enhancements of the catalyst. These results suggested that interactions between cyclohexanone and the amide groups were dominant in activating the cyclohexanone and the aldol reactions possibly occurred within the cavities of the octahedron. Evaluation of catalytic performance of Sm–PT1 in which those channels in Tb–PT1 were blocked upon cyanosilylation and aldol reactions as well as the adsorption experiments were also carried out for a comparison.